Mutants (33 page)

The right signal can even bring about the unexpected resurrection of organs long buried by evolution. Birds don’t have teeth, but their dinosaur ancestors certainly did. If a piece of ectoderm from a foetal chicken’s beak is grafted onto a piece of mesoderm from a foetal mouse’s mandible, and both are placed in the eye-orbit of a young mouse, the chicken tissue, which has not seen a tooth for sixty million years, suddenly begins to make them: hen’s teeth, shaped something like tiny molars, complete with dentine and enamel. This implies that the molecular signals used by
Tyrannosaurus rex
to make its mighty fangs are the same that a mouse uses to make its miniature molars. Signals that chickens just seem to have lost.

* * *

Perhaps it is also the retrieval of an ancient signalling system, partly buried by evolution, that causes some people to have extra nipples or even breasts. Humans and great apes have only two nipples but most mammals have many more. Sometimes extra nipples are little more than a small dark bump somewhere on the abdomen; at other times they are fully developed breasts. They are common: between 2 and 10 per cent of the population have at least one. In Europeans extra nipples or breasts are usually found somewhere below the normal ones, often in a line running directly down the abdomen. Japanese women, curiously, seem to get them above the normal breasts, often in the armpits.

S
UPERNUMERARY BREAST ON THIGH
.

These patterns of extra nipples may recollect an ancient ‘milk line’ – a row of ten pairs of teats that ran from the armpits to the thighs in some ancestral mammal. Armpit breasts are found in the lemur,
Gaelopithecus volans
, and the record number of nipples found on a single person seems to be nine (five on one side, four on the other). Wherever they are, extra breasts often work like normal ones, swelling and even lactating during pregnancy, and there are even accounts of women suckling children from supernumerary thigh-breasts. Extra nipples and breasts run in families, though the mutation (or mutations) that causes them has not been identified. However, a group of London researchers are attempting to determine the mutation behind a strain of mice that have eight nipples instead of the usual six. They have already dubbed the gene
Scaramanga
– for the villain of the James Bond film
The Man with the Golden Gun
who had, as a mark of his depravity, a supernumerary nipple on his upper left chest.

ARTEMIS EPHESIA

Breasts bring us back to Linnaeus. In 1761, made famous by
Systema naturae
, Linnaeus published one of his lesser-known works, a synopsis of the Swedish animals called
Fauna svecica
. The name was revolutionary: it was the first time that the word ‘fauna’ – from the Roman name for Pan-the-God – had been used to describe a work of this sort; a direct counterpart to the ‘floras’ that were already proliferating. As a frontispiece to this work Linnaeus chose a curious emblem, a representation of the Greek goddess Artemis, or Diana, of Ephesus. We don’t know
why he picked this particular emblem, but there are several possibilities.

Artemis Ephesia was, in the inexplicably duplicitous way of Greek deities, goddess of both nature and cities. In her original incarnation as the object of a cult that flourished in Asia Minor from around the sixth century BC, her image was hung on city walls to protect them from evil, while being surrounded by icons
of the country: garlands of vines and climbing animals such as lions, snakes, birds and harpies. Retrieved from the ruins of Ephesus, the eighteenth century made her into a symbol of wild-ness and of reason. The Jacobins even dedicated a Temple of Reason to her that once stood in Strasbourg, but is now gone. Perhaps this is why Linnaeus placed her at the front of his
Fauna
– as a symbol of the mastery of Reason over Nature, albeit a Swedish nature, in which, far from her Mediterranean home, Artemis stands among browsing reindeer.

A
RTEMIS
E
PHESIA IN
S
WEDEN
. F
RONTISPIECE OF
L
INNAEUS
1761
Fauna svecica
.

But perhaps she had another, more direct, meaning for Linnaeus as well. What is most striking about his Artemis are not the animals that surround her, but her four prominent breasts. In this she is a direct echo of the statues of her in antiquity, all of which are laden with a varying number of thoracic and abdominal protuberances. In the Renaissance these bumps were invariably interpreted as a case of extreme polymastia, but more sceptical modern scholars say they are more likely to have simply been strings of dates, bulls’ testicles, or perhaps just part of the cuirass in which the goddess was clad. Be that as it may, Linnaeus’ Artemis obviously has four fine breasts, and it seems quite possible that they are a direct allusion to one of his finest inventions, the Mammalia. For Linnaeus made the presence of mammary glands one of the defining features of what we are: members of that great class of creatures that embraces simultaneously the pygmy shrew and the blue whale.

There is a third possible source of Linnaeus’ Artemis, one that brings us back to where we started – the way in which we differentiate ourselves from the rest of brute creation. When
describing a species, Linnaeus did what taxonomists still do – he listed the things that distinguish it from all others. For all species, that is, but one: our own. When it came to
Homo sapiens
, instead of speaking of the number and kinds of teeth we have, the density of our hair, the distribution of our nipples, Linnaeus wrote only this:
Nosce te ipsum
. In a footnote he says that these are the words of Solon written in letters of gold upon the temple of Diana. Perhaps in choosing Artemis Ephesia as his icono-graphic symbol, Linnaeus is remembering and alluding to this account of the human species, the most concise possible: know thyself.

That is where Linnaeus’ discussion of
Homo sapiens
ends, but for a few strangely exigent epigrams in which he instructs us in the meaning of the new identity that he has given us. ‘Know thyself,’ he says, created by God; blessed with minds with which to worship Him; as the most perfect and wonderful of machines; as masters of the animals; as the lords of creation – all sentiments that today ring with the poignancy of certainties long since gone. Yet it is his parting shot that is most telling, and that could be taken as epigrammatic of much of what I have written here:

Know thyself, pathologically, what a fragile bubble you are, and exposed to a thousand calamities.

If you understand these things, you are man, and a genus very distinct from all the others.

[O
N AGEING
]

H
UNTINGTON DISEASE
is one of the nastier neurodegenerative syndromes. It usually first appears as a mild psychosis and does not seem especially serious. But, as the disease progresses, the psychotic episodes increase in frequency and severity. Motor-coordination also deteriorates, a characteristic rigidity of gait and movement sets in and then, eventually, paralysis. In the disorder’s final phase, which can take up to ten or twenty years to appear, the patient becomes demented and experiences neural seizures, one of which is eventually fatal. The disease is caused by dominant mutations that disable a protein used in synaptic connections of the brain’s neurons. For reasons that are not fully understood, the mutant form of the protein initiates a molecular programme that gradually kills the neurons instead.

L
UIGI
C
ORNARO
(1464–1566). T
INTORETTO
.

Huntington disease has several strange features. One is the way in which its symptoms become more severe from one generation to the next. This phenomenon, called ‘anticipation’, arises from a peculiarity of the Huntington gene itself and the mutations that cause the disease. The gene contains a region in which three nucleotides, CAG, are repeated over and over again. Most people have between eight and thirty-six of these repeats. Huntington disease mutations increase the number of repeats, so disordering the structure of the protein. Several mutations of this sort cripple the protein ever further over successive generations, increasing the severity of the disease.

Another oddity of Huntington is its frequency. It afflicts about 1 in 10,000 Europeans. This is very high – most dominant mutations that kill have frequencies of about one in a million. But Huntington disease can persist in a family for generations. In 1872, George Huntington, a New York physician, described the disorder from families in Long Island, New York. Among their ancestors was one Jeffrey Ferris who emigrated from Leicester, England, in 1634. He almost certainly had the disease, as do many of his descendants today. In South Africa, about two hundred Huntington’s patients are descended from Elsje Cloetens, the daughter of a Dutchman who arrived with Jan van Riebeeck to found the Cape Colony in 1652. A large group of Huntington’s patients who live near Lake Maracaibo, Venezula, are the decendants of a German sailor who landed there in 1860.

How can so lethal a disorder transcend the span of so many generations? In 1941 the brilliant and eccentric British
geneticist J.B.S. Haldane proposed an answer. He pointed out that, unlike most genetic disorders, the symptoms of Huntington disease usually appear in middle age. By this time most people with the defective gene have had their children – each of whom will have had a 50 per cent chance of inheriting the defective gene. Unlike most lethal dominant mutations that kill in childhood and so are never transmitted to the following generation, the Huntington mutation hardly impairs the reproductive success of those who bear it. Middle age is almost invisible to natural selection.

Few other disorders caused by a single mutation have such devastating effects so late in life. Yet the strangeness of Huntington disease is deceptive, for Haldane’s explanation of why it is so common also explains, with a little generalisation, why we, and most other animals, age. In this chapter I will argue that ageing is a genetic disorder, or rather, it is many genetic disorders, some of which afflict us all, others of which afflict only some of us. This point of view goes against the grain of most definitions of disease. Medical tradition distinguishes between ‘normal’ ageing, about which nothing much is done, and ‘age-related diseases’, such as arteriosclerosis, cancer and osteoporosis, that consume vast amounts of national health budgets. But this distinction is an illusion, a necessary medical fiction that allows physicians to ignore a disease that affects us all but which they are impotent to cure or even ameliorate. Properly understood, ageing is precisely what it seems: a grim and universal affliction.

IMPOTENT SELECTION

Ageing is the intrinsic decline of our bodies. Its most obvious manifestation is the increased rate at which we die as we grow older. An eight-year-old child in a developed country has about a 1 in 5000 chance of not seeing her next birthday; for an eighty-year-old it is about 1 in 20. Of course, it is possible to be killed by causes quite unrelated to ageing – violence, contagious disease, accidents – but their collective toll is quite small. Were it not for ageing’s pervasive effects, 95 per cent of us would celebrate our centenaries; half of us would better the biblical Patriarchs by centuries and live for more than a thousand years. We could see in the fourth millennium
AD
.