The timing and intensity of the seasons affect life around us, including the use of tools by birds, the evolutionary diversification of giraffes, and the behavior of our closest relatives, the primates.
Some scientists believe that early humans and their ancestors also evolved due to rapid environmental changes, but physical evidence to test this idea has been elusive—until now.
After more than a decade of work, we have developed a method to extract information about seasonal rainfall patterns from the jaws of living and fossil primates using tooth chemistry and growth.
We share our findings in a collaborative study just published in the Proceedings of the National Academy of Sciences.
Teeth are environmental time machines
During childhood, our teeth grow in microscopic layers, similar to the growth rings on a tree. Seasonal changes in the world around us, such as droughts and monsoons, affect our body chemistry. Evidence of this change is recorded in our teeth.
This is because the oxygen isotopic composition of drinking water naturally changes with temperature and precipitation cycles. During warm or dry weather, surface water accumulates more heavy oxygen isotopes. Lighter isotopes become more common during cool or humid periods.
These time and climate records are still locked in fossilized tooth enamel and can remain chemically stable for millions of years. But growth layers are usually so small that most chemical techniques cannot measure them.
To address this question, we collaborated with geochemist Ian Williams of the Australian National University, who runs the world-leading Sensitive High Resolution Ion Microprobe (SHRIMP) facility.
In our study, we collected detailed records of tooth formation and enamel chemistry from sections of two dozen wild primate teeth in equatorial Africa.
We also analyzed the fossilized two molars of an unusual great ape called Ape Lived in Kenya 17 million years ago. During this period, different groups of great apes lived in Africa, about 10 million years before our early ancestor, humans, evolved.
Dive into an ancient African landscape
Several aspects of our study contribute to understanding the link between environmental patterns and primate evolution.
First, we observed a direct relationship between historical rainfall patterns in Africa and primate tooth chemistry. This is the first test of applying an influential idea in archaeology and geoscience to wild primates: Teeth can record the finer details of seasonal environmental changes.
We were able to record the annual rainy season in West Africa and determine the end of the drought in East Africa. In other words, we can “see” the storms and seasons that occur in an individual’s early life.
This leads to another important aspect. We provide the largest record of primate oxygen isotope measurements collected to date from diverse environments in Africa that may have resembled ancestral humans.
Finally, we have been able to reconstruct annual and semi-annual climate cycles, as well as significant environmental changes, from information in the fossilized teeth of both apes.
Our observations support the following hypothesis: ape Certain features were developed to accommodate seasonal climates and challenging landscapes. For example, it had special dental features for feeding hard objects and a longer growth cycle for its molars than earlier apes and monkeys — consistent with the idea that it ate more seasonal food.
We conclude our work by comparing the data from ape An early study of hominin and monkey fossils from the same region of Kenya. Our detailed microsampling shows how sensitive tooth chemistry is to subtle climate changes.
A previous study of more than 100 fossil teeth missed the most interesting part of the oxygen isotopic composition in the teeth: the large seasonal changes in the landscape.
Read more: What teeth can say about the lives and environments of ancient humans and Neanderthals
Research potential closer to home
This novel approach to research, coupled with our ape fossil discoveries and modern primate data, is critical for future studies of human evolution – especially in Kenya’s famous Turkana Basin.
For example, some researchers have proposed that seasonal differences in foraging and stone tool use contributed to the evolution and coexistence of hominids in Africa. This idea is difficult to prove or disprove, in part because it is difficult to tease out seasonal climate processes from the fossil record.
Our approach can also be extended to animal remains in rural Australia to further understand historical climatic conditions, as well as prehistoric environmental changes that shaped Australia’s unique modern landscape.
Read more: Archaeology can help us prepare for future climates — not just looking back