KIDS on the street seem taller than ever; the Japanese grow three to four inches in 50 years; Scandinavian families fret about the towering stature of their daughters. It would be easy to conclude that Homo sapiens is destined to become a species of basketball players – but the truth is more complex. The growth spurt we are seeing in affluent parts of the world is fuelled by better nutrition. Until this effect kicked in, humans were smaller than at any point in our history or that of our ancestors, stretching back some two million years. Virtually all anthropologists agree that we are shorter, lighter and less robust than our forebears. And our brains are the smallest they have been for 200 000 years.
The shrinking process began 200 000 years ago and has continued in fits and starts ever since. It gathered pace at the end of the Pleistocene ice age, 10 000 years ago, but slowed to a halt a few thousand years ago. Now we are seeing an apparent reversal of the trend. But this growth spurt is due to nutrition rather than any genetic changes, so it is unlikely to continue indefinitely; indeed there are already signs that it is petering out in places like Japan. If it stops, will the older trend of shrinkage reassert itself? Is Homo sapiens destined to walk smaller than ever in centuries to come? (see Diagram)
The best hope of finding out is to identify the causes of past bouts of shrinkage. But this is where anthropologists’ unanimity over the fact that humans have shrunk does its own disappearing act. Some point to cultural changes; others to a shift in reproductive strategy or poor nutrition. Still others blame global warming that ended the Pleistocene ice age.
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Shaping up
Any journey through the thicket of opinions must start with a different question: what do the varied shapes of humans living today reveal about our body’s capacity to adapt to different environments and climates? The subject has a long history, founded on two “rules” named after the scientists who first stated them. In 1847 C. Bergmann, a Swedish anatomist, stated that in a geographically widespread species, populations in warmer parts of the range will have smaller bodies than those in colder parts of the range. Thirty years later, J. A. Allen, a British biologist, stated that populations of a geographically widespread species living in warm regions will have longer extremities than those living in cold regions. But progress in testing these empirical rules, working out their origins and applying them to human prehistory was surprisingly slow. An upsurge of frank racial anthropology in the 1960s and 1970s deterred some researchers. Others were put off by the sparseness of the human fossil record and the fragmentary nature of many skeletons.
No longer. In recent years, researchers have redoubled their efforts to understand body variations in both ancient and modern humans, producing findings that are very revealing about human shrinkage. One protagonist is Christopher Ruff, an anthropologist at Johns Hopkins University, Baltimore. Perhaps more than anybody else, Ruff has begun to show how the body shapes of our ancestors speak eloquently of their former circumstances in the world – and perhaps of the shape of humans to come.
In his analyses, Ruff viewed the human body as a cylinder, the diameter of which represents the width of the body, or, more specifically, the width of the pelvis, and the length of the cylinder represents stature. The link between anatomy and climate is governed by the balance between our abilities to produce heat and dissipate it; or, in Ruff’s terms, by the ratio between the volume of the cylinder, or body mass, and its surface area. In hot climates, a high ratio – that is, a small body mass relative to surface area-facilitates heat loss. In cold climates, a low ratio – that is, a high body mass relative to surface area – allows heat retention. Simple geometry shows that surface area to body mass ratio is high when the cylinder is narrow, and low when it is wide. This is the basis of Bergmann’s rule.
From this analysis you can predict that people living at low latitudes will have narrow bodies and a linear physique, those at high latitudes will have wide bodies and a relatively bulky physique. Ruff’s survey of 71 populations around the globe fitted the prediction very well, with equatorial Nilotics in Africa epitomising one body form and Inuits the other. Ruff also validated Allen’s rule, finding that tropical people have longer, thinner limbs – better for losing heat – than folk at high latitudes. This difference in limb proportions enhances the linear look of tropical people and the stocky appearance of high-latitude populations.
But Ruff’s cylinder model predicted something else, too: people living in a similar climate zone should have the same body width irrespective of their height. This is because it is width, not height, that determines ratios of surface area to body mass. With cylinders of fixed width, the ratio of surface area to body mass does not alter with length.
Ruff believes this may help to explain the difference in stature between Nilotic people, whose average height is more than six feet, and Mbuti Pygmies, who are on average two feet shorter. Humans rely heavily on sweating to cool down. Nilotics live in open environments where sweating is efficient, whereas Mbuti Pygmies, like most Pygmy populations, live in moist, humid forests where the air is still and sweating is inefficient. So their best tactic is to limit the amount of heat generated during physical exertion by reducing the volume of the cylinder. Because the width of the cylinder remains constant, this implies a reduction in length or stature.
Needless to say, the equations linking body width to latitude doesn’t always hold true in its simplest form. For instance, the link is less exaggerated in New World populations than it is in the Old World. And there are some odd paradoxes, too, such as the Pecos Pueblo American Indians of New Mexico, who experienced a relatively warm climate. Ruff has shown that these people had an average body width only slightly less than the Inuits – and yet their limb proportions were not too different from those of equatorial populations.
The explanation may lie in the speed of the body’s adaptation to new climates. Amerindians have been in the New World for at least 6 000 and perhaps as long as 30 000 years. The original colonisers of the Americas came from northeastern Asia, and would have had bodies like modern Inuits. As they moved south through the New World, populations would have begun to adapt to local conditions, but only gradually. Moreover, different parts of the body would be expected to adapt at different speeds, with limb proportions changing faster than body width.
The lesson, says Ruff, is that nothing is simple when looking at body proportions in different places and through different times in the past. Even so, many anthropologists would agree that from early in the history of Homo, some 2 million years ago, to the appearance of archaic Homo sapiens, 300 000 or 400 000 years ago, there was a steady increase in robustness, which then reached a plateau. Here we are talking about immensely strong people with thick skulls and heavily muscled limbs. Early anatomically modern humans, which appeared 200 000 years ago, were significantly less robust than archaic Homo sapiens, but much more so than people today. The first modern people in Europe were also more linear, because of their African origin.
According to some anthropologists, there was a gradual decline in the robustness of early modern humans that lasted for a long time. Then came a dramatic reduction starting at the end of the ice age, 10 000 years ago. But not in all populations. Australian Aborigines, Patagonians and Fuegans, for instance, are still relatively robust in their skull and skeletal anatomy. But where it occurs, the loss of robustnesss happened principally between 10 000 and 5000 years ago, and then halted. So, too, did reductions in brain size, from around 1500 to 1300 cubic centimetres, the size of teeth and jaws, and height.
Different characteristics shrank at different rates during these five thousand years. For example, from his studies of Australian populations, Peter Brown of the University of New England, Armidale, reports reductions of 4.5 per cent in teeth size, 6 to 12 per cent in face size, 9.5 per cent in brain size and 7 per cent in stature. Similar changes can be seen in human remains from Europe and Southeast and West Asia.
Experts are divided over whether the brain began shrinking 30 000 years ago or only 10 000 years ago. Yet everyone seems to agree that until the nutritional effects of the past century or so kicked in, modern people were midgets on the human evolutionary stage, and tiny in comparison to their robust forebears.
What can be deduced from this rise and fall of robustness? Before the advent of the first modern humans, our ancestors were hunter-gatherers helped only by rudimentary technology. Muscles, not missiles, were their weapons, so it may have been important to be robust for this reason alone. But there are other, more intriguing explanations. Robert Foley of the University of Cambridge speculates that people got stronger because the abundance of big game allowed males to monopolise resources, which leads to a polygamous social system. As a result, there was intense competition – for game and for females – among males in different groups, which prompted the evolution of a bigger physique in males and females (bigger females being selected for because of their ability to produce bigger sons).
Why, then, did robustness wane as modern humans emerged, and continue to decline for tens of thousands of years? Not because they became more peaceable, says Foley, but because technological inventions – spears and stone tools – took the place of brute strength. And people were smarter, too, so guile rather than brawn might have filled the larder. Loring Brace, an anthropologist at the University of Michigan, Ann Arbor, has been arguing for a long time that technology, or culture, was the driving force behind diminishing human robustness. And when humans began to prepare and cook food teeth weren’t so vital and began to shrink. This happened at different times in different parts of the world.
But, as Peter Brown observes of the Australians, teeth may shrink even in the absence of food preparation. So other forces must have been at work. The end of the ice age brought many dramatic changes, not least the disappearance of economy-sized larder-fillers such as the mammoth and mastodon. So, suggests Foley, people fell back on two subsistence strategies, both of which encouraged a decline in body size. The first was agriculture; the second a shift to a different kind of social structure.
Early food production was hazardous. The archeological record shows that early farmers found it difficult to maintain a balanced diet, a sure way of keeping body size small. In the second strategy, the diminished game meant that individual hunters were unable to monopolise food resources, making polygamy much less feasible. The result? The guys got skinny because they didn’t need to compete so aggressively for females anymore.
The “nutritional stress” theory has the virtue of explaining why the post-ice age shrinkage coincided with a booming human population. Limited resources often lead to reduced body size, says Christopher Stringer of the Natural History Museum, in London. Nor is it just a question of there being less to go round. Robert Martin of the Anthropological Institute, Zurich, believes that the stress may have resulted chiefly from a shift to earlier weaning, a strategy for boosting reproductive success in the face of burgeoning competition. Early weaning inevitably leads to a reduction in brain size – because most brain growth occurs in the year or two after birth – though not, says Martin, necessarily to a reduction in body size.
Martin’s viewpoint separates him from most anthropologists, who argue that the brain merely shrank in response to reduced body size. People don’t like to think of the brain getting smaller regardless of body size, says Martin – especially not at a time when our behaviour was getting ever more complex. But nutritional stress and the advent of agriculture cannot be the whole story. Agriculture was developed at different times in different parts of the world, and in some places not at all, whereas shrinkage was universal. When Europeans landed in Australia and parts of the Americas, for instance, people were still hunting and gathering – yet they too underwent the same pattern of body shrinkage. Why?
Global shrinkage
Enter global warming. The one change that applied universally was the increase in global temperature that occurred at the end of the Pleistocene. That fact alone makes it a serious candidate as an agent of shrinkage, says Brown. Moreover, around the globe many non-human animals, such as wombats, shrank in this same period. And wouldn’t rising temperatures select for reduced body mass, which in humans would mean, among other things, reduced stature?
Maybe. But Ruff is not persuaded. According to his analysis, changes in body form began before the end of the ice age, making warming an unlikely explanation. What’s more, the Pleistocene was punctuated with several warm interglacials – one around half a million years ago, another between 125 000 and 75 000 years ago – and, says Ruff, there is no obvious shift in stature to accompany these global changes in temperature.
If cultural causes like technology or population growth, local environmental causes like loss of large game, and global causes like increased temperature, don’t explain universal shrinkage, where else can we look for an answer? To outer space, suggests Maciej Henneberg, of the University of The Witwatersrand, Johannesburg. In the most bizarre explanation of all, Henneberg cites data he collected with his colleague Graham Louw that imply that ultimate body size is influenced by the position of the Earth in its orbit when a person is born.
For instance, individuals born between February and July end up shorter and lighter than those born in the rest of the year. The difference is small, some half kilogram and 7 millimetres, but is statistically significant in their sample. The effect is not seasonal, because the effect is the same in northern and southern hemisphere, and it is not restricted to humans: Henneberg and Louw see it in Alsatian dogs in South Africa. They suggest that the effect may be the result of small differences in gravity or electromagnetic radiation as the Earth travels around the Sun.
How could this explain long-term shrinkage? When Henneberg was describing his observations recently to Robin McKie, science editor of The Observer newspaper, McKie mused on the possibility of long-term shifts in Earth’s orbit that would translate the yearly cycle to one seen over tens of thousands of years. Henneberg is intrigued by this astronomical explanation, but shrugs that he is just an anthropologist and has no way of finding out.
If, as most anthropologists believe, humans stopped shrinking a few thousand years ago, what does the future hold? For some populations, the near future is already here, brought to heel by the improved nutrition that comes with affluence. Increasing stature has been a fact in Europe and North America for almost a century, and in recent decades in Asia – not just Japan, but Indonesia and China too. A similar shift will eventually come to other countries, particularly in Africa and parts of Asia that have yet to clamber up the economic pole.
But what of the longer term future? Even if global warming has little influence on the stature of future generations, it may be a mistake to assume that tomorrow’s humans will stay tall forever. The forces of technological and social change that may have influenced human stature in the past could do so again. In a world dominated by information superhighways, will there be any advantage in being big?