How prehistoric humans spread malaria across the globe
A sidebar to ‘Oxford and the mosquito’
By John Garth
Mosquitoes are not the only reservoir for malaria. The disease also exists in an even more difficult-to-control species — Homo sapiens. And they appear always to have been a host for human malaria parasites.
Oxford DPhil researcher Leonie Raijmakers focuses on the Plasmodium vivax species of the malaria parasite. Its mitochondrial DNA mutates relatively slowly, allowing the evolutionary branching of the parasite to be studied with considerable clarity across vast periods of time.
We were actually carrying this disease a long time ago and it has been adapting to us and influencing much of our history
The key to this kind of ‘phylogenetic’ analysis is the fact that in any species, genes mutate at a regular average rate over long periods. That regularity furnishes a ‘molecular clock’ by which evolutionary biologists like Raijmakers can estimate when any given subspecies branched off from its parent. Raijmakers began by looking at Papua New Guinea but has used vivax data from blood samples of malaria patients from West Africa to Vanuatu, from North Korea to Madagascar.
For human evolution, phylogenetics has produced a map of migrations out of Africa with approximate dates of genetic variation and geographic dispersion spanning dozens of millennia. Comparing this with her ‘tree’ of Plasmodium vivax evolution, Raijmakers wanted to establish whether the parasite arrived in Papua New Guinea with the earliest humans from Africa 60,000 years ago, with the Austronesians 4,000 years ago, or with humans in the historical era. The vast gulfs of time between these migrations means there is little scope for ambiguity.
Whereas the Anopheles mosquito that now carries vivax was present in the region all along, the dates for the arrival of malaria and humans overlap. That, says Raijmakers, strongly suggests that malaria accompanied the migration of the first anatomically modern humans 60,000 years ago via their various stepping stones from Africa. The pure phylogenetic analysis is correlated to archaeological evidence and comparable to the progress of tuberculosis — Raijmakers works under the umbrella of Oxford’s Social Sciences division, in the Research Laboratory for Archaeology and History of Art.
‘We were actually carrying this disease a long time ago and it has been adapting to us and influencing much of our history,’ she says.
The research in prehistory has vital current application. ‘Understanding how malaria spread, how it is still spreading today and how we interact with both malaria and the mosquitoes is very important to then inform eradication programmes, and understand what the challenges are going to be.’