Humans like us have existed for around 200,000 years -- moving from hunter-gathers to early settlements to the large-scale civilization we know today. And how much evolution has occurred in our species over that time span... or even over the past few of thousand years? Most people seem to assume that there hasn't been much evolution (if any) at all and that humans have essentially stopped evolving -- but nothing could be further from the truth.
Humans are changing and many changes are recent enough that they can be documented in great detail. The real questions lie in how much of the change has been genetic and genuine examples of microevolution and how much has been natural variation due to ambient circumstances. People getting heavier would be an example of the latter, but we do have cases of the former as well.
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. ...
A less well-known trend is that we have been becoming less muscular, almost certainty 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.
Christopher Ruff of the Johns Hopkins University School of Medicine in Baltimore, Maryland, has travelled the world taX-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 percent reduction over a mere 4000 years.
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."
Source: New Scientist, March 19, 2011
What's significant about all of this is the extent to which human civilization itself is propelling human evolution. In any species, evolution is driven by the need to adapt to the environment, but humans are unique in the extent to which we shape our own environments and thus the extent to which we shape our own evolution -- though we are largely ignorant of what we are doing and of the consequences of our actions.
Not all of this is an improvement for us. Women, for example, are not having as many children as they used to and this is having an impact on breast cancer rates. Women today are exposed to far more estrogen than our hunter-gatherer ancestors.
We can't check fossilized breasts, but we can measure the thickness of the skull just above the eyes -- something that is determined by estrogen levels. Today the thickening of that are of women's skulls is 50% more common than it was just 100 year ago. That indicates lower levels of estrogen today than in the past and that, in turn, is a sign of higher breast cancer rates today than in the past.
There are other physical changes that are more mysterious in origin. We seem to have acquired anew 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 percent 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 Riihli'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.
And it gets worse. You may have heard of spina bifida, a very serious condition that leads to an early death of an infant. Well, there is a more mild condition known as spina bifida occulta which is basically the same problem, but one which most never realize they have. The affected vertebrae are at the very bottom of the spine, so there are no clear problems until later in life when there is some back pain...
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 AD79, 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 percent had an unclosed 51 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, SS. was open in about 90 per cent of Pompeians, compared with nearly 100 percent in people alive today.
Researchers think that this is connected to a general weakening of the bones overall. But whereas vigorous exercise can cause a person's arm or leg bones to thicken up to the same level as our hunter-gatherer ancestors, nothing will cause your spine to get reconstructed on its own. The former is more a matter of environment; the latter is more genetic. So we have two similar and related issues that actually vary in terms of how much they are produced by environment and genetics.
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