The African savannahs are probably much older than previously assumed: it was previously assumed that large parts of Africa were covered by dense forests until around ten million years ago. However, new studies show that savannah grasses were already growing in various places in East Africa 21 million years ago. The landscape in which our ancestors developed was therefore more open than expected. This sheds new light on the development of upright gait. Skeletal finds of hominoids living in Uganda 21 million years ago indicate that they already showed adaptations to an upright posture.
In the savannah landscapes of today’s Africa, mainly grasses grow that have adapted well to hot, sunny conditions. They use the so-called C4 photosynthetic pathway, with which they can effectively carry out photosynthesis even in dry conditions. In contrast, so-called C3 plants can make better use of less strong sunlight and still make up the largest part of the vegetation in middle and high latitudes. C4 plants are likely to have independently evolved from C3 plants multiple times when environmental conditions required adaptation to strong sunlight and drought. So far, however, it has been assumed that they only began to spread around ten million years ago.
Grassland instead of dense forest
However, a team led by Daniel Peppe from Baylor University in Texas has now found evidence that C4 grasses were already present in various places in East Africa 21 million years ago. “Our results show that as early as 21 million years ago, C4 grasses were locally widespread and contributed to heterogeneous habitats ranging from forests to lightly wooded open grasslands,” the team reports. Peppe and his team analyzed soil samples from nine sites in Kenya and Uganda that were dated to be between 17 and 21 million years old. In it, they found numerous biomass signatures from C4 grasses.
“These data push back the earliest 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 by dense forests 21 million years ago. The shady conditions would have favored C3 grasses. The fact that C4 grasses were already present at that time suggests that the landscape was more open than previously assumed, so that a lot of sunlight could reach the ground and only a few trees provided shade. The evaluations show that the areas were seasonally dry, which favored the emergence of plants that were better adapted to the drought.
Early hominoids lived in open countryside
These findings also shed new light on the evolution of mammals, particularly hominoids, which include apes and humans. “Today’s hominoids are characterized by an erect trunk and versatile locomotion,” explains a team led by Laura McLatchy from the University of Michigan. “Previously it was assumed that these traits evolved to reach fruit on thin branches in forests.” The new finds now suggest different conclusions. McLatchy and her team studied early hominoid fossils from a site in Uganda called Moroto II. This is one of the sites Peppe and his team have identified as having C4 grasses 21 million years ago.
Using bone and tooth fossils, the researchers drew conclusions about the lifestyle and diet of Morotopithecus, a large primate species that is considered one of the oldest representatives of the hominoids. “The expectation was: We have this monkey with an upright back. He must live in forests and eat fruits. But as more and more information became available, the first surprise was that the monkey ate leaves. The second surprise was that he lived in open landscapes,” says MacLatchy. The angular shape of the molars reflects adaptations to fiber-rich foods such as leaves and grass. Chewing fruit molars, on the other hand, would typically be rounder. The thigh, trunk, and vertebrae bones are similar to those of modern-day primates that inhabit savannas and other open ecosystems.
Rethinking the origins of man
“Basically, when you put locomotion, diet and environment together, we discovered a new model for the origins of apes,” says MacLatchy. The combination of environmental data and skeletal analysis shows that open landscapes, believed to be an important factor in the evolution of upright gait, existed at least ten million years earlier than thought. “As such, we also need to reconsider our previous assumptions about human origins,” MacLatchy said. Her and Peppe’s work can 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., Science, doi: 10.1126/science.abq2835