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An artist’s rendering of Carpolestes, an early primate relative that lived in North America 56 million years ago. Carpolestes fossils indicate early primates co-evolved with flowering plants. Image: Sisyphos23/Wikicommons

One of the great origin stories in the history of mammals is the rise of primates. It’s a story that scientists are still trying to write.

In the early 20th century, anatomists believed primates—united by big brains, grasping hands and feet, and excellent vision, among other features—evolved in response to living in trees. In the 1970s, however, biological anthropologist Matt Cartmill realized an arboreal lifestyle alone wasn’t enough to explain primates’ unique set of characteristics. Plenty of mammals, like chipmunks, live in trees but don’t have nimble hands or closely spaced, forward-facing eyes that allow for good depth perception. Instead, Cartmill suggested these features evolved because early primates were insect predators. He noted that many modern predators, such as cats and owls, have forward-facing eyes because they rely on good vision to grab prey. In the case of early primates, Cartmill said, they hunted tree-dwelling insects.

Not long after Cartmill presented his explanation of primates’ roots, other researchers came up with an alternative idea: Primates evolved in step with the spread of flowering plants. Rather than relying on good vision and dexterity to nab bugs, early primates used these traits to carefully walk out to the ends of delicate tree branches to gather fruits and flowers, as well as the insects that pollinated flowering plants.

Physical anthropologists Robert Sussman and D. Tab Rasmussen of Washington University and botanist Peter Raven of the Missouri Botanical Garden review the latest evidence in support of this hypothesis in an article published online in the American Journal of Primatology.

The team suggests that the earliest primates and their extinct close relatives, a group called plesiadapiforms, weren’t strictly insect eaters and therefore the insect predation hypothesis doesn’t hold up. They point out that the molars of plesiadapiforms are rounder than the teeth of earlier mammals, which were sharp for puncturing bugs. The flatter teeth indicate plesiadapiforms were probably grinding fruits, nuts and other plant parts.

The switch to a plant diet coincides with the rise of rise of flowering plants. The earliest flowering plants show up in the fossil record roughly 130 million years ago and became the dominant type of forest plant by about 90 million years ago. Around 56 million years ago, global temperatures spiked and tropical forests spread around the world. About this time, many species of birds and bats emerged. Primates also diversified during this period. Sussman and his colleagues argue that while birds and bats could fly to the ends of branches to consume meals of fruit and nectar, primates took a different route, evolving adaptations that enabled them to be better climbers.

The skeleton of a 56-million-year-old plesiadapiform found in Wyoming provides further evidence of this scenario, the researchers say. Much of the early primate and plesiadapiform fossil record consists of teeth, but in 2002, scientists reported the discovery of the skull, hands and feet of Carpolestes simpsoni. The bones reveal that the species was a good grasper, with an opposable big toe and nails instead of claws. And the teeth indicate the creature ate fruit. But unlike living primates, C. simpsoni did not have forward-facing eyes, suggesting it didn’t have good depth perception. This is an important finding, Sussman and colleagues say. If primates evolved their characteristic features because they were visual predators, then you’d expect good vision to evolve in concert with good grasping. Instead, the C. simpsoni fossils suggest enhanced vision came later. Forward-facing eyes may have later evolved because it helped primates see through the cluttered, leafy environment of the forest canopy.

The team’s arguments rest heavily on evidence from plesiadapiforms. In the past, anthropologists have debated plesiadapiforms close connection to primates. However, Sussman and colleagues think the fossil evidence suggests the two groups shared a common ancestor, and thus the evolutionary trends seen in plesiadapiforms serve as a good guide for what happened in primates.

An artist’s reconstruction of Purgatorius, a probable primate ancestor. Image: Nobu Tamura/Wikicommons

Finding the earliest primates isn’t easy. The first members or our order probably lived about 65 million years ago and were rat-sized critters known mainly from teeth. With such scant evidence, researchers have had a hard time classifying these creatures and making connections to modern primates. Still, scientists have identified dozens of early primate, or probable primate, species. If you’re unfamiliar with our earliest origins, here are five primates to know.

Purgatorius: Discovered at Montana’s Hell Creek Formation, this shrew-sized mammal lived roughly 65 million years ago at the end of the Cretaceous period. Purgatorius‘ place in the primate family tree is debated. Aspects of the genus’ teeth align it with a group of extinct, primate-like mammals called plesiadapiforms. Some scientists say that the number and variety of teeth Purgatorius had makes it a possible common ancestor to primates and plesiadapiforms. Last week, paleontologists from Yale University announced they found  the first known Purgatorius ankle bones. The researchers say the fossils reveal the animal had flexible feet like modern tree-living mammals do, implying the earliest primates were indeed arboreal animals as scientists suspected.

Altiatlasius: A few molars and a jaw fragment are all that’s known of this small mammal discovered in Morocco. Many paleontologists consider Altiatlasius, which lived some 57 or 56 million years ago, to be the first true primate. How the ancient primate relates to modern primate lineages is unclear. While some researchers believe it’s similar to a group of primitive tarsier-like primates, others think it might be an ancient forefather of monkeys and apes.

Teilhardina: Named for the French paleontologist Pierre Teilhard de Chardin, Teilhardina has been found at North American and Asian sites dating to almost 56 million years ago. Scientists group the genus with the omomyids, a family of tarsier-like primates that emerged during the Eocene epoch some 56 million to 34 million years ago. Last year, scientists reported they had unearthed a cache of Teilhardina fossils in Wyoming’s Big Horn Basin that included the first evidence that early primates had nails instead of claws. The tips of the animal’s finger and toe bones were flattened, indicating the presence of fingernails, the researchers reported in the American Journal of Physical Anthropology.

Notharctus: This North American genus lived about 50 million years ago and belonged to a family of lemur-like primates called adapiforms. Notharctus had a long tail, leaped from tree to tree and snacked on leaves. A report published in PLOS ONE in January described fossils from this primate that indicate it would have had something like a cross between a fingernail and a claw on its second toe—kind of like modern lemurs, lorises and bush babies (or galagos) that all have a “grooming” claw on their second toe. But it’s not yet clear whether Notharctus was on its way towards evolving a true grooming claw, or on its way towards evolving a true nail.

Eosimias: Discovered in China, Eosimias lived about 45 million years ago. The size and shape of its teeth suggest it was the earliest ancestor of the lineage leading to monkeys and apes (and us!). Fossils of its feet suggest Eosimias walked on all fours like a modern monkey.

An artist's depiction of Afrotarsius (upper left) and other early primates from Africa. Illustration by Mark A. Klingler/Carnegie Museum of Natural History, Pittsburgh, Pa.

This week, I’m going to consider origin stories that go deeper into primate history than questions of when Homo sapiens evolved or when two-legged apes, or hominids, emerged.

Today, let’s go really far back, to a time some 40 million years ago known as the Eocene. Monkeys and apes weren’t even around yet, although their common ancestor was. But where? The discovery of a new species of Eocene primate is helping address that question.

Until about 20 years ago, the answer seemed obvious: Africa. That’s where the earliest fossil evidence was found, mainly from Egypt’s Fayum Depression. Starting in the 1990s, however, relevant fossils started popping up in Asia. Paleoanthropologists now consider a 45-million-year-old primate discovered in China, called Eosimias, to be the earliest anthropoid, the group of primates that includes monkeys, apes and humans. Eosimias was tiny, weighing less than half a pound. But it possessed certain dental and jaw characteristics that link it to living anthropoids.

The newly discovered species, named Afrasia dijijidae, dates to roughly 37 million years ago and was found in Myanmar. So far, all that’s known of Afrasia is based on four isolated teeth. But the nooks, crannies, crests and bumps on those teeth reveal a few things about where the ancestors of today’s monkeys and apes came from.

The species’ teeth are similar to those of the older Eosimias and other Asian species closely related to Eosimias. But the teeth’s size and shape are almost identical to those of a North African primate that lived at about the same time as Afrasia, approximately 38 million to 39 million years ago. It’s name is Afrotarsius. The findings are reported today by Jean-Jacques Jaeger of the University of Poitiers in France and colleagues in the Proceedings of the National Academy of Sciences.

The team suggests that the similarity in age between Afrasia and Afrotarsius indicates that a lineage, or lineages, of Asian anthropoids must have arrived in Africa only shortly before the appearance of Afrotarsius. If anthropoids had gotten to Africa much earlier, then Africa’s anthropoids would have evolved in their own direction, and millions of years later, you wouldn’t expect anthropoids in Asia and Africa to be so similar.

To get to Africa, anthropoids had to find a way across the Tethys Sea. The Tethys was a more sprawling version of the Mediterranean, drowning parts of northeastern Africa, the Middle East and West Asia. The small primates likely rafted over on giant mats of floating vegetation. Rafting may sound far-fetched, but researchers have suggested it’s how a variety of animals reached new land masses in the past. And around the same time that the ancestors of monkeys and apes left Asia for Africa, it appears some rodents did, too.

Come back on Wednesday for a look at the surprisingly European origins of the ancestor of Africa’s apes: chimpanzees, gorillas and humans.