I’ve always had a fondness for classic thunderbirds. The sleek, sporty chassis– the loud hum of the engine; the characteristic fins along the trunk. And even better if they had a bit of whitewall on the tires. Little did I know this predilection for “the Bird” would presage my discovery that I wouldn’t have been born if it weren’t for a thunderbird. A red thunderbird, to be precise.
You see, my father owned just such a beast. And it was this car that won him a date with my mother. Now, before we judge lest we be judged, let’s remember that cars were a major commodity in the mid-20th century dating scene. And let’s face it: look at this beauty. Who wouldn’t want to date the owner of this bad boy?
But it’s an ironic thought to realize that my entire existence hung in the air like a pair of giant fuzzy dice on the mirror of Life’s hot rod. And I’m sure many of us have similar stories, in which the meetings of our mothers and fathers hinged on some otherwise seemingly insignificant but very particular detail: A chance encounter, a friend’s introduction, or even a red sports car.
Although Stephan Jay Gould didn’t focus on individual organism’s lives like my paltry self but on species, he would probably nevertheless recognize my story as an example of “historical contingency,” which he first wrote about in his book, Wonderful Life: The Burgess Shale and the Nature of History. By historical contingency, we simply mean that events are influenced by chance and lead to substantial long term effects. Gould proposed historical contingency in the form of a thought experiment: If we rewound the tape of life, would it repeat itself or would each replaying be unique?
Gould was a proponent that Life’s tape would not replay in much the same way and that unique, largely unpredictable events– such as major extinctions– have completely changed the succeeding biological landscape. He, therefore, believed that historically contingent events could, and often did, have long-reaching effects on evolution and that we couldn’t ignore their influence in our studies.
Since the publication of Wonderful Life, scientists have argued back and forth about Gould’s thought experiment. Unfortunately, being a thought experiment, it is an untestable proposition. How can one possibly rewind the tape of Life? Many have also argued, with ample evidence, that although specific species or clades may be wholly different as a result of historically contingent events, evolution nevertheless has a tendency to fill environmental niches with organisms playing overlapping roles and sometimes even looking quite similar thanks to convergent evolution (e.g., ichthyosaurs and modern dolphins). In short, we would say that although the species differ over time, different organisms often fill the same “morphospace,”– aka, animal phenotypes.
Nevertheless, there have been other events in Life’s history, aside from extinctions, that have forever changed the trajectory of a clade. Another prime example is the two rounds of whole genome duplication that occurred in early vertebrates (our lineage). It’s safe to say that these whole genome duplication events, which provided ample new genetic material to play with and evolve, have opened up remarkable opportunity for our lineage, including dramatic changes to brain development itself. Suffice it to say, we did not have true brains before this point in time but just disparate groups of cerebral ganglia (groups of neurons). Fish might not seem that smart, but they are worlds apart from earlier chordates.
There is also considerable evidence that changes to climate and oxygen availability drove fishapods (predecessors to true tetrapods) out of the near-shore and onto land, where they ultimately found a smorgasbord of delectable insects to munch on and good reason to stay. Once again, had certain groups of lobe-finned fishes not already been living near-shore and had the climate not changed at just the right time, vertebrate tetrapods may never have ventured onto land and become fully terrestrial. For certain transitions, the stars must align (metaphorically).
But what about our species? Opponents of historical contingency are right to say that niches in the environment are often filled by organisms filling similar morphospace, but are individual species really as interchangeable as all that? Let’s do a similar thought experiment à la Gould using our own species, Homo sapiens:
Let’s rewind the tape of Life to 66 million years in the past. In this version, let’s say we give a tiny nudge to that giant asteroid that, in our timeline, created the Chicxulub crater and devastated life on earth, laying waste to the non-avian dinosaurs. Now instead, the asteroid is going to narrowly miss the earth, erasing the Cretaceous-Paleogene (K-Pg) extinction event altogether.
What happens to the mammals who were primed to fill all that available morphospace of big animals? As humans, we are not lions, tigers, or bears, but we are large primates. What happens to us? Do we stay small? Do we ever fully venture out of the trees knowing the range of sauropod predators that exist down there? Being largely arboreal, does the complexity of our societies continue to evolve or do we stagnate? Without leaving the trees, do we ever integrate hunting and meat-eating as a more regular part of our diets, supplying basic materials for our evolving big brains?
Mammals experienced a rapid, punctuated period of adaptation as they moved into and filled niches that non-avian dinosaurs once held. But without the K-Pg extinction, it is highly unlikely our species would exist today.
We are but one species. But in this thought experiment, think of the immensity of those implications. Would another animal come to fill our morphospace, control fire, design the wheel, publish books, invent computers, write this blog, or even form thought experiments in the first place?
Humans cannot be the exception; no species can. Yes, natural selection and adaptation are significant determinants in how life has evolved– and continues to evolve– on this planet. But we cannot ignore the mass extinctions, the whole genome duplications, and other “red thunderbirds” that have shaped Life’s history just as fundamentally.