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The earth was warmer 34 million years ago — and more amazing. The Pangea supercontinent split. Dinosaurs are long gone. But Antarctica had glacial forests. The rest of the continents seemed to be oppressed and tainted versions of today’s identity. Mammals were everywhere, especially primates and rodents. “From New York to Los Angeles, Canada, wandering through trees all over the place,” Seiffert says of North American primates. “But when this climatic event happened 34 million years ago, it all disappeared.”
Some scientists believe that the level of carbon dioxide in the atmosphere exceeded a critical threshold, the average air temperature dropped and Antarctica froze. As more sunlight reflected more ice, the temperature dropped more. The transition from the Eocene to the Oligocene has been described as a transition from “greenhouse” to “ice house”.
Then, in Africa, things got even worse. About 31 million years ago, volcanoes near the equator, in present-day Ethiopia, erupted with destructive toxic fumes and continuous flooding of molten basalt.
Fossil records from North America, Europe, and Asia are fairly well established between 11 million years before and after these events. Scientists were able to count fossils before and after the climate cooled to find out which animals existed and after, and what was lost. But, Seiffert says, “in this period of time, the record of African fossils is really weak.” This disagreement was of concern, so he tried to study the relationships between the fossil records that his group had.
For their study, Seiffert and de Vries focused on the 76 million-year-old family tree, when primates and rodents moved away. Specifically, they investigated the teeth of two suborders (histricognath and anomaluroid) and two suborders of primates (strepsirrhine and anthropoid). These clades created existing species such as capybara, flying squirrels with scaly tails, lemurs — and us.
Researchers decided to reconstruct the genealogical tree of the phylogeny or evolutionary relationships of these groups from 56 million years ago to 15 million years ago. Using teeth as a guide to “who’s who”, from fossils found in the late Eocene to Miocene survivors to their descendants, about 20 million years ago, they drew branches between lineages. When they were finished, a significant gap arose: the Miocene lineages arose from a small strange part of the earlier mammals. Researchers found that 63 percent of the lineages that existed in the late Eocene had not passed through the next era. About 30 million years ago, they concluded, these species must have become extinct thanks to their changing environment. “There’s really no other explanation,” Seiffert says. “They have to disappear.”
The diversity of lineages showed the group how many species were lost as a result of a changing climate, but not how different these species could be from each other, i.e. how many species were lost. anatomical diversity also disappeared. For example, says de Vries, imagine a scenario in which two types of birds disappear. These two species could be very similar, or they could be very different in terms of body types, genetics, or ecological niches. “If you have a hummingbird and a flamingo, that’s very different than if you were a pigeon and a pigeon,” he says.
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