evolution

The theory of evolution is one of the best-known scientific theories around. Try to make it through a day without using or hearing the word "evolution" and you'll see just how widespread this theory is.

Evolution is fascinating because it attempts to answer one of the most basic human questions: Where did life, and human beings, come from? The theory of evolution proposes that life and humans arose through a natural process. A very large number of people do not believe this, which is something that keeps evolution in the news.

In this article, we will explore the theory of evolution and how it works. We will also examine several important areas that show holes in the current theory -- places where scientific research will be working in the coming years in order to complete the theory. The holes are considered by many to be proof that the theory of evolution should be overthrown. As a result, quite a bit of controversy has surrounded evolution ever since it was first proposed.

Let's start off by taking a look at the basic principles of the theory of evolution, look at some examples and then examine the holes.

The Basic Process of Evolution

The basic theory of evolution is surprisingly simple. It has three essential parts:

• It is possible for the DNA of an organism to occasionally change, or mutate. A mutation changes the DNA of an organism in a way that affects its offspring, either immediately or several generations down the line.

• The change brought about by a mutation is either beneficial, harmful or neutral. If the change is harmful, then it is unlikely that the offspring will survive to reproduce, so the mutation dies out and goes nowhere. If the change is beneficial, then it is likely that the offspring will do better than other offspring and so will reproduce more. Through reproduction, the beneficial mutation spreads. The process of culling bad mutations and spreading good mutations is called natural selection.

• As mutations occur and spread over long periods of time, they cause new species to form. Over the course of many millions of years, the processes of mutation and natural selection have created every species of life that we see in the world today, from the simplest bacteria to humans and everything in between.

Billions of years ago, according to the theory of evolution, chemicals randomly organized themselves into a self-replicating molecule. This spark of life was the seed of every living thing we see today (as well as those we no longer see, like dinosaurs). That simplest life form, through the processes of mutation and natural selection, has been shaped into every living species on the planet.

Can such a simple theory explain all of life as we know it today? Let's start by understanding how life works and then look at some examples.

How Life Works: DNA and Enzymes

Evolution can be seen in its purest form in the daily evolution of bacteria. If you have read How Cells Work, then you are familiar with the inner workings of the E. coli bacteria and can skip this section. Here's a quick summary to highlight the most important points in How Cells Work:

• A bacterium is a small, single-celled organism. In the case of E. coli, the bacteria are about one-hundredth the size of a typical human cell. You can think of the bacteria as a cell wall (think of the cell wall as a tiny plastic bag) filled with various proteins, enzymes and other molecules, plus a long strand of DNA, all floating in water.

• The DNA strand in E. coli contains about 4 million base pairs, and these base pairs are organized into about 1,000 genes. A gene is simply a template for a protein, and often these proteins are enzymes.

• An enzyme is a protein that speeds up a particular chemical reaction. For example, one of the 1,000 enzymes in an E. coli's DNA might know how to break a maltose molecule (a simple sugar) into its two glucose molecules. That is all that that particular enzyme can do, but that action is important when an E. coli is eating maltose. Once the maltose is broken into glucose, other enzymes act on the glucose molecules to turn them into energy for the cell to use.

• To make an enzyme that it needs, the chemical mechanisms inside an E. coli cell make a copy of a gene from the DNA strand and use this template to form the enzyme. The E. coli might have thousands of copies of some enzymes floating around inside it, and only a few copies of others. The collection of 1,000 or so different types of enzymes floating in the cell makes all of the cell's chemistry possible. This chemistry makes the cell "alive" -- it allows the E. coli to sense food, move around, eat and reproduce. See How Cells Work for more details.

You can see that, in any living cell, DNA helps create enzymes, and enzymes create the chemical reactions that are "life."

In the next section, we'll discuss how bacteria reproduce.

How Life Works: Asexual Reproduction

Bacteria reproduce asexually. This means that, when a bacteria cell splits, both halves of the split are identical -- they contain exactly the same DNA. The offspring is a clone of the parent.

As explained in How Human Reproduction Works, higher organisms like plants, insects and animals reproduce sexually, and this process makes the actions of evolution more interesting. Sexual reproduction can create a tremendous amount of variation within a species. For example, if two parents have multiple children, all of the children can be remarkably different. Two brothers can have different hair color, different heights, different blood types and so on. Here's why that happens:

• Instead of a long loop of DNA like a bacterium, cells of plants and animals have chromosomes that hold the DNA strands. Humans have 23 pairs of chromosomes, for a total of 46 chromosomes. Fruit flies have five pairs. Dogs have 39 pairs, and some plants have as many as 100.

  The human chromosomes hold the DNA of the human genome. Each parent contributes 23 chromosomes.

• Chromosomes come in pairs. Each chromosome is a tightly packed strand of DNA. There are two strands of DNA joined together at the centromere to form an X-shaped structure. One strand comes from the mother and one from the father.
• Because there are two strands of DNA, it means that animals have two copies of every gene, rather than one copy as in an E. coli cell.

  Photo courtesy U.S. DOE, Human Genome Project

• When a female creates an egg or a male creates a sperm, the two strands of DNA must combine into a single strand. The sperm and egg from the mother and father each contribute one copy of each chromosome. They meet to give the new child two copies of each gene.
• To form the single strand in the sperm or egg, one or the other copy of each gene is randomly chosen. One or the other gene from the pair of genes in each chromosome gets passed on to the child.

Because of the random nature of gene selection, each child gets a different mix of genes from the DNA of the mother and father. This is why children from the same parents can have so many differences.

A gene is nothing but a template for creating an enzyme. This means that, in any plant or animal, there are actually two templates for every enzyme. In some cases, the two templates are the same (homozygous), but in many cases the two templates are different (heterozygous).

Here is a well-known example from pea plants that helps understand how pairs of genes can interact. Peas can be tall or short. The difference comes, according to Carol Deppe in the book "Breed your own Vegetable Varieties":

...in the synthesis of a plant hormone called gibberellin. The "tall" version of the gene is normally the form that is found in the wild. The "short" version, in many cases, has a less active form of one of the enzymes involved in the synthesis of the hormone, so the plants are shorter. We refer to two genes as alleles of each other when they are inherited as alternatives to each other. In molecular terms, alleles are different forms of the same gene. There can be more than two alleles of a gene in a population of organisms. But any given organism has only two alleles at the most. Shorter plants usually cannot compete with the taller forms in the wild. A short mutant in a patch of tall plants would be shaded out. That problem isn't relevant when a human plants a patch or field with nothing but short plants. And short plants may be earlier than tall ones, or less subject to lodging (falling over) in the rain or wind. They also may have a higher proportion of grain to the rest of the plant. So shorter plants can be advantageous as cultivated crops. Specific mutations or alleles are not good or bad in and of themselves, but only within a certain context. An allele that promotes better growth in hot weather may promote inferior growth in cold weather, for example.

One thing to notice in Deppe's quote is that a mutation in a single gene may have no effect on an organism, or its offspring, or its offspring's offspring. For example, imagine an animal that has two identical copies of a gene in one allele. A mutation changes one of the two genes in a harmful way. Assume that a child receives this mutant gene from the father. The mother contributes a normal gene, so it may have no effect on the child (as in the case of the "short" pea gene). The mutant gene might persist through many generations and never be noticed until, at some point, both parents of a child contribute a copy of the mutant gene. At that point, taking the example from Deppe's quote, you might get a short pea plant because the plant does not form the normal amount of gibberellin.

Another thing to notice is that many different forms of a gene can be floating around in a species. The combination of all of the versions of all of the genes in a species is called the gene pool of the species. The gene pool increases when a mutation changes a gene and the mutation survives. The gene pool decreases when a gene dies out.

One of the simplest examples of evolution can be witnessed in an E. coli cell. To get a better grip on the process, we'll take a look at what happens in this cell.

The Simplest Example of Evolution

The process of evolution acts on an E. coli cell by creating a mutation in the DNA. It is not uncommon for the DNA strand in an E. coli bacterium to get corrupted. An X-ray, a cosmic ray or a stray chemical reaction can change or damage the DNA strand. In most cases, a particular E. coli cell with mutated DNA will either die, fix the damage in the strand or fail to reproduce. In other words, most mutations go nowhere. But every so often, a mutation will actually survive and the cell will reproduce.

Imagine, for example, a bunch of identical E. coli cells that are living in a petri dish. With plenty of food and the right temperature, they can double every 20 minutes. That is, each E. coli cell can duplicate its DNA strand and split into two new cells in 20 minutes.

Now, imagine that someone pours an antibiotic into the petri dish. Many antibiotics kill bacteria by gumming up one of the enzymes that the bacteria needs to live. For example, one common antibiotic gums up the enzyme process that builds the cell wall. Without the ability to add to the cell wall, the bacteria cannot reproduce, and eventually they die.

When the antibiotic enters the dish, all of the bacteria should die. But imagine that, among the many millions of bacteria living in the dish, one of them acquires a mutation that makes its cell-wall-building enzyme different from the norm. Because of the difference, the antibiotic molecule does not attach properly to the enzyme, and therefore does not affect it. That one E. coli cell will survive, and since all of its neighbors are dead, it can reproduce and take over the petri dish. There is now a strain of E. coli that is immune to that particular antibiotic.

In this example, you can see evolution at work. A random DNA mutation created an E. coli cell that is unique. The cell is unaffected by the antibiotic that kills all of its neighbors. This unique cell, in the environment of that petri dish, is able to survive.

E. coli are about as simple as living organisms can get, and because they reproduce so rapidly you can actually see evolution's effects on a normal time scale. In the past several decades, many different types of bacteria have become immune to antibiotics. In a similar way, insects become immune to insecticides because they breed so quickly. For example, DDT-resistant mosquitoes evolved from normal mosquitoes.

In most cases, evolution is a much slower process.

The Speed of Mutations

As mentioned in the previous section, many things can cause a DNA mutation, including:

• X-rays
• Cosmic rays
• Nuclear radiation
• Random chemical reactions in the cell

Therefore, mutations are fairly common. Mutations happen at a steady rate in any population, but the location and type of every mutation is completely random. According to Carl Sagan in "The Dragons of Eden":

Large organisms such as human beings average about one mutation per ten gametes [a gamete is a sex cell, either sperm or egg] -- that is, there is a 10 percent chance that any given sperm or egg cell produced will have a new and inheritable change in the genetic instructions that make up the next generation. These mutations occur at random and are almost uniformly harmful -- it is rare that a precision machine is improved by a random change in the instructions for making it.

According to "Molecular Biology of the Cell":

Only about one nucleotide pair in a thousand is randomly changed every 200,000 years. Even so, in a population of 10,000 individuals, every possible nucleotide substitution will have been "tried out" on about 50 occasions in the course of a million years, which is a short span of time in relation to the evolution of species. Much of the variation created in this way will be disadvantageous to the organism and will be selected against in the population. When a rare variant sequence is advantageous, however, it will be rapidly propagated by natural selection. Consequently, it can be expected that in any given species the functions of most genes will have been optimized by random point mutation and selection.

According to the book "Evolution," by Ruth Moore, it is possible to speed up mutations with radiation:

So Muller put hundreds of fruit flies in gelatin capsules and bombarded them with X-rays. The irradiated flies were then bred to untreated ones. In 10 days thousands of their offspring were buzzing around their banana-mash feed, and Muller was looking upon an unprecedented outburst of man-made mutations. There were flies with bulging eyes, flat eyes, purple, yellow and brown eyes. Some had curly bristles, some no bristles...

Mutations fuel the process of evolution by providing new genes in the gene pool of a species.

Then, natural selection takes over.

Natural Selection

As you saw in the previous section, mutations are a random and constant process. As mutations occur, natural selection decides which mutations will live on and which ones will die out. If the mutation is harmful, the mutated organism has a much decreased chance of surviving and reproducing. If the mutation is beneficial, the mutated organism survives to reproduce, and the mutation gets passed on to its offspring. In this way, natural selection guides the evolutionary process to incorporate only the good mutations into the species, and expunge the bad mutations.

The book "Extinct Humans," by Ian Tattersall and Jeffrey Schwartz, puts it this way:

...in every generation, many more individuals are produced than ever survive to maturity and to reproduce themselves. Those that succeed -- the "fittest" -- carry heritable features that not only promote their own survival but are also passed along preferentially to their offspring. In this view, natural selection is no more than the sum of all those factors that act to promote the reproductive success of some individuals (and its lack in others). Add the dimension of time, and over the generations natural selection will act to change the complexion of each evolving lineage, as advantageous variations become common in the population at the expense of those less advantageous.

Let's look at an example of natural selection from How Whales Work.

The ancestors of whales lived on land -- there is evidence of the evolution of the whale from life on land to life in the sea (read How Whales Work for details), but how and why did this happen? The "why" is commonly attributed to the abundance of food in the sea. Basically, whales went where the food was. The "how" is a bit more perplexing: Whales are mammals, like humans are, and like humans, they lived and walked on solid ground, breathing air into their lungs. How did whales become sea creatures? One aspect of this evolution, according to Tom Harris, author of How Whales Work, is explained as follows:

To make this transition, whales had to overcome a number of obstacles. First of all, they had to contend with reduced access to breathable air. This led to a number of remarkable adaptations. The whale's "nose" moved from the face to the top of the head. This blowhole makes it easy for whales to breathe in air without fully surfacing. Instead, a whale swims near the surface, arches its body so its back briefly emerges and then flexes its tail, propelling it quickly to lower depths.

Photo courtesy Sea World Orlando

Odd as it seems that the whale's "nose" actually changed positions, the theory of evolution explains this phenomenon as a long process that occurs over perhaps millions of years:

• Random mutation resulted in at least one whale whose genetic information placed its "nose" farther back on its head.
• The whales with this mutation were more suited to the sea environment (where the food was) than "normal" whales, so they thrived and reproduced, passing on this genetic mutation to their offspring: Natural selection "chose" this trait as favorable.
• In successive generations, further mutations placed the nose farther back on the head because the whales with this mutation were more likely to reproduce and pass on their altered DNA. Eventually, the whale's nose reached the position we see today.

Natural selection selects those genetic mutations that make the organism most suited to its environment and therefore more likely to survive and reproduce. In this way, animals of the same species who end up in different environments can evolve in completely different ways.

Creating a New Species

Imagine that you take a group of Saint Bernards and put them on one island, and on another island you put a group of Chihuahuas. Saint Bernards and Chihuahuas are both members of the species "dog" right now -- a Saint Bernard can mate with a Chihuahua (probably through artificial insemination) and create normal puppies. They will be odd-looking puppies, but normal puppies nonetheless.

Given enough time, it is possible to see how speciation -- the development of a new species through evolution -- could occur among the Saint Bernards and the Chihuahuas on their respective islands. What would happen is that the Saint Bernard gene pool would acquire random mutations shared by all of the Saint Bernards on the island (through interbreeding), and the Chihuahuas would acquire a completely different set of random mutations shared by all of the Chihuahuas on their island. These two gene pools would eventually become incompatible with one another, to the point where the two breeds could no longer interbreed. At that point, you have two distinct species.

Because of the huge size difference between a Saint Bernard and a Chihuahua, it would be possible to put both types of dogs on the same island and have the exact same process occur. The Saint Bernards would naturally breed with only the Saint Bernards and the Chihuahuas would naturally breed with only the Chihuahuas, so speciation would still occur.

If you put two groups of Chihuahuas on two separate islands, the process would also occur. The two groups of Chihuahuas would accumulate different collections of mutations in their gene pools and eventually become different species that could not interbreed.

The theory of evolution proposes that the process that might create a separate Chihuahua-type species and Saint Bernard-type species is the same process that has created all of the species we see today. When a species gets split into two (or more) distinct subsets, for example by a mountain range, an ocean or a size difference, the subsets pick up different mutations, create different gene pools and eventually form distinct species.

Is this truly how all of the different species we see today have formed? Most people agree that bacteria evolve in small ways (microevolution), but there is some controversy around the idea of speciation (macroevolution). Let's take a look at where the controversy comes from.

Holes in the Theory

The theory of evolution is just that -- a theory. According to "The American Heritage Dictionary," a theory is:

A set of statements or principles devised to explain a group of facts or phenomena, especially one that has been repeatedly tested or is widely accepted and can be used to make predictions about natural phenomena.

Evolution is a set of principles that tries to explain how life, in all its various forms, appeared on Earth. The theory of evolution succeeds in explaining why we see bacteria and mosquitoes becoming resistant to antibiotics and insecticides. It also successfully predicted, for example, that X-ray exposure would lead to thousands of mutations in fruit flies.

Many theories are works in progress, and evolution is one of them. There are several big questions that the theory of evolution cannot answer right now. This is not unusual. Newtonian physics worked really well for hundreds of years, and it still works well today for many types of problems. However, it does not explain lots of things that were eventually answered by Einstein and his theories of relativity. People create new theories and modify existing ones to explain the unexplained.

In answering the open questions that still remain unsolved, the theory of evolution will either become complete or it will be replaced by a new theory that better explains the phenomena we see in nature. That is how the scientific process works.

Here are three common questions that are asked about the current theory of evolution:

• How does evolution add information to a genome to create progressively more complicated organisms?
• How is evolution able to bring about drastic changes so quickly?
• How could the first living cell arise spontaneously to get evolution started?

Let's look at each of these questions briefly in the following sections.

Question 1: How Does Evolution Add Information?

The theory of evolution explains how strands of DNA change. An X-ray, cosmic ray, chemical reaction or similar mechanism can modify a base pair in the DNA strand to create a mutation, and this modification can lead to the creation of a new protein or enzyme.

The theory of evolution further proposes that billions of these mutations created all of the life forms we see today. An initial self-replicating molecule spontaneously formed. It evolved into single-cell organisms. These evolved into multi-cell organisms, which evolved into vertebrates like fish, and so on. In the process, DNA structures evolved from the asexual single-strand format found in bacteria today into the dual-strand chromosomal format found in all higher life forms. The number of chromosomes also proliferated. For example, fruit flies have five chromosomes, mice have 20, humans have 23 and dogs have 39.

Evolution's mutation mechanism does not explain how growth of a genome is possible. How can point mutations create new chromosomes or lengthen a strand of DNA? It is interesting to note that, in all of the selective breeding in dogs, there has been no change to the basic dog genome. All breeds of dog can still mate with one another. People have not seen any increase in dog's DNA, but have simply selected different genes from the existing dog gene pool to create the different breeds.

One line of research in this area focuses on transposons, or transposable elements, also referred to as "jumping genes." A transposon is a gene that is able to move or copy itself from one chromosome to another. The book "Molecular Biology of the Cell" puts it this way:

Transposable elements have also contributed to genome diversity in another way. When two transposable elements that are recognized by the same site-specific recombination enzyme (transposase) integrate into neighboring chromosomal sites, the DNA between them can become subject to transposition by the transposase. Because this provides a particularly effective pathway for the duplication and movement of exons (exon shuffling), these elements can help create new genes.

Another area of research involves polyploidy. Through the process of polyploidy, the total number of chromosomes can double, or a single chromosome can duplicate itself. This process is fairly common in plants, and explains why some plants can have as many as 100 chromosomes.

The amount of research in this area is truly remarkable and is teaching scientists amazing things about DNA. The following links give you a taste of that research, and are interesting if you would like to learn more about these topics:

• Maize as a model for evolution
• Genomes Online Database
• Genome Evolution Search Engine Query
• Studies in Probabilistic Sequence Alignment and Evolution

you create a very large cage and put a group of mice into it. You let the mice live and breed in this cage freely, without disturbance. If you were to come back after five years and look into this cage, you would find mice. Five years of breeding would cause no change in the mice in that cage -- they would not evolve in any noticeable way. You could leave the cage alone for a hundred years and look in again and what you would find in the cage is mice. After several hundred years, you would look into the cage and find not 15 new species, but mice.

The point is that evolution in general is an extremely slow process. When two mice breed, the offspring is a mouse. When that offspring breeds, its offspring is a mouse. When that offspring breeds... And the process continues. Point mutations do not change this fact in any significant way over the short haul.

Carl Sagan, in "The Dragons of Eden," put it this way:

The time scale for evolutionary or genetic change is very long. A characteristic period for the emergence of one advanced species from another is perhaps a hundred thousand years; and very often the difference in behavior between closely related species -- say, lions and tigers -- does not seem very great. An example of recent evolution of organ systems in humans is our toes. The big toe plays an important function in balance while walking; the other toes have much less obvious utility. They are clearly evolved from fingerlike appendages for grasping and swinging, like those of arboreal apes and monkeys. This evolution constitutes a respecialization -- the adaptation of an organ system originally evolved for one function to another and quite different function -- which required about ten million years to emerge.

The fact that it takes evolution 100,000 or 10 million years to make relatively minor changes in existing structures shows just how slow evolution really is. The creation of a new species is time consuming.

On the other hand, we know that evolution can move extremely quickly to create a new species. One example of the speed of evolution involves the progress mammals have made. You have probably heard that, about 65 million years ago, all of the dinosaurs died out quite suddenly. One theory for this massive extinction is an asteroid strike. For dinosaurs, the day of the asteroid strike was a bad one, but for mammals it was a good day. The disappearance of the dinosaurs cleared the playing field of most predators. Mammals began to thrive and differentiate.

Example: The Evolution of Mammals
65 million years ago, mammals were much simpler than they are today. A representative mammal of the time was the species Didelphodon, a smallish, four-legged creature similar to today's opossum.

In 65 million years, according to the theory of evolution, every mammal that we see today (over 4,000 species) evolved from small, four-legged creatures like Didelphodon. Through random mutations and natural selection, evolution has produced mammals of striking diversity from that humble starting point:

• Humans
• Dogs
• Moles
• Bats
• Whales
• Elephants
• Giraffes
• Panda bears
• Horses

Evolution has created thousands of different species that range in size and shape from a small brown bat that weighs a few grams to a blue whale that is nearly 100 feet (30.5 m) long.

Let's take Carl Sagan's statement that "A characteristic period for the emergence of one advanced species from another is perhaps a hundred thousand years, and very often the difference in behavior between closely related species -- say, lions and tigers -- does not seem very great." In 65 million years, there are only 650 periods of 100,000 years -- that's 650 "ticks" of the evolutionary clock.

Imagine trying to start with an opossum and get to an elephant in 650 increments or less, even if every increment were perfect. An elephant's brain is hundreds of times bigger than an opossum's, containing hundreds of times more neurons, all perfectly wired. An elephant's trunk is a perfectly formed prehensile appendage containing 150,000 muscle elements (reference). Starting with a snout like that of an opossum, evolution used random mutations to design the elephant's snout in only 650 ticks. Imagine trying to get from an opossum to a brown bat in 650 increments. Or from an opossum to a whale. Whales have no pelvis, have flukes, have very weird skulls (especially the sperm whale), have blow holes up top, have temperature control that allows them to swim in arctic waters and they consume salt water rather than fresh. It is difficult for many people to imagine that sort of speed given the current theory.

Example: The Evolution of the Human Brain
Here is another example of the speed problem. Current fossil evidence indicates that modern humans evolved from a species called Homo erectus. Homo erectus appeared about 2 million years ago. Looking at the skull of Homo erectus, we know that its brain size was on the order of 800 or 900 cubic centimeters (CCs).

Modern human brain size averages about 1,500 CCs or so. In other words, in about 2 million years, evolution roughly doubled the size of the Homo erectus brain to create the human brain that we have today. Our brains contain approximately 100 billion neurons today, so in 2 million years, evolution added 50 billion neurons to the Homo erectus brain (while at the same time redesigning the skull to accommodate all of those neurons and redesigning the female pelvis to let the larger skull through during birth, etc.).

Let's assume that Homo erectus was able to reproduce every 10 years. That means that, in 2 million years, there were 200,000 generations of Homo erectus possible. There are four possible explanations for where the 50 billion new neurons came from in 200,000 generations:

• Every generation, 250,000 new neurons were added to the Homo erectus brain (250,000 * 200,000 = 50 billion).
• Every 100,000 years, 2.5 billion new neurons were added to the Homo erectus brain (2,500,000,000 * 20 = 50 billion).
• Perhaps 500,000 years ago, there was a spurt of 20 or so closely-spaced generations that added 2.5 billion neurons per generation.
• One day, spontaneously, 50 billion new neurons were added to the Homo erectus brain to create the Homo sapiens brain.

* In an absolutely fascinating experiment first reported in July 2002, scientists modified a single mouse gene and created mice with brains 50% larger than normal. This experiment shows that a point mutation can, in fact, have an immense effect on brain size. It is still unknown whether the larger brains make the mice smarter or not, but it is easy to imagine later mutations refining the wiring of these millions of new neurons. In another fascinating study, researches have identified minimal changes in an amino acid on a single gene that have a profound effect on speech processing in humans. It does appear that tiny changes in single genes can have very large effects on the species.

None of these scenarios is particularly comfortable. We see no evidence that evolution is randomly adding 250,000 neurons to each child born today, so that explanation is hard to swallow. The thought of adding a large package of something like 2.5 billion neurons in one step is difficult to imagine, because there is no way to explain how the neurons would wire themselves in. What sort of point mutation would occur in a DNA molecule that would suddenly create billions of new neurons and wire them correctly?* The current theory of evolution does not predict how this could happen.

One line of current research is looking at the effect of very small changes in DNA patterns during embryonic development. Any new animal, be it a mouse or a human, starts life as a single cell. That cell differentiates and develops into the complete animal. A tremendous amount of signaling happens between cells during the development process to ensure that everything ends up in the right place. Tiny changes in these signaling processes can have very large effects on the resulting animal. This is how the human genome, with at most 60,000 or so genes, is able to specify the creation of a human body containing trillions of cells, billions of carefully wired neurons and hundreds of different cell types all brilliantly sculpted into organs as diverse as the heart and the eyes. The book "Molecular Biology of the Cell" puts it this way:

Humans, as a genus distinct from the great apes, have existed for only a few million years. Each human gene has therefore had the chance to accumulate relatively few nucleotide changes since our inception, and most of these have been eliminated by natural selection. A comparison of humans and monkeys, for example, shows that their cytochrome-c molecules differ in about 1 percent and their hemoglobins in about 4 percent of their amino acid positions. Clearly, a great deal of our genetic heritage must have been formed long before Homo sapiens appeared, during the evolution of mammals (which started about 300 million years ago) and even earlier. Because the proteins of mammals as different as whales and humans are very similar, the evolutionary changes that have produced such striking morphological differences must involve relatively few changes in molecules from which we are made. Instead, it is thought that the morphological differences arise from differences in the temporal and spatial pattern of gene expression during embryonic development, which then determine the size, shape and other characteristics of the adult.

In other words, there just are not that many differences in the DNA of a human and a whale, yet humans and whales look totally different. Small collections of DNA mutations can have a very big effect on the final result.

Right now, the signaling mechanisms that wire up the 100 billion cells in the human brain are something of a mystery. How can the mere 60,000 genes in the human genome tell 100 billion neurons how to precisely wire themselves in the human brain? No one right now has a clear understanding of how so few genes can meticulously wire so many neurons. In a developing fetus in the womb, DNA is correctly creating and wiring up millions of cells per minute. Given that DNA does wire up a working human brain every time a baby is born, it may be the case that DNA has special properties that make evolution work more efficiently. As the mechanisms become better understood, the effects of DNA mutations during development will become better understood as well.

Question 3: Where Did the First Living Cell Come From?

In order for the principles of mutation and natural selection in the theory of evolution to work, there have to be living things for them to work on. Life must exist before it can to start diversifying. Life had to come from somewhere, and the theory of evolution proposes that it arose spontaneously out of the inert chemicals of planet Earth perhaps 4 billion years ago.

Could life arise spontaneously? If you read How Cells Work, you can see that even a primitive cell like an E. coli bacteria -- one of the simplest life forms in existence today -- is amazingly complex. Following the E. coli model, a cell would have to contain at an absolute minimum:

• A cell wall of some sort to contain the cell
• A genetic blueprint for the cell (in the form of DNA)
• An enzyme capable of copying information out of the genetic blueprint to manufacture new proteins and enzymes
• An enzyme capable of manufacturing new enzymes, along with all of the building blocks for those enzymes
• An enzyme that can build cell walls
• An enzyme able to copy the genetic material in preparation for cell splitting (reproduction)
• An enzyme or enzymes able to take care of all of the other operations of splitting one cell into two to implement reproduction (For example, something has to get the second copy of the genetic material separated from the first, and then the cell wall has to split and seal over in the two new cells.)
• Enzymes able to manufacture energy molecules to power all of the previously mentioned enzymes

Obviously, the E. coli cell itself is the product of billions of years of evolution, so it is complex and intricate -- much more complex than the first living cells. Even so, the first living cells had to possess:

• A cell wall
• The ability to maintain and expand the cell wall (grow)
• The ability to process "food" (other molecules floating outside the cell) to create energy
• The ability to split itself to reproduce

Otherwise, it is not really a cell and it is not really alive. To try to imagine a primordial cell with these capabilities spontaneously creating itself, it is helpful to consider some simplifying assumptions. For example:

• Perhaps the original energy molecule was very different from the mechanism found in living cells today, and the energy molecules happened to be abundant and free-floating in the environment. Therefore, the original cell would not have had to manufacture them.
• Perhaps the chemical composition of the Earth was conducive to the spontaneous production of protein chains, so the oceans were filled with unimaginable numbers of random chains and enzymes.
• Perhaps the first cell walls were naturally forming lipid spheres, and these spheres randomly entrapped different combinations of chemicals.
• Perhaps the first genetic blueprint was something other than DNA.

These examples do simplify the requirements for the "original cell," but it is still a long way to spontaneous generation of life. Perhaps the first living cells were completely different from what we see today, and no one has yet imagined what they might have been like. Speaking in general terms, life can only have come from one of two possible places:

• Spontaneous creation - Random chemical processes created the first living cell.
• Supernatural creation - God or some other supernatural power created the first living cell.

And it doesn't really matter if aliens or meteorites brought the first living cell to earth, because the aliens would have come into existence through either spontaneous creation or supernatural creation at some point -- something had to create the first alien cells.

Most likely, it will be many years before research can completely answer any of the three questions mentioned here. Given that DNA was not discovered until the 1950s, the research on this complicated molecule is still in its infancy, and we have much to learn.

The Future of Evolution

One exciting thing about the theory of evolution is that we can see its effects both today and in the past. For example, the book "Evolution" mentions this:

The earliest known reptiles are so amphibian-like that their assignment to one category or the other is largely a matter of opinion. In this area of life, however, there was no missing link; all the gradations from amphibian to reptile exist with a clarity seldom equaled in paleontology.

In other words, there is plenty of evidence, past and present, for some sort of evolutionary process. We see it in bacteria and insects today, and we see it in the fossil record through the development of millions of species over millions of years.

After thinking about questions like the three mentioned in the previous sections, different people come to different conclusions. In the future, there are three possible scenarios for the theory of evolution:

• Scientists will come to a complete understanding of DNA and show how mutations and natural selection explain every part of the development of life on this planet.
• Scientists will develop a new theory that answers the questions posed above to almost everyone's satisfaction, and it will replace the theory of evolution that we have today.
• Scientists will observe a completely new phenomenon that accounts for the diversity of life that we see today. For example, many people believe in creationism. In this theory, God or some other supernatural power intervenes to create all of the life that we see around us. The fossil record indicates that hundreds of millions of new species have been created over hundreds of millions of years -- Species creation is an intense and constant process with an extremely long history. If scientists were to observe the creation process occurring the next time a major new species comes into existence, they could document it and understand how it works.

Let's assume that the theory of evolution as currently stated is the process that did bring about all of the life that we see today. One compelling question is: "What happens next?" Evolution must be at work right now. Our species, Homo sapiens, only appeared about 40,000 years ago. What does evolution have in store for human beings, and how will the change manifest itself?

• Will a child appear one day whose brain is twice as big as any normal human brain? If so, what will be the capabilities of that brain, and how will it differ from the brain seen today? Or are our brains slowly evolving right now?
• Will children appear one day who have more than 23 chromosomes? If so, what will be the effects of the new chromosomes?
• Will man learn how to control or accelerate evolution through genetic engineering? Once we completely understand different genomes, will we be able to engineer evolutionary steps that lead to new species on a much faster schedule? What would those species look like? What would we design them to do?

These are all fascinating questions to think about. They reveal just how big an effect evolution can have. Given enough time, evolution could completely alter life on this planet by disposing of the species we see today and creating new ones.

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Elevators

Elevators have been around for over 150 years.

In the 1800s, new iron and steel production processes revolutionized the world of construction. With sturdy metal beams as their building blocks, architects and engineers could erect monumental skyscrapers hundreds of feet in the air.

But these towers would have been basically unusable if it weren't for another technological innovation that came along around the same time. Modern elevators are the crucial element that makes it practical to live and work dozens of stories above ground. High-rise cities like New York absolutely depend on elevators. Even in smaller multi-story buildings, elevators are essential for making offices and apartments accessible to handicapped people.

In this article, we'll find out how these ubiquitous machines move you from floor to floor. We'll also look at the control systems that decide where the elevator goes and the safety systems that prevent catastrophes.

Hydraulic Elevators

The concept of an elevator is incredibly simple -- it's just a compartment attached to a lifting system. Tie a piece of rope to a box, and you've got a basic elevator.

Of course, modern passenger and freight elevators are a lot more elaborate than this. They need advanced mechanical systems to handle the substantial weight of the elevator car and its cargo. Additionally, they need control mechanisms so passengers can operate the elevator, and they need safety devices to keep everything running smoothly.

There are two major elevator designs in common use today: hydraulic elevators and roped elevators.

Hydraulic elevator systems lift a car using a hydraulic ram, a fluid-driven piston mounted inside a cylinder. You can see how this system works in the diagram below.

The cylinder is connected to a fluid-pumping system (typically, hydraulic systems like this use oil, but other incompressible fluids would also work). The hydraulic system has three parts:

• A tank (the fluid reservoir)
• A pump, powered by an electric motor
• A valve between the cylinder and the reservoir

The pump forces fluid from the tank into a pipe leading to the cylinder. When the valve is opened, the pressurized fluid will take the path of least resistance and return to the fluid reservoir. But when the valve is closed, the pressurized fluid has nowhere to go except into the cylinder. As the fluid collects in the cylinder, it pushes the piston up, lifting the elevator car.

When the car approaches the correct floor, the control system sends a signal to the electric motor to gradually shut off the pump. With the pump off, there is no more fluid flowing into the cylinder, but the fluid that is already in the cylinder cannot escape (it can't flow backward through the pump, and the valve is still closed). The piston rests on the fluid, and the car stays where it is.

To lower the car, the elevator control system sends a signal to the valve. The valve is operated electrically by a basic solenoid switch (check out How Electromagnets Work for information on solenoids). When the solenoid opens the valve, the fluid that has collected in the cylinder can flow out into the fluid reservoir. The weight of the car and the cargo pushes down on the piston, which drives the fluid into the reservoir. The car gradually descends. To stop the car at a lower floor, the control system closes the valve again.

This system is incredibly simple and highly effective, but it does have some drawbacks. In the next section, we'll look at the main disadvantages of using hydraulics.

Pros and Cons of Hydraulics

The main advantage of hydraulic systems is they can easily multiply the relatively weak force of the pump to generate the stronger force needed to lift the elevator car (see How Hydraulic Machines Work to find out how).

But these systems suffer from two major disadvantages. The main problem is the size of the equipment. In order for the elevator car to be able to reach higher floors, you have to make the piston longer. The cylinder has to be a little bit longer than the piston, of course, since the piston needs to be able to collapse all the way when the car is at the bottom floor. In short, more stories means a longer cylinder.

The problem is that the entire cylinder structure must be buried below the bottom elevator stop. This means you have to dig deeper as you build higher. This is an expensive project with buildings over a few stories tall. To install a hydraulic elevator in a 10-story building, for example, you would need to dig at least nine stories deep! (Some hydraulic elevators don't require quite as much digging. Check out this site to learn about these systems.)

The other disadvantage of hydraulic elevators is that they're fairly inefficient. It takes a lot of energy to raise an elevator car several stories, and in a standard hydraulic elevator, there is no way to store this energy. The energy of position (potential energy) only works to push the fluid back into the reservoir. To raise the elevator car again, the hydraulic system has to generate the energy all over again.

The roped elevator design gets around both of these problems. In the next section, we'll see how this system works.

The Cable System

The most popular elevator design is the roped elevator. In roped elevators, the car is raised and lowered by traction steel ropes rather than pushed from below.

The ropes are attached to the elevator car, and looped around a sheave (3). A sheave is just a pulley with a grooves around the circumference. The sheave grips the hoist ropes, so when you rotate the sheave, the ropes move too.

The sheave is connected to an electric motor (2). When the motor turns one way, the sheave raises the elevator; when the motor turns the other way, the sheave lowers the elevator. In gearless elevators, the motor rotates the sheaves directly. In geared elevators, the motor turns a gear train that rotates the sheave. Typically, the sheave, the motor and the control system (1) are all housed in a machine room above the elevator shaft.

The ropes that lift the car are also connected to a counterweight (4), which hangs on the other side of the sheave. The counterweight weighs about the same as the car filled to 40-percent capacity. In other words, when the car is 40 percent full (an average amount), the counterweight and the car are perfectly balanced.

The purpose of this balance is to conserve energy. With equal loads on each side of the sheave, it only takes a little bit of force to tip the balance one way or the other. Basically, the motor only has to overcome friction -- the weight on the other side does most of the work. To put it another way, the balance maintains a near constant potential energy level in the system as a whole. Using up the potential energy in the elevator car (letting it descend to the ground) builds up the potential energy in the weight (the weight rises to the top of the shaft). The same thing happens in reverse when the elevator goes up. The system is just like a see-saw that has an equally heavy kid on each end.

Both the elevator car and the counterweight ride on guide rails (5) along the sides of the elevator shaft. The rails keep the car and counterweight from swaying back and forth, and they also work with the safety system to stop the car in an emergency.

Roped elevators are much more versatile than hydraulic elevators, as well as more efficient. Typically, they also have more safety systems. In the next section, we'll see how these elements work to keep you from plummeting to the ground if something goes wrong.

Safety Systems

In the world of Hollywood action movies, hoist ropes are never far from snapping in two, sending the car and its passengers hurdling down the shaft. In actuality, there is very little chance of this happening. Elevators are built with several redundant safety systems that keep them in position.

The first line of defense is the rope system itself. Each elevator rope is made from several lengths of steel material wound around one another. With this sturdy structure, one rope can support the weight of the elevator car and the counterweight on its own. But elevators are built with multiple ropes (between four and eight, typically). In the unlikely event that one of the ropes snaps, the rest will hold the elevator up.

Even if all of the ropes were to break, or the sheave system were to release them, it is unlikely that an elevator car would fall to the bottom of the shaft. Roped elevator cars have built-in braking systems, or safeties, that grab onto the rail when the car moves too fast.

In the next section, we'll examine a built-in braking system.

Safety Systems: Safeties

Safeties are activated by a governor when the elevator moves too quickly. Most governor systems are built around a sheave positioned at the top of the elevator shaft. The governor rope is looped around the governor sheave and another weighted sheave at the bottom of the shaft. The rope is also connected to the elevator car, so it moves when the car goes up or down. As the car speeds up, so does the governor. The diagram below shows one representative governor design.

In this governor, the sheave is outfitted with two hooked flyweights (weighted metal arms) that pivot on pins. The flyweights are attached in such a way that they can swing freely back and forth on the governor. But most of the time, they are kept in position by a high-tension spring.

As the rotary movement of the governor builds up, centrifugal force moves the flyweights outward, pushing against the spring. If the elevator car falls fast enough, the centrifugal force will be strong enough to push the ends of the flyweights all the way to the outer edges of the governor. Spinning in this position, the hooked ends of the flyweights catch hold of ratchets mounted to a stationary cylinder surrounding the sheave. This works to stop the governor.

The governor ropes are connected to the elevator car via a movable actuator arm attached to a lever linkage. When the governor ropes can move freely, the arm stays in the same position relative to the elevator car (it is held in place by tension springs). But when the governor sheave locks itself, the governor ropes jerk the actuator arm up. This moves the lever linkage, which operates the brakes.

In this design, the linkage pulls up on a wedge-shaped safety, which sits in a stationary wedge guide. As the wedge moves up, it is pushed into the guide rails by the slanted surface of the guide. This gradually brings the elevator car to a stop.

Safety Systems: More Backups

Elevators also have electromagnetic brakes that engage when the car comes to a stop. The electromagnets actually keep the brakes in the open position, instead of closing them. With this design, the brakes will automatically clamp shut if the elevator loses power.

Elevators also have automatic braking systems near the top and the bottom of the elevator shaft. If the elevator car moves too far in either direction, the brake brings it to a stop.

If all else fails, and the elevator does fall down the shaft, there is one final safety measure that will probably save the passengers. The bottom of the shaft has a heavy-duty shock absorber system -- typically a piston mounted in an oil-filled cylinder. The shock absorber works like a giant cushion to soften the elevator car's landing.

In addition to these elaborate emergency systems, elevators need a lot of machinery just to make their stops. In the next section, we'll find out how an elevator operates under normal conditions.

Making the Rounds

Many modern elevators are controlled by a computer. The computer's job is to process all of the relevant information about the elevator and turn the motor the correct amount to put the elevator car where it needs to be. In order to do this, the computer needs to know at least three things.

• Where people want to go
• Where each floor is
• Where the elevator car is

Finding out where people want to go is very easy. The buttons in the elevator car and the buttons on each floor are all wired to the computer. When you press one of these buttons, the computer logs this request.

There are lots of ways to figure out where the elevator car is. In one common system, a light sensor or magnetic sensor on the side of the car reads a series of holes on a long vertical tape in the shaft. By counting the holes speeding by, the computer knows exactly where the car is in the shaft. The computer varies the motor speed so that the car slows down gradually as it reaches each floor. This keeps the ride smooth for the passengers.

In a building with many floors, the computer has to have some sort of strategy to keep the cars running as efficiently as possible. In older systems, the strategy is to avoid reversing the elevator's direction. That is, an elevator car will keep moving up as long as there are people on the floors above that want to go up. The car will only answer "down calls" after it has taken care of all the "up calls." But once it starts down, it won't pick up anybody who wants to go up until there are no more down calls on lower floors. This program does a pretty good job of getting everybody to their floor as fast as possible, but it is highly inflexible.

More advanced programs take passenger traffic patterns into account. They know which floors have the highest demand, at what time of day, and direct the elevator cars accordingly. In a multiple car system, the elevator will direct individual cars based on the location of other cars.

In one cutting-edge system, the elevator lobby works like a train station. Instead of simply pressing up or down, people waiting for an elevator can enter a request for a specific floor. Based on the location and course of all the cars, the computer tells the passengers which car will get them to their destinations the fastest.

Most systems also have a load sensor in the car floor. The load sensor tells the computer how full the car is. If the car is near capacity, the computer won't make any more pick-up stops until some people have gotten off. Load sensors are also a good safety feature. If the car is overloaded, the computer will not close the doors until some of the weight is removed.

In the next section, we'll look at one of the coolest components in an elevator: the automatic doors.

Doors

The automatic doors at grocery stores and office buildings are mainly there for convenience and as an aid for handicapped people. The automatic doors in an elevator, on the other hand, are absolutely essential. They are there to keep people from falling down an open shaft.

Elevators use two different sets of doors: doors on the cars and doors opening into the elevator shaft. The doors on the cars are operated by an electric motor, which is hooked up to the elevator computer. You can see how a typical door-opener system works in the diagram below.

The electric motor turns a wheel, which is attached to a long metal arm. The metal arm is linked to another arm, which is attached to the door. The door can slide back and forth on a metal rail.

When the motor turns the wheel, it rotates the first metal arm, which pulls the second metal arm and the attached door to the left. The door is made of two panels that close in on each other when the door opens and extend out when the door closes. The computer turns the motor to open the doors when the car arrives at a floor and close the doors before the car starts moving again. Many elevators have a motion sensor system that keeps the doors from closing if somebody is between them.

The car doors have a clutch mechanism that unlocks the outer doors at each floor and pulls them open. In this way, the outer doors will only open if there is a car at that floor (or if they are forced open). This keeps the outer doors from opening up into an empty elevator shaft.

In a relatively short period of time, elevators have become an essential machine. As people continue to erect monumental skyscrapers and more small buildings are made handicap-accessible, elevators will become an even more pervasive element in society. It is truly one of the most important machines in the modern era, as well as one of the coolest.

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do you know magnet works ?

Introduction to How Magnets Work

It all started when we went shopping for a magnet for a demonstration on liquid body armor. We wanted to show that a magnetic field could cause certain liquids to behave as solids. Along with the petri dishes and iron filings we needed, the Steve Spangler Science catalog had a neodymium magnet it described as "super strong." We ordered our supplies, hoping that the magnet would be powerful enough to create an effect we could capture on film.

Our homemade ferrofluid before and after exposure to a magnetic field

The magnet didn't just transform our iron-and-oil fluid into a solid -- sometimes, its pull on the fluid cracked the petri dish holding it. Once, the magnet unexpectedly flew out of a videographer's hand and into a dish full of dry filings, which required considerable ingenuity to remove. It also adhered itself so firmly to the underside of a metal table that we had to use a pair of locking pliers to retrieve it. When we decided it would be safer to keep the magnet in a pocket between takes, people wound up momentarily stuck to the table, a ladder and the studio door.

Magnetic Poles A magnet can have multiple north and south poles, and these poles always occur in pairs. There can be no north pole without a corresponding south pole, no south pole without a corresponding north.

Around the office, the magnet became an object of curiosity and the subject of impromptu experiments. Its uncanny strength and its tendency to suddenly and noisily jump from unwary grips to the nearest metal surface got us thinking. We all knew the basics of magnets and magnetism -- magnets attract specific metals, and they have north and south poles. Opposite poles attract each other while like poles repel. Magnetic and electrical fields are related, and magnetism, along with gravity and strong and weak atomic forces, is one of the four fundamental forces in the universe.

But none of those facts led to an answer to our most basic question. What exactly makes a magnet stick to certain metals? By extension, why don't they stick to other metals? Why do they attract or repel each other, depending on their positioning? And what makes neodymium magnets so much stronger than the ceramic magnets we played with as children?

Iron filings (right) align along the magnetic field lines of a cylindrical neodymium magnet.

To understand the answers to these questions, it helps to have a basic definition of a magnet. Magnets are objects that produce magnetic fields and attract metals like iron, nickel and cobalt. The magnetic field's lines of force exit the magnet from its north pole and enter its south pole. Permanent or hard magnets create their own magnetic field all the time. Temporary or soft magnets produce magnetic fields while in the presence of a magnetic field and for a short while after exiting the field. Electromagnets produce magnetic fields only when electricity travels through their wire coils.

Iron filings (right) align along the magnetic field lines of a cubical neodymium magnet.

Until recently, all magnets were made from metal elements or alloys. These materials produced magnets of different strengths. For example:

• Ceramic magnets, like the ones used in refrigerator magnets and elementary-school science experiments, contain iron oxide in a ceramic composite. Most ceramic magnets, sometimes known as ferric magnets, aren't particularly strong.
• Alnico magnets are made from aluminum, nickel and cobalt. They're stronger than ceramic magnets, but not as strong as the ones that incorporate a class of elements known as rare-earth metals.
• Neodymium magnets contain iron, boron and the rare-earth element neodymium.
• Samarium cobalt magnets combine cobalt with the rare-earth element samarium. In the past few years, scientists have also discovered magnetic polymers, or plastic magnets. Some of these are flexible and moldable. However, some work only at extremely low temperatures, and others pick up only very lightweight materials, like iron filings.

It takes a little effort for these materials to become magnets. We'll look at how it happens in the next section.

Making Magnets: The Basics

Many of today's electronic devices require magnets to function. This reliance on magnets is relatively recent, primarily because most modern devices require magnets that are stronger than the ones found in nature. Lodestone, a form of magnetite, is the strongest naturally-occurring magnet. It can attract small objects, like paper clips and staples.

By the 12th century, people had discovered that they could use lodestone to magnetize pieces of iron, creating a compass. Repeatedly rubbing lodestone along an iron needle in one direction magnetized the needle. It would then align itself in a north-south direction when suspended. Eventually, scientist William Gilbert explained that this north-south alignment of magnetized needles was due to the Earth behaving like an enormous magnet with north and south poles.

A compass needle isn't nearly as strong as many of the permanent magnets used today. But the physical process that magnetizes compass needles and chunks of neodymium alloy is essentially the same. It relies on microscopic regions known as magnetic domains, which are part of the physical structure of ferromagnetic materials, like iron, cobalt and nickel. Each domain is essentially a tiny, self-contained magnet with a north and south pole. In an unmagnetized ferromagnetic material, each of the north poles points in a random direction. Magnetic domains that are oriented in opposite directions cancel one another out, so the material does not produce a net magnetic field.

In an unmagnetized ferromagnetic material, domains point in random directions.

In magnets, on the other hand, most or all of the magnetic domains point in the same direction. Rather than canceling one another out, the microscopic magnetic fields combine to create one large magnetic field. The more domains point in the same direction, the stronger the overall field. Each domain's magnetic field extends from its north pole into the south pole of the domain ahead of it.

In a magnet, most or all of the domains point in the same direction.

This explains why breaking a magnet in half creates two smaller magnets with north and south poles. It also explains why opposite poles attract -- the field lines leave the north pole of one magnet and naturally enter the south pole of another, essentially creating one larger magnet. Like poles repel each other because their lines of force are traveling in opposite directions, clashing with each other rather than moving together.

Connecting the north pole of one magnet to the south pole of another magnet essentially creates one larger magnet.

Making Magnets: The Details

To make a magnet, all you have to do is encourage the magnetic domains in a piece of metal to point in the same direction. That's what happens when you rub a needle with a magnet -- the exposure to the magnetic field encourages the domains to align. Other ways to align magnetic domains in a piece of metal include:

• Placing it a strong magnetic field in a north-south direction
• Holding it in a north-south direction and repeatedly striking it with a hammer, physically jarring the domains into a weak alignment
• Passing an electrical current through it

Two of these methods are among scientific theories about how lodestone forms in nature. Some scientists speculate magnetite becomes magnetic when struck by lightning. Others theorize that pieces of magnetite became magnets when the Earth was first formed. The domains aligned with the Earth's magnetic field while iron oxide was molten and flexible.

Iron filings line up along the magnetic fields of four small magnets. After removing the magnet, the filings will continue to have their own weak magnetic fields.

The most common method of making magnets today involves placing metal in a magnetic field. The field exerts torque on the material, encouraging the domains to align. There's a slight delay, known as hysteresis, between the application of the field and the change in domains -- it takes a few moments for the domains to start to move. Here's what happens:

• The magnetic domains rotate, allowing them to line up along the north-south lines of the magnetic field.
• Domains that already pointed in the north-south direction become bigger as the domains around them get smaller.
• Domain walls, or borders between the neighboring domains, physically move to accommodate domain growth. In a very strong field, some walls disappear entirely.

The resulting magnet's strength depends on the amount of force used to move the domains. Its permanence, or retentivity, depends on how difficult it was to encourage the domains to align. Materials that are hard to magnetize generally retain their magnetism for longer periods, while materials that are easy to magnetize often revert to their original nonmagnetic state.

You can reduce a magnet's strength or demagnetize it entirely by exposing it to a magnetic field that is aligned in the opposite direction. You can also demagnetize a material by heating it above its Curie point, or the temperature at which it loses its magnetism. The heat distorts the material and excites the magnetic particles, causing the domains to fall out of alignment.

Next, we'll take a look at why magnetized materials attract specific metals.

Shipping Magnets Large, powerful magnets have numerous industrial uses, from writing data to inducing current in wires. But shipping and installing huge magnets can be difficult and dangerous. Not only can magnets damage other items in transit, they can be difficult or impossible to install upon their arrival. In addition, magnets tend to collect an array of ferromagnetic debris, which is hard to remove and can even be dangerous.

For this reason, facilities that use very large magnets often have equipment on site that lets them turn ferromagnetic materials into magnets. Often, the device is essentially an electromagnet.

Why Magnets Stick

If you've read How Electromagnets Work, you know that an electrical current moving through a wire creates a magnetic field. Moving electrical charges are responsible for the magnetic field in permanent magnets as well. But a magnet's field doesn't come from a large current traveling through a wire -- it comes from the movement of electrons.

Many people imagine electrons as tiny particles that orbit an atom's nucleus the way planets orbit a sun. As quantum physicists currently explain it, the movement of electrons is a little more complicated than that. Essentially, electrons fill an atom's shell-like orbitals, where they behave as both particles and waves. The electrons have a charge and a mass, as well as a movement that physicists describe as spin in an upward or downward direction. You can learn more about electrons in How Atoms Work.

A simplified view of an atom, with a nucleus and orbiting electrons

Generally, electrons fill the atom's orbitals in pairs. If one of the electrons in a pair spins upward, the other spins downward. It's impossible for both of the electrons in a pair to spin in the same direction. This is part of a quantum-mechanical principle known as the Pauli Exclusion Principle.

Even though an atom's electrons don't move very far, their movement is enough to create a tiny magnetic field. Since paired electrons spin in opposite directions, their magnetic fields cancel one another out. Atoms of ferromagnetic elements, on the other hand, have several unpaired electrons that have the same spin. Iron, for example, has four unpaired electrons with the same spin. Because they have no opposing fields to cancel their effects, these electrons have an orbital magnetic moment. The magnetic moment is a vector -- it has a magnitude and a direction. It's related to both the magnetic field strength and the torque that the field exerts. A whole magnet's magnetic moments come from the moments of all of its atoms.

An iron atom and its four unpaired electrons

In metals like iron, the orbital magnetic moment encourages nearby atoms to align along the same north-south field lines. Iron and other ferromagnetic materials are crystalline. As they cool from a molten state, groups of atoms with parallel orbital spin line up within the crystal structure. This forms the magnetic domains discussed in the previous section.

You may have noticed that the materials that make good magnets are the same as the materials magnets attract. This is because magnets attract materials that have unpaired electrons that spin in the same direction. In other words, the quality that turns a metal into a magnet also attracts the metal to magnets. Many other elements are diamagnetic -- their unpaired atoms create a field that weakly repels a magnet. A few materials don't react with magnets at all.

This explanation and its underlying quantum physics are fairly complicated, and without them the idea of magnetic attraction can be mystifying. So it's not surprising that people have viewed magnetic materials with suspicion for much of history. In the next section, we'll take a look at the powers ascribed to magnets, as well as what they can and can't do.

Measuring Magnets You can measure magnetic fields using instruments like gauss meters, and you can describe and explain them using numerous equations. Here are some of the basics:

• Magnetic lines of force, or flux, are measured in Webers (Wb). In electromagnetic systems, the flux relates to the current.
• A field's strength, or the density of the flux, is measured in Tesla (T) or gauss (G). One Tesla is equal to 10,000 gauss. You can also measure the field strength in Webers per square meter. In equations, the symbol B represents field strength.
• The field's magnitude is measured in amperes per meter or oersted. The symbol H represents it in equations

Magnet Myths

Every time you use a computer, you're using magnets. A hard drive relies on magnets to store data, and some monitors use magnets to create images on the screen. If your home has a doorbell, it probably uses an electromagnet to drive a noisemaker. Magnets are also vital components in CRT televisions, speakers, microphones, generators, transformers, electric motors, burglar alarms, cassette tapes, compasses and car speedometers.

In addition to their practical uses, magnets have numerous amazing properties. They can induce current in wire and supply torque for electric motors. A strong enough magnetic field can levitate small objects or even small animals. Maglev trains use magnetic propulsion to travel at high speeds, and magnetic fluids help fill rocket engines with fuel. The Earth's magnetic field, known as the magnetosphere, protects it from the solar wind. According to Wired magazine, some people even implant tiny neodymium magnets in their fingers, allowing them to detect electromagnetic fields [Source: Wired].

Image used under GNU Free Documentation License Transrapid train at the Emsland, Germany test facility

Magnetic Resonance Imaging (MRI) machines use magnetic fields to allow doctors to examine patients' internal organs. Doctors also use pulsed electromagnetic fields to treat broken bones that have not healed correctly. This method, approved by the United States Food and Drug Administration in the 1970s, can mend bones that have not responded to other treatment. Similar pulses of electromagnetic energy may help prevent bone and muscle loss in astronauts who are in zero-gravity environments for extended periods.

Magnets can also protect the health of animals. Cows are susceptible to a condition called traumatic reticulopericarditis, or hardware disease, which comes from swallowing metal objects. Swallowed objects can puncture a cow's stomach and damage its diaphragm or heart. Magnets are instrumental to preventing this condition. One practice involves passing a magnet over the cows' food to remove metal objects. Another is to feed magnets to the cows. Long, narrow alnico magnets, known as cow magnets, can attract pieces of metal and help prevent them from injuring the cow's stomach. The ingested magnets help protect the cows, but it's still a good idea to keep feeding areas free of metal debris. People, on the other hand, should never eat magnets, since they can stick together through a person's intestinal walls, blocking blood flow and killing tissue. In humans, swallowed magnets often require surgery to remove.

Photo courtesy Amazon Cow magnets

Some people advocate the use of magnet therapy to treat a wide variety of diseases and conditions. According to practitioners, magnetic insoles, bracelets, necklaces, mattress pads and pillows can cure or alleviate everything from arthritis to cancer. Some advocates also suggest that consuming magnetized drinking water can treat or prevent various ailments. Americans spend an estimated $500 million per year on magnetic treatments, and people worldwide spend about $5 billion. [Source: Winemiller via NCCAM].

Proponents offer several explanations for how this works. One is that the magnet attracts the iron found in hemoglobin in the blood, improving circulation to a specific area. Another is that the magnetic field somehow changes the structure of nearby cells. However, scientific studies have not confirmed that the use of static magnets has any effect on pain or illness. Clinical trials suggest that the positive benefits attributed to magnets may actually come from the passage of time, additional cushioning in magnetic insoles or the placebo effect. In addition, drinking water does not typically contain elements that can be magnetized, making the idea of magnetic drinking water questionable.

Some proponents also suggest the use of magnets to reduce hard water in homes. According to product manufacturers, large magnets can reduce the level of hard water scale by eliminating ferromagnetic hard-water minerals. However, the minerals that generally cause hard water are not ferromagnetic. A two-year Consumer Reports study also suggests that treating incoming water with magnets does not change the amount of scale buildup in a household water heater.

Even though magnets aren't likely to end chronic pain or eliminate cancer, they are still fascinating to study. To learn more about them, check out the links on the next page.

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