Apes were supposed to be tied to the trees. Around 21 million years ago, during a time paleontologists know as the Miocene, what’s now Uganda was thought to be a lush, humid forest where early apes clambered over canopy branches in search of fruit. But a pair of new studies have overturned the established view, changing the setting in which paleontologists think the first apes evolved.
East Africa’s early apes were not hanging onto forests as lush woodlands gradually gave way to grasslands. Instead, early apes were already taking advantage of more open habitats that shifted between wet and dry seasons through the year. Woodlands where trees stood along the margins of open, grassy patches, researchers reported earlier this year in Science, were established in East Africa by about 21 million years ago—10 million years earlier than previously thought. The findings dovetail with an April study in Science demonstrating that a fossil ape called Morotopithecus, which lived in eastern Africa 21 million years ago, foraged on seasonally dry patches of grassy plants that spread between the stands of trees. The new evidence is part of a broader rethink of how this critical part of the world set the stage for ape and early human evolution.
“Prevailing origin stories about apes suggested that apes first evolved in tropical forests,” says University of Michigan anthropologist Laura MacLatchy, lead author on the Morotopithecus study. The Miocene was supposed to be a “planet of the apes” where primates thrived in warm, dense jungle that spread over much of the planet. But this vision wasn’t so much based on the fossil record as where apes live today. “These compelling ideas centered on forests did come from projection what we know about the living apes back in time,” MacLatchy says. But the classic tale hadn’t been tested, which sent researchers back to the fossil record to look at the animals, plants, fossilized soils and other clues that could reveal the habitats Morotopithecus and other apes lived in.
Anatomically speaking, MacLatchy notes, Morotopithecus looks like an ape that would have been at home in the forests. The ape’s body shows signs of a relatively upright posture and adaptations related to climbing. But the ape’s teeth tell a different story. The teeth of Morotopithecus more closely resemble those of primates that eat leaves than those that eat fruit. The ape’s molars, in particular, are oval in shape with shearing edges suited to slicing through plants like grasses. And given that geochemical signatures of the food animals eat are taken up and preserved in their teeth, MacLatchy and co-authors investigated those traces to determine what sorts of plants the ape was eating.The signatures captured in the Morotopithecus teeth best matched plants that grew according to wet and dry seasons in grassland settings, not the fruit that was expected. The apes lived in an environment with a mix of trees to climb and patches of more open ground where they ambled around, eating leaves of the grasses that grew there. “It was very unexpected that an ape with upright, versatile climbing abilities was living in a seasonal woodland with open, grassy patches, rather than in close tropical forest,” MacLatchy says.
The studies on both eastern Africa’s ancient ecology and Morotopithecus have altered what researchers thought about the rise of early apes and the habitat history of the continent. “If you had asked me just a few weeks ago to describe the paleoenvironments of these early Miocene sites, I would’ve said it’s forest through and through,” says Natural History Museum of Utah paleoecologist Tyler Faith, who was not involved in the studies.
Rethinking the lives and habitats of early apes is just one part of a broader reinvestigation of how eastern Africa’s environments changed through the critical eras when apes, and eventually early humans, were evolving. Up until recently, modern habitats—from forests where living apes dwell to eastern Africa’s savannas—have been taken as proxies for the settings where apes and humans evolved. But now researchers are finding that the present is not, in fact, the key to the past. Even within the timeline of human history, stretching back about seven million years, prehistoric East Africa wasn’t anything like what we can observe on the continent today.
The mix of plants, animals and other organisms in modern East Africa only came together relatively recently and can’t be taken as a holdover from the early days of human evolution. When Faith and colleagues looked at communities of mammals that roamed east Africa during the last seven million years, for example, the researchers found that the modern assemblage of elephants, giraffes, antelopes and other herbivores didn’t come together until about 700,000 years ago. For millions of years before that, a greater number of truly big herbivores trundled across the region and in turn were preyed upon by a menagerie of carnivores that included sabercats and giant hyenas. Those differences are themselves indicators that the prehistoric habitats weren’t just like those around today, and the differing fauna would have affected everything including what sorts of plants grew, how fire affected the landscape and the way nutrients were cycled through the soil.
The precise details of how different these ecologies were, however, isn’t fully known. “One of the questions that keeps me up at night is: ‘How do you feed all these megaherbivores?’” Faith says, noting that some fossil sites include at least five species of elephant relatives. Those immense herbivores would have required a great deal of vegetation to fuel their bulk, hinting that the landscape produced an incredible amount of plant food, which in turn raises new questions about what those plants were, how they grew and how they shaped the habitats where animals—including early humans—evolved. The task ahead is to meet these habitats on their own terms, sifting the fossil record to reconstruct these environments from the soil up. “There may be no modern analogs among the habitats that remain in Africa,” MacLatchy says, requiring that experts question long-held assumptions as they continue uncover the continent’s prehistory.