Scientific American has looked at the fossil record and found it wanting. Instead of there being a regular pattern of mass extinctions (like the death of the dinosaurs) followed by the rise of new dominant life forms (like the rise of mammals), the two things don’t seem to be related at all. Mass extinctions don’t clear a path for explosions of new life, however much we want them to:
“A classic example is the suggestion that mammals could not radiate until dinosaur competitors were removed in the end-Cretaceous mass extinction event,” when an asteroid impact triggered the world’s fifth massive loss of species, says data scientist Jennifer Hoyal Cuthill of the University of Essex in England. But by analyzing fossil occurrences logged in the collaborative Paleobiology Database using a machine-learning algorithm, Hoyal Cuthill and her colleagues found that life’s most prominent pulses did not usually follow the world’s greatest biological wipeouts.
To track biodiversity through time, the researchers turned to the Paleobiology Database. The team selected more than more than 1.27 million fossil data points from the database to determine where and when prehistoric organisms occurred through the ages. The resulting graph told a more complex story than mass extinctions clearing the decks for rapid evolutionary radiations. “The two most extreme examples of radiation we identified were [near] the beginning of the Cambrian and Carboniferous periods,” Hoyal Cuthill says. The first, about 532 million years ago, was when animal life burst on the scene, and the predecessors of today’s arthropods, vertebrates and many other creatures evolved. The second, about 358 million years ago, occurred when some vertebrates evolved to live on land for the first time in their history.
Neither of these events was tied to a mass extinction. Instead, Hoyal Cuthill notes, “these are times when life is thought to have been diversifying into new ecological arenas.” During the Cambrian, animals began to feed on each other in more complex ways.
Of the “big five” mass extinctions over time, four were not immediately followed by evolutionary radiation. The one exception occurred as the Permian period gave way to the Triassic period: After the worst mass extinction of all time, about 252 million years ago, life quickly recovered in the early part of the Triassic.