Human evolution is closely related to the environment and landscape of Africa, from which our ancestors first emerged.
According to traditional scientific narratives, Africa was once a verdant idyll of vast forests stretching from coast to coast. About 21 million years ago, in these green habitats, the early ancestors of apes and humans developed traits that differed from their ape relatives, including an upright posture.
But as the story unfolded, Earth’s climate grew colder and drier, and the forests shrank. About 10 million years ago, grasses and shrubs that could tolerate drought conditions began to invade East Africa and replace forests. Our distant ancestors, the first hominins, emerged from the forest remnants of grassy savannas. This new ecosystem has brought radical changes to our line. We walked on our own two feet.
Researchers have long linked the expansion of African grasslands to the evolution of many human traits, including upright walking, tool use, and hunting. Despite the importance of this theory, it has been overturned by growing evidence from paleontological and paleoclimatic studies. In two recent papers, our multidisciplinary team of Kenyan, Ugandan, European, and American scientists concludes that it is time to discard this version of evolutionary history. Ten years ago, we embarked on a then-unique experiment in paleoanthropology. Several independent research groups have come together to create a regional perspective on the evolution and diversification of early apes. Called REAHE, the project stands for Research on Catarrhine and Hominoid Evolution in East Africa and is based on the premise that inferences from evidence from multiple sites are more powerful than interpretations from single fossil sites. We wondered if earlier explorers had forgotten the trees for the sake of the trees.
21 million year-old monkeys from Uganda
Based on the lifestyles of monkeys living today, scientists suggest that the first monkeys successfully foraged for fruit in dense forests thanks to several important anatomical innovations.
Monkeys have stable and straight backs. When the back is straight, the monkey no longer has to walk on small branches like a monkey. Instead, he can grab onto various branches with his arms and legs and distribute his weight over multiple supports. Monkeys are less likely to lose their balance by hanging under tree branches. In this way, they have access to the orchard at the edge of the crown. Otherwise, only for smaller species. But did this scenario apply to early apes? A 21-million-year-old site in Moroto, Uganda, is an ideal place to study this question. There, our REACH team found teeth and other remains of the oldest ape, Morotopithecus. There, scientists discovered the remains of skulls, teeth, and other skeletal remains.
Two bones in particular have helped us understand how this species moves. The lower vertebrae, discovered decades ago and kept at the Uganda National Museum, have been identified as bony attachments for back muscles, indicating that Morotopithecus had a strong back that was good for climbing trees.
Apes that lived 21 million years ago were accustomed to both grassy and forest habitats. Credit: Corbin Rainbolt
The opening itself largely confirmed this climbing activity. At Moroto, I found a monkey femur with a short diaphragm but a strong and very thick wall. This type of bone is characteristic of living monkeys and helps them climb up and down vertical trees.
Although both skeletal remains are consistent with the hypothesis of fruit-eating, forest-dwelling monkeys, I found a surprise when I found fragments of a monkey’s lower jaw in a fossil bed. Large canines are elongated and have well-developed ridges between the ridges. These ridges are ideal for cutting leaves, unlike the low, rounded, crushing teeth of bona fide frugivores. If apes’ skeletal adaptations evolved in forests to help them exploit fruit, why do the teeth of early apes with these locomotion characteristics resemble those of leaf eaters?
These discrepancies between our evidence and the history of the origin of the great apes lead us to question other assumptions. Did Motopithecus live in forest environments? Moroto’s environment
To determine the habitat of Morotopithecus, we reconstructed Moroto’s ancient climate and vegetation by studying fossil soils, called paleosols, and the microscopic plant remains in them.
(A) Forest ecosystems thought to host early treetop fruit-eating apes, (B) New insights into the reconstruction of herbaceous forest ecosystems and the evolution of early open-dwelling and foraging apes on the leaves.Credit: MacLatchy et al., Science 380, eabq2835 (2023)
Most non-tropical trees, shrubs, and grasses are classified as C3 plants based on the type of photosynthesis they perform. Tropical grasses that depend on a different photosynthetic system are called C4 plants. Importantly, C3 plants and C4 plants have different proportions of different carbon isotopes. This means that the carbon isotope ratios preserved in the Paleolithic can inform the composition of ancient plants. We measured three different carbon isotopic properties, each providing a different insight into plant communities. carbon produced by the decomposition of plants and soil microorganisms; carbon from vegetable wax; and nodules of calcium carbonate formed in the soil by evaporation.
Each proxy gives us slightly different metrics, but they all come together to tell one great story. Moroto was not a closed forest but a relatively open forest environment. We also found evidence of abundant C4 plant biomass (tropical grasses). It was a revelation. C4 grasses lose less water during photosynthesis than C3 trees and shrubs. Today, C4 grasses dominate the seasonally dry savanna ecosystems that cover more than half of Africa. But scientists don’t think the levels of C4 biomass measured at Moroto could have developed in Africa 10 million years ago. Our data shows that this happened twice as often as it did 21 million years ago.
Our colleagues Caroline Strömberg, Alice Novello, and Rahab Kinyanjui used another line of evidence to confirm the abundance of C4 pools in Moroto. They analyzed phytoliths, siliceous bodies made from preserved plant cells in paleomorphs. Their results supported open forest and grassland environments for this time and place. Taken together, this evidence fundamentally contradicts the traditional view of ape origins, which holds that apes evolved upright bodies to bear fruit in lush forests. In contrast, the earliest known upright ape, Motopithecus, ate leaves and lived in open, grassy forests. An example of a typical grass phytolith from a local paleo. Some of them indicate the presence of C4 pools. Credits: Alice Novello
New regional approaches to early ape habitats
With the REACH project, we used the same approach to restore habitats at eight other fossil sites in Kenya and Uganda that are about 16 to 21 million years old. After all, Motopithecus is just one of many apes that lived during this time.
We were surprised to find that the environmental indices measured at Moroto were not unique. Instead, it was a large east in Africa.
People in all our excavations have contributed to two important visions. First, the vegetation type was changed from closed canopy forests to open wooded grasslands. Second, each plot contains a mixture of C3 and C4 plants, with some plots having a high proportion of C4 turf biomass. Phytoliths from the same paleotype confirm the abundance of C4 weeds in several places.
The recognition that these diverse environments, especially open habitats with C4 pools, were ancestral to apes has forced us to reassess the evolution of apes as well as humans and other African mammals. Although some studies suggest that this habitat diversity exists in Africa, our project has been able to repeatedly confirm it in the habitats of early and contemporary apes.
The paleoenvironments of the nine fossil sites analyzed ranged from closed canopy to open forest and grassland environments. The inset map shows the geographic location of locations in East Africa. Credits: Dan Pepe.
The timing of collection in African grassland habitats underlies many evolutionary hypotheses, and our finding that it occurred much earlier than expected warrants reexamination of this idea. In terms of human origins, our research adds more and more evidence that the differences between us and apes in anatomy, ecology, and behavior cannot be explained simply by the emergence of grassland habitats. However, we are careful to remember that hominin evolution took millions of years. It is clear that the vast and magnificent grasslands of Africa played a major role in some of the many stages of human evolution.