Evolution
of Skin Color in Humans
Copyright ã 2000 James Michael Howard,
In October of 1996, I first posted (to some
internet news groups) my hypothesis that skin color evolved as a result of
testosterone. It was entitled "Skin color …perhaps Testosterone." My
explanation of human evolution suggests increased testosterone in human males
and females, compared to chimpanzee males and females, is directly involved in
human evolution. Therefore, I decided testosterone could be linked to skin
color. (If you wish to explore my explanation of human evolution in more
detail, please read "New Theory of Human Evolution Involving DHEA,
Melatonin, and Testosterone" and "Androgens in Human Evolution: A New
Explanation of Human Evolution," at the main DHEA page.) In 1996, I found
limited support of a connection, so I left further work on this connection for
later. The July, 2000, issue of "The Journal of Human Evolution,"
included an article, "Evolution of Human Skin Coloration," which
caused me to look again at a connection between testosterone and human skin
color (Jablonski, NG and Chaplin, G, J Human Evolution 2000; 39: 57-106).
Jablonski and Chaplin state: "Here we present new evidence indicating
that variations in skin color are adaptive, and are related
to the regulation of ultraviolet (UV) radiation penetration
in the integument and its direct and indirect effects on
fitness." The article concerns UV light and skin color. They also state:
"In all populations for which skin reflectance data were available for
males and females, females were found to be lighter skinned than males."
This indicates to me that testosterone, as I first suggested in 1996, may be directly
involved in skin color.
July 12, 2000, I posted "Evolution of
Skin Color" to some news groups. This consisted of "Long ago, I
decided skin coloration is due to levels of testosterone. Blacks produce more
testosterone than other races, hence, blacks have
darker skin. (I derived this from my explanation of human evolution.) Jablonski
and Chaplin just found that ‘In all populations for which skin reflectance data
were available for males and females, females were found to be lighter skinned
than males.’ This would support my hypothesis." The obvious question
quickly returned in response at the news group. Why are black women darker than
white men? The following is my answer to the question and is an extension of my
original hypothesis that testosterone is directly involved in skin color and
human evolution. I suggest the difference results from effects of testosterone
on melanocytes in utero.
It is known that testosterone and ultraviolet
light work together in stimulating melanocyte structure and function.
"Cultured skin receiving both UVL [ultraviolet light] and testosterone
illustrates a synergistic effect." (J Exp Zoo 1978; 204:
229, "Organ culture of mammalian skin and the effects of ultraviolet light
and testosterone on melanocyte morphology and function," Gilmcher, ME, et
al.). This could account for the findings of Jablonski and Chaplin,
mentioned in my July 12 post that "In all populations for which skin
reflectance data were available for males and females, females were found to be
lighter skinned than males." This may be a tanning effect,
however, testosterone is definitely involved in melanocyte function.
Healthy black males produce significantly
more testosterone than healthy white males (J Nat Cancer Instit 1986; 76: 421).
Melanocyes from black males grow differently from melanocytes from white males,
in culture. In this study, melanocytes are derived from foreskins, an area of
skin directly affected by testosterone. "At the ultrastructural level,
cultured melanocytes derived from black (negroid)
neonatal skin (B-M) had numerous mature rod-shaped stage IV melanosomes, while
white (caucasoid) skin-derived melanocytes (W-M) in culture contained no mature
melanosomes. Growth rate, cell yield, and in vitro lifespan for B-M were more
than twice that for W-M in pure melanocyte cultures in the presence of MGF
[melanocyte growth factor]. Our results suggest that MGF-dependent growth of
B-M differs from that of W-M." (J Cell Physiol 1988;
135: 262-8, "Growth characteristics of human epidermal melanocytes in pure
culture with special reference to genetic differences," Hirobe, T, et
al.). Melanocytes grown in culture, without testosterone added to the
culture media, inherently express a difference in growth potential between
black and white males.
Melanocytes derived from neonates already
exhibit differences in growth rate according to race. "Differences in size
and number of melanosomes attributable to race of the tissue donor were readily
apparent, and pigment content of melanocytes from both black and Caucasian
donors appeared to increase with time in culture. Newborn melanocytes
proliferated more rapidly and survived longer than did adult melanocytes, but
there were no consistent morphologic differences as a function of donor
age." (J Invest Dermatol 1984; 83: 370-6,
"Selective cultivation of human melanocytes from newborn and adult
epidermis," Gilchrest, BA, et al.).
I think the effects of
testosterone on melanocytes first occurs in utero. In utero, black
fetuses are exposed to higher levels of testosterone. "Serum testosterone
was modestly, but significantly, greater in the black than in the white
women." (J Clin Endocrinol Metab 1996; 81: 1023-6,
"Demonstration of a lack of racial differences in secretion of growth
hormone despite a racial difference in bone mineral density in premenopausal
women—a Clinical Research Center study," Wright, NM, et al.).
Therefore, I suggest melanocytes are stimulated by increased testosterone
during gestation in blacks. In respect to the specific question, this would
result in increased melanocyte stimulation in female, black fetuses. Male
fetuses are exposed to testosterone, from the fetal gonads, at a critical
period in utero that exerts effects on reproductive development. I make the
assumption that this period of exposure of male, fetal-derived testosterone
does not coincide with the constant supply of maternally-derived testosterone
that stimulates melanocytes. If that is the case, then it is possible that
black women exhibit darker skin than white males because of a difference in
timing of exposure of developing melanocytes to testosterone in utero. The
color of offspring of interracial unions would depend upon testosterone levels
of the mother and embryonic/fetal melanocytes, that is, the propensity of
melanocytes for producing melanin and the stimulation of melanocytes by
testosterone during the critical period.
According to my explanation of human
evolution, lower testosterone groups migrated away from the equator. Therefore,
it is the lower levels of testosterone that resulted in lighter skin in groups
living away from the equator. Advantages of darker skin near the equator and
advantages of lighter skin away from the equator are simply secondary
advantages.
I did not use this citation:
|
Am J Physiol 1987 Jan;252(1 Pt 2):R166-80 |
|
Morphology
and development of an apoeccrine sweat gland in human axillae.
Sato K, Leidal R, Sato F
Evidence is presented that in adult human axillae there exists a third type of
sweat gland tentatively designated as the apoeccrine sweat gland. This type of
gland shows a segmental or diffuse apocrinelike dilatation of its secretory
tubule but has a long and thin duct which does not open into a hair follicle.
The electron microscopy of its dilated segment is often indistinguishable from
that of the classical apocrine gland. The less remarkably dilated segment of
the apoeccrine gland tends to retain intercellular canaliculi and/or dark
cells. These apoeccrine glands are consistently present in adult human axillae
regardless of sex or race. In the axillae of the two 6-yr-old subjects, both
classical apocrine and eccrine glands were present but no apoeccrine glands
were found. Between 8-14 yr of age, the number of large eccrine glands with or
without partial segmental dilatation gradually increased. At 16-18 yr of age,
the number of apoeccrine glands increased to as high as 45% of the total
axillary glands. The data support the notion that apoeccrine glands develop
during puberty in the axillae from eccrine or eccrinelike sweat glands.
New Material:
Copyright 2006:
Testosterone
activates tyrosine hydroxylase (Neurosci Lett 2006; 396: 57-61). “Testosterone replacement therapy immediately
following castration prevents the decrease in TH [tyrosine hydroxylase]
levels.” (Brain Res Mol Brain Res 1992; 14: 79-82). Tyrosine hydroxylase is involved in human
pigmentation: “Our results support a direct function for tyrosine hydroxylase
in the melanosome via a concerted action with tyrosinase to promote
pigmentation.” (Exp Dermatol 2003; 12: 61-70).
Tyrosine hydroxylase has been “suggested” to be “tightly coupled to
melanin synthesis” (J Invest Dermatol 1990; 95: 9-15).
(Jablonski,
NG and Chaplin, G, J Human Evolution 2000; 39: 57-106). Jablonski and Chaplin
state: "Here we present new evidence indicating that variations
in skin color are adaptive, and are related to the
regulation of ultraviolet (UV) radiation penetration in
the integument and its direct and indirect effects on
fitness." The article concerns UV light and skin color. They also state:
"In all populations for which skin reflectance data were available for
males and females, females were found to be lighter skinned than males."
This
indicates to me that testosterone is involved in skin color.