Apes may have begun walking upright to eat leaves, not fruits

A breakthrough study from the University of Michigan challenges long-held beliefs about our ape ancestors and the evolution of their upright torsos. Previously, it was thought that the upright posture evolved as a means to pluck fruit from trees in dense forests. 

However, this new research indicates that a life in open woodlands, coupled with a diet rich in leaves, may have been the primary drivers behind the development of apes’ upright stature.

The findings not only provide valuable insights into ape origins but also push back the timeline of grassy woodlands from the previously believed 7 to 10 million years ago to 21 million years ago, during the Early Miocene epoch.

In order to reach fruit growing on the outer edges of trees, large apes must balance their weight on branches closer to the trunk, extending their limbs towards the prize. This task becomes significantly easier when the ape is upright, as it can then grab onto different branches using both hands and feet. 

When an ape’s back is horizontal, its hands and feet are generally positioned underneath its body, making it far more difficult to reach outward to the smaller branches—especially for larger-bodied apes.

This is how modern-day apes forage for fruit, and researchers Laura MacLatchy and John Kingston from the University of Michigan previously believed this to be the reason apes evolved to be upright.

However, new research focusing on a 21-million-year-old fossil ape called Morotopithecus, led by MacLatchy, suggests a different story. The team of researchers now believes that early apes primarily consumed leaves and inhabited seasonal woodlands with broken canopies and open, grassy areas. They propose that this environment, as opposed to fruit in dense forests, played a pivotal role in driving apes toward an upright posture.

These groundbreaking findings have been published in the journal Science, alongside a companion paper that delves into the paleo grassy woodland habitats, reinforcing the research’s conclusions.

“The expectation was: We have this ape with an upright back. It must be living in forests and it must be eating fruit. But as more and more bits of information became available, the first surprising thing we found was that the ape was eating leaves. The second surprise was that it was living in woodlands,” explained MacLatchy, a paleoanthropologist and professor in the U-M Department of Anthropology.

How the study was done

Two groundbreaking papers have emerged from a U.S. National Science Foundation-funded collaboration of international paleontologists, known as the Research on Eastern African Catarrhine and Hominoid Evolution (REACHE) project. 

These researchers, who each specialize in different aspects of early ape paleoenvironments, have made significant discoveries that could reshape our understanding of ape evolution and human origins.

The study’s leader, Laura MacLatchy, focused on the Moroto site in eastern Uganda, which dates back 21 million years. Here, the team, including University of Michigan researchers William Sanders and Miranda Cosman, examined fossils found in a single stratigraphic layer. 

These fossils include those of the oldest documented ape, Morotopithecus, as well as other mammals, ancient soils called paleosols, and tiny silica particles from plants known as phytoliths. By analyzing this evidence, the researchers were able to recreate the ancient environment in which Morotopithecus lived.

MacLatchy and fellow researcher John Kingston discovered that the plants in this landscape were “water stressed,” meaning they experienced seasonal periods of rain and aridity. As a result, for at least part of the year, apes had to rely on something other than fruit for sustenance. 

The findings suggest that Morotopithecus inhabited open woodlands interspersed with broken canopy forests consisting of trees and shrubs.

“These open environments have been invoked to explain human origins, and it was thought that you started to get these more open, seasonal environments between 10 and 7 million years ago,” MacLatchy said. “Such an environmental shift is thought to have been selected for terrestrial bipedalism—our ancestors started striding around on the ground because the trees were further apart.”

“Now that we’ve shown that such environments were present at least 10 million years before bipedalism evolved, we need to really rethink human origins, too.”

The first clue that these ancient apes were leaf-eaters came from their molars. The apes’ molars were very “cresty”: craggy with peaks and valleys. According to MacLatchy, molars like these are used for tearing fibrous leaves apart, while molars used for eating fruit are typically more rounded.

The researchers also studied the dental enamel of the apes and other mammals found in the same stratigraphic layer. They discovered that isotopic ratios in the enamel showed that the apes and other mammals had been consuming water-stressed C3 plants, which are more prevalent in open woodland or grassy woodland environments today. C3 plants are primarily woody shrubs and trees, while C4 plants are arid-adapted grasses.

“Putting together the locomotion, the diet, and the environment, we basically discovered a new model for ape origins,” MacLatchy said. “In anthropology, we care a lot about ape evolution because humans are closely related to apes and features like lower back stability represent an arboreal adaptation that may have ultimately given rise to bipedal humans.”

Early Miocene period and open woodlands

Contrary to previous beliefs that equatorial Africa was densely covered with forests during the Early Miocene, new research has revealed the existence of open seasonal woodlands and grasslands much earlier than the 7 to 10 million years ago previously assumed. 

A second paper, part of the Research on Eastern African Catarrhine and Hominoid Evolution (REACHE) project, utilized a set of environmental proxies to reconstruct the vegetation structure from nine fossil ape sites across Africa, including the Moroto site, during the Early Miocene.

These proxies demonstrated that C4 grasses were widespread during this time period, according to John Kingston, a biological anthropologist and associate professor in the University of Michigan Department of Anthropology. 

“This paper looks at all these sites, pulls all this data together, and says, ‘Look, no matter how you evaluate the data, there’s no way you can escape the fact that all these proxies are converging on the same place—namely, that these environments are open, and they’re open with C4 grasses,” Kingston said.

This groundbreaking research shows that these grasses were widespread and played a key role in shaping the evolution of different mammalian lineages, including the development of various ape lineages.

The nine sites, dispersed across eastern equatorial Africa, provide a “regional picture” of the landscapes during the Early Miocene, as Kingston explained. At this time, the East African Rift was forming, causing significant variations in topography and, consequently, regional climate and vegetation. 

“The landscape is just physically highly variable, and that, no doubt, is related to the vegetation heterogeneity,” Kingston added.

Reconstructing the ancient environment

To reconstruct the paleoenvironment at each location, the researchers employed carbon isotope analyses of ancient soil organic matter, plant wax biomarkers, and phytoliths found at each site. The carbon isotope analyses disclosed a diverse range of plants in the grasslands, from closed canopy to wooded grasslands. 

Wax biomarkers—remnants of the waxy material that protects leaves—indicated a variety of shrubs, trees, and grasses. Phytoliths—microscopic biosilica structures that provide plants with support and defense against herbivores—offered further evidence for the prevalence of C4 grasses.

Upon reconstructing the paleoenvironments at these sites using these proxies, the researchers discovered that C4 grasses were abundant across eastern equatorial Africa and played a crucial role in shaping the heterogeneous habitats. The data also pushes back the earliest evidence of C4 grass-dominated habitats in Africa and globally by over 10 million years.

“The findings have transformed what we thought we knew about early apes, and the origin for where, when and why they navigate through the trees and on the ground in multiple different ways,” said Robin Bernstein, program director for biological anthropology at the National Science Foundation. 

“For the first time, by combining diverse lines of evidence, this collaborative research team tied specific aspects of early ape anatomy to nuanced environmental changes in their habitat in eastern Africa, now revealed as more open and less forested than previously thought. The effort outlines a new framework for future studies regarding ape evolutionary origins.”

These findings not only challenge our understanding of early ape environments but also provide a new perspective on the factors that influenced ape evolution and human origins.

Ape evolution and human origins

The history of ape evolution and human origins is a complex and fascinating journey spanning millions of years. It starts with the divergence of the primate lineage from other mammals, leading to the emergence of early apes and ultimately, humans. Here is a brief overview of this journey:

1. Primates: Around 65 million years ago, after the extinction of dinosaurs, the first primates appeared. These early primates were small, tree-dwelling mammals with features like grasping hands and feet, forward-facing eyes, and relatively large brains.
2. Prosimians and anthropoids: Around 40 million years ago, the primate lineage split into two main branches: prosimians (lemurs, lorises, and tarsiers) and anthropoids (monkeys, apes, and humans). Anthropoids went on to evolve larger brains, more complex social behaviors, and advanced communication skills.
3. Early apes: The first apes, or hominoids, emerged around 25-30 million years ago in Africa and Asia. These early apes were adapted for life in the trees, with flexible shoulder joints and shorter spines that allowed for greater mobility.
4. Hominids: Around 15-20 million years ago, the hominoid lineage split into two groups: the lesser apes (gibbons) and the great apes (orangutans, gorillas, chimpanzees, and humans). The great apes share several features, such as larger brains and a more upright posture, which distinguish them from other primates.
5. Hominins: The human lineage, or hominins, diverged from the common ancestor with chimpanzees around 6-7 million years ago. The early hominins were bipedal, walking on two legs, and had small brains compared to modern humans.
6. Australopithecines: Around 4 million years ago, the australopithecines appeared in Africa. They had a mix of ape-like and human-like traits, including bipedal locomotion, small brains, and a protruding face.
7. Homo genus: The genus Homo emerged around 2.5 million years ago, with the appearance of Homo habilis. This species had a larger brain, smaller teeth, and made simple stone tools. Other early Homo species include Homo erectus (1.9 million years ago) and Homo heidelbergensis (600,000 years ago).
8. Neanderthals and Denisovans: Around 300,000 years ago, the Neanderthals (Homo neanderthalensis) emerged in Europe and Western Asia, while the Denisovans appeared in Asia. These human relatives were adapted to cold environments and had larger brains than modern humans.
9. Homo sapiens: Anatomically modern humans, Homo sapiens, evolved in Africa around 300,000 years ago. They had larger brains, more advanced tools, and complex language and culture. Around 70,000 years ago, Homo sapiens began to migrate out of Africa, eventually replacing other hominin species and populating the entire world.

The history of ape evolution and human origins is an ongoing area of research, with new discoveries continuously reshaping our understanding of our distant ancestors and the factors that drove their development.

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