Ancient infections: the parasites that shaped our health
By Kirsty MacLeod
Early humans started to migrate from Africa 100,000 years ago, taking with them early human tools, early human culture – and early human health problems. What happened to the parasites that plagued us in our ancestral homeland? Which did we leave behind and which only began to play a role in human health after our spread to other continents?
Combined interests in parasitology and anthropology led Dr Piers Mitchell of the University of Cambridge to the same questions. Speaking at the British Science Festival today, he unveiled new research that tracks the historical range of human parasites to paint a picture of what parasites affected us when and where, providing an image of how diseases spread throughout human evolution.
Mitchell’s study is the first to take a global approach to historical human parasitology, and as such, uses evidence from an exotic range of sources: fossilised human faeces, archaeological sites, and mummified human remains from Egypt, including the mysterious contents of canopic jars. In these samples were evidence of ancient diseases, in the form of equally ancient parasitic remains. This information was then integrated with parasitology data from modern non-human primates such as gorillas and chimpanzees to complete the picture of how these parasites spread across time.
This research has allowed Mitchell to pinpoint 16 parasites he calls “heirloom” parasites, those which humans were ancestrally exposed to in Africa, possibly during our evolution. Of these, six did not spread outside Africa. A further 12 parasites are “souvenir” parasites which early humans acquired, probably from wild animals, as they migrated north, west and east. These were picked up in the same way that we might pick up a souvenir on our travels, says Mitchell.
Crucially, this is the first time that research has determined which parasites humans may have co-evolved with, a finding that could have implications for modern medicine. The assumption is that if we evolved under exposure to heirloom parasites such as threadworms and liver flukes, we should be physiologically more capable of dealing with these than those which we came into contact with much later. An increasingly well-studied link between high incidence of parasite infection and low rates of allergic diseases in developing countries has led scientists to suggest that parasites could be used in the treatment of extreme allergies. Heirloom parasites that we should be better evolved to cope with would be ideal target species for such research.
A link between historical human association with a parasite and our physiological capability to deal with it clearly requires further testing – the list of heirloom parasites includes malaria and leishmaniasis, both of which can be fatal regardless of how many thousands of years humans have had to evolve resistance to them. If more innocuous heirloom parasites, however, could be used to treat allergic diseases, the impact of Dr Mitchell’s work on the history of human parasites could stretch well into the future.