TCC Anthropology Professor Dr. Kirsten Jenkins, research team conclude that apes may have evolved upright stature to eat leaves in open woodland habitats

 Note: Parts of this story, including the quotes from Dr. Laura McLatchy, are used, with permission, from a press release prepared by Baylor University. Group photo shared with permission from Dr. Dan Peppe and the research team. Caption:Co-authors David Fox (University of Minnesota), Bill Lukens (James Madison University), Kirsten Jenkins (Tacoma Community College), John Kingston (University of Michigan), Al Deino (Berkeley Geochronology Center), and Rahab Kinyanjui (National Museums of Kenya and Max Planck Institute) preparing to cross the Turkwel River in northern Kenya on their way to the west Turkana sites. This interdisciplinary team focused on many aspects of the geology and paleontology of the fossil sites allowing us to interpret paleoenvironments associated with ancient apes in eastern Africa. Image credit: Bill Lukens

Tacoma Community College (TCC) Anthropology Professor Dr. Kirsten Jenkins is a contributing author to two papers published today in the journal Science. The papers document the earliest evidence for locally abundant C4 grasses in eastern Africa, pushing back the date for grasses by 10 million years, and shows how C4 grasses and the open habitats where they flourished influenced early ape evolution, concluding that apes may have evolved their upright stature to eat leaves in grassy woodland habitats, and not to eat fruits in forests as scientists long believed.

The papers published in Science are “The evolution of hominoid locomotor versatility: Evidence from Moroto, a 21 Ma site in Uganda” and “Oldest evidence of abundant C4 grasses and habitat heterogeneity in eastern Africa.”

Started in 2013 and funded by the U.S. National Science Foundation, the research was conducted by an international team collectively known as the Research on Eastern African Catarrhine and Hominoid Evolution project, or REACHE. The project encompasses a collection of eight fossil sites situated around ancient extinct volcanoes in eastern Africa.

“I have been fortunate to visit nearly all of these sites in these studies, but I keep coming back to Rusigna and Mfangano Islands on Lake Victoria, Kenya, where I have been working since 2009,” Jenkins said. “For this research, I finished up my data collection for Moroto (Uganda) just before I was hired at TCC in 2018, but I continued the analysis, writing, and editing for these papers to this year, 2023! The work is ongoing still; there is always more to learn. I still have collections waiting for me back at the National Museum of Uganda and the National Museum of Kenya.”

Jenkins helped to write the original REACHE grant and helped to develop the fossil collection protocols that would be consistently used across sites for comparative purposes. Her role as an anthropologist was to study fossil taphonomy, assist with fossil identification, and collect samples in the field.

“Taphonomy is the study of how the fossils came to be buried and preserved,” Jenkins said. “In some cases, I can assess how organisms died.”

Jenkins says that fossil preservation can also give us clues to the environments where the organisms lived – and that’s what the REACHE project was all about.

Scientists have long theorized that apes developed upright posture to reach fruit growing in the forests where they lived. But what if early apes didn’t live in dense forests? The project’s findings push back the origin of grassy woodlands from between 7 million and 10 million years ago to 21 million years ago, during the Early Miocene. The findings suggest that these grassy woodlands, which included broken tree canopy and open, grassy areas, are in fact the setting in which apes evolved their upright stature. Early apes living in such conditions, including the 21-million-year-old fossil ape called Morotopithecus the team studied, would have eaten leaves.

"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," said Laura MacLatchy, a paleoanthropologist and professor in the University of Michigan Department of Anthropology.

“At Moroto Fossil Site, I was able to tell that some fossils showed marks characteristic of weathering in more open environments and being rolled by a stream,” Jenkins said.

The group 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.

“Phytoliths are hard microscopic silica parts that plants leave after they die,” Jenkins explained. “Usually most of a plant decays and doesn’t fossilize, though we occasionally get fantastic fossil leaves, sticks, trees, etc., at some of these sites. However, plants are always shedding phytoliths that can be identified by taxon, or type, under a microscope.”

The team wasn’t sure that phytoliths would even preserve from the early Miocene, but Dr. Rahab Kinyanjui at the National Museum of Kenya discovered some of the grass phytoliths at the sites. The team collected phytoliths and sediments and analyzed them in the lab. Ancient soils preserve clues to ancient environments; the geochemistry and structures of sediments can tell us about rainfall and, broadly, what kinds of plants were present.

“That’s one of the things I love about this research; it’s the small, unassuming things that tell us so much about the past,” Jenkins said. “Microscopic plant parts, structures in these sediments, subtilties in the shape of teeth, scratches and marks left on fossil bones; all of these things tell us about the ancient environments in which all these apes and other animals evolved.”

The team discovered that the plants living in this landscape were “water stressed,” meaning that they would have lived through seasonal dry periods. During those periods, apes would not have been able to rely on fruit for survival.

"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."

Jenkins agrees. “Understanding early ape evolution can help us better understand our own human lineage and it can be helpful for informing modern ape conservation efforts. The past can give us a window into the future.”

The team examined the ape’s molars and found that they were “cresty,” with jagged peaks and valleys. “Cresty” teeth are used for tearing fibrous leaves apart, while the molars used for eating fruit tend to be more rounded.

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. When it comes to dental enamel, you are what you eat; our bodies uptake the elemental signatures of our plant foods.

"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."

Jenkins believes the study should give us pause as we attempt to understand our own, later, human evolution.

“We have many more apes 21 million years ago in the early Miocene than we did in the late Miocene/early Pliocene or even today. One of these early Miocene apes, Morotopithecus, is even trending towards more upright behaviors – but it wasn’t becoming a biped like us. If being a biped is such a great adaptive response to more open environments, why didn’t that happen in the early Miocene when there was much more opportunity for evolution to experiment with so many different apes in these heterogenous environments? Right now I don’t have an answer to that question, but it certainly has me rethinking many of our ideas about how the non-human apes became what they are today and how we humans became what we are today.”

Applying Collaborative Writing and Research Lessons in the Classroom

At TCC, Jenkins shares lessons from her multi-year experience of co-writing two scientific papers in with her students.

“Collaborative writing and research are hard. There are so many different sets of analysis done by so many different colleagues. The lead authors of our research, Dr. MacLatchy and Dr. Dan Peppe, had the difficult task of really smoothing everyone’s contributions to tell a coherent story AND fit the paper into Science’s formatting requirements. We did several rounds of edits over the last 2-3 years.”

Jenkins estimates that, in the end, she may have a sentence or two in the main text and a few paragraphs in the supplemental materials.

“I’m very proud of how it all turned out. It really shows how a large collaborative team can bring together different types of data and ideas to understand how the past was. The work is ultimately very rewarding.”

Jenkins said there are two things she wants to drive home to her students no matter what they decide to pursue: collaboration is a worthwhile skill, and writing is a process.

“This wasn’t something AI could write for us; it was all of our original ideas brought together to challenge long-held paradigms. None of us could have produced this new knowledge alone. It was hard and it took a while, but it’s been very rewarding! ”