Are most South Indians the descendents of Northern Men and Southern Women ?

It suggests that many modern south Indians are descended from
southern-fringe women, but few from southern-fringe men-implying a
comprehensive conquest of the southerners by the northerners, who won
extra southern wives.

The long march of everyman
Dec 20th 2005
The Economist

It all started in Africa

OUT of Africa, always something new", wrote Caius Plinius Secundus, a
Roman polymath who helped to invent the field of natural history. Pliny
wrote more truly than he could possibly have realised. For one fine
day, somewhere between 85,000 and 60,000 years before he penned those
words, something did put its foot over the line that modern geographers
draw to separate Africa from Asia. And that something-or, rather,
somebody-did indeed start something new, namely the peopling of the
world.

Writing the story of the spread of humanity is one of the triumphs of
modern science, not least because the ink used to do it was so
unexpected. Like students of other past life forms, researchers into
humanity's prehistoric past started by looking in the rocks. The first
fossilised human to be recognised as such was unearthed in 1856 in the
Neander Valley near Dusseldorf in Germany. Neanderthal man, as this
skeleton and its kin became known, is now seen as a cousin of modern
humans rather than an ancestor, and subsequent digging has revealed a
branching tree of humanity whose root can be traced back more than 4m
years (see article).

Searching for human fossils, though, is a frustrating exercise. For
most of their existence, people were marginal creatures. Bones from
periods prior to the invention of agriculture are therefore excedingly
rare. The resulting data vacuum was filled by speculation scarcely
worthy of the name of theory, which seemed to change with every new
discovery. Then, in the 1980s, a geneticist called Allan Wilson decided
to redefine the meaning of the word "fossil". He did so in a way
that instantly revealed another 6 billion specimens, for Wilson's
method made a fossil out of every human alive.

Living fossils
In retrospect, Wilson's insight, like many of the best, is blindingly
obvious. He knew, as any biologist would, that an organism's DNA
carries a record of its evolutionary past. In principle, looking at
similarities and differences in the DNA sequences of living organisms
should allow a researcher to reconstruct the family tree linking those
organisms. In practice, the sexual mixing that happens with each
generation makes this tedious even with today's DNA-analysis
techniques. With those available in the 1980s it would have been
impossible. Wilson, however, realised he could cut through the problem
by concentrating on an unusual type of DNA called mitochondrial DNA.

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Mitochondria are the parts of a cell that convert energy stored in
sugar into a form that the rest of the cell can use. Most of a cell's
genes are in its nucleus, but mitochondria, which are the descendants
of bacteria that linked up with one of humanity's unicellular ancestors
some 2 billion years ago, retain a few genes of their own.
Mitochondrial genomes are easy to study for three reasons. First, they
are small, which makes them simple to analyse. Second, mitochondria
reproduce asexually, so any changes between the generations are caused
by mutation rather than sexual mixing. Third, in humans at least,
mitochondria are inherited only from the mother.

In 1987, Rebecca Cann, one of Wilson's students, applied his insight to
a series of specimens taken from people whose ancestors came from
different parts of the world. By analysing the mutational differences
that had accumulated since their mitochondria shared a common ancestor,
she was able to construct a matriline (or, perhaps more accurately, a
matritree) connecting them.

The result was a revelation. Whichever way you drew the tree
(statistics not being an exact science, there was more than one
solution), its root was in Africa. Homo sapiens was thus unveiled as an
African species. But Dr Cann went further. Using estimates of how often
mutations appear in mitochondrial DNA (the so-called molecular clock),
she and Wilson did some matridendrochronology. The result suggests that
all the lines converge on the ovaries of a single woman who lived some
150,000 years ago.

There was much excited reporting at the time about the discovery and
dating of this African "Eve". She was not, to be clear, the first
female Homo sapiens. Fossil evidence suggests the species is at least
200,000 years old, and may be older than that. And you can now do a
similar trick for the patriline using part of the male (Y) chromosome
in the cell nucleus, because this passes only from father to son.
Unfortunately for romantics, the most recent common ancestor of the
Y-chromosome is a lot more recent than its mitochondrial equivalent.
African Adam was born 60,000-90,000 years ago, and so could not have
met African Eve. Nevertheless, these two pieces of DNA as they have
weaved their ways down the generations have filled in, in surprising
detail, the highways and byways of human migration across the face of
the planet.

Sons of Adam, daughters of Eve
Detail, however is not the same as consensus, and there are two schools
of thought about how people left Africa in the first place.
Appropriately, some of their main protagonists are at the rival English
universities of Oxford and Cambridge. The Oxford school, championed by
Stephen Oppenheimer, believes that the descendants of a single
emigration some 85,000 years ago, across the strait of Bab el Mandeb at
the southern end of the Red Sea, are responsible for populating the
rest of the world. The Cambridge school, championed by Robert Foley and
Marta Mirazón Lahr, agrees that there was, indeed, a migration across
this strait, though probably nearer to 60,000 years ago. However, it
argues that many non-Africans are the descendants of at least one
subsequent exodus.


Both schools agree that the Bab el Mandebites spread rapidly along the
coast of southern Arabia and thence along the south coast of Asia to
Australia, though Dr Oppenheimer has them turning inland, too, once
they crossed the strait of Hormuz. But it is in describing what
happened next that the two versions really part company, for it is here
that the descendants of the Oxford migration run into the eruption of
Toba.

That Toba devastated South and South-East Asia is not in doubt. Thick
layers of ash from the eruption have been found as far afield as
northern Pakistan. The question is whether there were people in Asia at
the time. One of the most important pieces of evidence for Dr
Oppenheimer's version of events is some stone tools in the ash layer in
Malaysia, which he thinks were made by Homo sapiens. Molecular clocks
have a regrettable margin of error, but radioactive dating is a lot
more accurate. If he is right, modern humans must have left Africa
before the eruption. The tools might, however, have been crafted by an
earlier species of human that lived there before Homo sapiens.

For Dr Oppenheimer, the eruption was a crucial event, dividing the
nascent human population of Asia into two disconnected parts, which
then recolonised the intermediate ground. In the Cambridge version,
Homo sapiens was still confined to Africa 74,000 years ago, and would
merely have suffered the equivalent of a nuclear winter, not an
ash-fall of up to five metres-though Dr Ambrose and his colleagues
think even that would have done the population no good.

The Cambridge version is far more gentle. The descendants of its
subsequent exodus expanded north-eastwards into central Asia, and
thence scattered north, south, east and west-though in a spirit of
open-mindedness, Sacha Jones, a research student in Dr Foley's
department, is looking in the ash layer in India to see what she can
find there.

Which version is correct should eventually be determined by the
Genographic Project, a huge DNA-sampling study organised by Spencer
Wells, a geneticist, at the behest of America's National Geographic
Society and IBM. But both already have a lot in common. Both, for
example, agree that the Americas seem to have been colonised by at
least two groups. The Cambridge school, though, argues that one of
these is derived ultimately from the first Bab el Mandeb crossing while
the other is descended from the later migrants.

Both also agree that Europe received two waves of migration. The
ancestors of the bulk of modern Europeans came via central Asia about
35,000 years ago, though some people in the Balkans and other parts of
southern Europe trace their lines back to an earlier migration from the
Middle East. But the spread of agriculture from its Middle Eastern
cradle into the farthest reaches of Europe does not, as some
researchers once thought, seem to have been accompanied by a mass
movement of Middle Eastern farmers.

The coming together of two groups of humans can be seen in modern
India, too. In the south of the subcontinent, people have Y-chromosomes
derived almost exclusively from what the Cambridge school would
interpret as being northern folk (and the Oxford school as the western
survivors of Toba). However, more than 20% of their mitochondria
arrived in Asia with the first migration from Africa (or, according to
taste, clung on along the south-eastern fringes of the ash plume).

That discovery speaks volumes about what happened when the two groups
met. It suggests that many modern south Indians are descended from
southern-fringe women, but few from southern-fringe men-implying a
comprehensive conquest of the southerners by the northerners, who won
extra southern wives.

This observation, in turn, helps explain why Y-chromosome Adam lived so
much more recently than mitochondrial Eve. Displacement by conquest is
one example of a more general phenomenon-that the number of offspring
sired by individual males is more variable than the number born by
individual females. This means that more males than females end up with
no offspring at all. Male gene lines therefore die out faster than
female ones, so those remaining are more likely, statistically, to
converge in the recent past.

Successful male gene lines, though, can be very successful indeed.
Students of animal behaviour refer to the top male in a group as the
"alpha". Such dominant animals keep the others under control and
father a large proportion, if not all, of the group's offspring. One of
the curiosities of modern life is that voters tend to elect alpha males
to high office, and then affect surprise when they behave like alphas
outside politics too. But in the days when alphas had to fight rather
than scheme their way to the top, they tended to enjoy the sexual
spoils more openly. And there were few males more alpha in their
behaviour than Genghis Khan, a man reported to have had about 500 wives
and concubines, not to mention the sexual opportunities that come with
conquest. It is probably no coincidence, therefore, that one man in
every 12 of those who live within the frontiers of what was once the
Mongol empire (and, indeed, one in 200 of all men alive today) have a
stretch of DNA on their Y-chromosomes that dates back to the time and
birthplace of the great Khan.

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