Based on phylogenetic analysis of 18S rRNA sequences and
clade taxon composition, this paper adopts a biogeographical
approach to understanding 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 fundamentally
different patterns of evolution. T. cruzi is placed in a
clade with T. rangeli and trypanosomes specific to bats
and a kangaroo. The predominantly South American and Australian
origins of parasites within this clade suggest an ancient
southern super-continent origin for ancestral T. cruzi,
possibly in marsupials. T. brucei clusters exclusively
with mammalian, salivarian trypanosomes of African origin,
suggesting an evolutionary history confined to Africa, while
T. cyclops, from an Asian primate appears to have evolved
separately and is placed in a clade with T. (Megatrypanum)
species. Relating clade taxon composition to palaeogeographic
evidence, the divergence of T. brucei and T. cruzi
can be dated to the mid-Cretaceous, around 100 million years
before present, following the separation of Africa, South America
and Euramerica. Such an estimate of divergence time is
considerably more recent than those of most previous studies
based on molecular clock methods. Perhaps significantly,
Salivarian trypanosomes appear, from these data, to be evolving
several times faster than Schizotrypanum species, a factor
which may have contributed to previous anomalous estimates of
divergence times.
