Blog: The Scars of Evolution.

The Scars of Evolution

http://www.daylightatheism.org/2008/03/the-scars-of-evolution.html

Human beings, like all other species on this planet, have a history.
We came into existence through a process of slow, grinding
trial-and-error, occurring over geological time via the sieve of
differential survival. And like all species, our bodies and our genes
reflect and bear witness to that history. Far from being perfect,
one-time creations, we still bear the scars of the evolutionary
process that made us.

This post will discuss some of the lines of evidence which hint at
humanity's past. I won't repeat that well-known example of an
evolutionary vestige, the human appendix. Instead, I'm going to focus
on a few other examples that aren't as widely discussed.

? Toes. It's only because we're used to having toes that we don't
usually consider how strange they are. Why do our feet have these
stubby, non-functional digits on the ends? They can't grip nearly as
well as fingers, and we don't need them to balance or to walk. (Why
not just have a fused front of the foot?) By contrast, anyone who
observes other primate species can see that they have, not two hands
and two feet, but four hands, all of which are good for grasping. As
human beings gained the ability to stand and walk upright, our feet
lost their grasping function, but the digits themselves, though now
shrunken and largely useless, remain.

? Lanugo. This little-known developmental phenomenon is an important
clue to our mammalian past. Lanugo is a coat of fine, downy hair that
fetuses grow while in the womb, covering the entire body except for
the soles of the feet and the palms of the hands. Typically, lanugo is
shed by the seventh or eighth month of pregnancy, although premature
infants may retain it for several weeks after birth. The question is
why we grow it at all, and the theory of evolution can easily explain
this as a vestigial characteristic retained from our furry ancestors.

? Goosebumps. Fitting neatly together with lanugo is the vestigial
human trait called the pilomotor reflex. When a person is cold or
frightened, tiny muscles at the base of each hair contract, causing
body hair to stand on end. In animals with thicker fur, this is a
useful reflex: erect hairs trap air to create a layer of insulation,
and they also make the animal appear larger and more intimidating. In
humans, however, it is pointless. Like lanugo, goosebumps are a
giveaway clue indicating that relatively hairless human beings are
descended from furry progenitors.

? Hiccups. Yes, hiccups are a sign of humanity's evolutionary past. In
fact, unlike goosebumps or lanugo, which merely point to our shared
history with hairier mammals, hiccups point all the way back to the
time when humanity's ancestors were amphibians. According to this
article by Neil Shubin (HT: The Panda's Thumb), the hiccup reflex is
controlled by an area of the brain that we share with tadpoles. The
hiccup consists of a sharp inhalation followed by a closing of the
glottis (the valve at the top of the windpipe). In tadpoles, which
have this same reflex, the inhalation draws water into the mouth,
where the gills can process the oxygen it contains, but closes the
glottis so the water does not enter the lungs. For tadpoles, it's a
vital breathing reflex; in humans, it's a hiccup. And the same
measures that often arrest hiccups in human beings (inhaling carbon
dioxide, stretching the chest wall by taking a deep breath) also stop
the gill-breathing reflex in tadpoles.

? The true human tail. One of the most shocking - for creationists,
anyway - human atavisms is the true human tail. On rare occasions,
human infants are born with short, non-prehensile, but undeniably real
tails, up to several inches in length and containing nerves, blood
vessels, muscle fibers, and sometimes even extra vertebrae. They can
move through voluntary muscle contraction.

In fact, all human embryos grow tails while in the womb, and normally
they are reabsorbed before birth. The true human tail is the result
when this does not happen. Usually they are surgically removed,
although they are benign. For an evolutionary scientist, the reason
why we grow them is obvious: we are descended from an ancestor species
which had them. For creationists, who claim that human beings were
created complete and separate as we currently are, it must be
difficult to explain why we have so many vestigial structures that
link us to other species of mammals.

? The fused chromosome 2. It's long been known that human beings have
23 pairs of chromosomes, while the other great apes such as gorillas
and chimpanzees have 24. It is all but impossible that the lineage
that led to humans could have completely lost all this genetic
material and still produced a viable organism. Where, then, did the
extra chromosomes go?

Chromosomes are tipped with distinctive segments of DNA called
telomeres and have another special segment called a centromere in the
middle. Lo and behold, human chromosome 2 has a telomere on one end,
then an inactivated centromere, then a segment of telomeres in the
middle, then another centromere, then a final telomere - the structure
we would expect to find if two chromosomes had fused into one. When we
compare this chromosome to the two appropriate ape chromosomes, we
find a compelling match, indicating that this chromosomal fusion
occurred at some point after the human lineage split from our ape
relatives.

? The vitamin C pseudogene. Unlike most mammals, human beings can't
synthesize their own vitamin C; we must ingest it as part of our diet,
or else we get the disease of scurvy. Under the hypothesis of special
creation, humans were created this way from the beginning, so we
wouldn't expect evidence that we once had this ability but have since
lost it. However, according to evolution, we are descended from other
mammals, and since most mammals can make their own vitamin C, we'd
expect that human ancestors did have this ability at some point as
well. If this is so, our genes may preserve evidence of it.

Sure enough, human beings have a version of the vitamin C synthesis
gene, but ours is "broken", disabled by mutations. Our primate
relatives, who also lack this ability, also have broken versions of
the gene. Just as evolutionary theory would predict, the same
disabling mutations that exist in the human gene can be found in the
genes of chimpanzees, orangutans, and macaques - compelling evidence
that we are all descended from a primate common ancestor who incurred
this mutation at some point in the past. (It's likely that this
mutation wasn't selected against because all primate diets are rich in
fruit, providing abundant vitamin C.)

Taken together, the scars of evolution provide abundant evidence of
humanity's history. Like all species on this planet, we are not unique
special creations. We are one end result of a long process of mutation
sieved through selection, a countless series of adaptive compromises
and tradeoffs. Our very bodies testify to the natural forces that have
shaped us through the vast expanses of time.

March 10, 2008, 7:31 am

--
Bob.

.

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