Tue, Mar-22-11, 04:42
Modern bodies: Our 10,000-year makeover
From New Scientist
21 March, 2011
Modern bodies: Our 10,000-year makeover
Civilised living has transformed our bodies, from deep within our bones to the tips of our fingers
IN A basement storeroom at the University of Zurich in Switzerland, rows of cardboard boxes are stacked tidily on metal shelves. Although they are stamped with the logo of a banana import company, they contain, not fruit, but something more macabre.
Inside are the heads of two Egyptian mummies and bits of ancient bog bodies. More recent remains include bones from a medieval graveyard, and others from Swiss citizens who died in the early 20th century.
Some of the body parts bear marks of exotic or violent rituals: there are traces of gold leaf around one of the mummies' eyes, for example. Others show the ravages of diseases that have long since been eradicated. In this plain, windowless room lie the remains of 2000 individuals, all told.
The collection is overseen by anatomist Frank Rühli, who wants to reconstruct the people to whom they once belonged. He is discovering how the human body has changed over the last few thousand years, to chart how civilisation is resculpting our bodies. The modern western lifestyle continues to change not only our waistline but also our height, muscles, bones, blood vessels and hormones.
Some of those transformations could be genetic in origin, examples of recent microevolution in action. But there is reason to suspect that others are temporary changes wrought throughout our lives that would melt away if we returned to a Stone Age environment. The complex interplay of nature and nurture is hard to disentangle, but the sheer breadth and scale of the changes show the ease with which the human body can adapt to new habitats over short timescales.
Discovering how those adaptations are making us more vulnerable to certain diseases and less so to others is an important facet of evolutionary or Darwinian medicine, an emerging specialty that views health and disease through the lens of evolutionary theory. Last October, the university opened a Centre for Evolutionary Medicine with Rühli at its head. This approach sheds light on many common ailments that have arisen because our modern western lifestyle is so different from the one we evolved to suit, says Rühli.
Anatomically modern humans are thought to have arrived on the scene around 200,000 years ago. They lived as hunter-gatherers in small nomadic groups until around 10,000 years ago, when the advent of farming led to permanent settlements and, in fits and starts, the long slog to civilisation.
The idea that evolution could have been taking place in the past few thousand years goes against all received wisdom. Weren't we taught that natural selection operates over millions of years?
Yet recent evidence indicates that we have got this wrong. A gene that gives people the ability to digest milk after infancy, for example, was recently shown to have arisen and spread with the invention of dairy herding several thousand years ago.
The genetic evidence stems from samples taken from people alive today. By looking at how a gene's sequence varies among populations, we can work out how long ago it arose and chart its spread round the globe.
Arguably, information about the past can be obtained more directly from ancient human remains that have been preserved by accident or design. By comparing them with modern-day humans, we can work out just what civilisation has been doing to our bodies.
Perhaps the best-known difference is that westerners have got fatter, thanks to our calorie-rich diet and less active lifestyle. Obviously, that change would be reversed if we returned to hunting and gathering to find our food. A less well-known trend is that we have been becoming less muscular, almost certainly because we have been using our muscles less and less. Bones that no longer support large muscles can themselves become punier, so our shrinking musculature can be tracked in the fossil record. Our bones have become more spindly or "gracile", with the overall diameter shrinking as well as the dense outer cortex of the bone becoming thinner in cross-section (see diagram).
Christopher Ruff of the Johns Hopkins University School of Medicine in Baltimore, Maryland, has travelled the world to X-ray about 100 fossil leg bones going back over 3 million years. He also studied bones from three populations from the near-present: Native Americans from the American Southwest who lived about 900 years ago, and east Africans and US whites from the early to mid-20th century.
Between 2 million and 5000 years ago, Ruff's team documented an average fall in bone strength of 15 per cent. At that point, however, the trend accelerated, as there was another 15 per cent reduction over a mere 4000 years (Journal of Musculoskeletal and Neuronal Interactions, vol 5, p 202).
Ruff thinks gracilisation kicked in when we began to use tools that reduced physical exertion, starting with hand axes, through to ploughs and eventually cars. Our increasingly sedentary lifestyle means our survival has come to depend less and less on our strength.
How much of this process is due to genetic changes, and how much would be reversed if we returned to a Stone Age lifestyle? It's impossible to say, admits Ruff. "We don't know what genes control bone mass and there's no way we can go and sample these fossils and figure that out."
What we do know is that the body has an impressive capacity to respond to exertion over a single lifetime. Take professional tennis players: by looking at X-rays, Ruff's team has worked out that the humerus in their playing arm is more than 40 per cent stronger than the corresponding bone in the opposite arm. For comparison, non-athletes have only a 5 to 10 per cent difference. "That would suggest that if you were in the Stone Age and you were forced to travel longer distances and lift heavier things, you would probably develop stronger bones," he says.
It's an important finding, because it suggests that we retain our ancient capacity for strength - if only we work our bodies hard enough - and stronger bones mean fewer fractures. Broken hips were less common in the past, and are vanishingly rare in archaeological specimens, even accounting for the fact that lives were shorter then.
Civilisation is changing not only our physical features but also the size of our families, which alters women's hormone levels. Female hunter-gatherers would typically have had six or seven children and spent much of their adult lives pregnant or breastfeeding, both of which cut oestrogen exposure. In the west we have smaller families and it is now rare to breastfeed for more than a few months. Obesity, lack of exercise, the contraceptive pill and hormone replacement therapy also raise oestrogen levels. "For many reasons modern women are exposed to enormous amounts of oestrogen," says Israel Hershkovitz of Tel Aviv University in Israel. That is thought to be the main reason women today have a 1 in 8 chance of developing breast cancer over their lifetime.
Breast tissue does not fossilise, but there is a way that hormone levels can be tracked through history. Prolonged oestrogen exposure is thought to cause thickening of the skull on the inside, just above the eyes.
Using medical school collections, Hershkovitz's group has measured nearly 1000 skulls of women who were alive 100 years ago. The team also ran CAT scans on 400 living women. In a paper soon to be published in the American Journal of Human Biology, Hershkovitz and colleagues report this thickening to be 50 per cent more common than it was a century ago. Among women in their 30s, the prevalence has nearly quadrupled from 11 to 40 per cent.
There are other physical changes that are more mysterious in origin. We seem to have acquired a new blood vessel in our arms, called the median artery. In fact, this blood vessel is present in the embryo but according to textbooks it normally dwindles and vanishes around the eighth week of pregnancy, to be replaced by the ulnar and radial arteries. An increasing number of adults now have this artery, up from 10 per cent at the beginning of the 20th century to 30 per cent at the end.
Over the same period, a section of the aorta lost a branch that is one of several supplying the thyroid gland. One of those who has helped to document these changes is Rühli's former teacher Maciej Henneberg, now an anatomist at the University of Adelaide in South Australia. They could be due to differences in the diet and lifestyle of pregnant mothers, he speculates, or perhaps a relaxing of the forces of natural selection, thanks to modern medicine and welfare systems.
It is these kinds of uncertainties that leave some practising doctors less than impressed with the buzz around evolutionary medicine. We cannot know if evolutionary explanations are true, since we rarely have a complete picture of the past, points out retired American family physician Harriet Hall, who blogs as The SkepDoc. "Conventional medicine has a long record of successes," she says. "Evolutionary medicine hasn't proven that it has any real value."
That's not to say that documenting trends over time isn't useful, even if it only corrects the textbooks. If the arteries of a 20-year-old differ from those of a 90-year-old, it could affect how they should be treated medically. At the very least, surgeons need to know in order to operate safely.
Even our fingerprints have been changing over time. Henneberg's team took prints from 115 bodies donated to the University of Cape Town in South Africa (Perspectives in Human Biology, vol 4, p 229). They divided them into two groups: those who were born before 1920 and those born later. There were significant differences in their patterns: simple arches, tented arches and whorls were more common in the later group, and ulnar loops less so.
Fingerprints may seem like a trivial sort of change but one of Rühli's next projects may shed light on the origin of a serious disease. In the world's malarial zones a number of mutations have arisen that persist in the gene pool despite causing serious diseases, because they protect their carriers from malaria. One of these affects an enzyme in red blood cells called glucose-6-phosphate dehydrogenase (G6PD), and those who carry two copies of the mutation have a serious form of anaemia.
By studying ancient miners, Rühli's team hopes to narrow the window in time when the G6PD mutation could have arisen. On two occasions - once in 500 BC and once in AD 500 - a salt mine collapsed in what is now Iran, burying those working there and preserving their flesh. That has given us two samples from the same place preserved in the same way, 1000 years apart. Rühli's team is trying to get usable DNA from the salt mummies.
Sometimes these looks into the past shed light on our future risk of disease. One such case involves spina bifida, the birth defect that causes paralysis of varying severity depending on how high up the spine is affected. It happens when the embryo's neural tube, which develops into the spine and brain, fails to close up properly, leaving gaps in one or more vertebrae.
The incidence of spina bifida has been falling over the past couple of decades in most western countries, thanks to campaigns persuading pregnant women to increase their folic acid intake. But this could be obscuring a longer-term trend in the opposite direction.
There is a much milder and commoner form, known as spina bifida occulta, where the only affected vertebrae are in the sacral region at the bottom of the back, which stretches down from S1 to S5 (see diagram). Most affected people have no outward sign and don't even know they have it, although there is some evidence linking the condition with back pain and some rarer health problems.
There is now an array of evidence that spina bifida occulta has become more common. Some comes from work on human remains found by Henneberg at Pompeii, the Roman city buried when mount Vesuvius erupted in AD 79, in a project led by his wife, Renata Henneberg. Pompeii has provided a wealth of information since excavations began in the 18th century. "You find families trapped together with the mother trying to protect the children," says Maciej Henneberg. "These were the last moments of real people, frozen in time."
The Hennebergs looked at the rate of spina bifida occulta among the Pompeians. About 10 per cent had an unclosed S1 vertebra, compared with an estimated 20 per cent of people today. Vertebrae lower down the spine are now even more likely to be open. The bottom-most one, S5, was open in about 90 per cent of Pompeians, compared with nearly 100 per cent in people alive today.
True spina bifida is so rare it is impossible to gauge its past prevalence with any accuracy. When we look at spina bifida occulta, however, the spine as a whole seems to be taking on a more open structure - although folic acid may be able to counter this trend. What's behind the change? Henneberg thinks one possible explanation is the long-term gracilisation of the skeleton. Changes to genes controlling the long bones in the legs and arms could have knock-on effects on the spine. "There's less bone everywhere in the body," he says.
A different explanation is, again, that selection pressures on humans are easing. "There's no question that it's relaxing," says Henneberg. One hundred years ago, one-third of children died before the age of 5. Now practically everyone who's born survives."
This kind of talk often comes with dire warnings about weakening the human race, and in the past has triggered the eugenics movement. But today's evolutionists seem sanguine about the future. Rühli believes less selection pressure is not necessarily bad for a species' survival. "The more a [population] is under environmental pressure, the more it narrows the variability," he says. With humans today: "We see a higher degree of variability within the body. An increase in variability may be good."
Even if that means a rise in conditions like spina bifida occulta? If food or minerals become scarce in future, it might be better to have spindly bones, says Henneberg. "Who knows what the future might hold for the human race."