We live in a human world worried about extinction, and with a recent memory of nuclear arms races. In evolutionary terms, we're a little late to the party.
What leads to great evolutionary change? What, for instance, lead to the rise of the dinosaurs? The rise of the mammals? A common answer (and probably a correct one) is extinction events. The Permian extinction (pleasantly termed the ‘Great Dying’) of roughly 250 million years ago cleared 57% of biological families from the world, including 70% of land dwelling vertebrates and 81% of all marine species. The less extreme, but more famous Cretaceous extinction which ended the age of the Dinosaurs roughly 66 million years ago got rid of the largest predators on earth, allowing mammals to grow to previously unprecedented sizes.
This echoes the famous work of palaeontologist J John Sepkoski, who in a 1981 study called for a division of the history of oceanic life into ‘three great evolutionary faunas’ broken up by mass extinction events. His study used mass extinction events taking place 490 and 250 million years ago as dividing marks between the three faunas, named, in order, the Cambrian, the Paleozoic, and the Modern. This division of evolutionary life is not the earliest (John Phillips suggested a three system model in the 1800s) or accepted by all (Brenchley and Harper suggested two Pre-Cambrian faunas they call the Ediacaran and Tomottian), but it is certainly the most influential.
But now scientists at Umea University in Sweden and the Florida Museum of Natural History have identified a fourth evolutionary fauna, satisfying similar criteria to Sepkoski’s original study. But which mass extinction event demarcates this period of evolutionary history? None at all. Instead, this period was ushered in by a much more gradual period of change driven by interactions between organisms. Sounds a little dull, I agree. So let’s reframe it a little bit.
Bats vs Moths: An arms race
Human arms races are boring. A long, repeating story of governments pimping out their brightest and best to make a fancier, more destructive device to wave over their enemies’ heads like a big threatening stick instead of doing things which would make the world a better place. They’re all politics. Bats and moths don’t have politics.* They just want to eat, and not be eaten. So how does it work?
Bats eat moths. Bats find moths using echolocation, making high frequency calls largely outside of the range of human hearing and seeing how the sound waves bounce around the space they’re in. The moths that were best able to survive bats’ echolocation systems were ones who had developed ears which could hear the echolocation. So the most successful bats became ones which used stealth echolocation (yeah, nice), at frequencies that not even moths could hear. And then moths started making ultrasonic clicks. This defends them from bats, but the exact reason why isn’t known.**
That’s the essence of an evolutionary arms race. A predator, and a prey species coevolving in order to eat more and be eaten less. Of course, evolutionary arms races exist outside the worlds of predators and prey. They exist in the worlds of species competing for resources, species competing to not be eaten by the same predator, and a whole range of more ecologically complex interactions I have trouble understanding.
The new era
The new transition is posited to have occurred from 250 to 66 million years ago, with a new era (named in the paper as Mid Cretaceous-Quaternary) beginning around then, arising out of the Triassic-Lower Cretaceous eta. The evolutionary arms race that drove this was ocean wide, and is named the Mesozoic Marine Revolution, beginning around 150 million years ago. This was caused by the rise of marine predators, including bony fish, crustaceans, and molluscs. The rise of predators made the prey species become more mobile and develop some other, extremely impressive defence techniques.
The future of the distant past
Why was this study able to create a new era of evolutionary fauna? Especially considering that Sepkoski actually knew about the Mesozoic Marine Revolution, an idea that’s been around since the 70s? Simple: more data. Sepkoski just didn’t have the fossil record data to prove that the fauna had changed enough to constitute a different evolutionary epoch. As more palaeontological data (a fancy term for new fossils, more or less) is uncovered, there may be far more evolutionary faunas than we understand there to be now. The history of life on earth is a fascinating topic for so many people, young and old alike, and this study is a reminder that while we can’t even see days into the future, we might be able to see hundreds of millions of years into the past.
Read more:
https://www.nature.com/articles/s42003-021-01805-y - study in Communications Biology
https://www.cambridge.org/core/journals/paleobiology/article/abs/factor-analytic-description-of-the-phanerozoic-marine-fossil-record/F2DC148BF90657CC5DF560D8E3A22BF6 - Sepkoski's original 1981 study
Cover image credit: Florida Museum of Natural History
*That I know of, anyway, it would be great if they did.
**My favourite theory is that it startles the bat, like the person you’re looking for in hide and seek screaming “BOO” at you when you find them and completely turning the tables, allowing the moth to flee.
Interesting. Question: how do the hyperoxic and other atmospherically different periods come into (affect?) hese periods? Are they important at all inthe big picture?