While one
thousand people are to having their
genomes mapped to understand how genes influence disease, it is also explaining
world-history:
The Genographic Project is focusing on DNA from people still
living in their ancestral homelands because they provide the crucial geographic
link between genetic markers found today and routes traveled long ago.
Population genetics using modern DNA can elucidate migration issues in historical
as opposed to deep prehistorical periods. As descendants of the
hunter-gatherers who first left humanity’s birthplace in east Africa some
65,000 years ago, they split off in different directions. Thus distinct
mutations accumulated in the DNA of each population. Like bread crumbs, these
genetic markers, passed on intact for millennia, can reveal the trail of the
original pioneers. All non-Africans share a mutation that arose in the
ancestors of the first people to leave the continent, for instance. But the
descendants of those who headed north and lingered in the Middle East carry a
different marker from those who went southeast toward Asia. Most of the world’s six
billion people, however, are too far removed from wherever their ancestors
originally put down roots to be useful to population geneticists. In its first
18 months, the project’s scientists have had considerable success, persuading
more than 18,000 people in off-the-grid places like the east African island of
Pemba and the Tibesti Mountains of Chad to donate their DNA. The first large
effort to collect indigenous DNA since federal financing was withdrawn from a
similar proposal amid indigenous opposition in the mid-1990s, the Genographic
Project has drawn quiet applause from many geneticists for resurrecting
scientific ambitions that have grown more pressing. As indigenous groups
intermarry and disperse at an ever-accelerating pace, many scientists believe
the chance to capture human history is fast disappearing.
The story of our map as of today, however, can be updated as
such that we know our ancestors Homo erectus left Africa around 1.8 million
years ago, settling in the tropical and subtropical zones of Central and East
Asia. They died out approximately 100,000 years ago, except perhaps on remote
islands in southeast Asia. The ancestors of the Neandenals left Africa around
500,000 years ago, but again their line died out. Today, humans are the only
hominid species to leave descendants down to the present day. Even the early
Homo sapiens who made it into the Middle East around 110,000 years ago died out
30,000 ago.The first humans to leave Africa likely followed the southern Indian
coastline, reaching Australia.
The ancestors of modern western Europeans-the members of the
Rl b clan-started wandering into Europe in earnest around 35,000 years ago,
they encountered another hominid living there-the Neandertals, who would die
out. Yet despite their advantage over the Neandertals, the newomers
however would also forced into retreat as well by the cold European
climate. As large ice sheets advanced across Scandinavia and Britain, the human
population beat a retreat to the best possible place to sit it out-the beaches
of the Mediterranean.
A few humans, ended up in Spain, Italy, and the Balkans,
where the ice was kept at bay by the warm waters of the Mediterranean.But there
was a stroke of good luck. The climate, for reasons that still aren't fully
understood, started to change around 15,000 years ago. The weather warmed up,
and the ice sheets began to retreat. Taking advantage of the situation, human
populations started to advance back into northern Europe and took their genes
with them. The current distributions of the European genetic lineages are
largely due to this "overwintering" and subsequent expansion, coupled
with the migrations of new haplogroups importing agriculture.
As humans were being pushed into a corner in Europe, their
cousins in Asia were enduring a similar on slaught. Although small groups did
live in the far north, for the most part Asians were forced into their own
enclaves during the last ice age. Glaciers spreading from mountain ranges-the
Hindu Kush, Tien Shan, and Himalaya hemmed them in to different corners of the
continent. Indians were isolated in the subcontinent, and East Asians were
restricted to their core territory farther east, perhaps in the area of
Vietnam, Cambodia, and southern China.
Because humans at this time lived in small huntergatherer
populations, genetic drift served to change gene frequencies over time, as
these small groups split off from each other and migrated into new territory.
Genetic drift, for instance, explains most of the genetic patterns we see for
the mtDNA and Y-chromosome markers. As humans moved through Eurasia, then, the
forces of genetic drift, climatic adaptation, and sexual selection combined to
change their physical appearance. At the same time, other changes were taking
place-changes in the languages they spoke, as well as cultural developments
that allowed them to survive in their new locations. Southeast Asians would
never need to create warm, fur-lined clothes, and Siberian hunters would not
need to worry about sunburn most of the year. The early wanderings of our
species helped to mold humanity's original African features-from language to
skin color-into local, specialized varieties, producing the wide diversity we
see around the world today. Most of the differences that distinguish us today
probably arose during this period, within the past 40,000 years. While all of
this was going on in Asia, the oldest continuously inhabited continent outside
of Africa, with evidence for a human presence starting around 50,000 years ago
is Australia.
Drawings discovered at Kimberley, in northwestern Australia,
have depictions of human and animal figures. They were clearly created by
humans, but it is unclear exactly when. How do we determine the age of a rock
scratching-what sort of clock could we use? Here nuclear-dating methods fail
us, since there is no way to carbon-date a figure etched in stone. Or is there?
It turns out that there is a way, and it comes in the form of an ancient insect
infestation. Archaeologists studying the artwork at Kimberley noticed an
abandoned wasp's nest covering one of the figures, which means that it must
have been built after the figure was scratched into the rock.The nest could
have been constructed a hundred years ago, of course, in which case it wouldn't
have been terribly useful for dating the ancient art. But when they tested it,
using a technique known as optical luminescence, they were surprised to learn
that it was about 17,000 years old, which dates back to the Paleolithic.
Moreover, the wasp's nest was built over a human figure with an elaborate
headdress, which makes this the oldest human depiction in the world (the
drawings in European caves are almost all animals). Richard Roberts, the
researcher who tested the nest, thinks the drawings may be substantially older
than this, since the climate in Kimberley 17,000 years ago was much drier and
would not have been a likely place for human habitation.
But farther south in New South Wales, near Sydney, human
skeletons at Lake Mungo have been dated to between 45,000 and 50,000 years ago.
This makes them the earliest modern humans ever found outside of Africa,
predating the evidence in Asia by roughly 10,000 years. Is it possible that
these people could have sailed directly from Africa to Australia, bypassing the
Arabian Peninsula, India, and southeast Asia?
This conundrum has long puzzled archaeologists, with extreme
views dominating the debate. Some have argued that modern Australians are the
descendants of Homo erectus in southeast Asia, suggesting that they entered
Australia 100,000 years ago or more. Others have theorized an influx of modern
humans within the past 10,000 years. The truth lies somewhere in the middle,
and genetics has garnered the clues necessary to answer the question.
Modern Australian Aborigines are not descended from erectus
or other human cousins. Their DNA, while quite different from that found in
people living in Asia due to their long period of isolation, is clearly part of
the modern human lineage. Like all other non-African lineages, Australian
Aborigines ultimately trace their ancestry back to Africa . So how did humans
make it to Australia without apparently passing through Asia?
Today we live in a benign, relatively warm period known as an
interglacial, but in the past the world was a much colder place. The same
effect that gave rise to the Bering land bridge and allowed people to walk into
the Americas from Siberia changed sea levels worldwide. A drop of 100 meters,
or more than 300 feet, has been documented at various times during the last ice
age because the water was tied up in the great ice sheets of the far north.
While this amount may not seem like much (it's not much taller than a small
skyscraper), the vertical difference is less important than what happened to
the land area. Australia is no longer an island continent. Rather, it is
joined to New Guinea as part of the ancient continent of Sundaland.
This is because the Torres Strait separating the two
countries is quite shallow, even though the currents are very strong. Even
today Australia and New Guinea retain similar animal and plant species in the
tropical zone, with oddities such as the velociraptor-like cassowary, a
five-foot-tall predatory bird found in both northern Queensland and across the
strait in the forests of New Guinea. The similar flora also reflects their
ancient history as joined landmasses. In the same way, other landmasses were
much larger 50,000 years ago. The western coast of India would have been as
much as 200 kilometers west of where it is today, and Sri.Lanka would have been
joined to it. Most of island Malaysia would have been joined to the peninsula.
Overall, the coast would have been very different. It is
likely that this is the reason that there is no trace of modern humans along
the early route to Australia . Their camps are underwater today, subsumed under
the rising sea levels of the past 10,000 years. It suggests that the best place
to look for evidence of an early human presence in Asia might not be inland,
but rather along the near-shore ocean bottom. Intriguingly, the earliest
evidence of modern humans in the Indian subcontinent has been found in a cave
near the Sri Lankan shore, suggesting a treasure trove of Paleolithic stone
tools may lie just under the water's surface.But if the archaeological evidence
fails to provide us with the clues we need to determine the route early
Australians took, perhaps genetics can find the trail in the people living in
Asia today.
Humans migrating from Africa may have passed through India en
route to populate the rest of southern Asia and Australia. But tests on several
hundred samples collected in southern India late 2005, showed a clear genetic
link to Australia. The genetic link between his Y chromosome and Australia came
in the form of a marker known as RPS4Y The lineage found in southern Indians is
directly ancestral to the same C lineage found in Australians. This tells us
that C appeared first along the coastal route leading to Australia from Africa,
and only later did it migrate inland to Mongolia, probably via the coast of
East Asia.
Haplogroup C3 seems to have continued its coastal journey all
the way into the Americas, where some North American populations have fairly
high frequencies of this lineage. These groups typically belong to those
speaking the Na-Dene languages, the second major linguistic family in North
America. From the age of the C3 lineages in North America it appears that the
ancestors of the Na-Dene speakers entered the continent only in the past 8,000 years.
The Indian and southeast Asian distributions clearly show
that the descendants of these early coastal migrants are a minority today. Most
people in India, and in fact, more than 95 percent of Y chromosomes and 50
percent of mitochondrial lineages in Eurasia, Australia, and the Americas,
trace their ancestry back to a different migration-the one that led ancestors
into Central Asia 40,000 years ago-an inland route through the Middle East.
This new group of migrants would become one of the most influential in the
spread of humans worldwide.
During the last ice age, when sea levels were dropping along
the coastal route that led to India, inland conditions were also changing.
Because of the generally drier conditions, the savannas of East Africa were
expanding into areas that had formerly been forest. The Earth's climate was
going through a period of erratic weather, with rapid increases in temperature
interspersed among the generally lower temperatures. The result was that humans
were pushed and pulled in many directions, their range alternately expanding
and contracting. There is evidence that the Sahara was sometimes wetter than it
is today, as well as drier. During the wetter phases savanna would have
intruded into today's desert zone, allowing humans and the animals they hunted
to move northward.
These early bands of hunting and gathering humans would have
been drawn into the expanding grasslands by a wetter climate, but worsening
conditions could also have pushed them out. The neurobiologist William Calvin,
who has written on the effects of climate change on early human evolution,
compares the Sahara during this period to a kind of pump, drawing in animals
from other regions during wetter phases and expelling them when the weather
turned drier.
During one of these ourward-pumping phases a small group of
hominids left Africa 45,000 to 50,000 years ago and entered the Middle East.
The exact route they took is unknown, although current Genographic work on the
populations of the Sahara-particularly those in Chad, Sudan, and Egypt -should
shed some light on this. By 45,000 years ago, this small band was well
established in the Middle East and left behind ample evidence in the form of
skulls, tools, and other archaeological material. There is also a unique genetic
marker called M89, the ancestor of much of the male population of the Northern
Hemisphere. M89 appears to have been accompanied on his journey through the
grasslands by a woman belonging to the mitochondrial DNA clan N, who also left
Africa for the Middle East around this time.
This was not the first time that humans had migrated to the
Middle East. Skulls and other fossilized remains belonging to the Homo sapiens,
dating to 100,000 years ago, have been found at Qafzeh and Skuhl caves. But
then they suddenly vanish about 30,000 years later. There is a long gap in the
fossil record between their disappearance and when humans reappear in the
Middle East about 50,000 years ago.Once they were established on the Asian
landmass, though, these people spread rapidly. According to the genetic
evidence, these inland populations moved quickly into Central Asia and India,
with one line leading to Phil's ancestors via markers M9 and M45. M89 and
haplogroup N's descendants also spread into India and reached East Asia soon
after. Early humans probably took only 10,000 years to colonize the majority of
the Asian landmass below the Arctic. This final frontier would have to wait
until Phil's ancestors made their icy trek 20,000 years ago.The inland
clans-M89 and mtDNA-N-soon expanded across regions formerly inhabited only by
the coastal clans defined by M130 and mtDNA-M. Why they were so successful is
unknown-perhaps their populations had developed cultural attributes that gave
them an advantage. Clearly, though, frequencies of the coastal lineages today
are a fraction of their former levels, with almost all Indian men tracing their
ancestry back to the incoming grassland populations.
Still open questions that we are currently looking into are:
Canada, the United States, Mexico, and the Caribbean: How
many waves of migration were there to the Americas, and was one of the earliest
migrations along the west coast? Could Europeans have migrated to the Americas
thousands of years ago? Is there a genetic signal from the expansion of
indigenous American agriculture-i.e., did the farmers or their culture move?
South America: Were there any migrations to South America
from the Pacific? Did the Inca Empire have a genetic Impact on
northwestern South America? How do we account for the extraordinary linguistic
diversity found in native South Americans? Have populations there been separate
for a long period of time? Can we find genetic signals of now extinct groups,
like the Arawak in the Caribbean , in today's admixed populations?
Europe: Was there any mingling between modern humans and
Neandertals during the European Upper Paleolithic? What route(s) did
modern humans take in their settlement of Europe? Did the Celtic
expansion of the mid-first millennium B.C. leave a genetic trail? Can we detect
historical migrations during the past 2,000 years in to day's European
populations Normans in Sicily , or the spread of Finno-Ugric languages in
Eastern Europe? Middle East and North Africa: Did important imperial conquests
have an impact on the genetic landscape of the conquered regions? For instance,
did Alexander the Great's armies leave a genetic trail? Where did the
Afro-Asiatic languages, including Arabic and Hebrew, originate? Who were the
aboriginal inhabitants of North Africa, and are the Berbers their direct
descendants? How much genetic exchange has there been across the Sahara?
Sub-Saharan Africa: the most genetically diverse part of the
world where questions are; Which African populations harbor the most ancient
genetic lineages, perhaps suggesting a geographic origin for modern humans? How
has European colonialism had an impact on the genetic patterns in Africa? Can
we trace the origins of the Bantu people-and their expansion across Africa
-from genetic patterns? Was there a separate domestication of cattle in
Africa? Did this lead to a population expansion?
India, probably the oldest continuously inhabited region
outside of Africa. Its 400-plus languages and complex social system make
sampling a tremendously challenging proposition. So we are asking: Where did
the Dravidian speakers originate? Were they the "first" Indians? What
role has the Indian caste system had in determining genetic patterns? Where did
so called, Indo-European speakers originate, and what languages were being
spoken prior to their spread in Europe and Asia?
Middle East and North Africa: Did important imperial
conquests have an impact on the genetic landscape of the conquered regions? For
instance, did Alexander the Great's armies leave a genetic trail? Where did the
Afro-Asiatic languages, including Arabic and Hebrew, originate? Who were the
aboriginal inhabitants of North Africa, and are the Berbers their direct
descendants? How much genetic exchange has there been across the Sahara?
West Asia: When and where did modern humans first colonize
the Arctic? Who were the first inhabitants of the Caucasus region, and why is
there such incredible linguistic diversity there? What role did the Silk Road
play in dispersing genetic lineages?
East Asia: How has the geography of China molded genetic
patterns there? Who were the aboriginal inhabitants of Indonesia? Was there
much genetic exchange with Australia? Was there any mingling with Homo erectus
as modern humans spread throughout Southeast Asia? What are the patterns of
genetic variation in New Guinea? Do they parallel the extraordinary linguistic
diversity there?
Australia/New Zealand: How do the genetic patterns in
Australia correlate with the Aboriginal songlines-their own oral histories? Can
one use genetics to trace the spread of the Polynesians from island to island
in the Pacific? Is it possible to obtain intact DNA from the remains of Homo
erectus and other extinct hominid species, allowing us to resolve many debates
in the field of paleoanthropology and early human history? Can ancient DNA be
used to observe changes in the genetic composition of a population over time?
Referred to as “Genome-wide detection of human copy number
variations” to be published soon in Genome Research,
shows a variation in the genetic make-up of humans that could lead to a
fundamental reappraisal of what causes incurable diseases, noticing differences
between African, Asian and European groups.