An extinct human relative found in a dark, cramped cave in South Africa was adept at both swinging in the trees and walking on two feet, making it unique among our known ancient forerunners, scientists say.

A fresh analysis of hand and foot bones of Homo naledi, the latest addition to the human genus, shows that while the creature’s foot resembles that of modern humans, its fingers are curved, in an unmistakable sign of arboreal living.



Researchers unveiled Homo naledi last month after discovering the remains of 15 individuals in the Rising star cave near Johannesburg. The bones have yet to be dated, making it impossible for scientists to work out where the species fits in the story of human evolution.

Homo naledi was small and slender with a tiny brain compared with modern humans. The adult males stood about 5ft, with the females a little shorter. From the first excavations, the hand and feet bones looked unusual, bearing the hallmarks of a creature that made and used tools, was an accomplished climber, but spent most of its time walking upright.



Scientists have now performed more detailed studies on a near complete right hand and more than a hundred pieces of foot bone and, in two papers published in the journal Nature Communications, reveal how extraordinary the remains are.



The wrist and thumb show that Homo naledi had a powerful grasp and was well-equipped for making and using stone tools. But these more modern features sit alongside highly curved fingers, a signature of early human ancestors that lived in the trees.



“That combination was really quite surprising,” said Tracy Kivell, who studied the bones at the University of Kent. “It shows you can have a hand that is quite specialised for manipulation and tool use in a species that is still using its hands for climbing, and moving around in the trees or on rocks.”



The team has yet to recover any stone implements near the remains of Homo naledi, but if the species did smash rocks together to make cutting and scraping tools, it did so without much in the way of brain power.



Homo naledi, discovered in South Africa, could combine land walking and tree swinging. Photograph: Mark Thiessen/National Geographi/PA

“It shows we have a much greater diversity in the fossils of human ancestors than we thought possible,” said Kivell.



Another team led by William Harcourt-Smith at the City University of New York analysed 107 pieces of Homo naledi foot bone. Writing in the journal, they describe how the foot is similar to those of Neanderthals and modern humans, but with a number of subtle differences. The toe bones are slightly curved, which may have helped Homo naledi a little when it took to the trees. The arch of the foot is low, or absent entirely, making Homo naledi flat-footed.



“It was unequivocally spending more time walking upright than not,” said Harcourt-Smith. “But you can imagine it spending time in the trees to gather fruit, or perhaps nesting in trees, or going there when there are predators around.” The curved toe bones are thought to be skeletal adaptations that Homo naledi inherited from its more arboreal ancestors and had not lost.



Until the bones can be dated, one of the major questions surrounding Homo naledi will remain: did the species emerge millions of years ago and live in successful isolation, perhaps even overlapping with modern humans? That is one possibility. Another is that Homo naledi is an evolutionary side-branch, a sister species of a known human ancestor, such as Homo erectus.



“You can imagine this lineage emerging early on, close to the origins of the Homo genus, and hanging on for a long period of time,” said Harcourt-Smith. “But that’s speculation. Evolution is messy. There is lots of experimentation going on, and lots of dead ends.”