Environmental change gave rise to the great apes in Africa – 04/17/2023 – Science

The environmental changes that paved the way for the origin of humanity’s ancestors in Africa may have happened much earlier than previously thought, according to two studies that have just been published.

According to the analyses, East Africa already had areas of open vegetation and a strong presence of tropical grasses around 20 million years ago. And new primate fossils found in the region would indicate that the ancestors of great apes and humans had evolved an upright torso because of these new open habitats by this time.

The data in favor of this hypothesis are in the latest edition of the American specialized magazine Science. To draw a new picture of the transformations that affected the region from the Miocene onwards (a geological epoch that began 23 million years ago), the international team of scientists analyzed characteristics of the soil, plant remains and prehistoric fauna.

In studies of the evolution of African great apes and the human lineage, which is directly derived from them, there is great controversy about the role of open vegetation areas. Until now, most experts believed that the tropical areas of Africa were dominated by large dense forests until at least 10 million years before present.

From then on, geological and climatic changes that would have brought more aridity and seasonality (that is, stronger differences between the times of the year, especially the rainy and dry seasons) would have “broken” the dense forests.

Savannah areas then emerged, with more sparse trees and the expansion of tropical grasses, also designated as C4 grasses by botanists (a very common example today in Brazilian pastures is brachiaria). These are plants that have adapted to the new environmental conditions and have expanded across much of the African continent.

One possibility is that this fragmentation of the great primordial forests in Africa has forced some primates to adapt to terrestrial locomotion, so that they could, for example, search for food resources by going from one tree to another. This process could be one of the triggers for the origin of hominins, the lineage that gave rise to humans.

However, the new studies greatly complicate this scenario by showing that something very similar may have already happened at the origin of the great apes —the broader group that includes both Homo sapiens and chimpanzees, gorillas and orangutans.

The works, which were coordinated by Laura MacLatchy and John Kingston, from the University of Michigan in Ann Arbor (USA), indicate that there were already several places with an abundance of C4 grasses in East Africa (in Uganda and Kenya) around 20 million from years ago. It is possible to infer this not exactly based on fossils of such grasses —which, in general, do not exist—, but based, among other things, on the chemical composition of ancient soils.

That’s because C4 grasses formed their leaves and stems using a different ratio of forms of the chemical element carbon compared to other types of plants. It is like a stamp or a signature, which is preserved in the soil even after the plant has decomposed. In addition, sediments from the early Miocene also harbor so-called phytoliths, tiny mineral grains produced by plants that have a different shape depending on the type of plant. Thus, it is possible to infer the presence of C4 grasses even without preserving the plant.

The last piece of the puzzle are the fossils of a large primate (over 30 kg) called Morotopithecus. Also around 20 million years old, it was identified by the authors of studies at a paleontological site in Uganda and has two very important characteristics.

First, everything indicates that its torso was erect, like that of current chimpanzees and gorillas, which strongly suggests that it belongs to the group of great apes. (Suffice it to compare the posture of a chimpanzee with that of a capuchin monkey when they are walking: the great ape’s chest is raised in relation to the ground, while the Brazilian monkey has a 100% quadrupedal posture, not much different from that of a cat or dog.)

Secondly, the Morotopithecus it has a dentition that is apparently adapted for the consumption of leaves. And, to boot, the chemical composition of its fossils indicates a diet that incorporates plants that suffered water stress —in other words, a relative lack of water, indicating a relatively drier environment.

Putting all this data together, the researchers propose that the typical posture of the great apes would have evolved, at first, precisely to face the new habitats, allowing them to climb up and down the trees more easily. On the other hand, their dentition, at first, would be useful to take advantage of drier leaves and vegetable matter, with less dependence on fruits, for example.

This does not necessarily mean a direct line with the evolution of bipedal posture, which would be exclusive to hominins like us, explained Laura MacLatchy to the Sheet.

“Although the fossil record is fragmented, the data we have on East Africa over the last 20 million years is pretty good. And we don’t have any fossil evidence of bipedal posture before about 7 million years ago,” she says. “So it’s unlikely that this posture evolved very gradually over that entire period. However, there may have been multiple independent evolutionary experiments related to the upright trunk involving great apes.”