Goosebumps

"The arrectores pili contract not only under the above emotions, but from the application of cold to the surface."  "We see the same action in our own goose-skin during the chill before a fever-fit." — Charles Darwin, The Expression of the Emotions in Man and Animals, 1872¹

"Goose bumps and the muscles that make them serve no useful function, at least in humans." — Jerry Coyne, Why Evolution Is True, 2009²

Summary

The parent article, Vestigial Structures, separates the dead claim that a vestige is useless from the live claim that it is a reduced copy of something fuller in an ancestor, and gives the live claim three steps: homology, reduction, and an ancestral distribution on a tree built from other characters.  This article runs that test on the smallest muscle in the list.

• What they are. Gray's Anatomy of 1918 describes "minute bundles of involuntary muscular fibers, termed the Arrectores pilorum," attached to the hair follicles, which "diminish the obliquity of the follicle and elevate the hair."³ They are smooth muscle, fired by the sympathetic nervous system, on two triggers: cold and strong emotion.⁴
• The two claims. Coyne writes that goosebumps "serve no useful function, at least in humans," and in the same paragraph that "our vestigial goose bumps are produced by exactly the same stimuli" as a cat's bristling fur.² The first is the useless claim; the second is the homology claim. Coyne's own definition says a trait "can be vestigial and functional at the same time,"² so a function answers only the first.
• The muscle has a job. In 2020 Yulia Shwartz and colleagues showed in mice that "APMs and sympathetic nerves form a dual component niche" for hair-follicle stem cells, that the muscle holds the nerve in place, and that two weeks of cold drove new hair growth as well as goosebumps.⁵ A 2014 review found that in irreversible pattern baldness the muscle detaches from the shrinking follicles, though "it has not been defined whether APM regression in irreversible hair loss was a cause or effect of the disease."⁶
• The reflex's thermal value in people is unmeasured. A 2015 physiology review says heat conservation by human piloerection "is usually regarded as insignificant and rudimentary."⁴ No measurement was found in either direction, and the creationist claims that erect vellus hair "helps to conserve body heat" and that the contractions "generate heat" carry no number either.⁷
• Emotional goosebumps are real. Benedek and Kaernbach filmed the forearms of fifty students and recorded visible piloerection in 24% of film-audio trials and 11% of music trials, tied to "the state of being moved or touched."⁸⁸
• What is reduced is the hair, not the muscle. Kamberov and colleagues counted follicles in five skin regions of humans, chimpanzees and macaques in 2018: "average human and chimpanzee hair densities did not differ significantly in any region, despite the very different external appearances of these species."⁹ The difference is terminal hair against vellus hair, and the arrector pili attaches to vellus follicles too.⁶
• The creationist replies. Creation Ministries International, Menton and Mitchell list functions and answer the useless claim.¹⁰⁷ David DeWitt names the reduced-homologue definition and answers it: "there is no evidence that the goosebump response has been reduced from that of our human ancestors."¹¹ Fazale Rana, an old-earth creationist, cites the Cell paper and concludes that goosebumps are "produced as a by-product."¹²

Verdict: the arrector pili is not a vestigial muscle, and the reflex is not reduced either; the reduction is in the hair.  Homology is not in dispute; CMI's own reply speaks of "a common designer, using the 'goosebump concept' in many different creatures."⁷  The third step, whether vellus hair is the derived state of a once-furred lineage, is a question about human hairlessness that a reflex shared with every monkey and ape cannot answer.  Goosebumps are weak evidence for either side, with one residue the design side has not explained: why a fear reflex is wired to hair that cannot bristle.

Part 1: What goosebumps are

Almost every hair on the body sits in a follicle with a small band of smooth muscle attached to it.  Gray's Anatomy of 1918 puts the anatomy in two sentences: the arrectores pilorum "arise from the superficial layer of the corium, and are inserted into the hair follicle, below the entrance of the duct of the sebaceous gland.  They are placed on the side toward which the hair slopes, and by their action diminish the obliquity of the follicle and elevate the hair."³  Because the muscle pulls the follicle upright and tugs on the skin around it, the surface buckles into the bump that gives the reflex its name.

The muscle is not under voluntary control.  Tansey and Johnson's 2015 review for physiology students describes the circuit: "In response to increased sympathetic nerve discharge the arrector pili muscles at the base of tiny hairs in the skin contract and cause the hairs to become upright"; and the triggers: "Piloerection is a known reaction to cold and also strong emotional stimuli."⁴

Darwin knew all of this in 1872.  In The Expression of the Emotions he credits "Kölliker's interesting discovery" of "minute, unstriped, involuntary muscles, often called arrectores pili," and adds that "the vast number of these minute muscles over the whole body of a hairy quadruped is astonishing."¹  He had seen the cold trigger on his own animals: "my mules and dogs, brought from a lower and warmer country, after spending a night on the bleak Cordillera, had the hair all over their bodies as erect as under the greatest terror.  We see the same action in our own goose-skin during the chill before a fever-fit."¹

In a furred animal the reflex does two things: erect fur holds a thicker layer of still air against the skin, and it makes the animal look bigger.  Darwin opens his chapter with the second: the appendages "are erected under the excitement of anger or terror," and "the action serves to make the animal appear larger and more frightful to its enemies or rivals."¹  His first example is the nearest relative: the keeper at the London Zoo reported of the chimpanzee and orang that "when they are suddenly frightened, as by a thunderstorm, or when they are made angry, as by being teased, their hair becomes erect."¹

In a person the same circuit fires on the same two triggers, and the hairs it raises are mostly vellus: short, fine and unpigmented.  Torkamani and colleagues note that "the proximal end of the APM surrounds the whole follicle in the bulge region of both terminal and vellus hairs,"⁶ so the muscle is present on the fine hairs, and the bump appears whether or not there is a visible hair to raise.

Part 2: The two claims, kept separate

The vestige argument about goosebumps is two claims that are usually stated together.  Laura Spinney's 2008 New Scientist list, which both Answers in Genesis and Creation Ministries International answered, put it this way: "In birds or mammals with feathers, fur or spines, this creates a layer of insulating warm air in a cold snap, or a reason for a predator to think twice before attacking.  But human hair is so puny that it is incapable of either of these functions."¹³  Coyne's textbook statement a year later adds the second claim: in other mammals the muscles "raise the fur for insulation when it's cold, and cause the animal to look larger when it's making or receiving threats."²

Rana's 2020 reply for Reasons to Believe states the structure more clearly than most of its proponents: life scientists "see goosebumps as compelling evidence that human beings have an evolutionary origin because: (1) goosebumps serve no useful purpose in humans today and (2) the same physiological process that causes goosebumps in humans causes hair and fur of other animals to stand erect when they feel cold or threatened."¹²  Claim (1) is the useless claim; claim (2) is the homology claim, which on the parent's test needs homology, a reduction, and a derived state on a tree built from other characters.

The two claims are independent, and Coyne says so: "A trait can be vestigial and functional at the same time.  It is vestigial not because it's functionless, but because it no longer performs the function for which it evolved."²  So a function for the muscle does not touch claim (2); everything in Part 3 bears on claim (1) only.

Two historical notes.  Darwin did not call goosebumps a rudiment; his 1872 problem was different.  He read emotional hair-raising in animals "not as a power acquired for the sake of some advantage, but as an incidental result, at least to a large extent, of the sensorium being affected,"¹ and then confessed: "We here encounter a great difficulty.  How can the contraction of the unstriped and involuntary arrectores pili have been co-ordinated with that of various voluntary muscles for the same special purpose?"¹  His answer was mainly inherited habit, the idea that a nervous response strengthened in parents passes to offspring, which no one holds now.  He added natural selection only at the end:  "Nor must we overlook the part which variation and natural selection may have played".¹  And Wiedersheim's 1895 catalogue of human vestiges, the source of the "86" the parent article counts, does not list the muscle.  It lists the hair: "Man is the least hairy of all the Primates; indeed, his skin may be called almost smooth,"¹⁴ and in the conspectus, "The primitive hairy covering or lanugo.  Vestiges of vibrissæ."¹⁴  The words "arrector" and "pili" do not appear in the book.  The muscle joined the popular lists later, as a rider on the hair claim.

Part 3: What the muscle does in a person

The stem-cell niche

The finding that changed the textbook line came from Ya-Chieh Hsu's laboratory at Harvard in 2020, in mice, where the follicle, its muscle and its sympathetic nerve can be labelled and deleted one at a time.  Their summary: "we show that APMs and sympathetic nerves form a dual component niche to modulate hair follicle stem cell (HFSC) activity.  Sympathetic nerves form synapse-like structures with HFSCs and regulate HFSCs through norepinephrine, whereas APMs maintain sympathetic innervation to HFSCs."⁵  Remove the muscle and the nerve retracts from the stem cells.  The muscle is scaffolding for the nerve.

The scaffolding has a measurable effect on hair.  When the team killed the sympathetic nerve with a toxin, the muscles were unharmed but the hair cycle stalled:  "By P30, hair follicles reached full anagen throughout the control back skin, whereas hair follicles in the sympathectomized skin remained mostly in telogen."⁵  When they killed the muscle instead, with a toxin aimed at smooth-muscle cells, "the sympathetic innervation to HFSCs was lost concomitantly following APM ablation," and "Similar to sympathectomized mice, these APM-ablated mice also displayed a delay in anagen entry."⁵  That second test used three mice per group.  The authors draw the conclusion themselves:  "It is possible the APM is evolutionarily conserved due to its indispensable role as a hub to attract and maintain sympathetic innervations in the skin."⁵  On their reading the muscle is kept for the niche, and the goosebump comes with it.

They then asked whether cold, which fires the nerve, does more than raise the hair.  "Mice displayed the classical goosebumps reaction in response to cold (Figure 6D).  Moreover, mice exposed to cold entered anagen precociously to produce new hairs within less than 2 weeks (Figures 6E and 6F)."⁵  Nine mice per group.

Three cautions.  This is mouse work; the only human data in the paper is that "addition of procaterol onto cultured human HFSCs promoted their growth, suggesting the pathway has a conserved function in regulating human HFSCs."⁵  Cold-induced hair growth has not been shown in a person.  And the authors did not think they were overturning the vestige claim; their introduction says the unit "is highly conserved across mammals including in humans, where piloerection has lost its role in thermoregulation, raising the possibility of additional functions."⁵  The paper answers Coyne's first sentence and assumes his second.

Baldness

The 2014 review by Torkamani, Rufaut, Jones and Sinclair opens with the textbook line and departs from it: "The APM mediates thermoregulation by contracting to increase air-trapping, but was thought to be vestigial in humans.  The APM attaches proximally to the hair follicle at the bulge, a known stem cell niche."⁶  In the reversible forms of hair loss the muscle stays attached to the shrinking follicle; in pattern baldness it does not: "the APM remains connected to the bulge of miniaturized follicles in AA and TE, which are reversible, but not in irreversible male and female pattern AGA."⁶  Bald scalp shows "significant decreases in muscle volume and an extensive fat infiltration around the residual APM of hair follicles in AGA patients."⁶

The limit is stated by the reviewers: "It has not been defined whether APM regression in irreversible hair loss was a cause or effect of the disease."⁶  The correlation is real; the direction is not known.  An argument that says "shrink the muscle and the hair falls out" runs the arrow one way the data do not yet support.  The Shwartz team offers a mechanism that would run it that way, and labels it a guess:  "It is possible that in such skin, loss of APMs leads to the loss of sympathetic nerves, making HFSCs more difficult to activate."⁵  The mouse ablation test makes the guess reasonable.  No one has tested it in human scalp.

Sebum

Gray's 1918 text already says that "contraction of the muscle thus tends to squeeze the sebaceous secretion out from the duct of the gland,"³ and Menton and Mitchell repeat it.  Torkamani's review, a century later, still lists it under "It has been speculated that the APM might have a role in sebum secretion."⁶  Plausible geometry, not a measured function.  The same review files a second structural role, holding the follicles of the scalp together, under "Hypothesized" functions:  in human scalp the follicles grow in small groups called follicular units, and, as the review reports, Poblet and colleagues described the muscle as "the ribbon on a bunch of flowers," because the follicles splay apart only above its insertion.⁶  That is an anatomical observation, and the review says the role "has not been thoroughly studied."

Heat and insulation

Here the creationist replies, quoted in Part 5, and the physiology reviews diverge, and neither side has a number.  Creation Ministries International says erect hair "helps to conserve body heat by trapping more air" and that "the muscle contractions generate heat";⁷ Menton says the contraction "produces heat."¹⁰  Tansey and Johnson say the opposite: "As humans possess relatively little hair and are often clothed, heat conservation through piloerection is usually regarded as insignificant and rudimentary.  However, piloerection may become more important in conjunction with shivering potentially enhancing the effectiveness of the shivering response."⁴  On heat production they class piloerection with vasoconstriction as responses "which are energetically inexpensive."⁴  A response that costs little energy releases little heat.

"Usually regarded" is a consensus, not a measurement, and no study measuring the insulating value of erect vellus hair on human skin, or the heat output of the arrector pili, was found for this article.  So Spinney's "incapable" is an inference from the size of the hair, CMI's "helps to conserve body heat" is an assertion, and the experiment that would settle it, skin heat loss with and without pilomotor activation, has apparently not been run.

Emotion

The one thing the reflex plainly still does in people is fire under emotion.  Benedek and Kaernbach put a camera on the forearms of fifty psychology students and played them sixty audio tracks from music and films.  "Pre-selected stimuli elicited piloerection equally often (17%) as self-selected stimuli (18%; χ2 = 0.07, ns), but film audio tracks significantly more often elicited piloerection (24%) than music tracks (11%, χ2 = 11.71, p < .001)."⁸  The response "was primarily accompanied by an increase of phasic electrodermal activity and increased respiration depth," and "emotional piloerection represents a valuable indicator of the state of being moved or touched."⁸

Whether that is a function or a by-product is not settled.  The paper's own framing is that cold-induced piloerection "is considered as a relic of the thermoregulatory response of our furred ancestors in evolution," and it notes hair rising "in the course of courtship of male chimpanzees (Nishida, 1997)."⁸  Spinney had suggested a "minor new role": "This could serve as a signal to others."¹³  Nobody has tested whether other people can see it.

Part 4: What is reduced, and what is not

The homology claim needs a reduction.  For a whale's pelvis or a horse's splint the reduction is in the bone itself.  For goosebumps it has to be located, and the comparative data locate it somewhere other than the muscle.

Kamberov and colleagues took full-thickness skin from five body regions (forehead, back, chest, forearm, thigh) of seven humans, four chimpanzees and eight rhesus macaques, and counted follicles and sweat glands.  "Although human hair cover is greatly attenuated relative to that of our close relatives, we find that humans have a chimpanzee-like hair density that is significantly lower than that of macaques."⁹  In the body: "macaques have approximately two- to 21-fold higher average hair density than either chimpanzees or humans," while "average human and chimpanzee hair densities did not differ significantly in any region, despite the very different external appearances of these species".⁹  Their conclusion: "the absence of visible fur cover in humans does not result from a loss of hair follicles per se, but instead favors a model of evolutionary changes in the morphogenesis of the individual hair follicles, which resulted in a shift from the large, pigmented terminal hair type characteristic of fur to the microscopic, unpigmented vellus hair type".⁹

Put that beside Torkamani's observation that the muscle wraps vellus follicles as well as terminal ones.⁶  A human forearm carries about as many follicles as a chimpanzee's, and the muscles on them fire on the same two triggers Darwin listed.  The follicle count is not reduced.  The muscle count has not been made:  in human scalp the follicles grow in groups that share one muscle,⁶ so a follicle count is not a muscle count, and no one has counted muscles across primates.  The histology that exists does not describe a shrunken muscle.  George Chaplin, Nina Jablonski and two colleagues surveyed the primate skin literature in 2014, most of it William Montagna's microscope work, and tabled the muscle on the body as "Strong" and "Widespread" in the chimpanzee and the same in Homo sapiens, and "Strong" in the gorilla.¹⁵  That is a word from a microscopist, not a measurement of size or force, but it is the same word for all three.  The reflex is present and fires; Benedek's cameras saw it in a quarter of the film trials.  What is reduced is the hair each muscle pulls on, from a pigmented terminal shaft to a vellus hair that cannot trap air or make anyone look bigger.

That reframes the case.  Goosebumps are a full-strength reflex acting on reduced hair, and the vestige argument about goosebumps is the argument about human hairlessness with the muscle as its most visible symptom, which is how Wiedersheim had it in 1895.

Two gaps.  No study measures the size, force or number of arrector pili muscles across primates; the evidence that the human muscle is not reduced is a histologist's descriptive word, the same for human, chimpanzee and gorilla, not a measurement.  And Kamberov's sample is four chimpanzees, two of them for the chest, which the authors flag as an outlier.

Part 5: The creationist replies, in their own words

Creation Ministries International's one-minute video of 2016 states the position: "Since humans have minimal hair, they often dismiss goosebumps as a 'non-functional relic' of our supposed evolutionary past.  However, human goosebumps are not useless.  Even with the small amount of hair we have, when it stands on end, it helps to conserve body heat by trapping more air.  Also, the muscles associated with goosebumps help squeeze oil onto the skin and the hairs help prevent the oil glands from becoming clogged.  Moreover, the muscle contractions generate heat, of which more can be generated by shivering.  Goosebumps, therefore, do not necessarily support the idea we are related to hairy animals.  But they do provide evidence for a common designer, using the 'goosebump concept' in many different creatures."⁷

Shaun Doyle's CMI reply to Spinney, 2008, concedes more: "the relative hairlessness of humans makes this reflex seem pointless, if that were its function in humans.  However, goose bumps are linked to emotional responses in humans, and may serve to heighten emotional reactions, according to Spinney."¹⁶  His conclusion: "a unique function for goose bumps in humans is hardly a problem for design, and is at the very least an equally valid explanation of their origin."¹⁶

David Menton's chapter in The New Answers Book 3 gives the function list: the erector pili "is in a position to help squeeze oil from the sebaceous glands," it is "supplied with nerves of the sympathetic nervous system," and its contraction "produces heat."¹⁰  Elizabeth Mitchell's 2011 Answers in Genesis piece makes the same list: "the erector pili muscles tugging on follicles cause the protective oil from attached sebaceous glands to ooze onto the skin.  They also generate some heat while giving us goose bumps."¹⁷

David DeWitt's 2008 Answers in Genesis reply to Spinney is the one that meets the modern definition.  He names it: "The classification of goosebumps as a vestigial organ stems from a revised definition of vestigial. ... If an organ or response is reduced compared to its ancestral version or takes a slightly different form, it can be considered vestigial.  Under the revised definition, because human hair is "puny" and the thermoregulatory impact reduced, goosebumps can be considered vestigial."¹¹  And answers it: "Once again, this example must assume that humans share a common ancestor with animals in order for goosebumps to be considered vestigial.  In this case, there is no evidence that the goosebump response has been reduced from that of our human ancestors."¹¹

Fazale Rana's 2020 post at Reasons to Believe is old-earth, not young-earth, and is included because it is the creationist response that engages the Cell paper.  He states both evolutionary claims correctly, quoted in Part 2, and argues from the first: "What if goosebumps in humans serve a useful function?  If they do, that function undermines the idea that goosebumps are a vestige of our evolutionary history".¹²  He is careful about the transfer from mice: "it is not clear at this juncture if hair growth in humans under these conditions would have any real benefit.  On the other hand, there is no evidence to the contrary.  We don't know."¹²  His conclusion: "because the muscle serves multiple purposes, it helps explain why these intact, fully functional muscles are found in human skin, with goosebumps produced as a by-product of the arrector pili's association with hair follicles and sympathetic nerves."¹²

Jerry Bergman and George Howe's 1990 book Vestigial Organs Are Fully Functional could not be obtained, and Bergman's 2018 ICR piece lists "goose bumps" among the claimed vestiges without discussing them.¹⁸

Which claim each reply answers

CMI, Menton and Mitchell answer the useless claim, and their three functions are of unequal weight.  Sebum squeezing has been "speculated" since Gray.  Heat production is asserted against a physiology review that calls the response "energetically inexpensive."  Air trapping by vellus hair is asserted against a review that calls it "insignificant and rudimentary," and neither side has measured it.  None of the three touches homology, and CMI's closing line, "a common designer, using the 'goosebump concept' in many different creatures," grants the correspondence and offers a rival reading of it rather than a test.

Doyle concedes that the reflex looks pointless for warmth and rests on the definition; "equally valid" is only as good as the design account's ability to say why the reflex should exist in this form, which the objections take up.

DeWitt fights on the right ground.  He says reduction is asserted and rests on the ancestry premise, which is the parent's step 3.  What he lacked in 2008 was a measurement, and Kamberov's 2018 count now supplies one: the response is not reduced, the follicle count is not reduced, the hair is.  That strengthens his sentence about the response and moves the dispute to the hair.

Rana answers the first claim with the best available evidence and concedes the second in his last sentence.  A "by-product" is exactly what the evolutionary reading calls the human reflex.  The two accounts differ on why the muscle is there, not on what the bump does.

Part 6: Running the three-step test

Step 1, homology.  The human arrector pili is the same muscle, attached at the same place on the follicle, fired by the same sympathetic nerve, on the same two triggers, as in every mammal Darwin and Shwartz describe; Shwartz calls the unit "highly conserved across mammals including in humans."⁵  No creationist source disputes it; CMI's "common designer" line assumes it.  Step 1 is met.

Step 2, reduction.  The muscle is present on vellus follicles, and the follicle count matches the chimpanzee's.  The reflex fires on cold and on emotion and can be filmed.  Nothing about the muscle or the reflex is shown to be reduced; the human muscle's size and force relative to other primates are unmeasured, and the histology describes it as "Strong", the word it uses for the chimpanzee and the gorilla.¹⁵  The hair is reduced, from terminal to vellus.  Step 2 is met for the hair and not for the muscle, and DeWitt's "no evidence that the goosebump response has been reduced" is correct as far as the response goes.

Step 3, ancestral distribution.  The muscle is not in every mammal.  Chaplin and colleagues' survey codes it absent in echidnas, pangolins, bats, shrews, otters, whales, sea cows and elephants, among others.¹⁵  Among primates the muscles "are mostly absent except on the tail in most strepsirhines, and are entirely absent in tarsiers and some lorisids," while "In anthropoids, the MAP are present on all of the non-glabrous skin."¹⁵¹⁵  So the character does sort the tree, and it puts humans where everyone already puts them, with the monkeys and apes; the authors read it as "a shared-derived condition probably related primarily to increased body size."¹⁵  It says nothing about the step that matters here, the split between humans and chimpanzees, because both carry the same strong, body-wide muscle.  The pattern also fits a design account in which the muscle is placed where large bodies need it, though no creationist source found discusses the distribution.  On the human-chimpanzee question the reflex is useless as evidence, and the reason has nothing to do with function:  a character both lines carry in the same form cannot show which way either changed.  What would bear on step 3 is the direction of the hair change: whether vellus-dominated skin is the derived state of a lineage that once had fur, or a created state.  That is decided by the genetics and development of hair follicles and by the fossil and molecular case for human-ape ancestry.  It is not decided by goosebumps, and a function for the muscle does not reach it.

So the parent's provisional verdict holds: the arrector pili is not vestigial as a muscle, and the ancestry of the reflex is a step-3 question the reflex cannot answer.  The sharpening this article adds is that the reflex is not reduced either.  The human goosebump is a complete reflex on incomplete hair.

Objections and responses

A function has been found, so goosebumps are not vestigial.  The function found is the muscle's, as a nerve scaffold, and the claim at issue is about the reflex and its ancestry.  Coyne's definition allows a trait to be "vestigial and functional at the same time."²  The Shwartz paper answers his first sentence and leaves his second untouched; its authors say human piloerection "has lost its role in thermoregulation" in the same paragraph that proposes the new function.

Erect hair still traps air, so goosebumps still insulate.  Possibly, and nobody has measured it.  Physiologists regard the effect as "insignificant and rudimentary."⁴  The claim needs a number: heat loss from skin with and without pilomotor activation.  Until someone produces one, "helps to conserve body heat" is an assertion with the burden on the side making it.

The contractions produce heat.  Smooth muscle at the base of a hair, firing briefly, is classed by the same review among the "energetically inexpensive" responses.⁴  Shivering is the heat-producing response, and the creationist sources say so themselves when they add that shivering follows.

Emotional goosebumps are a new function, so design explains them as well as evolution.  Emotional piloerection is real.⁸  But Darwin already read emotional hair-raising in animals as largely incidental,¹ Benedek's own framing is "relic," Rana's is "by-product," and Spinney's "signal to others" has never been tested.  A by-product is not a function on either account.  If someone shows that observers detect goosebumps and respond to them, the objection gains a function; until then it does not.

Calling the reflex reduced assumes common ancestry.  Correct for step 3, and this article agrees with DeWitt that that is where the argument lives.  But step 2 is a measurement, and it says the response is not reduced and the hair is.  The creationist side should say so plainly and then argue the hair question where it belongs.

The mouse result does not transfer to people.  Agreed, with one qualification.  The muscle-nerve-follicle unit is the same in human skin, human stem cells respond to the same adrenergic signal in culture,⁵ and human bald scalp loses the muscle.⁶  Cold-induced hair growth in people is untested, and Rana's "We don't know" is the right summary.

Plenty of furred mammals have no arrector pili, so the niche cannot be why the muscle exists.  This objection is sound as far as it goes, and it limits the design argument.  Tarsiers and some lorises have no arrector pili anywhere, lemurs keep it only on the tail, and bats, shrews and whales lack it,¹⁵¹⁵ yet the furred ones among them grow and replace hair.  So hair cycling does not need this muscle in every mammal, and the Shwartz team's "indispensable role" holds for mouse skin, where it was tested.  What the objection does not touch is the narrower claim:  in mouse skin, where it was tested, and possibly in human skin, which has the same arrangement,⁵ the muscle holds the nerve that tunes the stem cells, and removing it delays hair growth.⁵  That gives the human muscle a present job.  It does not show that the job is the reason the muscle is there, on either account.

Why would a designer wire a fear reflex to hair that cannot bristle?  This is the skeptic's objection to this article's verdict, and the strongest thing left in the case.  The evolutionary reading predicts the pairing: the circuit was built for fur and the fur went.  The design reading has two answers on record, "by-product" and "common designer," and neither predicts that the emotional trigger in particular should survive.  This residue is explained by one side and accommodated by the other.  It is not, on its own, evidence of ancestry, because humans share the reflex with every monkey and ape and so it cannot place them within that group; but it is a question the design side has not answered.

Issues and unanswered questions

• No measurement was found of the insulating value of erect vellus hair on human skin or of the heat output of the arrector pili. Both sides argue from words.
• No comparative measurement of arrector pili size, force or number across primates was found. The histological literature, as tabled by Chaplin and colleagues, calls the muscle "Strong" in humans, chimpanzees and gorillas, which argues against a reduced human muscle but is a description, not a number.
• The Shwartz result is in mice. Cold-induced hair growth has not been shown in people, and the human data are a culture assay.
• The baldness correlation has no established direction.
• Whether emotional piloerection is perceptible to others, and therefore could be a signal, has not been tested.
• Bergman and Howe 1990 could not be obtained, and no young-earth page citing the 2020 Cell paper was found; the creationist response that cites it is Rana's, and he is old-earth.
• The human hair sites that lack an arrector pili were not sourced.
• Kamberov's chimpanzee sample is four animals.

Verdict

The arrector pili is not a vestigial muscle.  It holds the sympathetic nerve against the hair-follicle stem cells, it is lost in irreversible baldness, and it is present in a person at about a chimpanzee's density.  The reflex is not reduced either: it fires on cold and on emotion, and it can be filmed.  What is reduced is the hair, and the goosebump is a complete reflex on hair too fine to do what the reflex does in a cat.

That settles the useless claim against the textbooks and settles nothing about ancestry.  Homology is agreed on both sides.  Reduction is real, and it is a fact about hair, so the vestige argument about goosebumps collapses into the argument about human hairlessness, where Wiedersheim had it in 1895.  The third step, whether vellus hair is the derived state of a once-furred lineage, is decided by the evidence for human-ape ancestry and by the genetics of hair morphogenesis, not by a reflex that every monkey and ape shares.  Goosebumps are weak evidence for either side.  The one residue the design side owes an answer for is the fear trigger: a reflex built to make an animal look bigger, preserved in an animal it cannot enlarge.

Sources