Last common ape ancestor was much smaller than thought

The last common ancestor of apes, including humans and chimpanzees, was much smaller than previously thought, about the size of a gibbon, according to new research published this week in the journal Nature Communications.

Researchers found that the proto-ape was likely small, and probably weighed about five kilograms. This goes against previous suggestions of a chimp-sized – and chimp-like — ancestor. Male adult wild chimpanzees can weigh between 40 and 60 kilograms.

The findings are fundamental to understanding the evolution of the human family tree.

“Body size directly affects how an animal relates to its environment, and no trait has a wider range of biological implications,” says the report’s lead author, Mark Grabowski, from Eberhard Karls University, in Tübingen, Germany. 

Grabowski says little is known about the size of the last common ancestor of all living apes, an omission he calls “startling”. 

“Body mass affects almost every aspect of an animal’s biology and ecology,” he explains. 

“Locomotion, behaviour, diet, social organisation, energy requirements, and a host of other vital biological and ecological characteristics are directly or indirectly tied to body mass.”

The finding has implications for a behaviour that is essential for large, tree-dwelling primates: “suspensory locomotion”, or overhand hanging and swinging.

The advent of this method of getting around could have been part of an “arms race” with a growing number of monkey species. Branch swinging allows an animal to get to a prized and otherwise inaccessible food – such as fruit on the edges of foliage.

Grabowski and coauthor William Jungers, from Stony Brook University in New York, suggest that the ancestor was already somewhat suspensory, and larger body size evolved later, with the adaptations occurring at separate points. 

Among living primates, humans are classified in the super-family Hominoidea, which comprises all apes, whether small, such as gibbons, or large, such as chimps and gorillas.

Known collectively as hominoids, the apes emerged and diversified during the Miocene Period, between about 23 million and 5 million years ago. Because fossils from this period are scarce, researchers do not know what the last common ancestor of living apes and humans looked like or where they originated.

Grabowski and Jungers compared body size data from modern primates, including humans, to recently published estimates for fossil hominins – extinct members of the Homo genus and other close human ancestral species — and a wide sample of other fossil primates, including Miocene apes from Africa, Europe, and Asia.

“There appears to be a decrease in overall body size within our lineage, rather than size simply staying the same or getting bigger with time, which goes against how we generally think about evolution,” Grabowski says.