Origins

Origins - The genus Mycobacterium has a slow rate of mutation, and this fact has enabled the development of hypotheses concerning the origins and evolution of Mycobacterium tuberculosis. There is some inferential reasonto suspect that the genus, as represented by Mycobacterium ulcerans, may have existed 150 million years ago in the Jurassic period.1 Gutierrez and her colleagues2 at the Pasteur Institute have concluded that the progenitor of M. tuberculosis emerged from an array of mycobacterial species about 3 million years ago, presumably infecting early hominids and other primates in prehistoric times.

It seems likely that all modern members of the M. tuberculosis complex evolved from a common ancestor 15,000–20,000 years ago.3,4 Mycobaterium bovis and other species in the complex split off from the central line at later times. Figure 1.1 presents the phylogenetic tree developed by Gutierrez and her colleagues. The hypothesized genome of the common progenitor more closely resembles M. tuberculosis than other mycobacterial species. Thus it is presented as a straight line from the hypothesized progenitor in Fig. 1.1. 

Genus Mycobacterium

Mycobacterium Tuberculosis

The positionin time of the common progenitor implies that whatever diversity had occurred during preceding millennia became severely constricted before giving rise to modern species. Present-day TB is caused by six or seven clades – strains with common ancestors – of M. tuberculosis, which have separate geographic origins.5–7 Dating methods applied to members of two of these strains suggest that they emerged in their present form between 250 and 1,000 years ago.6 The earliest archaeological evidence of human TB comes from Egyptian art and mummies; there is ample evidence of spinal TB (Pott’s disease) as early as 5,500 years ago.8–10 While early workers attributed these infections to M. bovis, there is now good evidencefrom studies of amplified DNA recovered from mummies that M. tuberculosis was the cause of disease in ancient Egyptians.11,12

There are unequivocal references to TB in the Old Testament books of Deuteronomy and Leviticus at the time when Jews were in exile in Egypt.13 In fact, although the archaeological record issparse or non-existent, there is reason to believe that TB was widespread, if not uniformly distributed, in Africa long before Arabians and Europeans entered the continent.14 There is general agreement that TB first appeared as a human disease in East Central Africa and that it travelled with early peoples as they migrated into Asia Minor and across the globe. There are imprecise prehistoric references to TB from India and China, but no archaeological evidence.15 Migrating early peoples reached the Americas across the land bridge connecting Siberia and Alaska and along its coast, perhaps in several waves, reaching as far south as Chile by 15,000 years ago. Tuberculosis was common in a number of western hemisphere locations before the arrival of Columbus and the Spanish conquistadores.16 As in Egypt, Andean mummies have yielded mycobacterial DNA. In both Africa and the Americas the TB epidemic wave seems to have crested and receded at early times, leaving naive populations susceptible to the reintroduction of TB by European colonizers.