Why did humans evolve ever-growing cranial hair?

How humans evolved to have head / beard hair
“that continues to grow longer than other animals, while losing hair elsewhere, is a topic that many anthropologists & biologists are still not sure about and there is no general consensus as to “why” yet.”

The following hypotheses are copied from the online references below.
They do not represent my original thoughts or anything to do with the LRT. Academic citations follow and can be accessed via the reference links.

The three main views are:

1) Evolution of the “Aquatic Ape.” (Ingram, 2000: Morgan 1997; 1982)

  • Infants, in order to hold onto their mothers in the water, would latch onto her hair. Limiting separation from the mother & increasing chances of survivability
  • Longer hair meant that infants / small children would need to swim less in order to get to their mother
  • Believed to be supported even further when you consider that aquatic mammals are almost always hairless, indicating that at one point, humans were highly “aquatic” mammals.

2) No real benefit, but used as a tool for “mate selection.” (Darwin, 1871; Cooper 1971)

  • The view held by many of the Darwin school of thought is that at first, “hairiness” was sexually attractive, but eventually “hairlessness” became more sexually attractive in most places (i.e. the face to see facial expressions & socialize better; Wong & Simmons 2001)
  • A sign of “virility” & “health” as can be seen in the mate-selection behavior of lions. Which is true even today as human diagnostic material for health (Klevay, 1972).

3) Practical evolutionary benefits for the human species specifically

  • A lot of body heat escapes from the head, probably the most important part of your body. Hair is a good insulator that can keep in heat. This increases survivability in colder climates. (Wong & Simmons 2001; Bubenick 2003). (Disputed but considered credible reason, especially when you compare hair length and types across different regions throughout history)
  • Protection against damaging UV rays (while still permitting adequate Vitamin D3 to come through) & some protection from free-radicals or other harmful particles. Because we became bi-pedal, the head was the main area exposed to the sun (as well as some of our back). Extending hair’s usefulness to even hot environments, while other body hair became less important with the development of sweat glands (Wheeler 1985).
  • Heightened “Situational Awareness” through “Touch sense.” A concept that may seem silly at first but has some evidence to support the theory. Though the hair is not “alive,” it is connected to the follicles & your nerves. In a nutshell, it may help to increase “sensory awareness” & “data gathering” of your environment, which would favor longer hair. This would be an asset in survivability (Kardong 2002; keratin.com 2010; Sabah 1974)
  • Though not a collegiate journal article, if reasonably credible, this small article is an interesting case for supporting hair & “Touch sense” in “recent history” & in combat-survival : http://www.sott.net/article/234783-The-Truth-About-Hair-and-Why-Indians-Would-Keep-Their-Hair-Long.

Other thoughts…

“Evolution selected for intelligence – and for hair. The person who radically shapes his hair, exploiting its continuous growth to demonstrate his on-going Neanderthal chic, is more likely to attract partners than the person whose hair is dull, lifeless and matted.”

“Darwin, noting that every human society, however primitive, invariably paints, tattoos, pierces and otherwise decorates its bodies, argued that, in the remorseless competition for sexual partners, we humans, during the evolutionary past, shed our hair to create a canvas on which to flaunt our creativity, flair and beauty.”

In a tweet:
“The reason we (mostly) still have head hair is mostly because it serves as a sun-screen – and the reason we still have pubic hair is because it traps pheromones.”

On the other hand…
“Left alone, our hair produces a three-foot, smelly, matted, greasy, bug-infested mass that will snag on trees and provide predators with a claw-hold.”

I prefer this one: “diagnostic material for health (Klevay, 1972).” 





Triassic origin of scales, scutes, hair, etc. as biting fly barriers?

During the Middle to Late Triassic

  1. Mammals developed fur/hair.
  2. Aetosaurs developed plates and horns beyond the earlier paired dorsal scutes.
  3. Crocodylomorphs developed large scales beyond the earlier paired dorsal scutes.
  4. Dinosaurs lost those paired scutes and developed placodes and quills. Ultimately these became scales and feathers.
  5. Turtles developed hard scales over a carapace and plastron.
  6. Lepidosaurs developed small scales.
  7. Pterosaurs and their Late Triassic sisters developed pycnofibers

All of these developed on the soft, naked skin
(think of a plucked chicken) that was a universal covering for Carboniferous and Permian tetrapods (Early forms retained large ventral scales inherited from finned ancestors, but these were lost by the Permian).

All of these extradermal structures have one thing in common.
They separated and/or protected the animal’s naked skin from the environment, one way or another. They developed by convergence. Dhouailly 2009 and other workers discussed the chemical similarities of the keratin found in these dermal structures. 

The question is:
What was different about the Triassic environment that was not present in earlier Carboniferous and Permian environments? We can 
eliminate heat, cold, UV rays, rain, aridity, etc. as possible reasons for the development of insulator structures because those factors had always been present. So what was new in the Triassic that affected all terrestrial tetrapods?

Flies and their biting, piercing kin.
“The earliest definitive flies known from the mid-Triassic of France, approximately 230 Ma (Krzemiski and Krzeminska, 2003)” according to Blagoderov, Grimaldi and Fraser 2007. The order Diptera (flies, mosquitos and kin) tend to land on large tetrapods for food, blood, etc. Scales, scutes, hair, feathers, etc. all separate flying insects from the naked skin of Triassic terrestrial tetrapods. Williams et al. 2006 even found mosquito repellents in frog skin. It is notable that, except for armored placodonts and mosasaurus (derived varanid lepidosaurs), aquatic and marine tetrapods also had naked skin with the thalattosaur, Vancleavea, a notable sermi-terrestrial exception. Is that because they had aquatic antecedents in the Triassic that were never affected by flying insects?

It’s not just the insect bite that drives this evolution,
it’s the appearance of new vectors for the rapid spread of disease that drives this evolution.

Interesting coincidence.
If this is not the case, this will take further study.

Figure 1. Lacertulus, a basal squamate from the Late Permian

Figure 1. Lacertulus, a basal squamate from the Late Permian

Carroll and Thompson 1982 report
on the Late Permian lepidosaur, Lacertulus (Fig. 1), “No scales dermal or epidermal are evident in the specimen.”

From the Dhouailly 2009 abstract:
“I suggest that the alpha-keratinized hairs from living synapsids may have evolved from the hypothetical glandular integument of the first amniotes, which may have presented similarities with common day terrestrial amphibians.

Concerning feathers, they may have evolved independently of squamate scales, each originating from the hypothetical roughened beta-keratinized integument of the first sauropsids. The avian overlapping scales, which cover the feet in some bird species, may have developed later in evolution, being secondarily derived from feathers.” Not realized by Dhouailly, the purported clade ‘Sauropsida’ is paraphyletic and a junior synonym for Amniota and Reptilia in the LRT.

Earlier we looked at the first appearances
of hair, quills, pycnofibers and hard scales in a three-part series here, here and here

Exceptionally, humans are terrestrial tetrapods
that have lost most of their hair, more or less returning to the primitive naked state. And yes, flies and mosquitos do bother humans. It is the price we pay for the benefits of naked skin. Clothing helps provide a barrier.

Just because an idea is proposed and a hypothesis is advanced doesn’t make it so. In science ideas have to be confirmed or refuted following their first appearance. If anyone has data concerning scales or other dermal structures in Carboniferous or Permian taxa, please make us aware of those.

Blagoderov V, Grimaldi D and Fraser NC 2007. How Time Flies for Flies: Diverse Diptera from the Triassic of Virginia and Early Radiation of the Order. American Museum Novitates 3572:1-39. DOI: 10.1206/0003-0082(2007)509[1:HTFFFD]2.0.CO;2
Carroll RL and Thompson P 1982.
A bipedal lizardlike reptile from the Karroo. Journal of Palaeontology 56:1-10.
Dhouailly D 2009.
A new scenario for the evolutionary origin of hair, feather, and avian scales Journal of Anatomy 214(4): 587–606. doi: 10.1111/j.1469-7580.2008.01041.x
Krzeminnski, W., and E. Krzeminska. 2003. Triassic Diptera: descriptions, revisions and phylogenetic relations. Acta Zoologica Cracoviensia (suppl.) 46: 153–184.
Maderson PFA and Alibardi L 2000.
The Development of the Sauropsid Integument: A Contribution to the Problem of the Origin and Evolution of Feathers. American Zoologist 40:513–529.
Rohdendorf BB, Oldroyd H and Ball GE 1974. The Historical Development of Diptera. The University of Alberta Press, Edmonton, Canada. ISBN 0-88864-003-X.
Williams CR, Smith BPC, Best SM and Tyler MJ 2006.
Mosquito repellents in frog skin. Biol Lett. 2006 Jun 22; 2(2): 242–245. doi: 10.1098/rsbl.2006.0448

The origin of feathers and hair (part 2: hair)

Yesterday we looked at reptile skin and scales, alpha and beta-keratins and examined the fossil record of scales, naked skin and pterosaur extra dermal membranes. Today we’ll take on mammal hair.

Mammals, like Megazostrodon, evolved in the Jurassic from synapsid reptiles, like Archaeothyris, that first appeared in the Late Pennsylvanian.

Dhouailly 2009 reports: “The synapsid lineage, which separated from the amniote taxa in the Pennsylvanian about 310 million years ago, may have evolved a glandular rather than a scaled integument, with a thin alpha-keratinized layer adorned with alpha-keratinized bumps. Those bumps may have even presented some cysteine-rich alpha-keratins, precursors of the hair-type keratins. In addition, the first synapsids may have developed both a lipid barrier outside the epidermis, similar to current amphibians living in xeric habitats, and some lipid complex with the alpha-keratins of the stratum corneum as in current mammals as a means to strengthen the barrier against water loss of the integument.”

So reptilian scales were never part of the mammal legacy — just naked glandular skin.

A dense coat of fur is found in all basal extant mammals, even those that lay eggs. Thus the origin of hair is to be found in the common ancestor of all living mammals, perhaps among therapsid-grade synapsids (Thrinaxodon Chiniquodon), or, more conservatively, perhaps right at the origin of early Jurassic mammals.

Dhouailly 2009 reports: “No intermediate form has ever been found between scales and hairs, resulting in only a few proposals of how mammalian hairs may have evolved from scales. These proposals were based on the development of sensory bristles in the hinge scale region of reptiles.”  Unfortunately basal reptiles and therapsids did not have scales (see below).

The traditional cynodont whisker hypothsis
Foramina (tiny holes) on the faces of basal gorgonopsians, therocephalians and cynodonts have been interpreted as providing passages for nerves and blood vessels supplying movable skin (subcutaneous muscles) and sensory vibrissae (whiskers). This would represent the first appearance of hair only to be followed by more and more hair spreading around the body. This essentially duplicates the new hypothesis on feather origin by Persons and Currie (2015, see that discussion tomorrow).

Unfortunately for this hypothesis,
the basal lizard, Tupinambis has similar rostral foramina, yet it lacks sensory vibrissae (Bennett and Ruben 1986).

An alternate mammal hair genesis hypothesis
Given that pelycosaurs and Estemmenosuchus were naked and had no hair, the origin of mammal-type hair must have occurred closer to mammals. On their way to evolving into mammals, taxa like Pachygenelus and Megazostrodon became progressively smaller in a rather common process known as phylogenetic miniaturization (the opposite of Cope’s Rule).

Due to their increased surface/volume ratio, smaller animals find it more difficult to internally thermoregulate because their insides are closer to their outsides. Having insulating fur when tiny would be helpful. That’s the traditional hypothesis for mammal hair genesis in tiny taxa, like Megazostrodon. Unfortunately the insulation hypothesis gives no reason for the first appearance of tiny sprigs of precursor hair, not yet plentiful enough to trap air (for insulation). Nor does it take into account that the smallest of all basal mammals, their newborns, are hairless.

Dhouailly 2009 reports: “Hairs [may have] evolved from sebaceous glands, with the hairshaft serving as a wick to draw the product of the gland to the skin surface, strengthening the barrier against water loss.”

Figure 2. An automobile driver can sense the presence of the curb on approach when a curb feeler is in place. This saves wear and tear on tires, just like individual hairs would touch the inside of burrows before the skin comes into contact.

Figure 2. An automobile driver can sense the presence of a curb on approach when a “curb feeler” is in place. This saves wear and tear on tires. Similarly individual hairs would touch the inside of burrows before the skin comes into contact.

The curb-feeler hypothesis
As others have noted, individual hairs provide tactile feedback. Those are especially useful to nocturnal and burrowing animals.

Naked mole rats provide a good analogy. Like therapsids, naked mole rats burrow, adjust their internal temperature to ambient temperatures, AND they have only a few whisker-like hairs that crisscross the body to form a sensitive array that helps them navigate in the dark. We know that certain small cynodonts were  also burrowers. That’s where we find them. We don’t know if they had whisker-like hairs that crisscrossed their body. Only the bones are preserved.

In this way,
individual hairs would have been like curb-feelers (Fig. 2), small wires that make a noise whenever a 1950s era automobile approaches a curb. Thus provided, basal mammals could have avoided multiple abrasions while running through their tunnels using their own curb feelers.

if that’s how hair started, once provided with the ability to grow hair, simply growing more hair would have provided incremental opportunities to spend more and more time outside of the burrow. Hair insulated mammals not only from ambient temperature, but from the environment at large, including the approach of winged insects like flies and mosquitoes. Note that those insects that finally developed the ability to burrow past the hair barrier, fleas, lost their wings in order to do so.

Navigation skills
learned in dark tunnels could be readily transferred to leaf litter in the open air at night (all the while avoiding the predatory gaze of hungry Jurassic dinosaurs).

Opossum tail showing rectangular eupelycosaurian scales

Figure 2. Opossum tail showing false scales. A couple of ‘curb feelers’ appear on the proximal tail.

The “scaly tail” of mammals,
like the opossum (Fig. 2), is actually, a criss-cross series of epidermal folds interspersed with hairs, not homologous with the scale of any other animal (Dhouailly 2009).

Figure 3. Naked mouse babies surround the furry mother mouse.

Figure 3. Naked mouse babies surround the furry mother mouse. The babies may be recapitulating evolution as they are naked and unable to maintain their own body temperature without a little help from mom.

The surprising origin of mammary glands
Dhouailly 2009 reports: “The mammary gland apparently derives from an ancestral sweat or sebaceous gland that was associated with hair follicles, an association which is retained in living monotremes, and transiently in living marsupials. The original function of the mammary gland precursor may not have been feeding the young, but as a means to provide moisture to the eggs.”

Tomorrow: dinosaur feathers.

Bennett AF and Ruben JA 1986. The metabolic thermoregulartory status of therapsids. In The Ecology and Biology of Mammal-like reptiles (Hottom, Roth and Roth eds) 207-218. Smithsonian Institution Press, Washington DC
Chudinov PK 1970. Skin covering of therapsids [in Russian] In: Data on the evolution of terrestrial vertebrates (Flerov ed.) pp.45-50 Moscow: Nauka.
Dhouailly D 2009. A new scenario for the evolutionary origin of hair, feather, and avian scales. Journal of Anatomy 214:587-606.
Persons WC4 and Currie PF 2015. Bristles before down: A new perspective on the functional origin of feathers.Evolution (advance online publication) DOI: 10.1111/evo.12634