Thursday, March 26, 2009

Are We the Pinnacle of Evolution?

The term “evolution” conjures the picture of initially unicellular life marching triumphantly towards greater size and increasing complexity --and of humans as the undisputed pinnacle of evolutionary history. This smug view is compounded by the widespread notion that evolution and progress are synonyms. It is common in textbooks and popular accounts to depict evolutionary series as ladders --from hyracotherium, the “dawn horse”, to the modern horse; or from Australopithecus to exalted Homo sapiens.

       But, as Stephen Jay Gould has shown in his book Full House, ladders are misleading. Hyracotherium is indeed the ancestor of modern horses, and, yes, there is a continuous line from him to present-day Equus. But the line twists and turns in time, branching endlessly so that the “dawn horse” is also the grandfather of countless other species, some living, but most extinct. The same is true of the line of descent going from Australopithecus, of “Lucy” fame, to modern humans. The line is not a line --it’s a bush. Neanderthals, who can also claim Lucy as their grandmother, are not our direct ancestors.

       Evolutionary lineages in general are not linear. Today’s living species, which we might represent as the outer leaves of an evolutionary tree, are attached to twigs, which are attached to larger twigs, which shoot off from branches, which sprout from larger branches, which emerge from an ancient common trunk going way back into the past --some 3.6 billion years-- to the first living organisms, a kind of bacterium.

       The ladder representation conveys the false idea that evolution is going somewhere --that those first bacteria somehow knew they were to become us. But if ladders had any truth in them then the lower rungs ought to be extinct to open the way for the young, so to speak. Bacteria, however, thrive today. What’s more, by their diversity, by their presence in every nook and cranny of the earth, by their longevity, and by their sheer numbers, bacteria are and always have been the dominant organisms in this planet, as Gould argues in Full House. If we go back far enough, you and I have a common ancestor who was a reptile; go back even further and we will find we are related to a fish. Yet reptiles and fishes are alive and well today. Not the same species, to be sure, but modern ones which may be our cousins many times removed. Humans are not the end point of evolutionary history --all species living today are.

       Natural selection, the motor of evolution, does not have a plan. The only criterion for survival is adaptation to existing conditions. The dinosaurs didn’t die out because they were evolutionary failures or because they were less perfect animals than present-day animals. In fact, dinosaurs have been one of the most successful groups in the history of life. They dominated macroscopic life for over 200 million years. Mammals, in contrast, have only been conspicuous for some 60 million years. The dinosaurs died because their environment changed abruptly when a very large meteorite or comet collided with the earth, some 65 million years ago.

       Consider another example of the progress fallacy. Mammoths, the hairy ancestors of modern elephants, were well adapted to life in the latest Ice Age. Hairless elephants are well adapted to present-day conditions. But a hairless elephant, as Gould points out, is not a cosmically better elephant. When another ice age comes --and it will--, a hairy elephant will be more likely to survive.

       And this brings on my final point. Possible hairy elephants of the frigid future will NOT be mammoths. The mammoth is dead and gone. If elephants ever have hairy descendants, those descendants will be new adaptations to cold weather. They may conceivably look somewhat like mammoths --with all the hair and stuff--, but the resemblance will stem from the fact that both species are solutions to a similar problem --like bats and birds. Extinction, as diamonds, is forever.

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