Bipedie is the technical term – but how did this typically human mode of locomotion come about? Shrinking forests may not have been the impetus for the evolutionary history of upright walking in our ancestors, a study in chimpanzees suggests. Because even in sparse tree stands they are no longer on the ground. Instead, our closest relatives have been observed to be most likely to get up on two legs when moving about in branches. Possibly the actual origin of the upright walk lies in the trees, the researchers say.
It is almost a symbol for the human line of development: while great apes usually walk on all fours and rarely stand up, our ancestors increasingly began to walk on two legs. This development has long been the focus of anthropology. It has been assumed that landscape changes in the African region of origin of the hominins about five million years ago played a role in the development of bipedy. At that time it became considerably drier there and the previously dense forests became increasingly lighter and finally turned into savannahs. This could have brought our ancestors more and more from the trees to the ground. The hypothesis states that they may then begin to walk upright more and more frequently on solid ground.
target chimpanzees
To provide observational data that sheds light on this possible story, an international team of researchers has now targeted the locomotor behavior of our closest relatives. Chimpanzees can best serve as a model for our Miocene and Pliocene ancestors. The focus of the study was a chimpanzee population that lives in an unusual environment for these great apes: the Issa Valley in western Tanzania is not characterized by dense forests, instead the animals live in a rather dry, mosaic-like landscape with sparse tree populations and open areas. The habitat thus resembles the forested savannah, which is assumed to be the home of the earliest human ancestors, the researchers explain. So they investigated whether Issa Valley chimpanzees show signs that this type of landscape encourages bipedal locomotion.
To this end, scientists conducted extensive observations of the movement behavior of a group of chimpanzees in various contexts. In particular, they recorded instances of upright posture and whether this was associated with being on the ground or in the trees. They also fundamentally analyzed the relationship between tree or soil-related behavior and vegetation. The scientists then compared the results with existing observation data of this species from chimpanzee populations living in forests in other regions of Africa.
"Since there are fewer trees in Issa than in typical tropical forests where most chimpanzees live, we actually expected to see more individuals on the ground than in the trees. And since many of the traditional drivers of biped locomotion seem to be tied to being on the ground, we thought we might also see more upright gait," says senior author Alex Piel of University College London. "But these expectations were not confirmed."
Upright in the trees
The team's analysis showed that despite their relatively open habitat, the Issa chimpanzees spent as much time in the trees as other chimpanzees living in dense forests. "Issa chimpanzees were no more terrestrial than chimpanzees living in forest habitats, suggesting that it is not a simple rule that fewer trees mean more time on the ground," the researchers write. Interestingly, as far as upright posture is concerned, they found that 85 percent of the time, the chimpanzees rose on two legs when moving through the branches of the trees in search of food.
At least as far as our closest surviving relatives are concerned, the study does not confirm the common assumption about the evolutionary history of upright walking, the researchers say. "Tree life may have been an integral part of the hominins' adaptive niche even as forests retreated," the researchers write. Piel says: "Our study suggests that late Miocene-Pliocene forest retreat about 5 million years ago was unlikely to have been a catalyst for the evolution of bipeds. Instead, trees probably remained a major factor in the development of this mode of locomotion. The search for food-producing trees could have been a driving force here," says the anthropologist.
As the researchers emphasize, however, the study results should only be regarded as indications: it remains a mystery why the ancestors of humans finally adopted an upright mode of locomotion. It seems clear that chimpanzees can only serve as a model to a limited extent, since they cannot be equated with the creatures that stood at the beginning of the evolutionary history of the human lineage. The researchers now want to provide more background information on the chimpanzees: "We now need to focus on how and why the chimpanzees spend so much time in the trees, even in the Issa habitat," says co-author Fiona Stewart from University College London .
Source: University College London, Article: Science Advances, doi: 10.1126/sciadv.add9752