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Biology

Increasing occurrence of an extra arm artery shows evolution at work

By Rich Haridy
October 11, 2020
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A new study suggests an increasing prevalence of a third forearm artery in humans over the past couple of hundred years
piotr_marcinski/Depositphotos
A sketch showing where the median artery can be found in a human forearm
Maciej Henneberg
A new study suggests an increasing prevalence of a third forearm artery in humans over the past couple of hundred years
piotr_marcinski/Depositphotos

A human fetus transitions through a number of developmental stages in the womb. Some of these involve ancient traits, such as tails or reptilian muscles, appearing and then disappearing. While others are more temporary embryonic structures that regress once their permanent replacements develop.

The median artery is one of those temporary embryonic structures. It runs down the human forearm but usually begins to regress, or disappear, around the eighth week of gestation as the two primary forearm arteries (the radial and ulnar arteries) begin to develop.

In a small volume of people the median artery does not regress. Persisting after birth, this third forearm artery is always present in a minority of people. But, a new study tracking the prevalence of the median artery in humans over the past 150 years is suggesting more and more people seem to be holding on to this embryonic relic.

“Since the 18th century, anatomists have been studying the prevalence of this artery in adults and our study shows it’s clearly increasing,” says corresponding author on the new study Teghan Lucas, from Flinders University. “The prevalence was around 10 percent in people born in the mid-1880s compared to 30 percent in those born in the late 20th century, so that’s a significant increase in a fairly short period of time, when it comes to evolution.”

To come to this conclusion, the researchers conducted an expansive literature review to establish the prevalence of the median artery in adults at different time periods over the past couple of centuries. The researchers also examined 78 upper limbs from recently deceased subjects.

A sketch showing where the median artery can be found in a human forearm
Maciej Henneberg

The study concludes there seems to be an increasing prevalence of the median artery in adults, and it is suggested this is a perfect example of natural selection guiding micro evolution in human beings.

“This is micro evolution in modern humans and the median artery is a perfect example of how we’re still evolving because people born more recently have a higher prevalence of this artery when compared to humans from previous generations,” says co-author Maciej Henneberg.

It’s unclear at this stage exactly what kinds of benefits may be conferred by retaining a functional median artery into adulthood. The additional blood flow could aid hand dexterity, or it could be advantageous in maintaining blood flow following damage to another artery in the forearm.

But, the researchers also hypothesize potential problems from a persistent median artery. Some prior studies have suggested the presence of the median artery can increase one’s chances of developing carpal tunnel syndrome.

“This increase could have resulted from mutations of genes involved in median artery development or health problems in mothers during pregnancy, or both actually,” notes Lucas. “If this trend continues, a majority of people will have median artery of the forearm by 2100.”

The new study was published in the Journal of Anatomy.

Source: Flinders University

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Rich Haridy
Rich has written for a number of online and print publications over the last decade while also acting as film critic for several radio broadcasters and podcasts. His interests focus on psychedelic science, new media, and science oddities. Rich completed his Masters degree in the Arts back in 2013 before joining New Atlas in 2016.
11 comments
ScienceFan
You can’t have so many random mutations in such a short time to go from 10% to 30% in 130 years. Must be a different mechanism. Could be food, could be higher likelihood of early childhood death in the past, could be environment/exposure. If not through methylation this is probably actually a step backwards. Cause and effect are also unclear. There may well be an underlying mechanism that has consequences other than survival chances because of changes in the forearm.
Brian M
as per ScienceFan's comment - Article doesn't say how natural selection is working here, its highly unlikely that those with the median artery get a lot better chance of surviving and reproducing than those that don't. So be interesting to to know what mechanism is occurring here, environmental? If so, is the mechanism causing other mutations?
JohnAshley
This trait had to begin somewhere and spread from there... most likely due to the greater intermingling of populations since the start oof the industrial age.
Username
Survival of the fittest as long been abandoned in our species, so has nay kind of selection. Humans are not very particular about who they are having children with. Like in the movie Idiocracy, the uneducated, unhealthy and un-intelligent reproduce at a grater rate. This new mutation is most likely a function of no selection.
Cody
If this is indeed an inherited genetic feature, and one where to investigate the changes in the genes involved, I would predict that this is the result of a degenerative mutation, in other words, a loss of genetic specificity. It is also unclear how natural selection could select for this particular mutation. However, if this is caused by a loss of information and is inherited, it is clearer how it could be increasing in the population regardless of whether or not it increases fitness.
Cody
In addition to my previous comment*

Similar to how you can breed wolves to chihuahuas but you can't breed chihuahuas back to wolves because selective breeding to chihuahuas involves a loss of genetic information.
Pablo
Now if we could just manage a few extras around the old ticker, we might corral heart disease...
anthony88
At such a rate, I predict humans will have extra thumbs in 150 years from 29 June, 2007.
Worzel
From ultrasound scans, I have been told that I have THREE kidneys, which although unusual is not unknown. What genetic advantage does this give ME? Well, one thing, is that I can, apparently, process alcohol more efficiently. I once drank a full bottle of whisky in an evening with NO noticeable ill effects whatsoever. Even when I do drink too much too fast, any effects disappear very rapidly. I've only ever had one hangover, and that was from mixing the grain and the grape, brandy and whisky combined, and in my early youth, one event that got me 'paralytic' and that took about a litre of mixed spirits, within a couple of hours, but no hangover! My paternal grandfather died from cirrhosis, so maybe that triggered my genetics to improve my alcohol processing. Who Knows? It's been shown, that in a Finnish village that suffered a famine, the grandchildren, and onwards still showed the effects in their genome 'tabs'. 'Tabs' (my term) are bits that used to be regarded as 'junk' until research and intelligence proved otherwise. I think the human body is far more complex than even most researchers realise, and the return of a past characteristic, is probably due to unnoticed environmental factors. After all, life has had hundreds of millions of years to develop. Human research into it, only maybe thousand or so.
foxpup
When an animal is domesticated, youthful traits tend to persist throughout its life. People use breeding selection to domesticate animals and we are doing the same, not so much by deliberate and controvercial ideas like eugenics, but we do it, nevertheless, unintentionally, by creating environments where people are more (or less) likely to procreate, based on variouis inherited traits. People who mature quickly and completely are more likely to think for themselves and cause trouble, which is discouraged in the "rat-race". People are encouraged to try to be as young as you can be for as long as you can at the expense of needed maturation. Then there's Hollywood's obsession with "little girls". Yea, there's a eugenics program going on, implemented by a mob of people and companies, not so much with any central control....taming the sheeple.
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