麻豆传媒

FOR a species honed by millions of years of evolution, we seem to be stuck
with a surprising number of features that are well past their sell-by date. Most
are useless but harmless accoutrements such as toenails and hairy legs, but
others are far from benign. We come equipped with the remnant of a tail, more
teeth than we have room for and an intestinal food trap that has kept anatomists
guessing for centuries. These vestigial bits and bobs not only appear to be good
for nothing, they also inflict pain and even death on millions of people
worldwide.

Take our surplus teeth鈥攖he third molars or wisdom teeth. Lurking at the
back of each side of the jaw, these are the last permanent teeth to erupt, and
most modern jaws cannot find space for them. In the lucky few, third molars
never emerge. For others they make a painful bid to join their fellows, often
during some stressful occasion such as exam time. Fortunately, at this age, the
jaw bone is still soft, making extraction comparatively easy.

But later, some time during middle age, unerupted wisdom teeth take on a
hard, brittle quality and attach rigidly to the bone. This makes them far more
difficult to remove without causing complications, ranging from nerve damage to
a broken jaw. To prevent all this, dentists have long debated whether to remove
wisdom teeth from every youthful jaw. Only recently have most of them agreed to
tolerate apparently trouble-free wisdom teeth.

But why do we have such dodgy dentition? At the root of the problem is a
mismatch between the number of teeth and size of our jaws. Anthropologists have
detected an evolutionary trend towards smaller jaws, but teeth do not seem to
have heard the news. In the 1970s, David Davies, an inquisitive scholar at the
University College dental school in London, attributed the 鈥渕assive鈥� jaws of
Eskimo women to their habit of chewing leather to make it pliant. The modern
diet, he surmised, could be too soft to give our teeth the exercise they need to
achieve their full potential.

Chewing power

Contemporary research supports Davies鈥檚 observations. 鈥淭he size and number of
human teeth that develop are genetically programmed,鈥� says Chris Stringer, head
of the human origins group at the Natural History Museum in London. 鈥淏ut jaw
size is largely determined by how much people use their jaws as they are growing
耻辫.鈥�

In our bodies, bones respond to the mechanical strain as muscles move against
them. 鈥淛aws almost certainly grow larger in response to chronic chewing, just as
bones in a tennis star鈥檚 serving arm grow to be as much as a third thicker than
in the other arm,鈥� says John Currey, a bone specialist at the University of
York.

One of the curious consequences of our mushy diet is that, were it not for
dentists, lots of Westerners would be dying in their prime from infections
triggered by impacted wisdom teeth. In parts of the world where dentists are
scarce, this selection pressure has fuelled an evolutionary trend to fewer
teeth. In parts of East Asia, says Stringer, people鈥檚 third molars show no signs
of erupting, and as much as a third of the population there have just two.

At the other end of the spinal column, evolution has seen off another
unneeded appendage鈥攁 tail. Some time in our distant past, the tail shrank
to leave the coccyx, a small triangle of fused and shrunken vertebrae. It gets
its scientific name from the Greek for cuckoo鈥攊t apparently resembles the
bird鈥檚 beak. But it鈥檚 of no use to us or any of our great ape relations who
share it, and it can be a real pain if you fall on it. 鈥淭here鈥檚 not much padding
to protect it, and so it is easily bruised by a fall,鈥� says Mike Adams, who
heads the spinal injury research group at the University of Bristol. 鈥淎nd it can
take a long time to heal.鈥�

And if orthopaedic specialists curse the coccyx, emergency surgeons loathe
the vermiform appendix. More than 50 000 appendicectomies are performed a year
in Britain alone, and the US has seen about 10 million since the turn of the
century.

Leonardo da Vinci was one of the first to illustrate this small tube, which
achieves an average length of about 9 centimetres. It empties into the caecum,
the beginning of the large intestine, at its junction with the small
intestine.

Troubles start with the appendix when something blocks the opening, often a
hardened lump of faecal matter. Prevented from emptying its mucus secretions,
the appendix swells, which in turn cuts off its blood supply. The bacteria
within multiply, worsening the inflammation. Eventually, if a surgeon doesn鈥檛
get there first, the beleaguered organ can burst and spread the infection
throughout the abdominal cavity.

But is it safe to say that the appendix is the most useless organ in the
world? Perhaps not, says Richard Williams, a pathologist at the University of
Melbourne and authority on the appendix. In humans, it is a 鈥渨ell-developed
structure鈥�, he says, shared only with our closest ape relatives. Although we
seem perfectly able to digest food without one, it is 鈥渁 complex rather than a
regressive structure鈥�. This implies that it has some value, he argues.

There is no shortage of suggested functions. One idea, dating from the 1900s,
is that the appendix helps us process plant food. Research, however, showed that
the appendix鈥檚 daily secretion of a millilitre or two of enzyme-containing fluid
is hardly impressive, and is soon lost in the wash of the caecum. In the 1930s,
some regarded the appendix as an anti-constipation device, designed to stimulate
the contraction of the colon. But people without an appendix proved to be no
more prone to blocked bowels than anyone else.

Present-day wisdom gives the appendix a role in immune surveillance, stopping
foreign invaders entering the large intestine. Research over the past decade has
shown that the structure is particularly rich in antibody-producing white blood
cells. 鈥淚t secretes antibodies into the gut and is clearly an immune organ,
although not a vital one,鈥� says John Cummings, a gastroenterologist at the
Medical Research Council鈥檚 Clinical Nutrition Unit in Cambridge.

The coccyx, too, is 鈥渘ot entirely useless鈥�, says Donal McNally, lecturer in
anatomy at the University of Bristol. 鈥淚t serves as an attachment point for
pelvic floor muscles.鈥� These handy muscles close off the anus and essentially
stop your guts falling out. And even wisdom teeth can prove useful, especially
if other molars wear down or decay. People blessed with the bone structure of a
Robert Redford are laughing鈥攕ometimes there鈥檚 room for even a fourth
molar. So do good-for-nothing organs exist? Who knows: perhaps you can never say
nothing鈥檚 no good.