The surprising origin of humanity's most deadly disease

Malaria is one of several deadly diseases that has jumped the species barriers from animals into humans.

It kills almost half a million people per year, and can be transmitted into humans by a single mosquito bite.

The most deadly form of malaria is caused by the Plasmodium falciparum protozoan parasite, one of several malaria-causing parasites.

But mosquitoes are simply the carriers, not the source. This has made tracing the parasite's origin challenging.

In order to understand how a disease evolves, it is crucial to know where and when it began affecting humans. This can help scientists unpick the miniscule genetic changes that could have made it so deadly.

In 2010 scientists made a breakthrough. By analysing western gorilla faecal samples that contained Plasmodium parasites, they found the human version - P. falciparum - was closely related to one of the three Plasmodium parasites that gorillas host.

It revealed, for the first time, that the deadliest form of human malaria came from gorillas, not chimps or other species of early-humans as others had previously proposed.

This is how it started: a mosquito bit an infected gorilla and then transmitted the parasite to a human with another bite. Once it was in humans it could then spread rapidly, as long as there were enough mosquitoes to pass the parasite from person to person.

But the story was still incomplete. In particular, it was not clear when the gorilla parasite jumped into humans.

The problem was that the faecal samples contained only snippets of the gorilla Plasmodium DNA. To get full genome sequences that would help pin down the time of transmission, the researchers needed blood.

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