THE EVOLUTION OF TRYPANOSOMES INFECTING HUMANS AND PRIMATES

Based on phylogenetic analysis of 18S rRNA sequences and clade taxon c omposition, this paper adopts a biogeographical approach to understand ing the evolutionary relationships of the human and primate infective trypanosomes, Trypanosoma cruzi, T. brucei, T. rangeli and T. cyclops. Results indicate that these parasites have divergent origins and fund amentally different patterns of evolution. T. cruzi is placed in a cla de with T. rangeli and trypanosomes specific to bats and a kangaroo. T he predominantly South American and Australian origins of parasites wi thin this clade suggest an ancient southern super-continent origin for ancestral T. cruzi, possibly in marsupials. T. brucei clusters exclus ively with mammalian, salivarian trypanosomes of African origin, sugge sting an evolutionary history confined to Africa, while T. cyclops, fr om an Asian primate appears to have evolved separately and is placed i n a clade with T. (Megatrypanum) species. Relating clade taxon composi tion to palaegeographic evidence, the divergence of T. brucei and T. c ruzi can be dated to the mid-Cretaceous, around 100 million years befo re present, following the separation of Africa, South America and Eura merica. Such an estimate of divergence time is considerably more recen t than those of most previous studies based on molecular clock methods . Perhaps significantly, Salivarian trypanosomes appear, from these da ta, to be evolving several times faster than Schizotrypanum species, a factor which may have contributed to previous anomalous estimates of divergence times.