ROB SCHMITZ, HOST:
For tens of thousands of years, early humans lived in small communities across Africa. New research suggests disease, in particular malaria, may have helped shape where those settlements were located. Here's science reporter Ari Daniel.
ARI DANIEL, BYLINE: The places on the African continent that early humans chose to inhabit helped shape our species from our genes to our behaviors. Eleanor Scerri is an evolutionary archaeologist at the Max Planck Institute of Geoanthropology in Germany.
ELEANOR SCERRI: If we want to understand how we got to where humans got to today - like, why are we the way we are? Why do we look the way we look? Obviously, that's a story that played out over a very deep timescale and over a very big area.
DANIEL: A lot of work considers the role of climate in influencing human settlement patterns. But Scerri believes that disease likely played a role, too, by causing people to avoid potential hot zones, say. That's been harder to investigate, though, since any genetic evidence of pathogens would've degraded long ago. In addition, says University of Cambridge evolutionary ecologist Andrea Manica...
ANDREA MANICA: The majority of diseases will actually not leave a trace in the remains of an individual.
DANIEL: So the researchers took a different approach. They decided to focus on malaria, a longtime lethal disease. They wanted to compare where malaria-carrying mosquitoes once lived and whether people occupied those areas or avoided them. First, they loaded up a set of climate models they developed for sub-Saharan Africa, looking back over the last 74,000 years.
MANICA: We can actually reconstruct the temperature, the precipitation, the type of vegetation that would grow in various parts of Africa.
DANIEL: With that information, they could then predict where mosquitoes would've preferred to live based on their modern habitats.
MANICA: And so we reconstructed where malaria could've been. And if it was there, it would've been a big problem.
DANIEL: The research team then considered where people were living, based on archaeological evidence of human settlements. The result was striking. For tens of thousands of years, people didn't tend to live in malaria hot spots.
MANICA: What we don't know is whether they were avoiding them or whether they were going there and dying. But basically, they were just not persisting in the areas where malaria would have been really, really problematic.
DANIEL: That is, disease helped sculpt human settlement patterns in the past, bringing populations together at times and separating them on other occasions. What if, however, ancient humans and mosquitoes simply preferred different climate conditions, so their geography would've had nothing to do with malaria? Well, Manica says that some 15,000 years ago, a key genetic mutation arose in West Africa, sickle cell anemia. Two copies of the gene are fatal, but one offers a critical degree of protection against malaria. And it's around that time that people's avoidance of the regions with the disease began to break down.
MANICA: It opened up a whole area of Africa that before was very challenging.
DANIEL: Especially in modern-day central West Africa. The research appears in the journal Science Advances.
SIMON UNDERDOWN: What the paper really neatly shows is it was an issue. Disease was - you know, it's always been an issue for humans. It is now, it was then, and it shapes how we live.
DANIEL: Simon Underdown is a biological anthropologist at Oxford Brookes University who wasn't involved in the research. And he says these results offer a window into the past and also the future as disease-carrying mosquitoes expand their range in today's changing climate.
UNDERDOWN: You can't suddenly evolve sickle cell. You know, that takes time. But what humans do is we can come up with cultural solutions to biological problems.
DANIEL: Like vaccines or antimalarials, because solving problems, that's very human, too.
For NPR News, I'm Ari Daniel.
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