Rewinding And Replaying Evolution

What are we left to think about human existence? With all the evidence pointing to our ape-like origins 6 Mya and our shared ancestry with everything down to cockroaches, slime molds, pond scum, tarantulas, pit vipers, foot fungus, and even viruses, the unique evolutionary status of H. sapiens may seem bleak and maybe even disgusting.

But even though we have lousy relatives, should we think less of our species? If we can forget about the aforementioned negative impacts we have made on the planet is it possible to still revel in our exquisite uniqueness? After all, it is remarkable what Mother Nature made from an ancestor with a small brain, clumsy hands, and no significant emotional or artistic forms of expression. We are both odd and wonderful, but are we unique for all time and in all of eternity? What if just one event happened differently in the course of Earth's history—would humans still have evolved?

If an asteroid had never wiped out the dinosaurs it is possible we would not be here today. The worldwide extinction event at the Cretaceous-Tertiary boundary spared small mammals and it is from these survivors that the entire mammalian radiation was born. Thanks to those early mammals, it is giraffes, not brontosaurs, that eat the leaves from the tallest trees and it is humans, lions, and hyenas, not tyrannosaurs, that sit atop the trophic pyramid.

Consider setting the clock back to zero, back to the Big Bang and the creation of this universe 14 billion years ago. Then set it in motion. What would the "redo" of history look like? Would it be the same, like a television rerun, or would it become a completely different episode?

Paleontologists Simon Conway Morris and Stephen Jay Gould (19412002) entered a famous debate on this issue. Simon Conway Morris argues that a redo of evolution would be a rerun because history is constrained by laws of physics and biology, and so forth; not all things are possible. Under normal environmental forces, life will adapt accordingly and there are only so many ways to do that. Morris emphasizes the numerous examples of convergent evolution like sharks and dolphins, and the evolution of saber-toothed cats in both the marsupial and placental mammal lineages.

Stephen Jay Gould saw the complete opposite view and argued that a replaying of evolution would result in a whole new episode of life. He called humans and the entire guild of living organisms alive today a "glorious accident" brought about by a unique unrepeatable sequence of events during Earth's history. Each time evolution is rewound and replayed and the lottery of events starts all over, the result will be an entirely new outcome with entirely unique organisms. Although he concedes that there are limits to the diversity that could result from such a hypothetical experiment, Gould argued that the chances are nearly impossible that today's world with today's creatures would inevitably evolve if Mother Nature was given another whirl. To Gould, examples of convergence are rare and underwhelming and the most important feature at stake in his argument, the uniqueness of humanness, humanity, and human consciousness, has not evolved in any other living being. From Gould's perspective, it is hard to imagine that anything like H. sapiens would evolve ever again.

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