Fossilised chompers help scientists sink their teeth into how climate impacted primate and human evolution

New international research is adding to what we know about the impact of climate on the evolution of primates including our own ancestors.

When we think about climate and evolution, it’s normally in the context of broad changes which happen over thousands or millions of years. But seasonal dynamics can also play a role (though these are also beholden to the bigger picture changes in the climate over time).

The research on ancient primate teeth was partially led by scientists at Griffith University and the Australian National University (ANU) and was published in the Proceedings of the National Academy of Sciences (PNAS).

The team examined the fossilised teeth of mammals from a 17-million-year-old site in Kenya. Included in the study were teeth from the large extinct ape Afropithecus turkanensis. Afropithecus would have been about 1.5 metres talland is regarded as the first hominoid (apes) with thick enamel on its teeth.

By looking at how the concentration of oxygen isotopes in the teeth varied, the team began building a picture of climatic behaviour, comparing with microsampled oxygen isotopes in modern African primates logged at weekly intervals.

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Using innovative ion microprobe sampling developed by a team led by ANU professor Ian Williams, the team was able to remove microscopic spots and measure oxygen isotopes recorded as the tooth was forming.

The technique, pioneered by Williams, is known as Sensitive High Resolution Ion Microprobe (SHRIMP), gave the researchers a glimpse through an environmental window through the fossilised teeth. The process is very precise as tooth enamel has daily growth lines that can be visualised with light microscopy.  

The researchers found that the teeth recorded details such as seasonal rainfall, environmental conditions and even animal behaviour. For example, heavier oxygen isotopes (more neutrons in the oxygen atoms’ nuclei) are associated with drinking water and foods found in arid regions or periods of lower rainfall.

Modern monkeys whose teeth were forming in the 1960s recorded in its oxygen isotopes an extreme rainfall event in one case and an extended drought in another.

Analysing the oxygen isotopes in teeth from Afropithecus, the team also compared their results with ancient rainfall patterns produced by cutting-edge climate models.

Their results showed that wet and dry seasons 17 million years ago were variable in their intensity. In periods where dry seasons were more extreme, the fossilised teeth showed indications that Afropithecus was well adapted in its tooth and jaw structures to consume hard foods during droughts and scarcity.

A diagram showing how oxygen isotope levels in the fossilised Afropithecus teeth indicate wet and dry periods. Credit: Smith, Green et al.

“This 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,” says Tanya Smith, professor in the Australian Research Centre for Human Evolution at Griffith University.

The study of isotope variation in modern primate teeth also showed that oxygen isotope levels in fossilised teeth often understate the level of ancient environmental change.

“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 sparse,” says Dr. Daniel Green from Columbia University. “Isotope values from Afropithecus and closely associated herbivores suggest that apes in this part of eastern Africa were living in a seasonal forest or woodland long before the origin of hominins. We may see the impact of that seasonal climate on the novel anatomical characteristics and behaviours of early apes.”

The researchers have been working on applying their approach to a study of living and fossil humans, including a landmark 2018 study of ancient seasonality looking at fossil teeth from Neanderthal children.

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