Evolution of Malaria

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QUESTION

how did malaria evolve?

ANSWER

Malaria in humans probably evolved independently several times, and both times likely due to a cross-over event from a closely related primate malaria. For example, Plasmodium vivax is evolutionarily closely related to several species of malaria found in macaque monkeys in south-east Asia, and so a cross-over of one of those species into human, with subsequent adaptation and speciation, is one hypothesis as to the origin of P. vivax. Conversely, some people argue that P. vivax emerged in Africa, due to the high prevalence of certain genetic traits in African populations (such as the Duffy negative antigen), which protect against P. vivax.

In contrast, P. falciparum is agreed to have emerged in sub-Saharan Africa, and likely in the Congo basin, though the exact source of its origin has been under recent scientific dispute. Until 2010, it was thought that P. falciparum had crossed over to humans from chimpanzees, as chimps are known to be infected with P. reichenowi, a species very closely related to P. falciparum. However, a paper was published in 2010 which had sampled Plasmodium parasites of gorillas and revealed new species of Plasmodium which are even more closely related to P. falciparum, suggesting that the cross-over occurred from gorillas to humans.

As you can see, humans are not the only primates to get malaria; many species of monkey and ape are also susceptible to Plasmodium species, and even lemurs have their own suite of Plasmodium parasites. Among the mammals, rodents also can get malaria, and bats are infected with Hepatocystis, a malaria-like parasite which also infects hippos, primates and rodents. However, no other species of mammal appears to be susceptible to Plasmodium/Hepatocystis, and the reasons for this are not entirely clear.

Plasmodium probably crossed over to mammals from birds or lizards, both of which are infected with a vast number of species of Plasmodium. It is unclear in which of these groups Plasmodium first emerged, though it likely evolved originally from another type of blood-borne parasite called Leucocytozoon, which infects birds and uses blackflies (genus Simulium) as vectors.

A sister group to Plasmodium, called Haemoproteus, also evolved from Leucocytozoon but utilises a variety of different vectors, including mosquitoes, biting midges (Culicoides), louse flies (Hippoboscidae) and tabanids (Tabanidae). Plasmodium, by contrast, exclusively uses mosquitoes as its vectors (apart from one species of lizard Plasmodium, P. mexicanum, which uses sandflies), but while mammalian Plasmodium is only transmitted by Anopheles mosquitoes, bird and lizard Plasmodium can be transmitted by Culex, Aedes, Culiseta, Anopheles, Mansonia and Psorophora. As such, understanding the patterns of vector and host switches within Plasmodium and related taxa can actually provide interesting insights into the genus’ evolutionary history.

How long has malaria existed?

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QUESTION:

How long has malaria been going on?

ANSWER:

The answer to your question depends on the kind of malaria as well as how exactly you define ‘malaria’. The parasites which cause all forms of malaria, in humans as well as other mammals and birds, belong to a group called Plasmodium; scientists believe, based on genetic information, that this genus evolved around 130 million years ago, which is before the dinosaurs went extinct! These ancient ‘malaria’ parasites probably infected lizards; some types of malaria still infect reptiles today.

Plasmodium parasites have since evolved to infect primates, including humans; some scientists argue that this ‘jump’ has probably occurred several times in evolutionary history, whereas other suggest it has only happened once; the debate on this will likely continue for some time!

In terms of when human malaria first evolved, the four main types of malaria that infects humans are P. vivax, P. malariae, P. ovale and P. falciparum; the first three likely either co-evolved with humans or at least first became associated with infecting humans very soon after anatomically modern humans evolved. This dates these types of malaria back to the Middle Stone Age, which started around 300,000 years ago in Africa.

P. falciparum, on the other hand, probably crossed over much more recently, and the most up-to-date genetic evidence suggests that it evolved from a type of malaria which is found in gorillas. Estimates for when this transfer occurred are shaky at best, but it might have only been in the region of 10,000 years ago.

For more reading on the debate regarding the origin and evolutionary histories of Plasmodium as a whole and human forms of malaria more specifically, the following scientific articles may be a good place to start:

Joy, DA; Feng X, Mu J, Furuya T, Chotivanich K, Krettli AU, Ho M, Wang A, White NJ, Suh E, Beerli P & Su XZ, (2003). ‘Early origin and recent expansion of Plasmodium falciparum’, Science 300 (5617): 318–21

Liu, W; Y Li, GH Learn, RS Rudicell, JD Robertson, BF Keele, JN Ndjango, CM Sanz, DB Morgan, S Locatelli, MK Gonder, PJ Kranzusch, PD Walsh, E Delaporte, E Mpoudi-Ngole, AV Georgiev, MN Muller, GM Shaw, M Peeters, PM Sharp, JC Rayner & BH Hahn (2010), ‘Origin of the human malaria parasite Plasmodium falciparum in gorillas’, Nature 467.

Yotoko KSC & Elisei C (2006), ‘Malaria parasites (Apicomplexa, Haematozoea) and their relationships with their hosts: is there an evolutionary cost for the specialization?’, Journal of Zoological Systematics and Evolutionary Research 44 (4): 265–73

How did malaria start?

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QUESTION:

When did malaria happen?

ANSWER:

It is believed that Plasmodium, the parasite which causes malaria in a wide variety of animals, first evolved in reptiles. Even today, reptiles are infected by species of Plasmodium that are related to those that infect humans. The parasite probably then evolved to infect birds, and then, more recently, to infect mammals. Many mammals can be infected with malaria-like parasites, but most commonly rodents (like rats and mice) and primates (including humans).

The exact origins of human malaria are less clear, and indeed, there are several different types of malaria, caused by different Plasmodium species, so it would be expected that there were different evolutionary origins for these different types. The most common and deadly form of human malaria, P. falciparum, was long believed to have crossed over about 500,000 years ago from a closely related chimpanzee malaria species called P. reichnowi, and evolved to infect humans.

However, a recent paper in Nature (Liu et al., “Origin of the human malaria parasite Plasmodium falciparum in gorillas,” in volume 467 and pages 420-425) has used molecular evidence, from almost 3000 samples and several genetic regions, has instead suggested that P. falciparum evolved from a type of malaria which is found in western lowland gorillas. However, the paper did not remark on when this cross-over might have occurred. No doubt more studies will be done on this subject in the near future, which will give us a better idea of when the first cases of truly “human” malaria might have occurred!