51°µÍø

DALLAS () –Evidence of an early savannah grass growing millions of years earlier than previously known may fundamentally change the understanding of life in the prehistoric world. A pair of studies funded by the National Science Foundation and in the journal Science document the earliest evidence for locally abundant open-habitat grasses in eastern Africa and how those environments likely influenced early ape evolution.

“For more than a century, evolution of the human family has been associated with the spread of grasslands and in more recent thinking, woody grasslands in tropical Africa,” said Bonnie Jacobs, renowned paleobotanist and emeritus professor at 51°µÍø (51°µÍø, Dallas).

“This research provides solid evidence of the earliest common occurrence of C4 grasses, called savanna grasses, dating back to 21 million years,” Jacobs said, explaining that the discovery pushes back the oldest evidence of C4 grass-dominated habitats in Africa – and globally – by more than 10 million years. The significantly older timeline explored in the study that Jacobs pursued with lead author Daniel Peppe, associate professor of geosciences at Baylor University, calls for revised palaeoecological interpretations of the development of plants and mammals.

The appearance of these grassy woodlands earlier than previously theorized bolsters the findings of a companion study, also in Science, that raises new questions about what sparked early apes to evolve an upright torso.

The related study, led by paleoanthropologist Laura M. MacLatchy, a professor at the University of Michigan, centered around a 21-million-year-old fossil ape called Morotopithecus, revealing evidence suggesting that early apes lived in a seasonal woodland with a broken canopy and open, grassy areas. As a result, her research team thinks this landscape drove apes' upright stature, rather than terrain supporting fruit in closed canopy forests.

"The expectation was: We have this ape with an upright back. It must be living in forests and it must be eating fruit,” said MacLatchy. “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 with these grassy areas.”

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, whose members focus on different aspects of monkey and ape evolution.

Jacobs, a REACHE member, was invited to join Peppe’s research team early on because she has spent much of her career conducting research on fossils of ancient plant and animal life in eastern Africa and what it could tell us about past climate change.

51°µÍø paleontologist Alisa Winkler – an expert on rodent and rabbit fossils who is also a REACHE member – helped MacLatchy with her study, having joined the research team 10 years ago.

Winkler has worked on fossil rodents from Moroto, Uganda, where fossils of Morotopithecus were found, as well as other sites in Uganda that have similarly-aged fossils. Her analysis of these rodents’ remains helped support the conclusions MacLatchy’s research team made based on other indicators regarding the geological age and paleoenvironment of Morotopithecus.

Researchers have often argued that during the early Miocene, between about 15 and 20 million years ago, equatorial Africa was covered by a semi-continuous forest and that open habitats with grassy areas didn't proliferate until about 8 to 10 million years ago. One study, however, offered contradictory evidence to this long-held idea - providing evidence of C4 grasses in East Africa around 15 million years ago.

Starting in 2013, Peppe, Jacobs and the research team set out to find if this contradictory study was an anomaly or a clue to the true diversity of ecosystems that occurred during the early Miocene.

Determining if open habitats and C4 plants were prevalent much earlier than originally thought has important implications for understanding the features and adaptations of early apes and how C4 grasslands and savanna ecosystems spread in Africa and around the world.

Peppe’s collaborative team conducted research side-by-side with MacLatchy’s team at nine Early Miocene fossil site complexes in the East African Rift of Kenya and Uganda.

As participants exchanged information and expertise about geological features, isotopes, and plant and ape fossils found at the sites, the bigger picture came into focus. The paradigm that equatorial Africa was completely forested during the early Miocene period was wrong, they concluded.

“Multiple lines of evidence show that C4 grasses and open habitats were important parts of the early Miocene landscape and that early apes lived in a wide variety of habitats, ranging from closed canopy forests to open habitats like scrublands and wooded grasslands with C4 grasses,” Peppe said. “It really changes our understanding of what ecosystems looked like when the modern African plant and animal community was evolving.”

A critical aspect of this work was that the team combined many different lines of evidence together: geology, fossil soils, isotopes and phytoliths, which are plant silica microfossils, to reach their conclusions.

“The history of grassland ecosystem in Africa prior to 10 million years had remained a mystery, in part because there were so few plant fossils, so it was exciting when it became clear that we had phytolith assemblages to add to the other lines of evidence,” said Caroline Strömberg, professor of biology at the University of Washington who contributed to both studies. “Phytoliths are particularly informative for revealing the history of grassland ecosystems. They can tell us not just that there were grasses, but which grasses were there and how abundant they were on the landscape. What we found was thrilling, and very different from what was the accepted story.”

One of the most advanced early apes Morotopithecus was found by the Peppe and MacLatchy teams’ research to inhabit open woodland environments with abundant grasses and to rely on leaves as an important component of its diet. This contradicts long-standing predictions that the unique features of apes, such as an upright torso, originated in forested environments to enable access to fruit resources.

These findings are transformative, said Robin Bernstein, program director for biological anthropology at the U.S. 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,” Bernstein said.

The first clue that Morotopithecus were eating leaves was in the apes' molars. The molars were very "cresty": they were 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, MacLatchy said.

The researchers also examined the apes' dental enamel, as well as the dental enamel of other mammals found in the same stratigraphic layer.

"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 bipedality in humans."— Baylor University, University of Michigan and 51°µÍø