New study changes perception of early apes and their environments

ASU researcher among team that found 'new model for ape origins'

April 13, 2023

Anthropologists have long thought that our ape ancestors evolved an upright torso to pick fruit in forests, but new research published this week in the journal Science suggests a life in open woodlands and a diet that included leaves drove apes' upright stature.

The finding sheds light on ape origins and pushes back the origin of grassy woodlands from between 7 million and 10 million years ago to 21 million years ago, during the Early Miocene. Artist-drawn image of prehistoric apes climbing a tree. Artistic rendering of the open woodland habitat reconstruction at Moroto II, with Morotopithecus bishopi with an infant on their back vertically climbing and a juvenile below. Rendering courtesy Corbin Rainbolt Download Full Image

The new research is centered around a 21-million-year-old fossil site called Moroto in eastern Uganda. There, the international research team, which includes Arizona State University researcher Rutger Jansma, examined fossils found in a single stratigraphic layer, including fossils of the oldest, clearly documented ape, Morotopithecus.

Also within this layer were fossils of other mammals, ancient soils called paleosols and tiny silica particles from plants called phytoliths. The researchers used these lines of evidence to recreate the ancient environment of Morotopithecus.

Previous research has supported the idea that apes with an upright back must be living in forests and eating fruit. Observations of modern-day apes show the primates reach out to fruit that grows on the spindly peripheries of trees. Large apes need to distribute their weight on branches stemming from the trunk, then reach out with their hands toward their prize. This is much easier if an ape is upright because it can more easily grab onto different branches with its hands and feet.

But as more research results from Morotopithecus became available, the first surprising thing researchers found was that the ape was eating leaves. The second surprise was that it was living in woodlands.

The first clue that these ancient apes were eating leaves was in the apes' molars. The molars were very “cresty and craggy,” with peaks and valleys. Molars like this are used for tearing fibrous leaves apart, while molars used for eating fruit are typically more rounded.

ASU researcher Jansma identified another primate from the Moroto assemblage — Rangwapithecus, a smaller, siamang-sized early ape that lived alongside the larger Morotopithecus.

“Its cheek teeth are long and narrow, with sharp crests that help to break down fibrous leaves, just like Morotopithecus,” said Jansma, a faculty associate with the School of Human Evolution and Social Change. “It is not usual to find several different apes together in the Miocene, but what is strange is that at least two of them were eating leaves. Today, there are only a handful of ape species left and most of them eat fruit. This study emphasizes the importance of fossil evidence to piece together ape evolution instead of only looking at their living descendants.”

The researchers also examined the apes' dental enamel, as well as the dental enamel of other mammals found in the same stratigraphic layer. They found that isotopic ratios — the abundance of two isotopes of the same element — in their dental enamel showed that the apes and other mammals had been eating water-stressed C3 plants that are more common in open woodland or grassy woodland environments today. C3 plants are primarily woody shrubs and trees, while C4 plants are arid-adapted grasses.

The research team also discovered that the plants living in this 21-million-year-old landscape were water stressed, meaning they lived through seasonal periods of rain and of aridity. This also indicates that, at least for part of the year, apes had to rely on something other than fruit to survive.

Together, these findings reveal that Morotopithecus lived in open woodlands punctuated by broken canopy forests composed of trees and shrubs.

Therefore, the research team suggests, early apes ate leaves and lived in a seasonal woodland with a broken canopy and open, grassy areas, which drove apes' upright stature, instead of fruit in closed canopy forests.

Ape habitats

Their results are bolstered by a companion paper published in the same issue of the journal examining the paleo grassy woodland habitats. These findings used 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 revealed that C4 grasses were widespread, and the general context of open seasonal woodland ecosystems were integral in shaping the evolution of different mammalian lineages, including and especially in this research, how different ape lineages evolved.

The two papers grew out of a U.S. National Science Foundation-funded collaboration of international paleontologists, collectively known as the Research on Eastern African Catarrhine and Hominoid Evolution project, or REACHE, each of whom focus on different aspects of early ape paleoenvironments.

“These open environments have been invoked to explain human origins, and it was thought that there was a more open, seasonal environment between 10 and 7 million years ago,” said lead researcher Laura MacLatchy, from the University of Michigan.

“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,” MacLatchy said. “Putting together the locomotion, the diet and the environment, we basically discovered a new model for ape origins. 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."

"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.

Research article: The evolution of hominoid locomotor versatility: Evidence from Moroto, a 21 Ma site in Uganda, Laura M. MacLatchy et al, Science. Companion article: Oldest evidence of abundant C4 grasses and habitat heterogeneity in eastern Africa, Laura M. MacLatchy et al, Science.

Julie Russ

Assistant director, Institute of Human Origins


ASU study examines why some environments have more species than others

Stalagmites tell the climate story about species diversity

March 21, 2023

Why do some regions of the world have so many species of plants while others have so few?

This is one of the great enduring research questions in the biological sciences, and there have been many ideas put forward as an answer. Close-up photo of a protea plant. Protea plant, native to South Africa Cape Floristic Region. Photo courtesy Kerstin Braun Download Full Image

One idea is that regions that have had relatively stable climates through the millennia will have greater numbers of species. In contrast, dramatic climate change of the type we are experiencing today will drive species extinct, thus winnowing away diversity.

A study“Climatic stability recorded in speleothems may contribute to higher biodiversity in the Cape Floristic Region.” Kerstin Braun, Richard M. Cowling, Miryam Bar-Matthews, Avner Ayalong, Tami Zilberman, Mark Difford, R. Lawrence Edwards, Xianglei Li, Curtis W. Marean. Journal of Biogeography. published in the Journal of Biogeography by an international transdisciplinary team of scientists, led by Arizona State University researchers Curtis Marean and Kerstin Braun, provides compelling evidence for this idea from the study of climate change from stalagmites.

The Cape Floristic Region on Africa’s southern tip has one of the most diverse floras on Earth despite having infertile soils and hot, dry summers. Normally, more productive, wetter regions have more diversity.

“To understand the reasons for this diversity, we need to develop long climate records from a large sample of regions in the world and study their climate stability relative to their floral species diversity,” said Marean, who is an ASU Foundation Professor, a research scientist with the Institute of Human Origins and School of Human Evolution and Social Change, and a Nelson Mandela University (South Africa) Honorary Professor.

To do that, Marean developed a program to find and study stalagmites throughout the Cape of South Africa and collaborate with a transdisciplinary team of scientists including South African botanist Richard Cowling of Nelson Mandela University.

Map of South Africa

Topography of South Africa with the location of the cave and major cities. Gray shading indicates the Cape Floristic Region and a dark grey outline marks the area dominated by winter rainfall (>60% rainfall between April and September). Image by Kerstin Braun PhD.

Why stalagmites?

Stalagmites can preserve long records of climate change; they grow in caves, sometimes for over tens to hundreds of thousands of years, and they preserve changes in stable isotopes of oxygen and carbon that depend on the rain water and vegetation above the cave site. They can also be accurately dated with a technique called uranium-thorium dating.

Marean and colleagues rescued a set of stalagmites destined for destruction by mining at Cape Limeworks in Robertson in the Cape Floristic Region. The stalagmites, which grew between about 670,000 and 240,000 years ago, were then studied by Braun, an assistant research scholar in the Institute of Human Origins and lead author of the study.

Stalagmite in a cave

Cape Lime Robertston cave. One of the studied stalagmites that grew in place for nearly 400,000 years. Photo by Curtis Marean

“We compared the Cape Limeworks records to those from other sites in similar summer-dry, Mediterranean-type climates but with lower diversities than the Cape Floristic Region," Braun said. "The results clearly show that the southwestern Cape was climatically much more stable through glacial-interglacial changes than the other areas."

The results are significant because the Cape experiences a Mediterranean climate with dry, hot summers and mild, wet winters. This means that temperatures are generally low during the main growth season.

Common hypotheses for the evolution of diverse floras were developed to explain the high diversity of the wet tropics and often postulate that warm and wet conditions are needed for the evolution of high diversity — for example, through increasing the speed of metabolism or interactions between organisms.

These hypotheses don’t apply in arid Mediterranean regions, yet many of the floras in those regions exceed those of many tropical rainforests in the number of species per area.

Researchers from Mediterranean climate regions have long recognized their exceptional floras and proposed alternative hypotheses for their evolution. Cowling thinks that environmental stability and low extinction rates are a major factor in the accumulation of species.

The study is the first time that a paleoclimate reconstruction spanning several hundreds of thousands of years backs up these claims.

Cowling notes that “These findings show that relative climate stability over evolutionary time explains patterns of diversity in Mediterranean regions: The more stable, the richer the flora, with the Cape at the head of the pack. It’s possible that the climate stability theory applies also to tropical rainforest regions.”

“Our results," said Marean, “provide a dire warning of the downstream impacts of rapid climate change that we are now experiencing. Our study shows that rapid climate change annihilates plant lineages, so the human-induced rapid climate change we see today will do the same with horrific consequences for the animals and humans that rely on those plants.”

This research was funded by the USA National Science Foundation, the Hyde Family Foundations, the John Templeton Foundation, Arizona State University, the European Commission, the South African National Research Foundation and Nelson Mandela University.

Julie Russ

Assistant director, Institute of Human Origins