The upright posture is a hallmark of the human lineage. It left our hands free to use tools and create all kinds of things, and it even influenced women to give birth to their children in pain. Anthropologists have long located the beginning of bipedalism in the great, arid African savannahs of about ten million years ago, where our ancestors would have been forced to stand upright to peer over the tall grasses for both potential prey and insects. predators.
However, two new studies published this Thursday in the journal ‘Science’ change the when and why of that important moment. They take it back further in time to around 20 million years ago and place it in a seasonal, open forest landscape. Nor did it happen on the ground as was believed, but between the branches of the trees, where the apes tried to reach the leaves to feed.
“This leads us to rethink how climate change and vegetation affected human evolution,” says one of the study’s authors, Laura MacLatchy, a professor in the Department of Anthropology at the University of Michigan (USA). ).
leaves instead of fruit
The investigation focused on an ape named Morotopithecus, which lived 21 million years ago, during the early Miocene, in what is now eastern Uganda. “We think that being upright was key for this ape to move and forage in trees with a large body size. With its back upright, an ape can cling to multiple branches with its arms and legs, distributing its body mass. He can even hang on to them, making him less prone to losing his balance. In this way, he accesses food that grows on the periphery of the treetops that would otherwise only be available to smaller species,” explains MacLatchy.
Fossils of Morotopithecus they were found in a single stratigraphic layer, along with those of other mammals, ancient soils (palaeosols) and small particles of plant silica called phytoliths. In this way, the scientists were able to recreate the landscape in which it lived, an open environment interrupted by seasonal broken-canopy forests made up of trees and shrubs. This means that for at least part of the year the fruit was not available, so the apes had to rely on something else to survive. What they were trying to reach were the leaves of the trees.
According to MacLatchy, the clue that these ancient apes ate leaves instead of fruit is in their molars. The teeth were very steep, with peaks and valleys, indicating that they were used for tearing fibrous leaves, while the molars used for eating fruit are usually more rounded.
Morotopithecus upper jaw
The researchers also examined ape tooth enamel, as well as tooth enamel from other mammals found in the same stratigraphic layer. They found that the isotopic ratios (the abundance of two isotopes of the same element) in their tooth enamel showed that the apes and other mammals had been eating water-stressed woody shrubs and trees, which are more common in open forests or covered forests. grass today.
a different landscape
The second paper in ‘Science’ reconstructs the vegetation structure of nine fossil ape sites in equatorial east Africa, including that of Morotopithecus, during the early Miocene, from a carbon isotope analysis of ancient soil organic matter, plant wax biomarkers, and phytoliths found at each site. It shows how arid-adapted grasses were “everywhere” at the time, which was instrumental in shaping the evolution of different lineages of mammals, including apes.
During the early Miocene, the East African Rift was forming. The Earth was breaking apart. As a result, the entire region was uplifted, causing great variation in topography, and therefore in the regional climate and vegetation. “There are mountains and volcanoes, there are cliffs and valleys,” describes John Kingston, co-author of the study and an anthropologist at the University of Michigan. “The landscape is physically very variable and this is undoubtedly related to the heterogeneity of the vegetation,” he adds.
For the authors, these findings have transformed what we thought we knew about the first apes and the origin of where, when and why they moved through the trees and on the ground in many different ways.
“We now know that the first record of these unique adaptations – the upright back – occurred in an ape living in a grassy open forest, where fruit would not have been available year-round. Instead, he ate leaves. So we have to rethink evolution,” summarizes MacLatchy.