Are humans still evolving? Have medical advances and the luxuries of modern life removed the pressures that natural selection might exert and so remove the driver for humans to change from generation to generation?
Several examples can be pointed to which indicate that we humans are still evolving from increasing head size to the reduction in frequency of wisdom teeth. And a new example has just been discovered!
Researchers at Flinders University and the University of Adelaide (both in Adelaide, Australia) have discovered that some adult humans today have three arteries that bring blood to their forearm and hand, while the rest have only two. And the percentage of the population that has three is increasing. These results were published in the Journal of Anatomy in September 2020.
We all had three arteries in our forearms when in utero which are called the ulnar artery, the radial artery, and the median artery. But most adults only have two because the median artery (the one in the middle) generally disappears as a fetus develops. However, sometimes it never disappears, and adults will have a functioning median artery. This variation in development is where the researchers focused their attention.
Anatomists (people who study anatomy) have noted that the median artery disappears sometimes, but not always, for well over a hundred years. Some of these anatomists examined adults born in the 1880s and noted that about 10 percent of them still had their median artery and 90 percent has lost theirs. Researchers from those two universities in Australia mentioned above read these reports and wondered if those numbers had changed. They examined the forearms of adults born in the late 1900s and found that about 30 percent retained their median artery while the percent that lost their median artery was down to 70 percent.
Looking at those numbers, it might not seem very dramatic, but a 20 percent difference over just a few generations is pretty significant in evolutionary terms. If this rate of change continues, everyone born 80 years or more from now will have a median artery as adults. When most people have a median artery as adults, medical standards will need to change. At that point, having a median artery will be the normal condition, and no longer be considered a variant as it is today.
No one is sure why this trait is being favored. Having three arteries that feed the hand does allow for increased blood supply. While it might be tempting to try and explain this change as a reflection of how important hands are to our survival, that idea is not supported by any data. In fact, hands have been important for human survival since before we evolved into humans, so even the idea is a bit weak. Whatever pressures are causing natural selection to favor the retention of a median artery into adulthood is an interesting question that stands open.
This example of the median artery shows that humans are still evolving and that natural selection continues to influence our species even if we don’t understand why it may be occurring.
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