Transition around 10,000 years ago, but not because of the arrival of human MTBC, which had been implanted many thousands of years before, but because of growing increases in population size and population density. Here again is where hunter-gatherers re-enter the picture as they began the switch from nomadic pursuits to a settled life of animal domestication and agriculture. The Neolithic Revolution took a few thousand years to implement all its dramatic changes – one of which, as emphasized in this first chapter, was the onset of warfare, a never-ending process that continues to kill and ravage today. Another derivative of the acceleration in growth of human populations and increasingly crowded surroundings with resultant multiplication of susceptible hosts was that human MTBC adapted to this stimulus by switching from a low-density infectious disease to a modern “crowd” disease; furthermore, a corollary consequence of an increasing number of potential victims unfailingly leads to higher virulence and shorter latency [36]. But crowd diseases (i.e., teeming people) caused by TB, as discussed later, do not really show up until around the 17th century.
Remember that the Holocene epoch, or current geologic period, began approximately 11,700 years ago, which is about the time humans had almost migrated from Africa into Europe, India, and China, and as Gagneux [30] correctly assumes, “human exploration, trade, and conquest” further broadened the distribution of people and increased the density of the growing populations. We are following the gospel that during the last 70,000 years humans became newly infected with M. tuberculosis and some must have become sick and died, but the accompanying long latency period, in theory, allowed people with hidden infection to migrate for many years and long distances before clinical signs of reactivation disease appeared; but bear in mind that the shortened human life expectancies during those hazardous years must have cut off TB latencies as well as lives. This leads to 2 important questions: when did humans migrate from Eurasia to the Americas, and did TB accompany those prehistoric migrations or did the disease recur much later through European contact?
Paulsen [39] concludes in his 1987 review that the evidence for the presence of TB in Native Americans and Alaskans in prehistoric North America remains inconclusive. And it looks like the jury is still out on that question. What remains unsettled, of course, is whether or not the heightened susceptibility of Native Americans to TB should have decimated the vulnerable inhabitants once virulent M. tuberculosis was brought along with the incoming migrants: which does not appear to have happened. Another Paulsen conclusion is that the evidence for the presence of TB in prehistoric South America has been “fairly well established.” Both conclusions, therefore, have stood the test of time, except to advance the current South American verdict from “fairly well” to “well established.”
North America: The melting of the giant ice caps of northern Canada opened up pathways to North America around 15,000 years ago. Several possible routes through or around (by boat) the remaining massive ice deposits towards the end of the Pleistocene epoch allowed humans to migrate from eastern Siberia to Alaska, then further southward. An early site at Clovis, New Mexico, for example, attracted considerable attention in the 1920s to 1930s, and is still a major reference center [40]. But several other locations have also gained archaeological importance: near Calgary, Canada, in south-central Oregon, Texas, and the Channel Islands of California. New evidence from the Wally’s Beach site in Canada, using improved radiocarbon dating and supplemented by data from Clovis, New Mexico, provides a more comprehensive understanding of the contribution of the 2,000-year hunting spree by humans in the extinction of at least 6 genera of megafauna, including mammoths, mastodons, gomphotheres, sloths, horses, and camels [41]. The extinction of these giant mammals was mainly caused by dramatic changes in climate and habitat; nevertheless, even though the human population at the time was small and its weapons primitive, hunting must have played a role. First solitary then groups of animals are believed to have been hunted from roughly “15,000–13,300 years ago.” But apparently no pre-Columbian European Contact with TB occurred in North America and Mexico, and throughout Mesoamerica, which comprises central Mexico south to the Central America isthmus countries that link North and South America.
Naturally, studies are underway to compare the genomes of modern Native Americans and Siberians. Findings by Dryomov et al. [42] and associates of the mitochondrial genome diversity support the hypotheses that there were “multiple streams of expansions to northern North America from northeastern Eurasia in late Pleistocene- early Holocene.” More definitive information about genomic dispersal in the Americas should soon be available.
South America: An early case of TB in pre-Columbian Peru was reported in 1973 in a child 8–10 years old with radiographic features of Pott’s disease plus lung, pleural and kidney disease [43]. Ziehl-Neelsen staining revealed many clumps of acid-fast bacilli and radiocarbon dating estimated death at approximately 700 A.D. Another Peruvian mummy with a radiocarbon age of 1,040 ± 4 years had pulmonary lesions with DNA unique to M. tuberculosis, as shown by extraction and identification techniques [44]. Another vertebral lesion was similarly identified from a pre-Columbian mummy from Arica, Chile [45]. These abnormalities provided circumstantial evidence for the presence of TB in pre-Columbian specimens, but given the lengthy differential diagnosis, definitive proof was impossible [46].
The still open question concerning the presence or absence of pre-Columbian TB seems to have been convincingly answered affirmatively in 2014 by Bos et al. [47]. Their excavations of 3 Peruvian mummies revealed mycobacterial genomes that proved that this particular and most unusual cause of human TB originated from seals and sea lions and evolved from a well-known ancient strain of non-human MTBC, which clearly differed from modern M. tuberculosis. All 3 specimens derived from the same period of historic Peruvian culture and had radiocarbon dates between 1028 and 1280 A.D. In addition, based on extensive genomic analyses, instead of clustering with other human strains, the Peruvian samples clustered with animal lineages, particularly M. pinnipedii – and provide “unequivocal evidence of human infection” by an animal – adapted strain of MTBC [47].
One possible sequence – there are not many others to go by – postulates that M. pinnipedii-infected seals and sea lions crossed the southern Atlantic Ocean, probably from Spain, where they colonized costal South American and (later) Australian waters. Because shore-based Peruvian and other neighboring humans had presumably been hunting and eating seals for thousands or more years, sooner or later the newly arrived seals liberated sufficient air-borne pathogen passengers to infect local humans and complete the zoonotic transfer of TB from seals to coastal humans, then from coastal humans to inland humans [47]. Previous observations have reported presumed air-borne M. pinnipedii transmission from seals to other mammals, including human animal keepers in a Netherland zoo [48].
Human Contact: Christopher Columbus certainly made human contact with indigenous natives in or near one of the present day Bahama Islands during his first voyage to the New World in 1492; he returned to Spain with a few gold nuggets, Indian captives, and 2 greatly unwanted gifts for the Old World: tobacco and syphilis (as many believe, but remains debatable). There is no evidence either his sailors or the aboriginal people he contacted had TB. In 1497, John Cabot landed in North America and ships from other countries soon followed. European contacts from Spain returned to South America and Mexico as conquistadores in the early 1500s, conquering and plundering the Aztec, Mayan, Toltec,