A fossilised foot discovered in eastern Africa belonged to a pre-human ancestor that was adapted to life in the trees.

Palaeontologists working in the Afar region of north-eastern Ethiopia uncovered eight bones from the front half of a single foot that date to 3.4 million years ago.

The foot is unusual for the positioning of its big toe, which juts out to the side like that seen in gorillas or chimpanzees. The orientation of the toe enables the foot to grasp branches, suggesting its owner was a creature at home in the forest canopy.

The age of the remains suggest the creature was a contemporary of another pre-human, or hominin species, Australopithecus afarensis, which had lost similar bone features in favour of other adaptations that committed it to walking on two feet.

The most complete remains of A. afarensis belong to the famous Lucy, which lived in the Afar region around 3.2 million years ago.

Bruce Latimer, professor of anthropology at Case Western Reserve University in Cleveland, Ohio, said he was "shocked" at the remains, which have been studied intensely since they were first uncovered in 2009.

The fossilised foot bones superimposed on the outline of a gorilla foot. The orientation of the big toe would have enabled the foot to grasp branches. Photograph: Nature

"It's an important species because it opens up a window into our past on how our foot evolved," Latimer said. "It was still partially arboreal, still spending a significant amount of time in the trees."

Analysis of sediments in the area revealed that what is now a dry, barren landscape was once crossed by rivers feeding into a lake with a wooded shore.

Yohannes Haile-Selassie, who worked on the fossils, said the remains demonstrated that our ancient ancestry was more complex than many researchers thought.

"Once walking on two legs – bipedality – evolved, we didn't think there was another species that had the capability to climb up trees living at the same time. We never expected another related species running around with the capability to climb up trees," he said.

The evolution of upright walking had a dramatic impact on human bone structure. Humans have long, hefty big toes that line up with the other toes, which are shorter and straighter than in other primates. Our feet have large, stable heels for striking the ground while walking, and a prominent arch that stiffens the middle of the foot and helps to transfer body weight towards the big toe.

Details of the foot bones, published in the journal Nature, describe a short big toe angled away from the foot, and long, slightly curved toes that are part way between those of apes and hominins.

"Until we find skulls and teeth of this organism, we're being cautious about describing it as a new species. Is it within the range of variation within the other fossils? I don't think so. I think it probably does represent a new species, but until we find more evidence, we're being cautious about it," Latimer said.

In an accompanying article, Daniel Lieberman, an evolutionary biologist at Harvard University, said more fossils would help scientists piece together how different skeletal features led our ancestors to take their first steps.

"We need more fossils to determine what sorts of bodies went with these feet, and to resolve which features evolved just once and which evolved multiple times. We also need to have a better understanding of how the anatomical variations we see in hominin feet affected the different species' ability to climb, walk and run," Lieberman wrote.

"Human evolution is often portrayed as a triumph of bipedalism, but who among us has not occasionally regretted our species' comparative clumsiness in the trees? I, for one, am pleased to know that some hominins retained feet well adapted for arboreality millions of years after we started to walk."

• This article was amended on 29 March 2012. The original described Lucy as "infamous". This has been corrected.