Human African trypanosomiasis

The history of sleeping sickness

Prehistory / Evolution

The evolution of trypanosomes dates back to some 500 million years ago. Most parasitologists believe that the Trypanosomatidae descended from free living forms that became ingested by early invertebrates such as leeches and arthropods and became gut parasites. Mammalians evolved 180 million years ago and 50 million years later prototypes of tsetse. When some insects became bloodsucking, the trypanosomatids adapted to mammalian hosts. The various prehominids followed 5 million years ago, another 125 million years later than tsetse.

Then, about at least 3 million years ago the climate became drier and parts of the forest that covered East Africa changed in savannah landscape. The prehominids were driven out the tropical rainforest for the savannah. They adopted walking upright to be able to look over the grass for predators, to carry loads of food and to be better able to run fast. As a result of living in the open, they became regularly exposed to the bites of the savannah tsetse (G. pallidipes), the vectors of trypanosomes of game animals. Since then, the way was open for tsetse and trypanosomes to adapt to hominids, not necessarily at one specific time and location but presumably in parallel, as a multifocal evolutionary process.

Initial infections of hominids presumably occurred in East Africa and must have been incidental animal-to-hominids transmissions. Since hunters and gatherers lived in small groups there was very little opportunity for man-to-man transmission.