When I talk about evolution, I mean the process by which living things changed from the first organism to all the ones we have now and new ones we'll have in the future. The key to that process is natural selection. Long before Darwin's Origin of Species, people knew about evolution. That's how we developed the goldfish and the chihuahua and the milk cow. People selected animals with the traits they wanted, and bred them with other animals with similar traits. The "selection" part is important because that's what makes any kind of evolution work. The "natural" part is important because it means this selection process doesn't require human intervention. Charles Darwin gets deserved credit for it, but the idea popped up independently in at least three minds. According to one source, the geologist James Hutton thought of it first, though he died without publishing it, and his work wasn't rediscovered until 1947. Darwin came next, but before he published, Alfred Russel Wallace thought of it, too, and the two men published it together. But evolution by natural selection became famous with the publication of Darwin's Origin of Species. It was the "natural" part that shook the world of biology. Everybody already knew about selection.

We don't have to dig up dinosaurs to appreciate how natural selection shapes animals. In Third World countries, feral dogs eke out a living scavenging human leftovers. The humans aren't intentionally selecting which of these dogs breed, but selection happens anyway. There are no feral Great Danes of Chihuahas, even though the occasional Dane or Chihuahua may find itself without a home, living among other feral dogs that are all kind of medium sized -- about forty pounds, and built pretty much like "Old Yaller." That's the kind of dog that best survives on man's garbage and whatever food it can find in the wild. The Hound of the Baskervilles can't find enough food, or if it can, is so large maybe it frightens humans, and they kill it. The humans aren't trying to shape a dog. They're just animals trying to stay safe. Very small dogs don't do too well, either, possibly because they can't compete with their larger brethren food, or maybe larger dogs kill them. Whatever the specific agents of selection, selection happens.

The dog that best copies its genes into viable offspring in an environment that includes indifferent humans is medium- sized and willing to suck up to people. But that dog is descended from its larger cousin, the wolf, which doesn't depend on human garbage to survive. The wolf is large enough to frighten a man and isn't particularly fond of humans. It rarely hunts them, probably because they're too dangerous, and it certainly doesn't fawn over them as domesticated dogs, even the feral ones, often do. The wolf's environment selects it to be about two or three times the size of the feral dog. Nature, including other dogs, people, food availability, and the rest of the dog's environment, shape the dog, whatever kind of dog it may be.

There are, amazingly enough, those who don't believe in evolution. Darwin's "Origin" left little room for doubt, and yet some people consider evolution flat-out wrong. Almost all believe in a little bit of evolution. Sure, natural selection can make a population of moths darker than it once was, but those things are still moths. The light and the dark ones can still interbreed. These people claim that while little changes are possible, the big changes that make new species are impossible. If they're talking about plants, they're completely wrong because flowers have been observed to evolve into new species in a single season. If they're talking about animals, it takes a little effort to rebut their arguments.

The best place to start the rebuttal, though, isn't with animals, but with microorganisms. They're tiny and numerous, and they can mature and reproduce in as little as half an hour. A single-celled critter absorbs nutrients, grows, and divides into two critters with identical genes -- well, maybe identical. Sometimes as the genes (composed of DNA) are copied, there's a mistake, and one organism is a little different from the other. Or maybe after it's perfectly copied, one gene gets changed by some chemical in its environment or by natural radiation. That genetic change is what we call a mutation. And when the gene changes, the gene product is likely to change, too. The gene may build a slightly different protein, which leads to a slightly different organism than its parent's genes built, even under identical environmental conditions. We end up with two different microorganisms because an organism is really the result of the interaction of its genes with their environment.

Let's say our microorganism likes to live in people and cause tuberculosis, and let's call it Mycobacterium tuberculosis because that's what the microbiologists call it. Zap it with the right antibiotic and it dies off and the disease goes away. The patient is cured. Unless somewhere along the line in the endless billions of copies of the original tuberculosis organism, a gene or two got copied wrong, and a few of the organisms that cause tuberculosis are built so that the antibiotic doesn't bother them. In a few days or weeks of antibiotic treatment, all the susceptible tuberculosis microbes die, leaving lots of food for the ones that tolerate the antibiotic, which grows and multiplies and quickly replaces the original kind. The tuberculosis that was almost cured makes a comeback. That's evolution, and it's given us strains of the tuberculosis microorganism that can be difficult or impossible to treat.

Or take an organism a little closer to humans, a fruit fly living in the forests of Hawaii. Natural "experiments" in Hawaii have produced significant changes in fruit fly populations in a period of about 140 years. There, in the mid-1800s, a volcano erupted, spewing lava down the mountainside, producing isolated bits of forest called kipukas separated from other forests by extensive lava fields which the flies couldn't cross. Because of this isolation, the flies within one patch of forest couldn't breed with flies in other patches, so mutations occurring in one group weren't spread to the others, and in the space of 140 years, substantial genetic differences arose between flies of the same species isolated in different kipukas. I asked Kenneth Kaneshiro, Director of the Center for Conservation Research and Training at the University of Hawaii, who's been studying the Hawaiian flies for decades, whether similar isolations in kipukas that occurred over longer times have led to documented evolution of new species in historical times. His answer was no. He said that within a few thousand years the forests grow back and the flies, not yet separate species, interbreed, short circuiting speciation.

But suppose the flies lived on an island that was split in two by an earthquake and a wide channel of water separated the two islands, leaving two populations of the same species isolated for for thousands of years, allowing their genetic makeup to become very differentt. Eventually they'd become different enough that they couldn't breed with one another and we'd have separate species. Maybe it would take ten thousand years or fifty thousand -- a long time by our usual standards, but not very long at all in the three billion years life has been evolving on earth. Three billion years is sixty thousand times that long.

So if it took fifty thousand years to come up with a new species of fly, nature could have done it all sixty thousand times over with a nearly unlimited number of species. And it certainly looks like it did just that. Wherever one species got split into isolated populations by any means, and they stayed isolated for long enough, those different populations evolved into new species, not terribly different from the original organism if the time span was relatively short, or wildly different if the time and the selective forces on the islands were different. Darwin's finches are all birds, for example. But in the old days, biologists could be pretty sure that birds evolved from reptiles, and pretty sure that apes evolved from monkeys, and that humans were a species of ape, and that somewhere in the distant past, some unknown microorganism started it all -- that all living things today are related to some founding organism.

That's very plausible science, and if it's true, we should all have genes made out of the same stuff. Even before Darwin, Gregor Mendel had found a hint of genes in the characteristic way his peas inherited certain traits. But it wasn't until the twentieth century that the gene was found to be a stretch (or disconnected stretches) of DNA, a chemical structure containing thousands or millions of organic bases conventionally represented by the letters A, C, G, and T. The gene's chemistry -- what it produces, how much it produces, and where it produces its products -- depends on the sequence in which those bases appear in the DNA and the environment surrounding those bases. That environment, of course, includes other genes and their products, and the non-coding DNA, which we once thought of as junk, but which now is known to be involved in turning genes on and off.

In the last few years the entire set of DNA sequences for the human being and many other animals have been determined. Enough of the chimpanzee genome has been examined that we can esitmate the genetic difference between chimpanzees and humans. It's less than two percent. Surprisingly, fifty or sixty percent of fruit fly genes have their human counterparts, not usually exact duplicates of human genes, but extremely similar. How closely related are we to "lesser" organisms? Closely enough that if you compare human and ape genes, you'll find that in many cases there are no differences at all between them. We're closely enough related to un-human-like creatures, too, that we can splice human genes into bacteria or yeast and have them work just fine in those organisms. That's how pharmaceutical companies make most of the insulin diabetics use these days. They splice human DNA into bacteria and let the bacteria produce human insulin.

We're all related. We all trace our ancestry back to some original creature that came to be by accident in the primordial environment, and by mistakes in copying its genes or damage to genes (mutations) and by natural selection of the organisms containing one set of genes over organisms containing a different set, we came up with the fruit fly and the gorilla and the human and the iguana.

Now different sets of genes make different species look different. A lizard may have green skin, a bird has feathers, spiders have eight legs, people four (or two if you insist on calling our front legs arms). But they also make animals behave differently. If you're born with elephant genes, you won't fly, no matter what the Disney movie says. If you're born with human genes in a normal human environment, you'll communicate with language. If you're five years old, and for some reason don't have a language yet, and you meet a whole bunch of other kids who don't have language, you'll form a society with them, and together you'll invent a language. When deaf kids in Nicaragua who'd lived isolated lives and had no language were put together in a school where the teachers expected to teach them to read lips in Spanish, the kids didn't do that very well, but they invented an incredibly sophisticated sign language their teachers could't understand. On a seemingly simpler level, our closest relatives, the chimpanzees, form families that don't include adult males. But in every culture, humans form families that do include adult males. Sea turtles just mate, and the female drops her eggs in a hole she digs in the beach, then leaves them to fare for themselves. Humans of both sexes care for their young for more than a decade. What's the difference? In the final analysis, sea turtles have sea turtle DNA, and humans have human DNA. And both we and the turtles got our DNA by evolution by natural selection. Those animals that behaved in ways that got their genes copied efficiently into the next generation became numerous; those that didn't went extinct. If you're built like a female sea turtle, you can efficiently copy your genes by dropping your eggs and leaving them behind as you swim off to sea. If you're a human female and leave your baby behind at birth, the baby dies and your genes don't go on generation after generation. So the only human females who passed their genes on were the ones that had genes that inclined them to care for their young.

Different animals found advantages in different kinds of behaviors, and those behaviors. The red- tailed hawk is extremely skillful at flying. It has the wings and feathers and eyesight that makes that possible, but it also has a brain that understands flying in ways human aeronautical engineers don't. And we humans have our own specialties, we make shelters, for example. That's not unique in itself. Termites and ants make shelters. But all the shelters of any given termite species are pretty much alike, made of the same materials in the same patterns. Humans, on the other hand, make endless kinds of shelters of a large variety of materials -- wood, brick, stone, aluminum, plastic, concrete, and so on. And they make them in a variety of forms. A gorilla or chimpanzee has the arms and the hands to make shelters from the materials around them, for example, branches or mud. But they don't make them. Their brains and emotions didn't evolve to give them the emotional push to do so or the specialized brain structures needed to make a shelter. Apparently it never occurs to them that by building a shelter they could keep dry during storms or warm when the winds howl. And even if they have such thoughts (they will take advantage of shelters people built) their brains wouldn't tell them how to go about it. But we humans have genes that virtually force us to learn language, or even to invent it, and to build tools that grow increasingly more complex as old tools are combined and modified to serve new purposes. Chimpanzees and gorillas will never make sailboats or computers or learn to calculate the height of an object using formal trigonometry. Humans do those things, and much more, and the only reason they can do such things is that they have human genes. Animals without human genes can no more write sonnets than humans can capture insects by echolocation. If we're "better" than other creatures in any evolutionary sense, it's only because there are so many of us, compared to many other animals, and that we live in such varied environments. But before we get too proud of our accomplishments, let's remember that there are far more bacteria in the world than humans, both in number and in total biomass.