Science Advances: Early humans probably started by walking upright in trees

A new study involving researchers at University College London suggests that humans' ability to walk on two legs (walking upright on two legs) may have evolved on trees, rather than on the ground as previously thought
.

In the study, published today in the journal Science Advances, researchers from University College London, the University of Kent and Duke University in the United States explored the behavior of wild chimpanzees living in the Isa Valley in western Tanzania in the Rift Valley region of East Africa – their closest relatives
at the moment 。 Known as "savanna-mosaics"—a mix of dry open land, few trees, and dense forests—these chimpanzees' habitats are very similar to those of our earliest human ancestors, and scientists chose them to explore whether such open landscapes would have encouraged bipedal walking in ancient humans
.

The study is the first to explore whether savannah mosaic habitats lead to increased time spent on the ground by Isa chimpanzees, and compares
their behavior to other studies by cousins in other parts of Africa that only live in forests.

Overall, the study found that Isa chimpanzees spent as much time in trees as other chimpanzees living in dense forests, although their habitats were more open and not as terrestrial (terrestrial-based)
as expected.

In addition, although researchers expect Issa chimpanzees to be more likely to walk upright in open savannah vegetation, where they do not move easily through the canopy, more than 85 percent of bipeds occur in
trees.

The authors say their findings contradict widely accepted theories that it was an open, dry savanna environment that encouraged our prehistoric human relatives to walk upright, and instead they may have evolved to walk on both feet in order to move
around trees.

Dr Alex Peele, an anthropologist at University College London, co-author of the study, said: "We naturally thought that since Isa had fewer trees than the typical tropical forest where most chimpanzees live, we saw more
chimpanzees on the ground than in the trees.
" Also, since many of the traditional drivers of bipeds, such as carrying objects or crossing tall grass, are ground-related, we think we'll naturally see more bipeds
here as well.
However, this is not what
we found.

"Our study shows that forest retreat and more open savannah habitats during the late Miocene-Pliocene period about 5 million years ago were not actually catalysts
for amphipod evolution.
" Instead, trees may remain crucial to its evolution — and finding trees that produce food may have been a driver of this trait
.
”

To confirm their findings, over the course of the 15-month study, the researchers recorded more than 13,700 instantaneous observations of positional behavior in 13 adult chimpanzees (6 females and 7 males), including nearly 2,850 observations of individual locomotor events (e.
g.
, climbing, walking, hanging, etc.
).

They then used the relationship between tree/land behavior and vegetation (forest vs woodland) to investigate association patterns
.
Similarly, they recorded every instance of the biped, and whether it was on the ground or in a tree
.

The authors note that walking on two feet is a defining characteristic
of humans compared to other "knuckle-walking" great apes.
However, despite the study, the researchers say why only humans first began walking on two feet in apes remains a mystery
.

Study co-author Dr Fiona Stewart (University College London Anthropology) said: "To date, numerous hypotheses about the evolution of bipeds suggest that ancient humans (human ancestors) came down from trees and walked upright on the ground, especially in drier, more open habitats
lacking tree cover.
" Our data simply does not support this
.

"Unfortunately, the conventional view that fewer trees equals more terrestrial organisms (land dwellers) has not been confirmed
by Issa data.
What we need to focus on now is how and why these chimpanzees spend so much time in trees – and that's what we're going to focus on next, to piece together this complex evolutionary puzzle
.
”