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.
Today's bloody brilliant fact: 35% of humans are born with vestigial muscles in the top lip that allow for mammalian whisker-wiggling. pic.twitter.com/hsymmituzD
— Jules Howard (@juleslhoward) June 19, 2017
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.
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.