Wooded grasslands flourished in Africa 21 million years ago – New research forces rethinking of ape evolution

    Wooded grasslands flourished in Africa 21 million years ago – New research forces rethinking of ape evolution , Human evolution is closely related to the environment and landscape of Africa, where our ancestors first appear

According to the traditional scientific narrative, Africa was once a verdant idyll of vast forests stretching from coast to coast. In these lush habitats, about 21 million years ago, the earliest ancestors of apes and humans first developed traits—including an upright posture—that distinguished them from their ape cousins.

But then, according to the story, the global climate cooled and dried, and forests began to decline. About 10 million years ago, grasses and shrubs that better tolerated increasingly dry conditions began to take over East Africa, replacing forests. The earliest hominins, our distant ancestors, ventured to leave the forest remnants that had been their home for the grass-covered savannah. The idea was that this new ecosystem pushed a radical change for our line: we became bipedal.

For a long time, researchers have been links the expansion of grasslands in Africa to the evolution of numerous human traits, including bipedalism, tool use, and hunting.

Despite the prominence of this theory, growing evidence from paleontological and paleoclimatological studies undermines it. On two recent papersour multidisciplinary team from Kenya, Uganda, Europe and American scientists concluded that it is time to finally reject this version of evolutionary history.

A decade ago, we began what was at the time a unique experiment in paleoanthropology: several independent research teams came together to build a regional perspective on the evolution and diversification of early apes. The project, called REACHE, short for Research on Eastern African Catarrhin and Hominoid Evolution, was based on the premise that conclusions drawn from evidence at many sites would be more powerful than interpretations from single fossil deposits. We wondered if previous explorers had missed the forest for the trees.

A monkey in Uganda 21 million years ago

Based on the lifestyles of living apes today, scientists have hypothesized that they first evolved in dense forests, where they successfully fed on fruitthanks to several key anatomical innovations.

Monkeys have stable, straight backs. Once the back is vertical, the monkey no longer has to walk on top of small branches like a monkey. Instead, it can grasp various branches with its arms and legs, distributing its body mass over multiple supports. The monkeys can even hang under the branches, making them less likely to lose their balance. In this way, they have access to fruits growing on the edges of tree crowns that would otherwise only be available to smaller species.

But is this scenario true for the earliest apes? A 21-million-year-old site in Moroto, Uganda, has become an ideal place to study this question. There, our REACHE team found teeth and other remains belonging to Morotopitekthe oldest ape for which scientists have found fossils of the skull, teeth and other parts of the skeleton.

Two bones in particular helped us understand how this species moved. A lower spine, discovered decades ago and curated by the Uganda National Museum, was already famous for its bony attachments for back muscleswhich shows that Morotopitek it had a stiff lower back suitable for climbing trees upright.

A discovery of our own largely confirmed this climbing behavior. At Moroto we found a fossil ape femur, which is short but strong, with a very thick shaft. This type of bone is characteristic of living monkeys and helps them climb up and down trees with a vertical torso.

Although both skeletal fossils are consistent with the frugivorous forest-dwelling ape hypothesis, we found something amazing when we found a fragment of the mandible of a monkey in the same layer of excavation. Its molars were elongated, with well-developed cutting crests passing between the cusps. These edges are perfect for cutting leaves, but they don’t look like the low, round, crushing teeth of committed fruit. If ape skeletal adaptations evolved in forests to aid fruit exploitation, why did the earliest ape exhibiting these locomotor characteristics instead have leaf-eater teeth?

Such discrepancies between our evidence and the traditional account of ape origins have led us to question other assumptions: Morotopitek do you live in a wooded environment at all?

The environment in Moroto

To understand Morotopitek habitat, we studied the chemistry of the fossil soils—called paleosols—and the microscopic plant remains they contain to reconstruct the ancient climate and vegetation of the Moroto.

Trees and most shrubs and non-tropical grasses are classified as C₃ plants based on the type of photosynthesis they perform. Tropical grasses that rely on a different photosynthetic system are known as C₄ plants. Importantly, C₃ plants and C4 plants differ in the proportions of the different carbon atoms isotopes they take over. This means that the carbon isotope ratios preserved in paleosols can tell us the composition of ancient vegetation.

We measured three different carbon isotope signatures, each providing a different perspective on the plant community: carbon derived from decomposing vegetation and soil microbes; carbon derived from vegetable waxes; and nodules of calcium carbonate formed in soils by evaporation.

Although each proxy gave us slightly different values, they came together in a remarkable story. The moro was not a closed forest habitat, but rather a relatively open forested environment. Moreover, we found evidence of abundant C₄ plant biomass—tropical grasses.

This discovery was a revelation. C₄ grasses lose less water during photosynthesis than C₃ trees and shrubs. Today, C₄ grasses dominate seasonally dry savanna ecosystems that covers more than half of Africa. But scientists didn’t think the levels of C₄ biomass we measured at Moroto had developed in Africa until 10 million years ago. Our data shows that this happened twice as far back in time, 21 million years ago.

Our colleagues Caroline StrombergAlice Novello and Rahab Kinyanjui used another piece of evidence to confirm the abundance of C₄ grasses in the Moroto. They analyzed phytoliths, small siliceous bodies created by plant cells preserved in paleosols. Their results support an open woodland and wooded grassland environment for this time and place.

Taken together, this evidence dramatically contradicts the traditional view of ape origins—that apes evolved upright torsos to reach fruit in forest canopies. Instead, Morotopitekthe earliest known upright walking ape, eats leaves and inhabits open forest with grassy areas.

A new, regional view of early ape habitats

Through the REACHE project, we applied the same approach to reconstruct habitats at eight other fossil sites in Kenya and Uganda, ranging in age from about 16 million to 21 million years old. After all, Morotopitek is just one of several monkeys that lived during this time period.

To our surprise, we found that the ecological signal measured at Moroto was not unique. Instead, it is part of a wider pattern in East Africa during this time.

Our isotopic proxies at each fossil locality contributed two important discoveries. First, vegetation types range from closed canopy forests to open wooded grasslands. And second, each site has a mixture of C3 and C4 vegetation, with some sites having a high proportion of C4 grass biomass. Phytoliths from the same paleosols again confirm that abundant C₄ grasses are present at multiple sites.

The realization that such a variety of environments, especially open habitats with C₄ grasses, was present at the dawn of apes calls for a reassessment not only of ape evolution, but also of humans and other African mammals. Although some studies suggest that such habitat variation is present in Africa, our project was able to confirm it repeatedly in the very habitats that early apes and their contemporaries inhabited.

Because the timing of the assembly of grassland habitats in Africa is the basis of many evolutionary hypotheses, our finding that they existed much earlier than expected calls for a recalibration of these ideas.

As for human origins, our research adds to a growing body of evidence that our divergence from apes—in terms of anatomy, ecology, behavior—cannot be explained simply by the emergence of grassland habitats. However, we cautiously remind ourselves that hominin evolution has unfolded over many millions of years. It is almost certain that the vast and majestic grasslands of Africa played an important role in some of the many steps on the way to becoming human.

Laura M. McClatchy is a professor of anthropology at the University of Michigan. Dan Pepe is an associate professor of geosciences at Baylor University. Kieran McNulty is a professor of anthropology at the University of Minnesota. This article was republished by The conversation under a Creative Commons license. Read on original article.