New Theory Says If We Traveled Back to the Beginning of Time, Humans Might Never Have Evolved
When you look at the evolutionary tree of life, it's comforting to follow the one strand that starts at "bacteria," passes through "fish" and "amphibians," makes it through each of the mass extinctions, and ends all the way at "humans." In the grand Pachinko game of life on Earth, humans not only made it, we made it to the top.
In 1989, Stephen Jay Gould published Wonderful Life, which focuses on a group of well-preserved fossils from just after the Cambrian Explosion, a period in which many new forms of life developed. Gould argues that many of the now-extinct species from that time were just as well-adapted to survive as the species that actually did, which implies that luck can sometimes play a bigger role in evolution than natural selection or beneficial adaptations.
Gould goes on to say that if we rewound the history of Earth back to the beginning and allowed it to play out again under slightly different circumstances, the evolution of life would be radically different, maybe even unrecognizable to what we know today.
This is due in part to the fact that natural selection relies on random mutations in DNA, and that the conditions that would produce a specific, beneficial mutation may be extremely rare.
In essence, Gould is arguing for the evolutionary equivalent of the Butterfly Effect.
There is another school of thought, however: "Convergent evolution."
This theory argues that different forms of life can independently develop similar features to survive in similar environments, and can do so again and again. Even if the first proto-frog was killed by bad luck (say, a lightning bolt), its evolutionary ancestors have the capacity to produce another, slightly different proto-frog that might survive.
This idea has been borne out in new research, published in Current Biology, which tracked the development of stick spiders across several islands in Hawaii.
The spider species, which are isolated to their individual islands and all descended from gold-colored ancestors, have independently evolved to become black or white-colored as a form of camouflage in multiple instances, giving credence to the idea that even if one specific species goes extinct, another similar species will probably arise in the same environment, despite the vicissitudes of fate.
It's still unclear whether humans (or something like us) were an inevitability here on Earth, but if we hope to find intelligent life on other planets, we'd better learn more about the detours and patterns in our own evolution.