Earth was hotter 34 million years in the past—and wonkier. The Pangea supercontinent had break up up. The dinosaurs have been lengthy gone. However Antarctica contained glacier-free forests. The opposite continents seemed like squished and smeared variations of their current selves. Mammals have been in all places—particularly primates and rodents. “From New York to Los Angeles, up into Canada, bopping round within the bushes far and wide,” says Seiffert of North America’s primates. “However when this climatic occasion occurred 34 million years in the past, all of them disappeared.”
Some scientists consider that atmospheric carbon dioxide ranges dropped previous a crucial threshold, inflicting common air temperatures to drop and Antarctica to freeze. Extra daylight reflecting off of extra ice made temperatures plummet additional. The transition from the Eocene to Oligocene has been described as a transition from “greenhouse” to “ice home.”
Then, in Africa, issues bought even worse. Round 31 million years in the past, volcanoes close to the equator, in present-day Ethiopia, exploded with devastating poisonous fumes and unrelenting floods of molten basalt.
North American, European, and Asian fossil data are fairly well-established through the 11 million-year stretch earlier than and after these occasions. Scientists might rely up the fossils displaying which animals existed earlier than the local weather cooled, and which after, and determine what went lacking. However, Seiffert says, “throughout this time interval, the fossil report in Africa is definitely actually patchy.” That discrepancy bugged him, so his staff has tried to parse the relationships between no matter fossil data they do have.
For his or her examine, Seiffert and de Vries centered on a household tree that goes again 76 million years, when primates and rodents diverged. Particularly, they studied the tooth of two suborders of rodents (hystricognath and anomaluroid) and two suborders of primates (strepsirrhine and anthropoid). These clades gave rise to present species like capybara, scaly-tailed flying squirrels, lemurs—and us.
The researchers resolved to reconstruct the phylogeny—or household tree of evolutionary relationships—of those teams from 56 million to fifteen million years in the past. Utilizing tooth as a information for “who’s who,” they drew branches between lineages extending from fossils discovered within the late Eocene to their progeny that survived into the Miocene, round 20 million years in the past. After they completed, a stark hole emerged: The lineages from the Miocene descended from an oddly small fraction of earlier mammals. The researchers discovered that 63 % of lineages that existed within the late Eocene by no means made it previous the following epoch. About 30 million years in the past, they concluded, these species should have vanished, due to their altering setting. “There’s actually no different clarification,” Seiffert says. “They should have gone extinct.”
Lineage variety gave the staff an enormous image view of what number of species have been misplaced to a altering local weather, however not of how completely different these species may need been from each other—in different phrases, how a lot anatomic variety went extinct, too. For instance, says de Vries, think about a situation by which two sorts of chicken go extinct. These two species might be very related, or they might be wildly completely different by way of physique sorts, genetics, or ecological niches. “When you have a hummingbird and a flamingo, that may be very completely different than should you had a pigeon and a dove,” she says.
