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HUMAN EVOLUTION, MIGRATIONS, CIVILIZATION, AND PARASITIC INFECTIONS

Human evolution and parasitic infections have run hand in hand, and thanks to the spinoffs from the Human Genome Project, we now know much more about the origins of the human race than ever before (197). Sometime, about 150,000 years ago, Homo sapiens emerged in eastern Africa (254) and spread throughout the world, possibly in several waves (252), until 15,000 years ago at the end of the Ice Age humans had migrated to and inhabited virtually the whole of the face of the Earth, bringing some parasites with them and collecting others on the way. For the purpose of this review, the parasites that infect humans can be classified as heirlooms or souvenirs. Heirlooms are the parasites inherited from our primate ancestors in Africa, and souvenirs are those that we have acquired from the animals with which we have come in contact during our evolution, migrations, and agricultural practices. The development of settlements and cities facilitated the transmission of infections between humans, and the opening up of trade routes resulted in the wider dissemination of parasitic infections. The slave trade, which flourished for three and a half centuries from about 1500, brought new parasites to the New World from the Old World (58); in more recent times, the spread of human immunodeficiency virus HIV and AIDS and the immunodepression associated with these conditions has resulted in the establishment of a number of new opportunistic parasitic infections throughout the world (5).

We are beginning to learn a lot about the past history of parasitic infections from studies of archaeological artifacts, such as the presence of helminth eggs or protozoan cysts in coprolites (fossilized or desiccated feces) and naturally or artificially preserved bodies; from such studies has emerged a new science, palaeoparasitology. Examples of some of these discoveries will be discussed later.

So vast is the field of human parasitology, and so many and far-reaching the discoveries made, that it is not possible to do justice to the whole subject. Therefore; only the most significant aspects and the most important parasites are considered under two major headings, the helminth worms and the protozoa.

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