快猫短视频

ARE YOU a glutton for hamburgers? Or more of a fish-and-chips person? Perhaps
you鈥檙e a vegan but sometimes can鈥檛 resist an egg. Whatever your dietary secrets,
Steve Macko will find them out鈥攁nd all he needs is a tiny lock of your
hair. Each strand contains telltale signs of your favourite foods, says Macko,
an organic geochemist at the University of Virginia in Charlottesville. 鈥淭hey
are pumping in from your breakfast right now.鈥� And once they are there, they鈥檙e
fixed鈥攁 lasting record of past meals.

Your eating habits may be no secret, but for archaeologists the chemical
signatures in hair are a great untapped resource, packed with clues to ancient
lives. Remains of food found at archaeological sites may be a misleading
indicator of diet, because some things are more perishable than others. But
excavated bodies, particularly mummies, often have a luxuriant head of hair. And
Macko鈥檚 latest tests show that ancient locks can show what their owners ate,
perhaps thousands of years ago, just as reliably as your hair says what you ate
last week.

Fruits of the sea

For archaeologists, diet is not just a question of what people had for
dinner. 鈥淚f you know what people ate, you know how they spent a vast amount of
their time,鈥� says Tamsin O鈥機onnell, a chemist at the Research Laboratory for
Archaeology and the History of Art at Oxford University. 鈥淚f they ate only
mussels or fish, you can say they were fishermen. If they ate barley and wheat,
you have farmers.鈥�

鈥淵ou are what you eat,鈥� says Macko. And hair, made of a protein called alpha
keratin, reflects that as much as any other tissue. The chemical signatures that
allow Macko and his colleagues to see what people have eaten are provided by
stable isotopes of carbon, nitrogen and sulphur, present in the amino acids that
make up keratin. In nature all three elements exist as a mixture of stable
forms. The light forms鈥攃arbon-12, nitrogen-14 and
sulphur-32鈥攑redominate. The heavier isotopes, burdened with extra
neutrons, are there in tiny, but measurable, amounts.

The proportion of heavy isotopes in food depends on where it came from and
its 鈥渢rophic level鈥濃�攊ts position in the food chain. In the case of plants,
it also depends on the biochemistry of photosynthesis. 鈥淲e cannot tell what kind
of bread a person ate but we can determine if they ate grains, or meat or fish
or vegetables. There is a great deal we can explain about a person鈥檚 diet
through hair,鈥� says Macko.

To prove the point, Macko and his colleague Michael Engel, a geochemist at
the University of Oklahoma, analysed hair from some of their undergraduate
students. Burning tiny samples of hair from each at 1000 掳C or more
generates a mix of gases, which can be separated by a gas chromatograph and
analysed in a mass spectrometer, which measures the amount of each isotope.

The amount of carbon-13 indicates broadly what sorts of plants the students
have eaten. When plants manufacture sugars during photosynthesis, they use one
of two sets of chemical reactions, driven by different enzymes. The reactions in
C3 plants鈥攚hich make up most of the world鈥檚 species鈥攄iscriminate
against the heavy isotope and end up with hardly any carbon-13 in their tissues.
C4 plants鈥攁 specialised group that includes important crops such as corn
(maize), sorghum, millet and sugar cane鈥攗se the two isotopes in proportion
to their concentration in the air. These carbon signals pass up through the food
chain so that hair from people who eat a lot of corn or corn-fed animals will be
higher in carbon-13 than hair from people who eat more wheat, or animals fed
grass and hay.

The nitrogen signature distinguishes vegetarians from meat-eaters. At each
step up the food chain, from plants to herbivores, to omnivores and carnivores,
the nitrogen-15 signal increases by a small but constant amount. 鈥淣o one yet
knows why, but it does,鈥� says 翱鈥机辞苍苍别濒濒. 鈥淚t has to do with the way metabolic
processes deal with nitrogen.鈥� Vegans have the lowest levels of nitrogen-15.
Vegetarians who eat eggs and milk are 鈥減seudo omnivores鈥� in terms of their
nitrogen signature, and enthusiastic meat eaters will have a much higher level
of nitrogen-15. People who eat a lot of fish have the highest levels of
nitrogen-15. That is partly because algae, which are at the base of the marine
food chain, have more nitrogen-15 in their tissues than land plants, but mostly
it reflects the longer food chains in the sea.

Large amounts of sulphur-34 in hair also reveal seafood eaters. Bacteria in
the sea that produce hydrogen sulphide discriminate against the heavy isotope,
so when the gas eventually escapes from the water, the ocean is left richer in
sulphur-34. Meanwhile the lighter isotope rains out over the land, altering the
balance of isotopes available to terrestrial plants.

Macko and Engel were not surprised to find a huge variation in isotope
signatures in their students鈥� hair. The variability is a sign of today鈥檚
鈥渟upermarket diet鈥�. 鈥淭here are lots of different foods and lots of different
eating habits,鈥� says Macko. If you plot the students鈥� combined carbon and
nitrogen signatures on a graph, they form a widely scattered cluster
characteristic of omnivores with lots of foods to choose from
(see Diagram).

A few stood out from the crowd. One was clearly a keen carnivore, with a
nitrogen signal that put him high up the food chain. He also had more carbon-13
than his classmates, suggesting that the animal that provided his meat lived on
a diet of C4 plants. He turned out to be a member of the college baseball team
who ate almost nothing but burgers鈥攎ade with meat from corn-fed cattle.
Another student stood out for her high nitrogen-15 signal and relatively low
carbon-13鈥攁 combination that made her a top carnivore, but not one eating
corn-fed meat. This student came from a family which ate a lot of fish. After
six months away from home, her signature fell into line with the rest of the
class.

And then there was the vegan. Or was she? 鈥淗er signal was odd,鈥� says Macko.
It was too rich in nitrogen-15 for someone so close to the bottom of the food
chain. 鈥淭hen she owned up to eating the odd slice of Virginia ham,鈥� says Macko.
鈥淥ur data were telling the truth.鈥�

Macko reckoned that if he could pick out the food foibles of his students
from their isotope signatures, then it ought to be possible to predict what
long-dead people had been eating from samples of their hair. Archaeologists have
hunted for clues to ancient diets among the stable isotopes of bone collagen.
But even under the best conditions, says Macko, collagen suffers from decay and
weathering after death. The method of extracting collagen is also difficult, and
means destroying sizeable amounts of bone. 鈥淔or archaeologists, parting with
bones is a big deal,鈥� says Engel. 鈥淏ut who cares about a strand of hair?鈥�

Macko and Engel believe that hair is a more reliable source of information
than collagen. Although hair is not always present at archaeological sites, when
it is there is often plenty of it. 鈥淵ou find it in dry areas, in frozen places
and in bogs,鈥� says Macko.

When Macko and Engel compared the chemistry of hair from modern people and
ancient mummies, they found that the amino acids in ancient keratin survived in
exactly the same proportions. They also bombarded hair with radiation to mimic
ageing. Still the amino acids remained the same. And if these proportions don鈥檛
change, then neither should the proportions of stable isotopes. 鈥淭he stability
of collagen can be suspect,鈥� says Macko. 鈥淏ut even in 5000-year-old hair, the
amino acids look the same as if it was cut from a living scalp today. So when
you do have hair, you know it鈥檚 well-preserved.鈥�

Hair is remarkably resilient. Age does not change it, nor do modern hair
treatments or natural dyes of the sort ancient people might have used.
O鈥機onnell鈥檚 experiments with shampoos showed that they make no difference to the
results. 鈥淵ou can compare modern and ancient hair without worrying about modern
treatments,鈥� she says. Dyes don鈥檛 seem to have any effect on the result either.
鈥淚 tried henna鈥攁 beauty treatment for thousands of years鈥攁nd that
didn鈥檛 make any difference,鈥� she says. Nor does it matter if the hair is brown
or grey鈥攐r which part of the body it comes from.

Hair on ancient corpses might have been treated in other ways. The ancient
Egyptian embalmers speeded the drying of bodies with natron (sodium carbonate),
a salt gathered from saltpans and lakes. And sometimes they anointed hair with
palm oil. Neither changes the chemistry of the hair, says Macko. 鈥淎lmost nothing
we do changes its composition. This makes hair the ideal artefact to address
questions of diet.鈥�

So far, Macko and Engel have analysed hair from two very different groups of
Egyptian mummies: ancient aristocrats who lived around 4000 years ago, and
Christian Copts from around the 8th century. From the Americas, they have
analysed some of the famous Chinchorro mummies from Chile, which are between 800
and 5000 years old. They also analysed hair from 脰tzi, the Neolithic man who
died on a high Alpine pass 5300 years ago.

Where possible, Macko analyses the remains of food from the same
archaeological sites, so he can compare hair signatures directly with the foods
people were eating rather than modern foods. This way, he doesn鈥檛 have to worry
about the influence of artificial nitrogen fertilisers, or the shift in carbon
signals as a result of burning fossil fuels. Food remains from the Chinchorro
sites include corn, a tuber called jiquina, the remains of shellfish and
sea-lion bones. In the case of the Copts, Macko had samples of many local
plants, including cow beans, wheat, and seeds from grapes, dates and
watermelons.

The hair from Egypt revealed that these two peoples鈥攕eparated by
thousands of years by race, religion and class鈥攁te very different diets.
The Copts, whose bodies were mummified naturally in the desert sands, represent
a cross section of society: rich, poor and middle-class. 鈥淭here was a huge range
of foods available鈥攁lmost like the supermarket diet,鈥� says Macko. 鈥淪ome
ate vegetables, some ate fish and some ate meat.鈥� In fact, the Copts鈥� eating
habits were not unlike those of today鈥檚 undergraduates.

Exclusive eating

The older mummies from Egypt鈥檚 Middle Kingdom period had a far more
restricted diet. These were the Egyptian elite, wealthy enough to afford the
best embalmers. Hair from these aristocrats gave a very uniform signal,
suggesting that they all ate much the same, including a significant amount of
meat. 鈥淲e don鈥檛 know if this reflects what they were allowed to eat or what they
chose to eat,鈥� says Macko.

The Chinchorro mummies came from three burial sites in the Atacama desert in
northern Chile. The people buried at Morro, on the coast, ate seafood and little
else. Their isotope signatures suggest that perhaps 95 per cent of their food
came from the sea. At Maderas Enco, 5 kilometres inland, people ate a wider
variety of food, but still about half of it was seafood. The third site was in
the Azapa valley, around 30 kilometres from the sea. With one exception, these
people had been eating a mixture of C3 and C4 plants and a little meat, perhaps
llama, plus a small amount of seafood. The odd mummy out had been eating almost
nothing but seafood. Who was this person, who clearly came from the coast but
was buried high up in the desert? 鈥淢aybe he was a travelling salesman, perhaps a
fishmonger from the coast,鈥� says Macko. Whoever he was, his hair shows that the
Chinchorros had dealings with people from quite distant settlements. 鈥淭his is
evidence you couldn鈥檛 have got any other way,鈥� says Macko.

And then there鈥檚 脰tzi the iceman. There is only one 脰tzi, so Macko and his
colleagues were lucky to have even a scrap of his hair. What they found has
raised a few eyebrows. The iceman鈥檚 isotope signature matches that of a modern
vegan who eats mostly C3 plants. Opinions vary on who 脰tzi was鈥攑erhaps a
shepherd or a shaman or a hunter. But he wore clothes made from skin and carried
weapons designed for hunting (鈥淏lood on the axe鈥�, 快猫短视频, 12
September, p 40). The wear on 脰tzi鈥檚 teeth suggests a diet rich in grains and
plant material. But was he really a vegan?

Hair grows about a centimetre a month, so a change of diet, even for a couple
of months, can leave its mark. 脰tzi鈥檚 isotope signature could correspond to a
time of plenty, with a good supply of grains, berries and fruits. Hair that grew
in the depths of winter might tell a different story, with more reliance on
dried meat to tide 脰tzi over the winter.

The speed with which isotope signatures appear in hair makes it ideal for
teasing out information on changes in diet over the course of a year or
two鈥攇iven a long enough strand of hair. By sampling at intervals along it,
researchers should be able to spot seasonal variations, with a switch to a
different source of food, or even a regular rotation of crops. 鈥淗air is
fantastic if you have a suspicion that people had a seasonal diet,鈥� says
翱鈥机辞苍苍别濒濒.

Macko is now keen to look at samples of hair from a wider range of ancient
people, including the Bronze Age bog bodies of Denmark. Most of these
sacrificial victims have fine heads of hair but little bone, which dissolved in
the acid bog waters. Some of these bodies still have the remains of their last
meal in their stomachs. But as Macko points out, a last meal can be very
different from the everyday diet. Hair from a collection of around two hundred
mummies discovered recently in the Peruvian rainforest could reveal much about
the lives of forest dwellers, whose diet consists mainly of forest fruits and
game.

But what Macko would really like is hair from people suspected of eating
their neighbours. 鈥淐annibals should be a step higher up the food chain from
people with a more normal human diet,鈥� he says. 鈥淭hey鈥檇 certainly be different
from those on a grocery-store diet.鈥�