It’s been 25 years since Gould’s Wonderful Life: The Burgess Shale and the Nature of History crystallized the debate over the importance of contingency in evolution, most famously illustrated by his metaphor of “replaying the tape of life”. If we could roll back the history of life on earth and restart it in the pre-Cambrian, would we see the same forms arise again? Would we have dinosaurs a few hundred million years later, and bipedal intelligent apes after a half billion years?
Gould’s answer was no — that the role of chance was too great, and because the forms of life do not represent optimum ideals built by perfectly plastic forms, but rather are kludges built atop limiting and enabling prior adaptations. Some people, like Simon Conway Morris, argue otherwise that there are ideal forms (including us bipedal anthropoids) upon which life will tend to converge. I favor Gould’s view rather strongly — it’s absurd to talk about evolution without an appreciation that all organisms are the product of history, and that what traits we have now are almost entirely contingent on what traits our ancestors had.
My favorite example of the error of limited thinking about adaptive ideal forms is a comparison of bony fish and squid. Both are fast swimming, active predators that are torpedo shaped in motion (physics constrains that!), but in detail everything is completely different. If the pre-Cambrian ancestor of all chordates were accidentally squashed, there is no reason to assume that the pre-Cambrian ancestor of all molluscs would then evolve a line of torpedo-shaped predators with brainy skull at the front and an undulating muscular body for propulsion. Why should they? They have a demonstrated capacity in our universe to evolve into torpedo-shaped predators with tentacles at the front and propulsion via a jet of water squirted out of a muscular mantle cavity.
The body plan of the ancestor dictates what capacities the descendant will have. There is no reason to assume that the world as we see it now was inevitable — that’s simply a failure of imagination and reason.
An article by Zach Zorich explores other examples of evolutionary contingency. Much of it is dedicated to that fascinatingly concrete example of actually being able to roll back the tape of life, the Lenski experiments, in which populations can be frozen and restarted at any time. In that case we seen on a molecular level that the evolution of specific biochemical problems is not inevitable at all, but depends entirely on the presence of prior mutations. History, and your parents, matter!
But I also like this example.
“Not everything is possible,” no matter the process, Wake explains. “Organisms evolve within the framework of their inherited traits.” Organisms can’t pass on mutations that kill them or prevent them from reproducing. In the case of Hydromantes salamanders, their ancestors had to overcome a serious limitation: To acquire their ballistic tongues they had to lose their lungs. That’s because their tongue partly derives from muscles that their predecessors instead used to pump air into the lungs. Now, that formerly small and weak muscle is much larger and stronger. It wraps like a spring around a tapered bone at the back of the mouth, and when the muscle squeezes, the bone generates the force that fires the tongue along with its bones out of the mouth. So, Hydromantes’ ancestor did not simply acquire a mutation and evolve a fast ballistic tongue. Instead, the adaptation followed a series of mutations that first enabled the creature to overcome its reliance on lungs for oxygen and buoyancy control. Each change was contingent on the one before it.
Chameleons, on the other hand, retain their lungs. Instead of re-tooling their lung anatomy, they have evolved a piece of collagen that allows them to catapult their tongues at prey. On the surface, salamander and chameleon tongues converge, but not upon closer inspection. It takes a chameleon 20 milliseconds to shoot its tongue at its prey, a positively glacial pace when compared to the Hydromantes’ five-millisecond firing time. Why are chameleons stuck hunting with such slow tongues? The answer is that they have encountered a kind of obstacle to convergent evolution. The chameleon’s tongue is fast enough to ensure their survival, but they lack the “framework of inherited traits” to evolve the salamanders’ deadlier ballistic anatomy. The chameleons have reached what biologists call an “adaptive peak.”
Before anyone says that ballistic tongues represent an example of convergence, too, I’ll agree…but with the caveat that this is an example of modifying extant traits in the tetrapod toolkit. Two vertebrates evolved ballistic tongues, because they share the trait of having tongues. Squid also have a high speed prey capture mechanism, only lacking tongues, they use a pair of arms.
As D’Arcy Wentworth Thompson put it, “Everything is the way it is because of how it got that way.” You can’t appreciate evolution unless you also recognize the path it takes…it’s all about the trajectory.