Groups, glaciation and the pox
[This is the pre-edited version of my Lab Report column for the February issue of Prospect.]
Blaming America for the woes of the world is an old European habit. Barely three decades after Columbus’s crew returned from the New World, a Spanish doctor claimed they brought back the new disease that was haunting Europe: syphilis, so named in the 1530s by the Italian Girolamo Fracastoro. All social strata were afflicted: kings, cardinals and popes suffered alongside soldiers, although sexual promiscuity was so common that the venereal nature of the disease took time to emerge. Treatments were fierce and of limited value: inhalations of mercury vapour had side-effects as bad as the symptoms, while only the rich could afford medicines made from guaiac wood imported from the West Indies.
But it became fashionable during the twentieth century to doubt the New World origin of syphilis: perhaps the disease was a dormant European one that acquired new virulence during the Renaissance? Certainly, the bacterial spirochete Treponema pallidum (subspecies pallidum) that causes syphilis is closely related to other ‘treponemal’ pathogens, such as that which causes yaws in hot, humid regions like the Congo and Indonesia. Most of these diseases leave marks on the skeleton and so can be identified in human remains. They are seen widely in New World populations dating back thousands of years, but reported cases of syphilis-like lesions in Old World remains before Columbus have been ambiguous.
Now a team of scientists in Atlanta, Georgia, has analysed the genetics of many different strains of treponemal bacteria to construct an evolutionary tree that not only identifies how venereal syphilis emerged but shows where in the world its nearest genetic relatives are found. This kind of ‘molecular phylogenetics’, which builds family trees not from a traditional comparison of morphologies but by comparing gene sequences, has revolutionized palaeonotology, and it works as well for viruses and bacteria as it does for hominids and dinosaurs. The upshot is that T. pallidum subsp. pallidum is more closely related to a New World subspecies than it is to Old World strains. In other words, it looks as though the syphilis spirochete indeed mutated from an American progenitor. That doesn’t quite imply that Columbus’s sailors brought syphilis back with them, however – it’s also possible that they carried a non-venereal form that quickly mutated into the sexually transmitted disease on its arrival. Given that syphilis was reported within two years of Columbus’s landing in Spain, that would have been a quick change.
Having helped to bury the notion of group selection in the 1970s, Harvard biologist E. O. Wilson is now attempting to resurrect it. He has a tough job on his hands; most evolutionary biologists have firmly rejected this explanation for altruism, and Richard Dawkins has called Wilson’s new support for group selection a ‘weird infatuation’ that is ‘unfortunate in a biologist who is so justly influential.’
The argument is all about why we are (occasionally) nice to one another, rather than battling, red in tooth and claw, for limited resources. The old view of group selection said simply that survival prospects may improve if organisms act collectively rather than individually. Human altruism, with its framework of moral and social imperatives, is murky territory for such questions, but cooperation is common enough in the wild, particularly in eusocial insects such as ants and bees. Since the mid-twentieth century such behaviour has been explained not by vague group selection but via kin selection: by helping those genetically related to us, we propagate our genes. It is summed up in the famous formulation of J. B. S. Haldane that he would lay down his life for two brothers or eight cousins – a statement of the average genetic overlaps that make the sacrifice worthwhile. Game theory now offers versions of altruism that don’t demand kinship – cooperation of non-relatives can also be to mutual benefit – but kin selection remains the dominant explanation for eusociality.
That was the position advocated by Wilson in his 1975 book Sociobiology. In a forthcoming book The Superorganism, and a recent paper, he now reverses this claim and says that kin selection may not be all that important. What matters, he says, is that a population possess genes that predispose the organisms to flexible behavioural choices, permitting a switch from competitive to cooperative action in ‘one single leap’ when the circumstances make it potentially beneficial.
Wilson cites a lack of direct, quantitative evidence for kin selection, although others have disputed that criticism. In the end the devil is in the details – specifically in the maths of how much genetic common ground a group needs to make self-sacrifice pay – and it’s not clear that either camp yet has the numbers to make an airtight case.
The discovery of ice sheets half the size of today’s Antarctic ice cap during the ‘super-greenhouse’ climate of the Turonian stage, 93.5-89.3 million years ago, seems to imply that we need not fret about polar melting today. With atmospheric greenhouse gas levels 3-10 times higher than now, ocean temperatures around 5 degC warmer, and crocodiles swimming in the Arctic, the Turonian sounds like the IPCC’s worst nightmare. But it’s not at all straightforward to extrapolate between then and now. More intense circulation of water in the atmosphere could have left thick glaciers on the high mountains and plateaus of Antarctica even in those torrid times. In any event, a rather particular set of climatic circumstances seems to have been at play – the glaciation does not persist throughout the warm Cretaceous period. And it is always important to remember that, with climate, where you end up tends to depend on where you started from.