Category: anatomy

Evolution of a human artery in modern times?

January 20, 2025 • 11:20 am

This article, published in the Journal of Anatomy four years ago, was also highlighted in ScienceAlert this January 18, which is how Matthew Cobb found it.  And although the results aren’t new, I find them interesting from an evolutionary point of view and sure didn’t know about them before. (I’m not sure why ScienceAlert chose to highlight them this week.)

The paper (and the shorter popular summary) describes an Australian study of a variable trait: an extra artery in the forearm and hands of humans called “the median artery”.  It is present in fetuses, where it feeds the growing arm and hand, but regresses during development so that it’s not usually present in newborns. However, in a substantial number of cases—now about 30%—it remains as a functioning artery in adults.  The paper describes a present study of the incidence of this “vestigial artery” in modern adult Australians, and compares this incidence with that seen in adults going back to the late 19th century. There has been a marked increase in persistence—threefold!—over that period.  What we don’t know is why this is happening.  It could be strong natural selection, an environmental change we don’t understand, or both.

You can see the paper by clicking on the title below, or download a pdf here.

First, here’s what the artery looks like in an adult (caption from the paper). I’ve put a red oval around the artery:

Median artery and superficial palmar arch (anterior dissection of the left lower forearm, wrist and hand) – Median artery accompanied the median nerve and completed the superficial palmar arch laterally.

Now although the artery feeds the arm and hand, we don’t know whether it actually benefits those who have it.  The authors and ScienceAlert appear to favor natural selection as the reason for the increase over time, but we don’t know that. To know for sure, we’d have to do long-term studies of the reproductive output of individuals having the artery versus those lacking it, or perhaps genetic studies (see below). We don’t have that data and therefore cannot say anything about natural selection.

Further, perhaps its increased persistence into adulthood is due to some environmental effect. We have no data on that, either. All we can say, and we can’t even say that with a high degree of confidence, is that the percentage of adults having the artery seems to have increased drastically over time.

But I’m getting ahead of myself. The authors dissected 78 arms of Australians aged from 51 to 101 years who died between 2015 and 2016, determining how many of them had the persisting median artery.  Individuals were excluded who might have skewed the studies, including individuals with only the hands and not arms examined, people who had carpal tunnel syndrome (possibly caused by persistence of the artery), and examinations using angiography, which has a greater ability to detect arteries.  Exactly a third of adults (33.3%) showed the artery.

The authors then went back and scoured the literature, using data on adults from 47 published papers going back to 1897. Using data from that arms in individuals who died at a known age, we have a dataset of individuals born from about 1846 to 1997—a span of roughly 150 years, or about 5 human generations.  That’s a remarkably short span of time from an evolutionary viewpoint.

Nevertheless, they found a significant increase over this period of the proportion of individuals having a median artery nearly tripled—from about 10% to 30%. Here’s the most relevant graph plotting the percentage of individuals showing the artery as adults born between 1880 and 2000. (There’s considerable scatter because sample sizes at each date are small.). The authors gives a probability of less than 0.0001 that this temporal trend would be due to chance, so it’s highly statistically significant (they don’t specify whether they’re testing the regression coefficient or the correlation coefficient, but it doesn’t really matter with p values that low.

They also extrapolate this trend and say that one “could predict that the median artery will be present in 100% of individuals born in the year 2100 or later.”  It will then no longer be a persisting fetal trait, but a trait that persists throughout life, and the persisting adult trait could no longer be seen as “vestigial”, like persisting wisdom teeth in some people.

The authors do suggest that environmental factors could play a role in this increase, but also that it could be due to natural selection. Such selection, to cause such a strong change in just a few generations, would have to be strong! The ScienceAlert article plays up the selection part, saying this:

“This increase could have resulted from mutations of genes involved in median artery development or health problems in mothers during pregnancy, or both actually,” said Lucas.

We might imagine having a persistent median artery could give dexterous fingers or strong forearms a dependable boost of blood long after we’re born. Yet having one also puts us at a greater risk of carpal tunnel syndrome, an uncomfortable condition that makes us less able to use our hands.

Nailing down the kinds of factors that play a major role in the processes selecting for a persistent median artery will require a lot more sleuthing.

Indeed, a TON of more sleuthing. What would be required to show selection would be either or both of two things:

1.) Show that, over a long period of time, individuals with median arteries as adults leave more offspring than individuals lacking these arteries. This is how the Framingham Heart Study, which began in 1948, showed that there appeared to be natural selection in women for reduced height, increased stoutness, reduced total cholesterol levels, and lower systolic blood pressure. Further, there appears to have been selection for women to produce their first child earlier and to reach menopause later. This is what I tell people who ask me, as they inevitably do when I lecture on human evolution, where our spercies is going. Not that exciting, is it? But of course the time span of such studies are necessarily limited.

2.) Find the genes responsible for the persistence of the artery and show, by population-genetic analysis, that those genes leading to persistence have been undergoing positive selection. This would be even harder because we have no idea what those genes are.

Absent those two types of studies, all we can say is that we have a putative case of evolution occurring over a short period of human evolution.

Caveats: The authors offer these caveats, and I have one more:

Limitations of the present study include the fact that the number of whole cadavers that were available for the study was not adequate. In addition, our search of the literature may have missed some publications not listed in Google Scholar. Finally, the definitions of ‘persistent median artery’ may have differed somewhat among the various published studies included in the present study.

Finally, as far as I can determine from looking at a few of the papers they cite in the older literature, the samples of arms came not just from Australia, but from other countries like Brazil and South Africa. Given that we know that at present populations from different places differ in the persistence of the artery, this could also throw some bias into the data. However, to create a time course this significant, I don’t think that using arms from different places could be the explanation, for it would require that arms from older people tended to come from places which had a lower incidence of the artery in general.

h/t: Matthew Cobb

You have vestigial muscles that moved the whiskers of your ancestors

June 19, 2017 • 9:30 am

This is the kind of post I envisioned writing—once every few weeks or so—when I started this website. My intention was to use the site to publicize new evidence for evolution. Not that we need any to show that that well evidenced theory is true, of course, but to support the book and alert people to cool new findings. But, as I’ve said, things got out of hand, and so we have cats, food, travels, religion, and so on. So let’s go back to our roots today. . .

Matthew, on the job as always, sent me this tw**t and asked me if I’d mentioned this in the “vestigial structures” section of Why Evolution is True.

I told him I hadn’t even heard of this, but, sure enough, I found the following in the “Human vestigiality” article in Wikipedia (worth looking at):

In many non-human mammals, the upper lip and sinus area is associated with whiskers or vibrissae which serve a sensory function. In humans, these whiskers do not exist but there are still sporadic cases where elements of the associated vibrissal capsular muscles or sinus hair muscles can be found. Based on histological studies of the upper lips of 20 cadavers, Tamatsu et al. found that structures resembling such muscles were present in 35% (7/20) of their specimens.[50]

Naturally I went to the cited source: a short paper by Yuichi Tamatsu et al. in Clinical Anatomy in 2007 (reference and link below; free access if you have the legal Unpaywall application). That paper shows what to me (and I’m not an anatomist) looks like vestigial muscles that are the remnants of muscles that move the whiskers in our mammalian relatives—and in our whiskered ancestors.

Mammalian whiskers are called “vibrissae” and most are are movable (their function is at least partly tactile–touch–though they may have other functions). As for how and why they move, here’s the Wikipedia entry:

The follicles of some groups of vibrissae in some species are motile. Generally, the supraorbital, genal and macrovibrissae are motile, whereas the microvibrissae are not. This is reflected in anatomical reports that have identified musculature associated with the macrovibrissae that is absent for the microvibrissae. A small muscle ‘sling’ is attached to each macrovibrissa and can move it more-or-less independently of the others, whilst larger muscles in the surrounding tissue move many or all of the macrovibrissae together.

Amongst those species with motile macrovibrissae, some (rats, mice, flying squirrels, gerbils, chincillas [sic], hamsters, shrews, porcupines, opossums) move them back and forth periodically in a movement known as whisking, while other species (cats, dogs, racoons, pandas) do not appear to. The distribution of mechanoreceptor types in the whisker follicle differs between rats and cats, which may correspond to this difference in the way they are used. Whisking movements are amongst the fastest produced by mammals. In all whisking animals in which it has so far been measured, these whisking movements are rapidly controlled in response to behavioural and environmental conditions. The whisking movements occur in bouts of variable duration, and at rates between 3 and 25 whisks/second. Movements of the whiskers are closely co-ordinated with those of the head and body.

You might remember that we evolved from a shrewlike ancestor, and thus probably an ancestor that could move its whiskers.

In their paper, Tamatsu et al contrast the whisker muscles (muscles of the “sinus hairs”, another name for vibrissae) with those of regular body hairs. The latter have smooth “arrector pili” muscles that can erect each hair involuntarily during times of cold or fear, giving us goose bumps. As I note in WEIT, these are probably vestigial in humans, as we have no need to look bigger by erecting our hairs (versus cats, who bush out when they’re threatened), and erecting our body hair in the cold doesn’t provide much thermal insulation since we’re “naked apes”. Arrector pili appear to be remnants from mammalian ancestors who could really use these muscles adaptively, and thus they give testimony to our evolution.

Here’s a drawing showing the arrector pilus, the orange-red band affixed to the hair follicle at center left:In contrast, whiskers are attached to special “capsular muscles” and can be moved voluntarily; unlike the smooth arrector pili, they are striated, or “striped”, as voluntary muscles are.

Tamatsu et al. looked for these capsular muscles by dissecting 20 cadavers (11 males and 9 females) and doing scanning electron microscopy of sections of the upper lip.  They found what looked like capsular “whisker muscles” in 4 males and 3 females, or 35% of the sample. I won’t go into detail, but will just show a few of the photos they present as evidence, along with their captions (indented). Note the striated muscle in (b), which you can see better two pictures down:

Sections through the upper lip. a: Light micrograph of a section of the upper lip region of a 76-yearold female. The right side of the figure is medial and the left side is lateral. Arrows indicate the location and direction of muscle fascicles. The fascicles diverge from the orbicularis oris layer and course to the dermis (D). SC, subcutaneous tissue; ML, muscle layer. Azan-Mallory stain. b: Magnified light micrograph of the rectangular, outlined area shown in (a). The arrows indicate the course of a muscle fiber having a striated pattern and collagen fibers. This bundle courses to a hair follicle (HF). [Scale: bar = 1 mm in (a) and 0.5 mm in (b)]

Higher magnification light micrograph (c) and SEM micrograph (d) of the rectangular, outlined area shown in (b). Arrows indicate an attachment area between the hair follicle and collagen fibers continued from the muscle fiber located in the subcutaneous tissue. [Scale bars: 250 microns in both photos]

Here the striated muscle is seen clearly:

High magnification light micrograph of the small rectangular outlined area shown in (b). A striated pattern can be seen in the muscle fiber. Scale bar = 25 microns

The authors conclude that these muscles are similar to those that move the whiskers in mammals that have them:

. . . in the sections that displayed vertical sections of hair follicles it was observed that these muscle and collagen fascicles surrounded the outer half of a hair follicle. This configuration is different from arrector pili muscles of body hairs that attach to the follicle at a single point, but shows similarity to muscle slings of mystacial vibrissae reported by Dorf (1982). According to his report, muscle slings of mystacial vibrissae embraced the follicles. Furthermore, we found many blood vessels containing blood cells near these follicles. Because of their thin walls and large diameter, these blood vessels were assumed to be veins, which are known to be associated with sinus hair follicles. Our findings of the structural characteristics of these muscles and follicles, which bear similarities to those of sinus hairs, led us to conclude that the observed muscle fascicles are a vestigial muscle of sinus hair. A 35% incidence supports this conclusion, given that regressive organs do not exist in all individuals.

Vestigial structures like wisdom teeth and ear-moving muscles often are missing in many individuals—one of the signs that the feature is of no or little use.  Now you’re probably asking yourself, if you’re a man, “If I had a mustache, could I move it if I were one of the individuals that had these muscles?” Or, if you have a mustache, you’ve probably already tried to move it as you read this. But no dice, for a mustache is not the equivalent of mammalian whiskers. A ‘stache is made of regular body hair and thus, while it could be moved by arrector pili, its hairs cannot be moved voluntarily.

Let’s take the finding of Tamatsu et al. as tentative but very suggestive, as the authors knew what kind of muscle to look for and found it. And we have no vibrissae. And the muscle is present in only a fraction of individuals, and in females, too. This looks to be an anatomical remnant of our whiskery ancestry: a vestigial trait that testifies to the fact of evolution.

____________

Tamatsu, Y.; K. Tsukahara, Kazue; M. Hotta and K. Shimada. 2007. “Vestiges of vibrissal capsular muscles exist in the human upper lip“. Clinical Anatomy. 20(6): 628–31.