Aeon is a nonprofit science and technology magazine that occasionally has some good pieces, though I’m not a frequent reader. However, several readers called my attention to a new piece by Dan Falk, a Canadian science writer, about whether or not human evolution was inevitable. Click the screenshot to go to the article:
Falk poses a strict dichotomy that many of you will have encountered before: was the evolution of humans (or “humanoids”: human-like creatures with high intelligence) inevitable, or was it due to a “stroke of luck”? This is the same debate that was once raged between Simon Conway Morris and Stephen Jay Gould. In his book Wonderful Life on the creatures of the Burgess Shale, Gould emphasized that humans were a “contingent” product, and if the tape of life were rewound, things could have turned out very differently. The early chordate Pikaia, for instance, could have gone extinct through “bad luck”, and hence no vertebrates and no us. The message of Gould’s book was that humans (and perforce other beasts) are here by accident.
In contrast, Simon Conway Morris, who is a Christian, has taken the view that the evolution of humans on Earth was inevitable, and if the tape of life were rewound, we’d still evolve. It’s pretty clear that his views are conditioned by his faith, because of course he sees God as having directed the process, with humans the end-product made in His image. (Also, Conway Morris’s argument based on evolutionary convergence—the repeated and independent evolution of some body plans, like those of fish, dolphins, and ichthyosaurs—doesn’t support his argument, as human intelligence is a one-off, not seen in any other animal.)
But Falk, like many others writing on this subject, is confused by the meaning of “contingency” and “accident”. I discuss all this in Faith Versus Fact, but I’ll give a precis here.
First, you have to clarify by what you mean by “rerunning the tape of life”. Do we mean starting the Earth over and over again with every molecule and particle in the same position, and then seeing what happens? If you’re a determinist, then I’ll maintain that “contingencies” and “accidents” are really determined circumstances, and so everything would pretty much evolve as it did over and over again—with one caveat (see below).
If you mean starting the Earth under different conditions, like different temperatures or different configurations of seas and continents, then of course evolution would not necessarily be repeatable, for the environment—a crucial component of natural selection—would be different. And that means that all we can say to the question “Was human life inevitable?” is “Who knows?”
Finally, you need to define what you mean by “humans”? Do you mean anything with the intelligence of humans, like hyper-cerebral octopi? Do the creatures have to have syntactic language? Do they have to recognize and worship God? (I suspect that Conway Morris would say “yes” to the last question.) Again, based on the one-offness of human intelligence, I don’t think we can say that human evolution was inevitable, even if you accept Conway Morris’s argument and the voluminous data he’s compiled on convergence in other animals. Again, we get a “who knows?” answer.
Now let’s answer the question as pure determinists. If you start the Earth over again with every molecule and particle in place, then evolution will repeat itself more or less exactly, as there are no contingencies, no “bad luck,” no accidents. All those things, like the asteroid striking the Earth, supposedly killing the dinosaurs, were inevitable consequences of the laws of physics.
Unfortunately, Falk, like many, mistakes “unpredictability” for “true indeterminism,” so he sees evolution as the result of a series of “accidents”. To wit:
But even if evolution has a direction, happenstance can still intervene. Most disruptive are the mass extinctions that plague Earth’s ecosystems with alarming regularity. The most catastrophic of these, the Permian-Triassic extinction, occurred about 250 million years ago, and wiped out 96 per cent of marine species, along with 70 per cent of land-dwellers. Gould examined the winners and losers of a more ancient mass extinction, the Cambrian-Ordovician extinction, which happened 488 million years ago, and found the poster child for biological luck – an eel-like creature known as Pikaia gracilens, which might be the precursor of all vertebrates. Had it not survived, the world could well be spineless.
But those mass extinctions were determined by physical laws, and are “happenstance” only in the sense that a well-informed human couldn’t predict them. Likewise this:
With the planting and harvesting of wheat and barley, and the domestication of livestock, it can seem like a short and perhaps inevitable step to the walls of Jericho and the pyramids of Giza. This doesn’t remove contingency from the equation. ‘There is nothing inevitable about the origins of agriculture,’ Chazan says. ‘It didn’t have to happen – but once it happened, it’s irreversible.’
Under determinism, of course, the origins of agriculture had to happen, and would happen again (along with the evolution of humans) under a true rerun of the tape of life.
So if we had a true rerun of the tape of life, with all the starting conditions repeated exactly, then, it seems, evolution would have repeated itself, and we’d have all the species, living or dead, that actually did evolve.
With one caveat.
And that is the caveat that if there is any true indeterminism in the history of life, of the quantum sort, then no, evolution wouldn’t repeat itself, even under identical starting conditions.
One such indeterminism is mutation. If mutation involves quantum effects, such as cosmic rays, then the process of mutation is fundamentally indeterministic. And if mutation is fundamentally indeterminstic, then it’s entirely possible that evolution, which is fueled by mutations, would also be indeterministic, and would not go the same way on a rerun. We don’t know the answer. Another spanner in the works: mutations are recurrent: the same mutations often happen over and over again, especially in large populations. So even if a single mutation changing a single DNA base is a single individual indeterminate, it may be statistically determined, so that again, evolution would go the same way under a rerun.
Finally, we’re only talking about Earth here. If you take other planets into account, all bets are off. There’s simply no way to answer the question of whether, somewhere in the vast Universe, humanoid creatures would inevitably evolve.
1). If by “rerun of evolution” we mean “a starting condition repeated exactly, with every molecule in place,” then yes, the evolution of all species, including humans, was inevitable—but only if mutations are not fundamentally indeterminate occurrences.
2). Under all other scenarios, including indeterminate mutations, the answer to the question “Was the evolution of humans inevitable?” must be “How the hell do I know?”