The findings may provide insights into how the brain evolves during periods of significant climate change.
Thirty million years ago, the landscapes of northern Africa underwent a transformation. Dense, leafy canopies yielded to grassy savannas due to droughts, temperature fluctuations, and a decline in atmospheric carbon dioxide. As the environment changed, so did the animals that called it home.
In a recent publication in the Journal of Mammalian Evolution, Alannah Pearson, a doctoral candidate at the Australian National University, and David Polly, a Professor of Earth and Atmospheric Sciences within the College of Arts and Sciences at Indiana University Bloomington, explored the case of one such family: Cercopithecidae, or Old World monkeys. These primates are named for the geographic regions they once inhabited: stretches of Africa, Asia, and Southern Europe formerly called the ‘Old World’ by European colonizers. The researchers found that the temporal lobe of the brain, responsible for functions such as facial recognition and language, increased in size in these monkeys at key stages between 30 to 40 million years ago.
“There was a shift that occurred at that evolutionary point,” explained Pearson. As forest coverage receded, monkeys were forced to adapt to open environments and began to live in larger groups. Larger groups meant more socializing, which Pearson hypothesized would be reflected by changes in the size of the temporal lobe. What they found was that the temporal lobe did get bigger in the Cercopithecidae lineage and that it happened much faster than anticipated.
Studying ancient brains was no easy task, said Pearson. The average brain decomposes in just ten years, so obtaining brain samples of species that died millions of years ago was not an option. Instead, Pearson turned to the fossil record. They found a wealth of examples of primate skulls and wondered if these would provide enough information to estimate the size of these individuals’ temporal lobe.
Pearson tested this hypothesis in 2020 by comparing the skulls of 23 primate species to the brains of their living representatives. For all the species tested, they found that the dimensions of a primate’s skull were a suitable way to estimate the size of their temporal lobe.
So, for their 2023 study, Pearson focused directly on skulls. They analyzed digital micro-CT scans of 222 skulls, each requiring up to eight hours. “The most challenging part is the hours it takes me to segment the scan data,” said Pearson. Digital debris like rock and dirt had to be removed manually from each segment of the scan, which usually numbered around 1,000. “With these very early monkeys, there are a lot of parts that don’t exist anymore. So, you have to ask, is this actually a piece of rock? Or is it a piece of bone that's come loose?” said Pearson.
The effort was worth it. Pearson’s results are the first to quantify how the temporal lobe changed in Old World monkeys and suggest that these changes were directly related to changes in behavior.
“I think it is quite correct to suppose that changes in brain anatomy suggest changes in behavior or cognition,” agreed Tom Schoenemann, a Cognitive Science professor in the College who was not involved in the study. “It is just difficult to know precisely the nature of any behavioral or cognitive change,” he said. Schoenemann cautioned that some information is lost when using the skull as a proxy for the brain and that, further, brain morphology can’t always predict an individual’s behavior.
Still, Pearson’s research used advanced methodologies to get as close to understanding morphological and cognitive changes as possible.
"The 3D data processing allows you to do [a study like this] in a much more sophisticated and detailed way,” reflected Polly, Pearson’s doctoral advisor who leads a team of researchers studying vertebrate evolution in the College. “New methodologies address questions in sometimes more meaningful ways,” he said, in this case that of how species adapt to climate change.
Just as Pearson’s Old World monkeys adapted to droughts, temperature changes, and the resulting shift from tropical forests to savannas, organisms will also need to adapt to the environment created by anthropogenic climate change.
“The global temperature could change by 5-10 ºC [...] and there will always be someone who says, ‘that doesn’t matter’,” said Polly. “So, the question is how much does it matter? What reorganizations change? How fast did they change in the past? How much time was there to either become extinct or adapt?”
Pearson’s case study in Old World monkeys offers an example of how one species’ brain reorganized to adapt to a changing environment and may be indicative of a broader trend, suggested Schoenemann.
“This general idea that environmental shifts to more open environments may have led to modifications of brain areas, because of those areas' mediation of particular types of cognition,” he said, “I think is reasonably applied to humans also.”
