For decades, scientists thought that the story of human evolution was fairly straightforward: We and our primate ancestors evolved in Africa over millions of years, then started crossing continents and traversing seas to reach all the places we’re found today. Simple (ish).
“There were a lot of questions,” said K. Christopher Beard, a paleontologist at the University of Kansas. “What caused it was the biggest kind of cosmic question, because we always want to answer ‘why?’ But even things like ‘when?’ and ‘how?’ were a mystery.”
Decades later, “the full story is only now emerging,” Beard said. And a new discovery could help fill in the gaps.
In a study published Thursday in the journal Science, Beard and his colleagues at the Chinese Academy of Sciences in Beijing report on an “incredible cache” of fossils from 10 previously unknown species uncovered in China’s Yunnan province. These fossils help illuminate a new story of our evolution: one in which our primate ancestors evolved in Asia, sailed across a narrow sea to Africa, then were pushed to extinction on their home continent because of drastic climate change. Some of the only primates that survived were the ones whose fossils were just uncovered — primitive creatures that were closer to lemurs than apes and humans living today.
“It’s a little complicated,” Beard said, almost sheepishly.
You don’t say.
This more convoluted version of our history begins in the Eocene, some 40 million years ago. At this time, Earth’s climate was hot and humid, and the continents were just beginning to move into the positions they hold today. India was zooming headlong toward the bottom of Asia (the inevitable collision would one day give rise to the Himalayas). An inland sea flooded the center of the Eurasian land mass. And Africa was an island continent, separated from Asia and Europe by a narrow stretch of ocean.
Early anthropoid (humanlike) monkeys were flourishing in Asia at that time. But they also, somehow, found a way to migrate across the watery barrier to Africa. And since monkeys don’t really swim, scientists’ best theory about their migration is — I kid you not — that they sailed across on rafts made of trees.
“You’re laughing,” Beard said, “but it’s now known that this happened repeatedly. Because of the greenhouse conditions, a lot of monsoons were hitting Asia at the time. When that happens, rivers would flood, riverbanks erode. A half an acre of land with a bunch of trees growing out of it falls into a river and floats out to sea.”
“And if there are a bunch of monkeys hanging out in the trees when that happens,” he continued, “suddenly those monkeys become sailors.”
It was a good thing, too, because climate records show that dramatic changes started ravaging the Earth soon after. Around 34 million years ago, the warm, wet climate of the Eocene gave way to the cooler, drier Oligocene epoch. Tropical forests receded, open plains and deciduous trees sprouted across the Eurasian continent. Life for monkeys in Asia suddenly became very, very hard.
That’s evident in the fossils Beard and his colleagues found in Yunnan province. These tropical tree-dwellers had been pushed south to stay with the dwindling tropical forests. They were almost all strepsirrhines (lemurlike) primates; the only anthropoid fossil came from a tiny, very primitive member of the group.
“This was more or less the anthropoids’ last stand that [the fossils] are capturing,” Beard said.
The fossils “fill a gap,” in our understanding of our evolutionary history, Stony Brook University primatologist John Fleagle, who was not involved in the study, told the Christian Science Monitor. They illustrate “a whole aspect of primate evolution that wasn’t clearly documented before.”
They also help pinpoint exactly when “the plot shifted” from Asia to Africa. “Everything that happens subsequently leads to Africa becoming center stage,” Beard said.
Even as Asian anthropoids were dying out in droves, the population of their seafaring African relatives exploded. The species spread and diversified, developing swiftly into the vast variety of primates we know today, from little masked vervets to huge, powerful gorillas to australopithecines such as the famous “Lucy” and, eventually, to us.
Exactly why these primates were so successful is a question for further study, Beard said. It may have been pure chance — evolution rolled the dice in two places, and only one game worked out well. Or it could be that Africa, which was closer to the Equator and less climatically chaotic than Asia, was just a better place to try to survive.
Whatever the reason African monkeys were able to hang in there, we should be glad they did.
“If these monkeys had not been in Africa right before this big chill,” Beard said, “then it's an open question whether or not we would be here today thinking about it.”
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