London: Researchers have found that the last common ancestor of apes and humans was likely small, probably weighing about 12 pounds -- about the size of a gibbon.
The findings challenged the earlier suggestions that the last common ancestor was similar to modern chimpanzees in both size and appearance.
"Body size directly affects how an animal relates to its environment, and no trait has a wider range of biological implications," said lead author Mark Grabowski, Assistant Professor at the University of Tubingen in Germany.
Among living primates, humans are most closely related to apes, which include the lesser apes (gibbons) and the great apes (chimpanzees, gorillas, and orangutans) -- together known as hominoids.
These hominoids emerged and diversified during the Miocene period, between about 23 million to five million years ago.
For the study, published in the journal Nature Communications, the team compared body size data from modern primates, including humans, to fossil hominins and a wide sample of fossil primates including Miocene apes from Africa, Europe and Asia.
The results suggested that the ancestor already had the ability of "suspensory locomotion" -- overhand hanging and swinging, before becoming too large to walk easily on branches.
While the development of suspensory locomotion helped them to get to prized but otherwise inaccessible food, the larger body would let them engage in direct confrontation with monkeys when required, the researchers said.
In addition, a group of early human relatives called australopiths were found to be smaller than their ancestors, and early humans continued to remain small till the arrival of Homo erectus -- ancestor to modern humans, who lived at least 1.8 million years ago, the research revealed.
"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 said.