The
popular idea that Northern Europeans developed light skin to absorb more UV
light so they could make more vitamin D – vital for healthy bones and immune
function – is questioned by UC San Francisco researchers in a new study
published online in the journal Evolutionary Biology.
Ramping
up the skin's capacity to capture UV light to make vitamin D is indeed
important, according to a team led by Peter Elias, MD, a UCSF professor of
dermatology. However, Elias and colleagues concluded in their study that
changes in the skin's function as a barrier to the elements made a greater contribution
than alterations in skin pigment in the ability of Northern Europeans to make
vitamin D.
Elias'
team concluded that genetic mutations compromising the skin's ability to serve
as a barrier allowed fair-skinned Northern Europeans to populate latitudes
where too little ultraviolet B (UVB) light for vitamin D production penetrates
the atmosphere.
Among
scientists studying human evolution, it has been almost universally assumed
that the need to make more vitamin D at Northern latitudes drove genetic
mutations that reduce production of the pigment melanin, the main determinant
of skin tone, according to Elias.
"At
the higher latitudes of Great Britain, Scandinavia and the Baltic States, as
well as Northern Germany and France, very little UVB light reaches the Earth,
and it's the key wavelength required by the skin for vitamin D
generation," Elias said.
"While
is seems logical that the loss of the pigment melanin would serve as a
compensatory mechanism, allowing for more irradiation of the skin surface and
therefore more vitamin D production, this hypothesis is flawed for many
reasons," he continued. "For example, recent studies show that
dark-skinned humans make vitamin D after sun exposure as efficiently as
lightly-pigmented humans, and osteoporosis – which can be a sign of vitamin D
deficiency – is less common, rather than more common, in darkly-pigmented
humans."
Furthermore,
evidence for a south to north gradient in the prevalence of melanin mutations
is weaker than for this alternative explanation explored by Elias and
colleagues.
In
earlier research, Elias began studying the role of skin as a barrier to water
loss. He recently has focused on a specific skin-barrier protein called
filaggrin, which is broken down into a molecule called urocanic acid – the most
potent absorber of UVB light in the skin, according to Elias. "It's
certainly more important than melanin in lightly-pigmented skin," he said.
In
their new study, the researchers identified a strikingly higher prevalence of
inborn mutations in the filaggrin gene among Northern European populations. Up
to 10 percent of normal individuals carried mutations in the filaggrin gene in
these northern nations, in contrast to much lower mutation rates in southern
European, Asian and African populations.
Moreover,
higher filaggrin mutation rates, which result in a loss of urocanic acid,
correlated with higher vitamin D levels in the blood. Latitude-dependent
variations in melanin genes are not similarly associated with vitamin D levels,
according to Elias. This evidence suggests that changes in the skin barrier
played a role in Northern European's evolutionary adaptation to Northern
latitudes, the study concluded.
Yet,
there was an evolutionary tradeoff for these barrier-weakening filaggrin
mutations, Elias said. Mutation bearers have a tendency for very dry skin, and
are vulnerable to atopic dermatitis, asthma and food allergies. But these
diseases have appeared only recently, and did not become a problem until humans
began to live in densely populated urban environments, Elias said.
The
Elias lab has shown that pigmented skin provides a better skin barrier, which
he says was critically important for protection against dehydration and
infections among ancestral humans living in sub-Saharan Africa. But the need
for pigment to provide this extra protection waned as modern human populations
migrated northward over the past 60,000 years or so, Elias said, while the need
to absorb UVB light became greater, particularly for those humans who migrated
to the far North behind retreating glaciers less than 10,000 years ago.
The
data from the new study do not explain why Northern Europeans lost melanin. If
the need to make more vitamin D did not drive pigment loss, what did? Elias
speculates that, "Once human populations migrated northward, away from the
tropical onslaught of UVB, pigment was gradually lost in service of metabolic
conservation. The body will not waste precious energy and proteins to make
proteins that it no longer needs."
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