Book 2 Listening (1108796), страница 17
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The species mayhave been a tool-maker, as its hand allowed for a human-like grip.Sediba's pelvis, an upright butterfly shape, is likewise reminiscent of the human one. Ittherefore sheds light on a longstanding debate: whether it was bipedalism or giving birth tobabies with large heads that drove changes in the shape of the human pelvis.
Adult sediba(and therefore, presumably, their babies) had small heads, which indicates that walkingupright was the advantage brought by the modern pelvis.All of which makes sediba more similar to modern humans than are otheraustralopithecines - and more similar, even, than Homo habilis, until now seen as one of theearliest humans. The consensus had been that habilis was a transitional form betweenAustralopithecus and Homo erectus. Dr Berger posits that sediba may have evolved directlyinto Homo erectus, leaving habilis as an evolutionary sideline, and not even part of the genusHomo.
Slowly, then, the origin of the strange assemblage of characters that makes a humanbeing human is emerging. As the oracle said, the beginning of wisdom is: know thyself.September 10 2011)Script 31. Human evolutionThe skinny on skin colourHomo sapiens became black to beat cancerSHAVE a chimpanzee and you will find that beneath its hairy coat its skin is white.Human skin, though, was almost always black - at least it was until a few thousand years agowhen the species began settling in parts of the world so far north that the sunshine was tooweak to allow dark skin to synthesise enough vitamin D. This means that, sometime afterchimps and people parted ways, the colour of human skin changed.
And that, in turn, musthave required an evolutionary pressure.One suggestion often preferred is that the melanin in black skin, by absorbing ultravioletlight which might otherwise damage DNA and cause mutations, protects against skin cancer.Certainly, white-skinned people who move to the tropics a re more at risk of such cancersthan they would have been had they stayed at home.But critics of this hypothesis point out that most types of skin cancer, specifically thebasal-cell and squamous-cell carcinomas that are the commonest varieties of the disease,tend to affect older people (who have already reproduced and are thus, in Darwinian terms,expendable) and are often not lethal anyway.
Malignant melanoma, the one variety which is64both lethal and affects all age groups, is rare.However, a study by Mel Greaves, of the Institute of Cancer Research, in Britain, justpublished in the Proceedings of the Royal Society, settles the question in favour of cancerbeing the driving force. Dr Greaves does so by reviewing the clinical data about thoseAfricans who do not have black skin because they are albino.Albinism has a variety of genetic causes, but they all have the same consequence - arestricted or non-existent ability to synthesise melanin. The phenomenon is not well studjedin Africa, not least because of widespread prejudice against albinos, who are ostracised inmany parts of the continent.
Dr Greaves nevertheless managed to assemble 25 relevantstudies, and they do not make pretty readmg.One, conducted in Nigeria and published in 1980, found that half of the 512 albinoswhom the researchers followed had developed skin cancer of some sort by the time theywere 26. Another, carried out a few years later in Tanzania, showed that half of 125participants were afflicted by the age of 20. A third, from Soweto in South Africa, suggestedthat that an albino African has a thousandfold greater risk of developing skin cancer thandoes his normally pigmented neighbour. And a fourth estimated that fewer than 10% ofalbinos in equatorial Africa survive into their thirties-with the strong inference that what iskilling them is skin cancer.Nor is the cancer in question always malignant melanoma.
Basal-cell and squamouscell carcinomas are not, for African albinos, the relatively harmless diseases of old age whichdata collected in the rich world suggest. In Africa, they kill- quickly. Presumably they wouldhave done the same to any human forebear who had had the evolutionary temerity to shedhis hairy coat without replacing it with a suitably dark undercoat of melanin-laden skin.Why humans became naked apes is still a mystery. Explanations range from ease ofheat loss to the selection of mates by the quality of their (now visible) skin. Dr Greaves'sstudy, though, removes any doubt about why, having done so, a change of skin colour wasessential.
Economist March 07, 2014Script 32. Human evolutionTime's arrowsSome pieces of ancient weapons may illuminate modern man's evolutionTHE shards of stone pictured below, which have an average length of about 30mm, or1.2 inches, may provide an insight into the evolution of the human psyche. They werediscovered at Pinnacle Point, on South Africa's southern coast, by Kyle Brown of theUniversity of Cape Town and Curtis Marean of Arizona State University, and they areestimated to be 71,000 years old.Such shards are known as microliths.
They are made by heating a suitable lump of rockin a fire, and then bashing it, in order to flake pieces off its surface. They are believed to havebeen employed mainly as arrow heads-and were so used in Scandinavia as recently as9,000 years ago. From about 40,ooo years ago microliths are common. Before that date, onlyone set of examples, from about 6o,ooo years ago, had been found.
This fact has been usedfor support by those who think the human psyche evolved separately from, and more recentlythan, the physique of Homo sapiens.Both fossil evidence and DNA analysis using molecular clocks (estimates of historicalmutation rates) agree that Homo sapiens is 150,ooo-2oo,ooo years old. It is only in the pastso,ooo-6o,ooo years, however, that it has really taken off. Some ascribe that late take-off tochance. Others think the human mind crossed a threshold at that time, and the flourishing ofhumanity is the consequence. The battleground for this debate is the handful of artefacts thatpredate 6o,ooo years ago-which is also the moment when Homo sapiens left Africa andstarted the rise that has now established the species on every continent.The discovery of these particular microliths, which Dr Brown and Dr Marean report inthis week's Nature, shows that people 71,000 years ago were able to conceive of makingthem, to act on that conception and to use the result.
That suggests they had bows andarrows, a sophisticated form of weapon.65This finding thus adds weight to the argument of those who believe that alive at thattime were not, psychologically, very different from those alive today. That their culture wassimpler because there were fewer of them, and inventions needed time to accumulate, notbecause they were less clever.The existence of these ancient microliths may also have a bearing on a relatedargument, over why human psychology is different from that of other species. Onemanifestation of that difference, in the view of some, is extreme altruism - extreme in thesense that people will occasionally lay down their own lives for the sake of others.Such self-sacrifice is most often seen in war, and a controversial hypothesis proposedby a few evolutionary biologists is that it did indeed evolve in the context of warfare, at thetime when the invention of weapons such as bows and arrows first made it possible for onegroup of humans to annihilate another.
In those circumstances, heroic self-sacrifice topreserve a band of relatives might make evolutionary sense, since an individual's genescould still be passed on collaterally, through surviving members of the band. That impulse,the theory goes, is still felt today, even though comrades-in-arms are not always bloodrelations. Such thoughts are a heavy burden for a handful of stones to bear, but that is oftenthe fate of fossil signs of human activity. Each discovery, though, does bring the truth a littlecloser.
members of Homo sapiens (From The Economist, November 10, 2012)Script 33. The demographic transitionMore or lessWhy, as people get richer, do they have fewer children?One of the most significant phenomena of modern history is the demographic transition:as people get richer, they have smaller families. This slowing of reproduction with economicdevelopment is the reason why Thomas Malthusian prediction of disaster, caused by thehuman population outstripping its supply of food, is unlikely ever to come true. In the shortterm, Malthusian doom has been evaded by innovations that increased the food supply.
Butin the long term it is likely to be a ceiling on demand that helps to save humanity. The world'spopulation, now some 7 billion, is expected to level out at a little over 10 billion towards theend of the century.Why the demographic transition happens, though, is obscure - for this reaction by Homosapiens to abundance looks biologically bonkers. Other species, when their circumstancesimprove, react by raising their reproductive rate, not curtailing it. And work just published byAnna Goodman of the London School of Hygiene and Tropical Medicine and her colleagues,in the Proceedings of the Royal Society, suggests what humans do is indeed bananas.Dr Goodman has shown that the leading explanation advanced by biologists for thetransition does not, in the context of the modern world, actually deliver the goods.
Thisexplanation is that, according to circumstances, people switch between two reproductivestrategies. One, known to ecologists as "R-selection", is to produce lots of offspring but investlittle in each of them. This works in environments with high infant mortality. The other, knownas "K-selection", is to have only a few offspring but to nurture them so that they are superbspecimens and will thus do well in the competition for resources and mates, and producemore grandchildren for their parents than their less well-nurtured contemporaries. Thedemographic transition, according to this analysis, is a shift from R-type to K-type behaviour.To test this idea, Dr Goodman turned to Sweden - specifically, to a group known as theUppsala Birth Cohort.
These people (there are about 14,000 of them) were born between1915 and 1929 in Uppsala University Hospital. They and their descendants havesubsequently been tracked by Sweden's efficient system of official records. Among otherthings these records show their income and socioeconomic status (which, crucially, are alsoknown for the parents of members of the original cohort), how many children have been bornto cohort members and their descendants, and when. These data were Dr Goodman's rawmaterial.If the R/K interpretation is correct (the letters stand for the rate of reproduction and the"carrying capacity", or resource richness, of the environment), then an advantage of some66sort for the socioeconomically privileged should show up as the generations succeed oneanother.
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