An international team of scientists has shown in a new study that fossil primate teeth can offer insights into the roles seasonal climates and behaviors may have played in human and primate evolution.
The study examined oxygen isotopes in teeth from a 17-million-year-old site in northwest Kenya’s Turkana Basin. These included teeth from an enigmatic large-bodied ape known as Afropithecus turkanensis. The research was just published in the Proceedings of the National Academy of Sciences,
To put the fossils into context, the researchers also measured oxygen isotopes in modern primate teeth from across equatorial Africa, and analyzed the isotopic signals they produced. The team, led by Daniel Green, a postdoctoral scientist at Columbia University’s Climate School, and Tanya Smith of Australia’s Griffith University, found that the modern oxygen isotopes (natural variants of oxygen that differ by mass) recorded details of rainy seasons and droughts, environmental conditions like altitude and vegetation, and variations in primate behavior.
In the modern teeth, the researchers found that teeth could even indicate human alterations to the natural landscape, like the damming of a river, and document specific meteorological events. Measurements from modern monkey teeth recorded an extended drought in one case in Ethiopia, and an extreme rainfall event in another case in Uganda. Working with primatologists, the researchers were able to ascertain that the teeth were forming when these climatic events occurred.
With the context provided by modern primates, the authors then analyzed teeth from Afropithecus found at a site called Kalodirr, and compared their results with 17 million-year-old rainfall patterns simulated by climate models.
The researchers showed that the ancient apes experienced dry and wet seasons of variable intensity over time, possibly influenced by natural changes in Earth’s orbit around the sun. These results also suggested that specialized jaw adaptations—thick enamel and strong chewing muscles—would have helped Afropithecus consume hard foods during seasons of drought or resource scarcity.
“The effects of climate variation on the earliest African apes are poorly understood, because detailed records of seasonal variation from this early period, the Miocene, are more sparse,” said Green. “Oxygen isotope compositions from Afropithecus and closely associated herbivores suggest that ape behavior and anatomy in this part of eastern Africa were shaped by seasonal environments long before the origin of hominins.”
Another key finding of the team’s research is that studies of oxygen isotopes in fossil hominins—our ancestors—have thus far underestimated the chemical variations in their teeth. For this reason, it has been hard to estimate how their behaviors may have varied by season on the savannas in Africa.
Smith said the research “has broad significance, because seasonal changes in resource availability are thought to have influenced the evolution of great apes, early hominins, and modern humans.”
The study leveraged innovations in high-resolution chemical measurement techniques driven by Ian Williams of the Australian National University, who coauthored the paper. Teeth grow in microscopic temporal increments analogous to rings in trees, but during childhood, tooth increments are formed daily, rather than annually. Thus with innovations provided by Williams’ work, the researchers were able to recover seasonal changes in body and environmental chemistry from millions of years ago.
Green, Smith, and Williams have been working to refine and apply these techniques to living and fossil humans for the past five years. In 2018, their work yielded the first detailed assessment of ancient seasonality from the teeth of Neanderthal children. The team is currently planning to sample African fossil hominin teeth from the Turkana Basin.
The paper was coauthored by Janaina Ávila of Griffith University; Susanne Cote of the University of Calgary; Wendy Dirks of Durham University; and Daeun Lee and Christopher Poulsen of the University of Michigan, Ann Arbor. The research was funded by Harvard, Columbia, and Griffith universities.
Adapted from a press release by Griffith University.
