What if our bipedalism had appeared in the trees?

A female chimpanzee (Pan troglodytes) and her baby stand on a tree branch in Gombe Stream National Park in Tanzania.

The hypothesis that our ancestors started to walk on two legs when they moved from an environment composed of forests to that of the more open savannahs is called into question by observations of wild chimpanzees made by British and American scientists.

In a study published in the journal Science Advances(in English), paleoanthropologists led by Rhianna Drummond-Clarke of Kent University have analyzed the behaviors of a group of chimpanzees living in an area of ​​Tanzania's Issa Valley that exhibits both types environment.

The compilation of observational data tends to show that the chimpanzees studied do not exhibit more bipedal behaviors in the savanna-mosaic, an open environment dotted with trees. Quite the contrary.

• Humans are the only properly bipedal primate, walking on two feet, although some others such as chimpanzees, orangutans and gorillas can stand occasionally despite walking on all fours most of the time. time.
• Bipedalism is also a characteristic trait of our oldest fossil ancestors called hominins.
• The oldest traces of bipedalism in human ancestors date back 4.2 million years and are associated with Australopithecines.
• Why humans are the only great apes to walk on two feet remains a mystery to this day.

Reconstruction of the appearance of an Australopithecus sediba on display at the University of Michigan Museum of Natural History.

There are several hypotheses to explain bipedalism that are not mutually exclusive. It is a subject of study that is very relevant in the sense that we do not really know why we adopted this mode of locomotion, notes anthropologist Michelle Drapeau from the University of Montreal, who has not participated in the study.

Thus, many theories are put forward to explain why our ancestors began to stand to move around:

• To save energy when traveling;
• To transport objects or to eat;
• To see over tall grass.

These hypotheses all have one thing in common: they are based on the fact that our ancestors descended from trees to begin to walking upright on the ground when their habitat became more open and drier as dense forests disappeared to make way for a savannah environment as a result of climate change.

Acacia trees in the savannah of Tanzania.

The Pre Drapeau however thinks that this point alone cannot explain the transition to bipedalism. In past environmental reconstructions, the oldest species largely lived in areas that were still fairly forested, she notes, adding that even reconstructions are subject to disagreement among scientists.

“In general, however, it can be said that there were still many fairly closed areas that we would call for forest, although there were also somewhat more open areas, mosaic environments. »

— Michelle Drapeau, University of Montreal

To explore the paths leading to the appearance of bipedalism, the British-American team therefore analyzed the behavior of wild chimpanzees living in a territory including a savannah-mosaic: an open habitat with few trees and patches of dense forest.

The goal of the scientists was to find out if the openness of the savanna-mosaic landscape encourages bipedalism in these great apes, which could have encouraged another era of bipedalism in the first hominins.

So, over a period of 15 months, the researchers observed a group of 13 wild adult chimpanzees. According to the researchers, it is likely that over the millennia this population of chimpanzees adapted to the landscape in a manner similar to that of hominins a few million years ago.

This study is the first of its kind to examine whether mosaic savannah habitats explain the increased time spent on the ground by chimpanzees in the Issa Valley. It also made it possible to compare its results with those of other studies carried out on their cousins ​​living only in the forest in other regions of Africa.

In Overall, the study found that chimpanzees from Issa spent as much time in trees as other chimpanzees living in dense forests, despite their more open habitat, and that they weren't more earthly than expected.

Additionally, although researchers expected Issa's chimpanzees to walk more vertically in open savannah vegetation, where they cannot move easily via tree canopy, more 85% of occurrences of bipedal behaviors have been observed in trees.

Furthermore, contrary to researchers' assumptions, Issa's group is no more terrestrial than groups that live in dense forest in other parts of Africa.

What is interesting in this article is that the researchers compare a group of chimpanzees that exploit both types of habitat (mosaic-savannah and forest), remarks La Pre Drapeau, who adds that the idea of ​​a bipedalism arboreal is not new.

Professor Drapeau explains that other work carried out with orangutans also tends to show that bipedalism has an arboreal origin, as well as the reconstructions of the diet of the first hominins made through the analysis of fossilized bones.

“It is believed that early hominins had a diet mostly based on vegetables such as fruits and leaves produced by trees and bushes, and fewer grasses and plants produced on the ground. »

— Michelle Drapeau, University of Montreal

The authors of the work are nevertheless surprised by their results which contradict the theories which suggest that it was an open and dry savannah environment which encouraged our distant relatives to walk upright.

We naturally thought that because the Issa Valley has fewer trees than typical rainforests, where most chimpanzees live, we would see the individuals more often on the ground than in the trees , says anthropologist Alex Piel of University College London.

“Furthermore, since many of the traditional factors of bipedalism (such as carrying objects or seeing over tall grass, for example) are associated with being on the ground, we thought that we would naturally see more bipedalism there. But that's not what we found.

—Alex Piel of University College London

Thus, the traditionally accepted idea that fewer trees equal more terrestriality is not supported by data collected in the Issa Valley.

If I had been asked to predict the results, I would certainly have said that they [chimpanzees) would move bipedally more on the ground than in the trees. It's a surprise, acknowledges Professor Drapeau.

However, Professor Drapeau calls for caution in interpreting the results, since the number of individuals studied (13) is small. She says we need to see if these observations hold true in other groups.

“Be careful not to generalize , since not all groups of chimpanzees are the same. Other factors, such as predation, must also be considered. »

— Michelle Drapeau, Université de Montréal

The researcher points out that, as the study points out, bipedal behaviors in monkeys remain very rare. If we break down the number of hours they are biped with those where they are not, it's very little, she notes.

We must now understand why these chimpanzees spend so much time in the trees, important information for piecing together our own complex evolutionary puzzle, concludes for his part Alex Piel.