History of Malaria Parasite And Its Global Spread
Time Line For Origin of Malaria
Half a billion years ago
Existence of pre-parasitic ancestor
150 million to 200 million years ago
Early Dipterans, ancestors of mosquitoes, appear
130 million years ago
Two-host life cycle in Dipterans and vertebrates evolves
130 million years ago
Divergence of the bird and mammalian malaria parasites
100 million years ago
Lineage of P. malariae, P. ovale, and P. vivax diverges
~5 million years ago
P. falciparum evolves
2-3 million years ago
Divergence of P. vivax from P. cynomolgi
4000-10000 years ago
Lethal strain of P. falciparum appears
4000-5000 years ago
Anophelines in Africa develop highly anthropophilic habits
Man and Malaria seem to have evolved together. It is believed that most, if not all, of today's populations of human malaria may have had their origin in West Africa (P. falciparum) and West and Central Africa (P. vivax) on the basis of the presence of homozygous alleles for hemoglobin C and RBC Duffy negativity that confer protection against P. falciparum and P. vivax respectively.
The ancestors of the malaria parasites have probably existed at least half a billion years ago. Molecular genetic evidence strongly suggests that the pre-parasitic ancestor for malaria parasite was a choroplast-containing, free-living protozoan which became adapted to live in the gut of a group of aquatic invertebrates. This single-celled organism probably had obligate sexual reproduction, within the midgut lumen of a host species. At some relatively early stage in their evolution, these "premalaria parasites" acquired an asexual, intracellular form of reproduction called schizogony and with this, the parasites greatly increased their proliferative potential. (This schizogony in the RBCs of humans causes the clinical manifestations of malaria). Among the invertebrates to which the ancestors of the malaria parasites became adapted were probably aquatic insect larvae, including those of early Dipterans, the taxonomic order to which mosquitoes and other blood-sucking flies belong. These insects first appeared around 150 million to 200 million years ago. During or following this period, certain lines of the ancestral malaria parasites achieved two-host life cycles which were adapted to the blood-feeding habits of the insect hosts. In the 150 million years since the appearance of the early Diptera, many different lines of malaria and malaria-like parasites evolved and radiated. The malaria parasites of humans evolved on this line with alternate cycles between human and the blood-feeding female Anopheles mosquito hosts. Fossil mosquitoes have been found in geological strata 30 million years old.
P. falciparum is found to be very closely related to a malaria parasites of chimpanzees, P. reichenowi and these two are more closely related to the malaria parasites of birds than to those of other mammals. The lineage of these parasites possibly occurred around 130 million years ago, nearly about the same time as the origin of the two-host life cycle involving blood-feeding Dipterans and land vertebrates. The separation of the lines that led to P. falciparum and P. reichenowi probably occurred only 4 million to 10 million years ago, overlapping the period in which the human line diverged from that of the African great apes. The modern, lethal strains of P. falciparum probably emerged about 4,000 years ago, after agriculture took roots in Africa.
P. malariae, P. ovale, and P. vivax diverged over 100 million years ago along the lineage of the mammalian malaria parasites. P. ovale is the the sole known surviving representative of its line and causes infection only in humans. P. malariae was a parasite of the ancestor of both humans and African great apes and had the ability to parasitize and cross-infect both host lineages as they diverged around five million years ago. P. malariae is found as a natural parasite of chimpanzees in West Africa and P. brazilianum that infects New World monkeys in Central and South America is morphologically indistinguishable from P. malariae. P. malariae, like P. ovale, is the only confirmed and extant representative of its line. P. vivax belongs to a group of malaria parasites like P. cynomolgi, that infect monkeys. The time of divergence of P. vivax from P. cynomolgi is put at 2-3 million years ago.
End of the last glacial period and warmer global climate heralded the beginnings of agriculture about 10000 years ago. It is argued that the entry of agricultural practice into Africa was pivotal to the subsequent evolution and history of human malaria. The Neolithic agrarian revolution, which is believed to have begun about 8,000 years ago in the "Fertile Crescent," southern Turkey and northeastern Iraq, reached the western and Central Africa around 4,000 to 5,000 years ago. This led to the adaptations in the Anopheles vectors of human malaria. The human populations in sub-Saharan Africa changed from a low-density and mobile hunting and gathering life-style to communal living in settlements cleared in the tropical forest. This new, man-made environment led to an increase in the numbers and densities of humans on the one hand and generated numerous small water collections close to the human habitations on the other. This led to an increase in the mosquito population and the mosquitoes in turn had large, stable, and accessible sources of blood in the human population, leading to very high anthropophily and great efficiency of the vectors of African malaria. Even though the practice of agriculture had developed throughout the tropics and subtropics of Asia and the Middle East up to several thousand years before those in Africa, simultaneous animal domestication in Asia probably prevented the mosquitoes from developing exclusive anthropophilic habits. In most parts of the world, the anthropophilic index (the probability of a blood meal being on a human) of the vectors of malaria is much less than 50% and often less than 10 to 20%, but in sub-Saharan Africa, it is 80 to almost 100%. This is probably the most important single factor responsible for the stability and intensity of malaria transmission in tropical Africa today.
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