![]() ![]() This requires, however, that the divergence among hosts and parasites approximates to the same timescale. schaeffi, which infests chimpanzees, one might assume that human and chimp lice have cospeciated with their hosts as family heirlooms ever since they diverged from a common ancestor. Given that the closest relative of the human head and body louse, Pediculus humanus, is P. Lice and nits have been found in textiles, hair and combs excavated from archaeological sites. The primary reservoirs of the Ebola filovirus, the SARS coronavirus and the Nipah paramyxovirus, however, seem to be in fruit bats (flying foxes). The market for 'bushmeat' has led to Ebola virus outbreaks in Africa from butchering primates, and to the SARS outbreak in China from eating small carnivores, such as civet cats. Today's novel viral infections are more likely to originate from exotic species than from animals that were domesticated long ago. The influenza pandemic of 1918–1919 came from birds, and the 1968 influenza strain could be an avian-porcine recombinant. These new acquisitions originate from evolutionarily distant host species. Most human pandemic infections were acquired horizontally very recently on the evolutionary timescale, even though diseases such as typhus, measles and smallpox first occurred in prehistoric times. Thus a shared habitat, rather than a shared ancestry, is important for the acquisition of many infections. When we domesticated ruminants, and animals such as dogs, cats and rats 'domesticated' us for the rich pickings around human habitation, we acquired many infections from our new neighbors. Further opportunities for horizontal crossover of microbes and parasites from animals to humans arose when humans spread out of Africa. Ashford argues that the great apes became more specialized forest dwellers at the same time that early hominids explored the savannah, and that human gut parasites resemble those of omnivorous baboons more than those of chimps because humans, like baboons but unlike chimps, are omnivorous. Human DNA might show 98% similarity to that of chimps, but we share less than 50% of our microbes and parasites with them. ![]() The majority of zoonoses, however, remain in their animal reservoirs and, so far as their sojourn in humans goes – even with limited human-to-human transfer (as with Ebola or severe acute respiratory syndrome (SARS)) – we can regard them as 'temporary exhibits'. They have diverged from their progenitors in the original host: for example, measles is now distinct from rindepest. The new acquisitions were initially derived from zoonotic infections but have flourished as self-sustaining infections in the human population. Writing on infections of humankind, Tony McMichael and I have called those that cospeciated with their hosts 'family heirlooms' and those that crossed over from other hosts in recent evolutionary time 'new acquisitions'. I shall examine this unfolding story in the context of what we know about microbial infections, and will look at the promiscuity of viruses through the lens of modern molecular technology and I will add my own speculation on why naked apes have pubic hair. It is a tale of infidelity that I shall begin with the recent research on lice of David Reed and colleagues and of Mark Stoneking's group who, on the basis of phylogenetic analysis, have speculated that we may have acquired a clade of head lice from another hominid species and pubic lice from gorillas they have also suggested that lice might help determine the date when humans adopted clothing. Here, I discuss how we may in the course of prehistory have acquired the lice, and how other infections may, like the typhus bacillus, come to be shared by us and the animal species with which we are in close contact. Written in 1934 and subtitled The biography of a bacillus, it tells the tale of that dreaded disease typhus, its reservoir in rats and its transmission among humans by lice. Hans Zinsser's Rats, Lice and History is a classic in microbiology. ![]()
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