A plant-based diet full of leaves may have helped apes stand upright

Snacking on leafy greens may have had more of an impact than scouring treetops for fruit.
An artistic rendering of the open woodland habitat reconstruction at Moroto II with Morotopithecus bishopi vertically climbing with infant on back and Mount Moroto, an active, in background.

An artistic rendering of the open woodland habitat reconstruction at Moroto II with Morotopithecus bishopi vertically climbing with infant on back and Mount Moroto, an active, in background. Corbin Rainbolt

Two new studies are shedding light on not only early hominid evolution, but are turning back the evolutionary clock on how early grassy woodlands appeared on the African continent.

The first new study, published April 13 in the journal Science, suggests that life in the open woodlands of Africa and a leafy diet may have influenced the upright stature of humans’ ape ancestors. 

[Related: Ancient DNA confirms Swahilis’ blended African and Asian ancestry.]

Anthropologists had long believed that our ancestors evolved an upright torso to pick fruit in forests, since some of our favorite produce grows on the spindly peripheries of trees. Large apes would have needed to distribute their weight on the branches stemming up from the trunk and then reach up with their hands to grab the fruit. Performing this task is easier if an ape is upright, since it can grab the branches better with their hands and feet. If an ape’s back is horizontal, the hands and feet are typically underneath the body, which makes it harder to move outward to the smaller branches of the tree. 

However, new research using a 21-million-year-old fossilized ape called Morotopithecus suggests that early apes actually ate the leaves in a seasonal woodland with a broken tree canopy and open grassy areas. The team believes that this landscape, and not fruit in closed canopy forests, possibly drove the ape’s upright structure.   

“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,” co-author and University of Michigan paleoanthropologist Laura MacLatchy said in a statement.

Both papers grew out of a collaboration of international paleontologists called the Research on Eastern African Catarrhine and Hominoid Evolution project (REACHE). MacLatchy’s study focused on the a 21-million-year-old site in eastern Uganda called the Moroto site. Here, the team found fossils in a single rock layer. Fossils of other mammals and evidence of plant life were found in this layer and these lines were used to recreate Morotopithecus’ environment.

In a companion paper, also published April 13 in Science, another team used environmental proxies to reconstruct nine fossil ape sites across Africa, including the Moroto site during the early Miocene. The proxies revealed that grasses were actually all over the area 21 million years ago, instead of the previously determined 7 to 10 million. 

The team then found that the plants in this landscape were “water stressed,” which means that they lived in seasonal periods of rain and of aridity. These shifts mean that apes would have had to rely on something other than fruit to survive. These findings indicate that Morotopithecus likely lived in an open woodland that was punctuated by broken canopy forests made up of shrubs and trees. 

[Related: The ‘granddaddy’ of all early hominins walked on Earth a lot longer than we thought.]

“For the first time, we’re showing that these grasses are widespread, and it’s this general context of open seasonal woodland ecosystems that were integral in shaping the evolution of different mammalian lineages, including and especially in our case, how different ape lineages evolved,” study co-author and University of Michigan biological anthropologist John Kingston said in a statement.

The nine sites in both studies are scattered across eastern equatorial Africa, which is an area large enough for the team to develop a better regional picture of what these landscapes looked like 23 million to 16 million years ago during the early Miocene. At this time, the East African Rift forming the region saw huge change in topography. This upheaval as the Earth was pulling apart resulted in regional climate and its vegetation.  

“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.”