Our muscles can vary a lot from person to person, in terms of size, shape, structure. Some people even have muscles that are completely missing in others! The muscles that exhibit the most extreme variation are often mostly useless, having been important in our evolutionary past but no longer serving any real function. As such they can disappear with no real consequence, and may eventually become extinct.
But all of this variation doesn’t appear to be random. Some researchers have examined four muscles in the hand and arm and they found that the changes they are undergoing appear to be an example of evolution in action. One appears to be developing a new, beneficial form. Another seems to be taking on a new function and the other two seem to be being deliberately selected against. Our arm is continuing to evolve, and here’s how.
Flexor digitorum supeficialis-V
The first of these evolving muscles they identified was the FDS-V. This muscle is attached to the little finger and helps – as the name suggests – flex it (flexion meaning contract). It’s missing in about 20% of people and some only have it in one arm! Those without the muscle don’t have a weaker grip than those with it and having bulky muscles in the hands could interfere with our dexterity. Given all this, it seems obvious why it is beginning to disappear in humans.
However, these researchers noted that 13% of people have variants of the FDS-V. Although the regular FDS-V may offer no advantage when compared to people without it, they found that some of these variants did improve the strength of their grip. So if the fifth finger is used a lot by modern humans natural selection may favour these variants of the muscle, rather than removing it entirely. The researchers pointed to typing as an example of a modern activity where the little finger has to do almost as much work as the others.
So if typing begins to influence our chances of reproduction, natural selection would favour these variants of the FDS-V
The anconeous is a muscle in the elbow that has no abbreviation, so I have to type it out in full each time (thus giving my FDS-V more work). It helps extend the elbow joint, but the triceps is the main muscle which does this. As such the anconeous brings nothing to the table. In some species it does seem to help stabilise the elbow joint, but that’s a joint which is naturally quite stable in modern humans. In other words, it appears to be a rather useless model.
So we might expect it to begin to disappear, like the FSD-V? Not quite. The authors noted that the elbow joint isn’t as stable as normal whilst crawling. Thus the anconeous may be a crucial muscle for helping babies move around. So we’ll keep it, even though it serves no real function in adults (although you could always start crawling around, just to get a ripped anconeous). Unfortunately there have been few studies of infant crawling, so the ultimate fate of the anconneous is still a mystery
Palmaris longus & anconeus sextus
These are the two muscles which the researchers believe are being removed from the body via natural selection as neither appears to serve any real function. As a result they are often completely missing, and when they are present they are in a very reduced, weak form when compared to most other mammals.
The palmaris longus helps flex the wrist (remember, flexion means contract). However, a bunch of other muscles also flex the wrist (and do so with more power), rendering the palmaris longus redundant. As such surgeons often harvest it for tissue to help repair other muscles in the body. Assuming you have one, it’s missing in 3 – 60% of people (depending on ethnic group). The researchers behind all this recent hand research note that when it is present, it’s in a very small and weak state compared to other animals. As such they hypothesise it is on the way out
The anconeus sextus (AE) is pretty much the same story. Like the regular anconeous, it helps stabilise the elbow. However, it does so during extension, which isn’t an activity as important to baby crawling. As such it has no real redeeming value which would explain why it is absent in 70% of people!
The a stronger variant of the FDS-V might help our little finger grow type, the anconeus might stick around to help babies crawl but the palmaris longus and AE won’t. It would be interesting to look back at the frequency of these muscles in a few generations to see if these predictions hold true. After all, evolution waits for no man, woman or species.
Capdarest-Arest, N., Gonzalez, J. P., & Turker, T. (2014). Hypotheses for Ongoing Evolution of Muscles of the Upper Extremity. Medical Hypotheses.