This study presents new findings concerning the evolution of the
human pathogens, Trypanosoma brucei and T. cruzi,
which suggest that these parasites have divergent origins and fundamentally
different patterns of evolution. Phylogenetic
analysis of 18S rRNA sequences places T. brucei in a clade comprising
exclusively mammalian trypanosomes of African
origin, suggesting an evolutionary history confined to Africa. T. cruzi
(from humans and sylvatic mammals) clusters with
trypanosomes specific to Old and New World bats, T. rangeli and
a trypanosome species isolated from an Australian
kangaroo. The origins of parasites within this clade, other than some of
those from bats, lie in South America and Australia
suggesting an ancient southern super-continent origin for T. cruzi,
possibly in marsupials; the only trypanosomes from
this clade to have spread to the Old World are those infecting bats, doubtless
by virtue of the mobility of their hosts.
Viewed in the context of palaeogeographical evidence, the results date
the divergence of T. brucei and T. cruzi to the
mid-Cretaceous, around 100 million years before present, following the
separation of Africa, South America and Euramerica.
The inclusion in this study of a broad range of trypanosome species from
various different hosts has allowed long
phylogenetic branches to be resolved, overcoming the limitations of many
previous studies. Moreover, T. brucei and the
other mammalian tsetse-transmitted trypanosomes appear, from these data,
to be evolving several times faster than T.
cruzi and its relatives.