Soon After Dinosaur Decimation, Our Primate Ancestors Began Pouncing on Prey

A hunger for crunchy insects may have led our early primate ancestors to develop the signature traits that we inherited.

Small primates like Teilhardina, which lived in the forest canopy of ancient China 55 million years ago, were early adopters of some of our evolutionary family’s most famous characteristics. Looking much more like lemurs than monkeys, they had forward-facing eyes, grasping hands and nails that would become hallmarks of many later primate species—including ourselves. How such traits evolved in the first place has long been controversial among fossil primate experts, but a new hypothesis suggests that sneakily hunting up in the treetops had a lot to do with it.

Investigations of human origins often center on our ancient hominin relatives such as “Lucy,” but many of the characteristics that make us unique among mammals are much, much older. “Humans represent a mosaic of traits that have evolved at various points throughout the 66-million-year history of primates,” says University of Toronto paleontologist Mary Silcox.

Our gifts of depth perception, dexterous hands and more can be drawn back to a time much closer to the dinosaurian heyday than the present. The first primate with a grasping toe dates back to 56 million years ago, the first big increase in brain size occurred 55 million years ago and the number of teeth we have can be traced back to 30 million years ago, Silcox points out. But exactly why our early ancestors came to have such telltale traits has been debated by experts for decades.

Working from what’s known about living primates and experiments involving how squirrels climb trees, Yonghua Wu, a biologist at Northeast Normal University in Changchun, China, and colleagues have proposed that many of our classic primate traits came from quietly sneaking up on prey in the trees. Their study, published earlier this year in Science Advances, proposes that key primate traits like nails, the ability to leap and forward-facing eyes all evolved because early primates were ambush hunters. Past researchers have suggested that hunting for insects was crucial to primate origins, but the specific way primates might have captured their meals was left unclear. What the new study adds is the notion that using stealth to hunt small prey, specifically, was the catalyst for some big primate changes. “I really like that paper, because it introduced some new ideas to a topic that was getting a bit stale,” Silcox says.

Taken individually, many of our primate traits might seem strange. Forward-facing eyes were present in primates by 55 million years ago, but paleoanthropologists have often wondered why. If this ocular innovation was so important to moving in the trees, for example, then why do other arboreal animals like squirrels have eyes on the sides of their heads? Most primates also have nails rather than claws, but these would seem to make climbing harder.

To investigate how early primates were hunting, Wu and colleagues turned to clues found among living species. Lemurs, bush babies and tarsiers all roughly resemble early primates and share traits like nails instead of claws and eyes that face forward. Lemurs and other primitive-seeming primates are also accomplished leapers, able to easily bound from trunk to trunk or branch to branch. And leaping isn’t just useful for getting around. Leaping allows some insect-eating primates to catch their prey unaware. Being able to climb, leap and ambush prey, Wu and colleagues propose, might explain why primates evolved to be different from other tree-dwelling mammals.

Nails would be quieter than claws while sneaking up on prey, for example, and the ability to leap would allow early primates to get the jump on their prey, with binocular vision being critical for gauging the distance. To test this idea, Wu and colleagues didn’t look to fossils—they looked to genes involved in what primates digest and how those genetic markers have changed over time. “For primates, we used the molecular approach to reconstruct their ancestral diet,” Wu says. While the very earliest primates that lived at the end of the Age of Dinosaurs likely had genes that assisted the digestion of fats, carbohydrates and proteins indicative of an omnivorous diet, the researchers reconstructed the last common ancestor of all living primates as having a diet richer in fat—a clue that the primates of 55 million years ago had shifted to eating more animal prey and were hunting.

To prove that primates were quietly climbing and leaping to pounce on unsuspecting prey, the researchers ran an experiment with squirrels—some with intact claws, some of which had been trimmed. They found that claws were much noisier when the squirrels were climbing. Early primates with nails might have traded in extra grip for relative quiet to sneak up on prey. In all, Wu says, “Orbit convergence may help promote the accurate judging of prey distances, grasping hands and feet with claw reduction may be adapted for reducing the noise made in favor of stalking, and leaping may be used as a means to launch a rapid attack.”

But the case of how early primates evolved some of their most characteristic traits isn’t closed just yet. The new study is based primarily on living primates, Silcox, who was not involved in the study, notes. Comparison to the fossil record is essential for testing the ideas in the study. Grasping hands and feet seem to have evolved before primates had forward-facing eyes, for example, and so linking them as results of the same way of living doesn’t quite work. And traits like binocular vision could be helpful for other reasons. “Short-distance 3-D vision that comes with convergent orbits is really useful for tasks like picking out fruit from a leafy background,” Silcox says. Living in trees was important to these changes, but one single pressure or reason may not explain everything.

Paleontologists are going to keep digging. Researchers are still seeking to fill the gap between the earliest primates of 66 million years ago and the earliest euprimates, or “true primates,” which started to thrive about 55 million years ago. “The biggest remaining mysteries are about what was happening in that period,” Silcox says, which will rely on as-yet-undiscovered fossils. Given the tiny size of early primates—some with bones smaller than a grain of rice—paleontologists are looking for fossil needles in a geological haystack spanning tens of millions of years. But with a sense of what to look out for, as well as new tools such as micro-CT scans and new techniques that can remove small bones from even the hardest rocks, paleoprimatologists are getting closer to grasping the full story.

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.