Hominin round-up

Our tenacious companions.

Male human head louse, Pediculus humanus capit...
Male human head louse, Pediculus humanus capitis (credit: Wikipedia)

Until recently humans and lice were inseparable and the same goes for all primates, and nearly all mammals. However, unlike fleas, which happily will suck any blood that is going provided it is easily tapped, lice are tailored to their hosts. Should a baboon louse, for instance, get into your short and curlies it will almost certainly die. In any case, again unlike fleas, the louse cannot leap: they spread through intimate contact. The human head louse spreads especially well among nursery- and infant-school children, as any parent knows, because lessons often involve them literally getting their heads together. Less well known is that Pediculus humanus eschew soiled or greasy hair and it is the well-scrubbed kids who suffer and spread ‘beasts on the head’. Conversely, the clothes louse that carries typhus and other infections is deterred by regular laundry and ironing. And then there is the  or trouser shrimp Phthirus pubis, which is not caught from public toilets…

Such is the variety of primate lice that they clearly have evolved, and their specificity suggests that this evolution paralleled that of their hosts in some way. The growing speed and falling cost of DNA sequencing permits this co-evolution to be followed in some detail (Dunn, R. 2012. Intimately yours. New Scientist, v. 216, 3 November issue, p. 36-39). The obvious link to check is that with lice on our nearest living relatives, chimpanzees. Though we do not share them, chimp lice (Pediculus schaeffi) are genetically as close to our head lice as we are to chimps themselves. In fact using a molecular clock technique gives roughly the same date for the last common louse as it does for the last common ancestor of chimpanzees and humans; around 6 Ma. Yet human head lice have clearly evolved into three populations that are genetically very different: one occurs everywhere; another occurs only in Asia and the Americas; while, curiously, Nepal and Ethiopia share a third. The two lesser lice have been speculatively assigned to an ancestral louse that infested possibly Denisovans, crept onto to modern humans –through intimacy either linked to genetic signs of interbreeding or through combat – and then journeyed on with them (some Ethiopians originated from people who returned to Africa from Asia).

DNA comparison between human head- and clothes lice shows that they are closely related despite having very different habits. Yet the molecular clock for clothes lice suggests that they evolved between 83 to 170 ka ago: it seems that hominins went unclothed for a very long time, developing clothing just in time to migrate to higher latitudes as sea-levels fell while climate cooled. But were we furry in earlier times. This is where the pubic louse offers insight. It is very different from head lice that could have roamed over an entire furry body. A subspecies exploiting the only other abundantly hairy place once humans became naked did not evolve. Human pubic lice are anatomically similar to the gorilla louse, and this is where matters become interesting as intimate contact would be needed for these slowly creeping, crab-like creatures to move from the one primate to another. Astonishingly the molecular date for that exchange is around 3.3 Ma, before the genus Homo entered the scene. Moreover, to choose pubic hair cover rather than join with head lice suggests that our bipedal hominin ancestor was already mostly naked at that time.

Molecular clock running fast

Animation of the structure of a section of DNA...
Animation of the structure of a section of DNA. (credit: Wikipedia)

Much as geoscientists and archaeologists love to tag their finds with a date, the history of so-called ‘absolute’ dating records lots of problems, the latest being that new or improved methods can cast doubt on all preceding chronologies (http://earth-pages.co.uk/2012/06/14/disputes-in-the-cavern/). Now, it seems, palaeoanthropologists have been stunned by the scientific equivalent of a sand-filled sock: the rate at which mutations build up in human and presumable hominin DNA may well be slower than previously assumed (Gibbons, A. 2012. Turning back the clock: slowing the pace of prehistory. New Scientist, v. 338, p. 189-191). This is the rate used to estimate the times of divergence of living species from the differences in their DNA, hitherto usually that from mitochondria. Recent work on whole genomes from related humans hints at a rate half as fast as that currently at the core of hominin evolutionary timing. A further problem is that, unlike the decay of radioactive isotopes, mutation rates are unlikely to remain constant.

Anne Gibbons reports news from the famed Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany that the error bar for the divergence of anatomically modern humans from Neanderthals and Denisovans lies between 170 and 700 ka. Combined with the uncertainties in stratigraphic and radiometric dating of fossils, a palaeogenetic framework for hominin evolution seems set for a period of yet more turmoil. It even throws doubt on the chronologies for large-scale migrations and separation of now widely-spread living populations that have been based on mitochondrial and Y-chromosome DNA. The oft-cited <70 ka diaspora from Africa to Asia, based on molecular clocks, may have been considerably earlier, perhaps tallying better with the geochronological evidence from the Middle East for 100 ka and more. The divergence of moderns from a last common ancestor that we shared with Neanderthals and Denisovans (probably Homo heidelbergensis) could be between 400 to 700 ka; more aligned with fossil evidence.

But matters go serious awry with more distant links. The human-chimpanzee divergence from a common precursor with the new estimates is pushed back more than 5 Ma from the 4-7 Ma using the old rate of genetic change to 8 to 10 Ma. That does not fit well with the stratigraphic appearance of the most chimp-like hominin fossil (Sahelanthropus) around 6-7 Ma, and stymied by a dearth of fossils of early knuckle-walking proto-chimps. The new human-orang utan estimate is way back too. One way out is to boldly assume – groundlessly – that early primates evolved a sight faster than we do, and that the pace slowed around the time of the human-chimp split and again with the emergence of modern humans. Enthusiasts cite the likelihood of longer timing to sexual maturity: mutations arise only after birth so longer times between generations force down the annual mutation rate. Such flaws can only inflame debate, though that has less purpose than the antagonists might imagine without more bones and better geochronology.

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