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Austin Cline

Ongoing Human Evolution

By January 19, 2012

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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 higher 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 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 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.

Comments
January 19, 2012 at 3:17 pm
(1) Eric O says:

For quite a while, I’ve been fascinated by biocultural evolution – genetic change in a population that can be linked to cultural practices.

The development of the sickle-cell gene is another example of “recent” human evolution. The prevailing theory is that when humans started cutting down trees in tropical areas for the purposes of creating agricultural land, we unintentionally created a habitat where malaria-carrying mosquitos could thrive. Widespread malaria led to selective pressure on the local human population; people with the sickle-cell gene were more resistant to the parasite and so these people tended to be more reproductively successful. Of course, there are well-known drawbacks of the sickle-cell gene, but in places where malaria is endemic, the benefits outweigh the costs.

January 20, 2012 at 5:33 pm
(2) Cousin Ricky says:

I believe that the “51″ in the quote should be “S1″ (ess one), and the “SS” should be “S5″ (ess five).

January 20, 2012 at 6:49 pm
(3) Ricardo says:

“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. ”

In fact, lower levels of estrogen are linked to a lower incidence of breast cancer.

January 22, 2012 at 10:23 am
(4) Austin Cline says:

You’re right, I typed that in incorrectly…

January 20, 2012 at 6:58 pm
(5) OZAtheist says:

An interesting article Austin which should give evolution denyers pause if they bother to read it.

One of the most common retorts we get from creationists when it is suggested that we evolved from other species is – “Why are we no longer evolving then”? This statement is usually delivered in an emphatic and triumphant way as if it is an undeniable truth without need of verification. I have been at the receiving end of these words many times.

I, of course, challenge the veracity of the above assertion, and go on at some length about the evidence supporting evolution that exists inside our own bodies – a little googling provides lists of these indicators. The above article now gives me some more material to use.

A sobering thought has occurred to me a number of times when contemplating the future of our species and what evolution may produce in the future. Whereas evolution is the mechanism that ensures the survival of the fittest, modern medicine ensures the survival of the not so fit, and thus works in the opposite direction.

I would love to be able to gaze into the future and see what the consequence of this progression is. The high standard of morality that the majority of us humans have, (those who’s morality is not compromised by religious belief that is), prohibits us from denying help to those that need it so we are to use a common expression; “stuck between a rock and a hard place”.

January 20, 2012 at 9:14 pm
(6) kathryn says:

This is very interesting, and worrisome. Are we devolving?

Another related phenomenon is the early onset of puberty in girls and delayed onset of puberty in boys. It’s suspected to be linked to obesity, and to hormone-mimicking chemicals in plastics. BTW, these chemicals are also affecting fish and other animals. Do such hormonal changes alter the DNA, so that they are passed on to offspring?

January 23, 2012 at 11:51 am
(7) P Smith says:

OZAtheist says: “One of the most common retorts we get from creationists when it is suggested that we evolved from other species is – ‘Why are we no longer evolving then’?”

Korean, Chinese and some other Asian languages use the same word for blue and green, which means the inability to differentiate colurs goes back hundreds or even a thousand years. But the Japanese, who mixed with the Ainu from Kamchatka (and diverged from those who entered the Americas, carrying the Haplogroup C gene) do have distinct words for blue (aoi) and green (midori).

As well, the ancient Greeks described colours differently than you or I, such as the sky. Colour blindness is most common in Caucasians (especially men), second most in Asians, least in Africans. The Mongols spread their genes (again, Haplogroup C) across Eurasian to the Urals, but not much further west during their conquests of the 13th century.

http://serendip.brynmawr.edu/exchange/node/61

http://www.omim.org/entry/303800?search=color%20blindness&highlight=color%20blindness%20colour

http://www.hhmi.org/senses/b130.html

That barely qualifies as an idea, not a hypothesis, and never mind “proof”. But evolution and isolation as a cause of colour blindness, and genetic mixing as a reduction of it, makes more sense as an explanation for colour recognition than “godidit”.

OZAtheist says: “This statement is usually delivered in an emphatic and triumphant way as if it is an undeniable truth without need of verification.”

It’s delivered with the same stupid snootiness as “Were you there?” Those who know the least assume they know much, and those who know much admit they still have more to learn.

———-

kathryn says: “This is very interesting, and worrisome. Are we devolving?”

Don’t you know Mark Mothersbaugh and the boys proved this 30 years ago?

http://shorttext.com/QLxWKzQ
http://www.youtube.com/watch?v=62lN2NQsHPw

.

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