Evolution of ancient humans may have been influenced by malaria

Some of the smallest organisms might have influenced our own evolutionary path.

A new study has found that the distribution of malaria in Africa in the last 70,000 years could have influenced where our ancient ancestors were living and as a result how populations were interacting. This in turn would’ve had implications for how Homo sapiens evolved and migrated out of Africa.

Dr Michela Leonardi is our biodiversity modeller in the Plants Under Pressure group. In collaboration with the University of Cambridge, she also looks at how ancient human populations responded to changes in the climate over tens of thousands of years. She was involved in the new study, now published in the journal Science Advances.

“We may not think about it today in England, but malaria is one of the most problematic diseases worldwide, and it has really shaped our evolution as humans,” says Michela. “It was thought that it mostly impacted the last 10 to 20 thousand years of our story. Instead, what our results suggest is that humans likely avoided areas where the risk of contracting malaria was higher way earlier than that.”

“I wasn't expecting this kind of story to come out.”

To confirm whether the lack of overlap between where mosquitoes and ancient humans were living in the ancient past is actually causation, and not just correlation, the scientists independently reconstructed their distribution, so the results on humans could not impact the results on mosquitoes and vice versa. The team also looked at how this shifted over time. As expected, humans started to live more in malaria-prone places when they became more sedentary.

They also found that at around 60,000 years ago there was an increase in overlap roughly corresponding with when humans successfully migrated out of Africa, and again about 12,000 years ago when there was a change in climatic conditions and humans became sedentary.

A complex picture of human evolution

Historically, it was thought that humans evolved in a single region as our ape-like ancestors descended from the trees and started walking upright through the African savannas. But recent research is showing that this theory is probably too simplistic.

Instead, most scientists now think that our own species evolved across a much wider region within Africa through the interactions of multiple populations living in different parts of the continent. The theory goes that as these separate populations met, they exchanged knowledge, technology and genes which eventually culminated in the emergence of modern Homo sapiens.

How these distinct populations were distributed across the continent, and what was allowing or preventing them from meeting, has become a key question.

Most focus on this has looked at the physical or climatic barriers that might have prevented ancient human movements. This would include things like rivers, mountains or deserts. But this latest study has instead looked at how disease might have shaped these interactions.

“It’s not easy to study diseases in the past,” explains Michela. “There are diseases that leave traces, for example, in our skeletons. But because we don’t have a lot of skeletal remains from the ancient past, it’s not that easy to find individuals with these characteristics.”

“In this case, though, we are talking about a disease that doesn’t leave a trace in our body remains, which makes it even harder.”

To get around this, Michela’s colleagues at the Max Plank Institute of Geoanthropology and the University of Cambridge, especially Margherita Colucci, first author of the study, turned instead to the animals that carry the diseases.

Mapping ancient distributions

Malaria is caused by a range of different Plasmodium parasites, which themselves can infect several different mosquito species.

By mapping the current distribution of three African mosquitoes most responsible for transmitting malaria, the researchers could build up a picture of which climatic environments they live in. This was then combined with data on past climate across the continent to produce historic maps of where these mosquito species were most likely to live, and by proxy where most malaria would have been found.

It is at this point where Michela’s work on ancient human distributions came in. In a previous piece of work, she had been able to reconstruct a map of where humans would have been able to live in Africa in the last 120,000 years.

“If you put the maps over the last 74,000 years of humans and mosquitoes next to each other, we see that basically the reconstruction of places where humans were living doesn’t really overlap with places where malaria was most likely to be,” says Michela. “Humans appear to have avoided places where malaria was most likely.”

To further explore whether, for example, people were avoiding areas likely to have malaria for other reasons, researchers want to explore other diseases and see if they can find similar patterns. For now though the study helps to show a different dimension to how and where people lived that has not received much attention.

“This study opens up new frontiers in research on human evolution,” says Professor Eleanor Scerri from the Max Planck Institute. “Disease has rarely been considered a major factor shaping the earliest prehistory of our species, and without ancient DNA from these periods it has been difficult to test.”

“Our research changes that narrative and provides a new framework for exploring the role of disease in deep human history.”