Unit 11. Human evolution

Script 29. Human evolution and palaeobotany

Grassed up

A cherished theory about why people walk upright has just bitten the dust.

Africa’s great grasslands are one of that continent's most famous features. They are also reckoned by many to have been crucial to human evolution. This school of thought holds that people walk upright because their ancestors could thus see farther on an open plain. Forest primates do not need to be bipedal, the argument continues, because the trees limit their vision anyway.

As "Just So" stories go, it is perfectly plausible. But some go further and argue that the transition took place when the savannahs themselves came into existence, replacing the pre-existing forest and forcing human ancestors to adapt or die out. Fossil evidence suggests humanity's upright stance began to evolve between 6m and 4m years ago. So the question is, did that coincide with the formation of the savannah? A paper in Geology, by Sarah Feakins, of the University of Southern California, suggests not.

Dr Feakins studied sediment cores from the Gulf of Aden, a place where offshore winds deposit detritus from a goodly part of the east of the African continent. In these, she discovered plant molecules that date back between 12m and 1m years. Such molecules contain carbon, and carbon atoms come in various isotopes, whose ratios give away their history. In particular, the ratio of 12C to 13C can tell you what sort of plant made the molecule in question.

Plants in rainforests tend to discriminate against 13C. Those in modern African grasslands are less selective and 13C is thus more abundant in their molecules. Dr Feakins was therefore able to ask when these grasslands came about.

To her surprise, they seem to have been there even 12m years ago. Close examination of the cores shows that the nature of the grass changed over the millennia, as species that were adapted to dry conditions took over from those that prefer wetter weather, but savannah of some form there always was.

The climatic change she observed was already known about. It was the reason people suspected forests had given way to savannah. But, contrary to that suspicion, Dr Feakins has shown that early humanity's east African homeland was never heavily forested, so the idea that people were constrained to walk upright by the disappearance of the forests is wrong.

Perhaps it was more pull than push - a preexisting, but empty ecological niche crying out to be filled by an enterprising species that could make the transition. But perhaps those who seek an ecological explanation of this sort are, as it were, barking up the wrong tree. (From The Economist, February 16, 2013)

Script 30. Human evolution

You look familiar

Another piece of humanity's family tree is fitted into place.

The opening scene of Mel Brooks's film "History of the World: Part One" dispenses with human origins in one line: "And the ape stood, and became man." Would that it were that easy for palaeontologists to sort out. The transition to humanity is generally agreed to have occurred between Australopithecus, a genus of small-brained, bipedal primates whose most famous member is a fossil nicknamed "Lucy", and the big-brained species Homo erectus. But pinning down when precisely this took place, and which of the various australopithecine species were involved, has been challenging. Now the most human-like australopithecine found to date is clarifying things - and staking a claim to be the species from which early humans evolved.

Fossils of the new species, Australopithecus sediba, were discovered in 2008 in a cave in South Africa. Initial research, led by Lee Berger of the University of the Witwatersrand, in Johannesburg, concluded that the species came too late in the fossil record to be the ancestor of the Homo lineage. This week, however, a range of new research into sediba, again led by Dr Berger, has been published in Science. These studies conclude that sediba did in fact predate Homo erectus and, moreover, that parts of its anatomy are surprisingly similar to modern man.

The fossils examined in the Science papers are of an adolescent boy and an adult woman. They are well preserved, and encased in sediments that allow uncommonly precise dating. They lived 1.977m years ago, predating the appearance of Homo erectus by 77,000 years. The period is an especially muddled one for palaeontology, being full of fragmentary fossils that are difficult to assign either to Homo or to Australopithecus. The sediba fossils, by contrast, have some of the most complete features in the early human record.

The new studies centre on the most telling bits of anatomy in the story of human evolution: the brain, pelvis, hands and feet. The brain itself does not fossilise, but the inside of the cranium retains an impression of its contours. The researchers mapped these with high-powered x-ray beams to create a three-dimensional model of the surface of sediba's brain. They found that its size was on a par with other australopithecines, but its shape was more like that of a human brain. Specifically, the frontal lobes, which are the seat in modern humans of higher cognitive functions such as abstract reasoning, looked more humanlike in sediba's brain than they do in the brains of other australopithecines. That suggests the neurological changes which gave rise to humanity may have predated the brain's expansion - an event that had, hitherto, been regarded as crucial to the emergence of humans.

The hands, feet and pelvis of sediba indicate that it both climbed trees and walked upright, though with a different gait from that of humans or chimpanzees. The species may have 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. It therefore sheds light on a longstanding debate: whether it was bipedalism or giving birth to babies 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 walking upright was the advantage brought by the modern pelvis.

All of which makes sediba more similar to modern humans than are other australopithecines - and more similar, even, than Homo habilis, until now seen as one of the earliest humans. The consensus had been that habilis was a transitional form between Australopithecus and Homo erectus. Dr Berger posits that sediba may have evolved directly into Homo erectus, leaving habilis as an evolutionary sideline, and not even part of the genus Homo. Slowly, then, the origin of the strange assemblage of characters that makes a human being human is emerging. As the oracle said, the beginning of wisdom is: know thyself. (From The Economist, September 10, 2011)

Script 31. Human evolution

The skinny on skin colour

Homo sapiens became black to beat cancer.

Shave 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 ago when the species began settling in parts of the world so far north that the sunshine was too weak to allow dark skin to synthesise enough vitamin D. This means that, sometime after chimps and people parted ways, the colour of human skin changed. And that, in turn, must have required an evolutionary pressure.

One suggestion often proferred is that the melanin in black skin, by absorbing ultraviolet light which might otherwise damage DNA and cause mutations, protects against skin cancer. Certainly, white-skinned people who move to the tropics are more at risk of such cancers than they would have been had they stayed at home. But critics of this hypothesis point out that most types of skin cancer, specifically the basal-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 is both lethal and affects all age groups, is rare.

However, a study by Mel Greaves, of the Institute of Cancer Research, in Britain, just published in the Proceedings of the Royal Society, settles the question in favour of cancer being the driving force. Dr Greaves does so by reviewing the clinical data about those Africans who do not have black skin because they are albino.

Albinism has a variety of genetic causes, but they all have the same consequence - a restricted or non-existent ability to synthesise melanin. The phenomenon is not well studjed in Africa, not least because of widespread prejudice against albinos, who are ostracised in many parts of the continent. Dr Greaves nevertheless managed to assemble 25 relevant studies, and they do not make pretty reading.

One, conducted in Nigeria and published in 1980, found that half of the 512 albinos whom the researchers followed had developed skin cancer of some sort by the time they were 26. Another, carried out a few years later in Tanzania, showed that half of 125 participants were afflicted by the age of 20. A third, from Soweto in South Africa, suggested that that an albino African has a thousandfold greater risk of developing skin cancer than does his normally pigmented neighbour. And a fourth estimated that fewer than 10% of albinos in equatorial Africa survive into their thirties - with the strong inference that what is killing them is skin cancer.

Nor is the cancer in question always malignant melanoma. Basal-cell and squamous- cell carcinomas are not, for African albinos, the relatively harmless diseases of old age which data collected in the rich world suggest. In Africa, they kill - quickly. Presumably they would have done the same to any human forebear who had had the evolutionary temerity to shed his 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 of heat loss to the selection of mates by the quality of their (now visible) skin. Dr Greaves's study, though, removes any doubt about why, having done so, a change of skin colour was essential. (From The Economist, March 01, 2014)

Script 32. Human evolution

Time's arrows

Some pieces of ancient weapons may illuminate modern man's evolution.

The shards of stone pictured below, which have an average length of about 30mm, or 1.2 inches, may provide an insight into the evolution of the human psyche. They were discovered at Pinnacle Point, on South Africa's southern coast, by Kyle Brown of the University of Cape Town and Curtis Marean of Arizona State University, and they are estimated to be 71,000 years old.

Such shards are known as microliths. They are made by heating a suitable lump of rock in a fire, and then bashing it, in order to flake pieces off its surface. They are believed to have been employed mainly as arrow heads - and were so used in Scandinavia as recently as 9,000 years ago. From about 40,000 years ago microliths are common. Before that date, only one set of examples, from about 60,000 years ago, had been found. This fact has been used for support by those who think the human psyche evolved separately from, and more recently than, the physique of Homo sapiens.

Both fossil evidence and DNA analysis using molecular clocks (estimates of historical mutation rates) agree that Homo sapiens is 150,000-200,000 years old. It is only in the past 50,000-60,000 years, however, that it has really taken off. Some ascribe that late take-off to chance. Others think the human mind crossed a threshold at that time, and the flourishing of humanity is the consequence. The battleground for this debate is the handful of artefacts that predate 60,000 years ago - which is also the moment when Homo sapiens left Africa and started the rise that has now established the species on every continent.

The discovery of these particular microliths, which Dr Brown and Dr Marean report in this week's Nature, shows that people 71,000 years ago were able to conceive of making them, to act on that conception and to use the result. That suggests they had bows and arrows, a sophisticated form of weapon. This finding thus adds weight to the argument of those who believe that members of Homo sapiens alive at that time were not, psychologically, very different from those alive today. That their culture was simpler because there were fewer of them, and inventions needed time to accumulate, not because they were less clever.

The existence of these ancient microliths may also have a bearing on a related argument, over why human psychology is different from that of other species. One manifestation of that difference, in the view of some, is extreme altruism - extreme in the sense 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 proposed by a few evolutionary biologists is that it did indeed evolve in the context of warfare, at the time when the invention of weapons such as bows and arrows first made it possible for one group of humans to annihilate another. In those circumstances, heroic self-sacrifice to preserve a band of relatives might make evolutionary sense, since an individual's genes could 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 blood relations.

Such thoughts are a heavy burden for a handful of stones to bear, but that is often the fate of fossil signs of human activity. Each discovery, though, does bring the truth a little closer. (From The Economist, November 10, 2012)

Script 33. The demographic transition

More or less

Why, 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 economic development is the reason why Thomas Malthus’s prediction of disaster, caused by the human population outstripping its supply of food, is unlikely ever to come true. In the short term, Malthusian doom has been evaded by innovations that increased the food supply. But in the long term it is likely to be a ceiling on demand that helps to save humanity. The world's population, now some 7 billion, is expected to level out at a little over 10 billion towards the end of the century.

Why the demographic transition happens, though, is obscure - for this reaction by Homo sapiens to abundance looks biologically bonkers. Other species, when their circumstances improve, react by raising their reproductive rate, not curtailing it. And work just published by Anna 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 the transition does not, in the context of the modern world, actually deliver the goods.

This explanation is that, according to circumstances, people switch between two reproductive strategies. One, known to ecologists as "R-selection", is to produce lots of offspring but invest little in each of them. This works in environments with high infant mortality. The other, known as "K-selection", is to have only a few offspring but to nurture them so that they are superb specimens and will thus do well in the competition for resources and mates, and produce more grandchildren for their parents than their less well-nurtured contemporaries. The demographic 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 the Uppsala Birth Cohort. These people (there are about 14,000 of them) were born between 1915 and 1929 in Uppsala University Hospital. They and their descendants have subsequently been tracked by Sweden's efficient system of official records. Among other things these records show their income and socioeconomic status (which, crucially, are also known for the parents of members of the original cohort), how many children have been born to cohort members and their descendants, and when. These data were Dr Goodman's raw material.

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 some sort for the socioeconomically privileged should show up as the generations succeed one another. Dr Goodman's analysis shows that it does, but in a way that is not translated into any obvious evolutionary advantage.

Reducing family size certainly creates what look, on the face of things, like more competitive descendants. Children, grandchildren and great-grandchildren alike get better marks at school, are more likely to go to university and have higher incomes as adults. What these competitive individuals do not do, though, is go on to compete in the one arena which matters in a Darwinian sense: reproduction. If anything, the tendency towards smaller, more socially successful families tends to feed back on itself over the generations, and the contribution of the K-selected to the gene pool therefore shrinks.

To biologists, this is all very puzzling. If K-type behaviour is not delivering the goods then it should never have come about in the first place. But there may be an explanation: that the psychological make-up which encourages K-type behavior worked in the past but is not appropriate to modern circumstances.

This does seem plausible. In large parts of the world, better hygiene, nutrition and medicine have almost abolished child mortality, meaning the advantage of K over R is diminished. Education is available free to all. And harem-formation, which would have been an option in the past for many K-selected males, is frowned on these days. In other words, the disadvantages of being R-selected have disappeared.

The upshot is that the demographic transition may be the result of a mismatch between ancient psychology and the modern world. In that, it would be like the epidemic of obesity which results from stoneage appetites meeting capitalist abundance. Unlike obesity, though, small families do no harm to the individuals involved. In fact this particular mismatch may actually be all that stands between humanity and ecological disaster. (From The Economist, September 1, 2012)