Open landscapes shaped human evolution

African savannas may be much older than previously assumed: it was previously assumed that large parts of Africa were covered with dense forests until about ten million years ago. However, new studies show that savannah grasses were already growing in various places in East Africa as early as 21 million years ago. So the landscape in which our ancestors grew up was more open than expected. This sheds new light on the evolution of upright walking. Skeletal finds of hominins living in Uganda 21 million years ago indicate that they did indeed show adaptations to the upright position.

In the savannah landscapes of Africa today, mainly grasses grow that are well adapted to hot sunny conditions. They use the so-called C4 photosynthetic pathway, by which they can photosynthesize effectively even in dry conditions. In contrast, the so-called C3 vegetation can make better use of less strong sunlight and still make up the bulk of the vegetation cover in the middle and high latitudes. C4 plants likely evolved independently from C3 plants several times when environmental conditions required adaptation to strong sunlight and drought. However, so far it is assumed that they began to spread only about ten million years ago.

grasslands rather than dense forests

However, a team led by Daniel Pape of Baylor University in Texas has found evidence that C4 weeds were already present in various places in East Africa 21 million years ago. “Our results show that 21 million years ago, C4 grasses were locally widespread and contributed to heterogeneous habitats ranging from forests to lightly forested open grasslands,” the team says. Peppe and his team analyzed soil samples from nine sites in Kenya and Uganda that were dated between 17 and 21 million years ago. In it, they found several biomass signatures from C4 weeds.

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“These data push back the first evidence of C4 grass-dominated habitats in Africa by more than 10 million years,” the authors said. It was previously assumed that East Africa was covered with dense forests 21 million years ago. Shady conditions favored C3 grasses. The fact that C4 grasses were already present at that time indicates that the landscape was much more open than previously assumed, so much sunlight could reach the ground and only a few trees provided shade. Assessments indicate that the areas were seasonally dry, resulting in plants that were more drought-adapted.

The first humans lived in the open countryside

These results also shed new light on the evolution of mammals, especially hominins, which includes apes and humans. “Today’s hominins are characterized by an erect torso and versatile movement,” explains a team led by Laura McClatchy of the University of Michigan. “It was previously assumed that these traits evolved for fruit on thin branches in forests.” New findings now suggest different conclusions. McClatchy and her team studied early human fossils from a site in Uganda called Moroto II, one of the sites identified by Peppe and his team as containing C4 weeds 21 million years ago.

Using fossil bones and teeth, the researchers came to conclusions about the lifestyle and diet of Morotopithecus, a large species of primate considered one of the most ancient representatives of humans. “The expectation was: We have this monkey with an erect back. It must live in forests and eat fruit. But as more and more information became available, the first surprise was that the monkey ate leaves. The second surprise was that it lived in open landscapes,” says McClatchy. . The angular shape of the molars reflects adaptations to high-fiber foods such as leaves and grass. On the other hand, chewing fruit molars will be more round. The femurs, trunk, and vertebrae are similar to those of modern primates that live in savannas and other open ecosystems.

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Rethinking human origins

“Essentially, when you combine movement, diet, and environment together, we’ve discovered a new model for the origins of monkeys,” McClatchy says. Combining ecological data with skeletal analysis shows that open landscapes, thought to be an important factor in the evolution of upright gait, existed at least ten million years earlier than previously thought. “As such, we also need to re-examine our previous assumptions about human origins,” McClatchy said. Her and Peppe’s work could form a basis for future studies.

Sources: Daniel Peppe (Baylor University, Texas, USA) et al., Science, doi: 10.1126/science. abq2834
Laura MacLatchy (University of Michigan, USA) et al., Sciences, doi: 10.1126/science. abq2835

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