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Life’s Speed Limit

Mass extinctions have decimated the diversity of life on Earth numerous times. Almost all ocean life was lost during the aptly named “Great Dying” 252 million years ago. And after an extinction was triggered by a massive asteroid strike 66 million years ago, the evolutionary tree of dinosaurs had been chopped down to a single branch: modern birds.

Nevertheless, after extinction, recovery begins. And no matter the circumstances of what caused the extinction, scientists have found that the recovery speed limit — the time it takes for species diversity to reach its pre-extinction heights — is about 10 million years.

What scientists didn’t know was why. Researchers at The University of Texas at Austin Jackson School of Geosciences have hunted the fossil record for answers. They found that evolution — specifically, how long it takes surviving species to evolve traits that can help fill open ecological niches or create new ones — could be behind the recovery speed limit.

The study, published April 8, 2019, in the journal Nature Ecology & Evolution, zeroed in on how microscopic life forms called forams recovered after Earth’s most recent mass extinction (the one that snuffed out the dinosaurs). The asteroid impact that triggered the extinction is the only event in Earth’s history that brought about global change faster than present-day climate change. So, the authors said the study could offer important insights on recovery from ongoing, human-caused extinction events.

The idea that evolution was behind the recovery speed limit was proposed 20 years ago, but this study is the first to find solid evidence for it in the fossil record. Foram fossils are prolific in ocean sediments around the world, allowing the researchers to closely study species diversity without any large gaps in time.

Lead author Christopher Lowery, a research associate at the University of Texas Institute for Geophysics, said that the team found a close association between foram complexity and the recovery speed limit, which points to evolution as the key factor behind the speed control.

“We see this in our study, but the implication should be that these same processes would be active in all other extinctions,” Lowery said. “I think this is the likely explanation for the speed limit of recovery for everything.”