Earth was warmer 34 cardinal years ago—and wonkier. The Pangea supercontinent had divided up. The dinosaurs were agelong gone. But Antarctica contained glacier-free forests. The different continents looked similar squished and smeared versions of their contiguous selves. Mammals were everywhere—especially primates and rodents. “From New York to Los Angeles, up into Canada, bopping astir successful the trees each implicit the place,” says Seiffert of North America’s primates. “But erstwhile this climatic lawsuit happened 34 cardinal years ago, they each disappeared.”
Some scientists judge that atmospheric c dioxide levels dropped past a captious threshold, causing mean aerial temperatures to driblet and Antarctica to freeze. More sunlight reflecting disconnected of much crystal made temperatures plummet further. The modulation from the Eocene to Oligocene has been described arsenic a modulation from “greenhouse” to “ice house.”
Then, successful Africa, things got adjacent worse. Around 31 cardinal years ago, volcanoes adjacent the equator, successful present-day Ethiopia, exploded with devastating toxic fumes and unrelenting floods of molten basalt.
North American, European, and Asian fossil records are beauteous well-established during the 11 million-year agelong earlier and aft these events. Scientists could number up the fossils showing which animals existed earlier the clime cooled, and which after, and fig retired what went missing. But, Seiffert says, “during this clip period, the fossil grounds successful Africa is really truly patchy.” That discrepancy bugged him, truthful his squad has tried to parse the relationships betwixt immoderate fossil records they bash have.
For their study, Seiffert and de Vries focused connected a household histrion that goes backmost 76 cardinal years, erstwhile primates and rodents diverged. Specifically, they studied the teeth of 2 suborders of rodents (hystricognath and anomaluroid) and 2 suborders of primates (strepsirrhine and anthropoid). These clades gave emergence to existing taxon similar capybara, scaly-tailed flying squirrels, lemurs—and us.
The researchers resolved to reconstruct the phylogeny—or household histrion of evolutionary relationships—of these groups from 56 cardinal to 15 cardinal years ago. Using teeth arsenic a usher for “who is who,” they drew branches betwixt lineages extending from fossils recovered successful the precocious Eocene to their progeny that survived into the Miocene, astir 20 cardinal years ago. When they finished, a stark spread emerged: The lineages from the Miocene descended from an oddly tiny fraction of earlier mammals. The researchers recovered that 63 percent of lineages that existed successful the precocious Eocene ne'er made it past the adjacent epoch. About 30 cardinal years ago, they concluded, these taxon indispensable person vanished, acknowledgment to their changing environment. “There's truly nary different explanation,” Seiffert says. “They indispensable person gone extinct.”
Lineage diverseness gave the squad a large representation presumption of however galore taxon were mislaid to a changing climate, but not of however antithetic those taxon mightiness person been from 1 another—in different words, however overmuch anatomic diverseness went extinct, too. For example, says de Vries, ideate a script successful which 2 kinds of vertebrate spell extinct. Those 2 taxon could beryllium precise similar, oregon they could beryllium wildly antithetic successful presumption of assemblage types, genetics, oregon ecological niches. “If you person a hummingbird and a flamingo, that is precise antithetic than if you had a pigeon and a dove,” she says.