Friday, February 10, 2012
Why Evolution Looks like It Has an Overall Direction
When directional selection occurs, evolution has a direction. It rapidly chooses the winners. But directional selection usually does not last very long. One would not expect evolution to take a consistent, overall direction over the course of millions of years. What it takes to be a winner at one place and time is not what it takes at another.
And yet, in the popular imagination, the idea persists that humans are the “most evolved” species and that evolution pushes “upward” toward a human-like state. Worms are inferior to us because they did not evolve as much. Chimpanzees are inferior to us also, but not as inferior as worms. Ask somebody about evolution, and they will probably either say it is a plot of the Devil or else they will tell you a story that puts humans up at the top of the stairs. Literally. When the publisher was designing the cover to the paperback version of my first Encyclopedia of Evolution, the artist depicted stairs, with monkeys at the bottom and humans at the top. One cannot blame the artist, who merely drew what he or she had been taught to believe. The publisher, not the author, decides the cover images of a book, but this publisher immediately changed the image when I told them that it was scientifically wrong. More recently, an editor rejected a book manuscript because, he said, evolution is an inevitable upward progression, and since my book did not say this, I obviously did not know what evolution was.
This up-the-stairs version of evolution is the one that harmonizes most easily with old creationist views. In the Middle Ages, the natural world was depicted as a scala naturae, or ladder of nature. Rocks were at the bottom, simple organisms above the rocks, mammals and birds above the simpler animals, humans above all other animals, angels above humans, God above angels.
To many early evolutionary scientists, and to many people today, the scala naturae still exists, only God and the angels have been lopped off the top, and evolution has been substituted for creation. This is essentially the pre-Darwinian version of evolution that was proposed by French biologist (inventor of the word biology) Jean Baptiste de Lamarck. Simple organisms were continually forming themselves out of the mud, then modifying themselves into ever more complex forms until they became human, or something like a human.
Scientists today understand that there is no single, upward staircase of evolutionary progress. All species are equally evolved. Worms are very good at being worms. They can make a living in ways that humans cannot. Chimpanzees are very good at being chimpanzees. In some ways, bacteria can be seen as the most successful life form on this planet. There are more bacteria, and more kinds of bacteria, than any other group of organisms. They live in a greater range of conditions, and have persisted for three and a half billion years. The human species, in contrast, can live in only a narrow range of conditions, and has done so for only about 100,000 years, about 0.003 percent as long as bacteria have existed.
There is, however, no denying that life has seemed to make progress. Three and a half billion years ago, only bacteria (and probably viruses as well) existed. By a billion years ago, there were complex cells as well. Five hundred million years ago, almost all life forms lived in the oceans. By four hundred million years ago, some life forms lived on land. Bigger and more complex organisms have evolved over time. An evolutionary story is unfolding. If this isn’t progress, what is?
The reason that evolution seems to lead to progress is that evolution accumulates successes. Bacteria were successful. So also were the more complex cells that evolved from them. Natural selection kept both of them. Plants have been successful. So have animals. Natural selection has kept both of them. The Tree of Life gets bigger and bigger. Also, natural selection capitalizes upon whatever works, whether it is greater complexity or greater simplicity, whether it is life on land or life in the sea. Natural selection capitalizes upon opportunity. Each step toward greater complexity opened up new possibilities. Over evolutionary time, successes and complexities have accumulated. Some of the stories have vanished, such as the saga of the dinosaurs. But most of them are still here, to astonish us today as we behold and study them.
Check out the new YouTube video available at http://youtu.be/SsunNzKrj_s (Channel, StanEvolve). This week it is Charles Darwin explaining that humans, bananas, grapes, and chickens all have a common evolutionary ancestor.
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This entry appears in my book Life of Earth: Portrait of a Beautiful, Middle-Aged, Stressed-Out World, published by Prometheus Books.