SCIENCE
Human Migration: ‘Out of Africa’
By Dr. Rizwana Rahim
P. O. Box 60267, Chicago
IL 60660-0267
The completely sequenced human genome shows that
we -- some six and a half billion Homo sapiens sapiens
on earth -- are 99.9% genetically alike. The rest
(0.1%) accounts for our differences (facial features
or physiognomy, color of our skin, eyes, hair, etc).
But where did we all come from is still being debated.
The view held by most paleo-anthropologists is that
modern humans came ‘out of Africa’ roughly
200 thousand years ago (TYA) – not in just
one big wave but several small ones over time. And,
this has been increasingly supported by genetic
information, in addition to fossil remains found
around the world. [The other view, ‘multi-regional’
theory, that evolution occurred concurrently in
different places around the world, lacks similar
genetic support].
In these population genomics studies, two genetic
approaches are involved. One deals with analysis
of DNA in mitochondria, which are mostly responsible
for cellular metabolism and energy. Each cell has
many mitochondria in the cytoplasm outside the nucleus.
Each mitochondria contain its own DNA, and since
it is completely sequenced (only 16,500 bases; much
smaller than nuclear DNA) it helps us follow the
mutations it accumulates over time. While nuclear
DNA of father recombines with mother’s, mtDNA
does not. Instead, mtDNA with its accumulated mutations
is passed intact from the mother to the child. Even
though focusing on a small fraction of mitochondrial
DNA (mtDNA) where mutation rate is highest (‘the
hot spot’ or ‘D-loop’, about 7%
of total mtDNA) is criticized, but the rest of mtDNA
(15,345 bases) is not all that big either to follow
and normalize the rate either.
The other approach deals with Y-chromosome (the
chromosome found only in males); most of it is transferred
from father to son.
These two lines of evidence go all the way to our
‘initial’ ancestors. Mutations occurring
at steady frequency over time in subsequent generations
over thousands of years and all around the world
are the telltale signs or the markers used in tracing
the history. So, following the markers in populations
around the world, one can make some ‘educated’
guess of the generalized human wanderlust.
Comparing mtDNA of women around the world, it was
found that the diversity in African women was twice
as much as found elsewhere. This led the scientists
to suggest that modern humans could have lived twice
as long in Africa as anywhere else. Based on other
information and extrapolating back in history, they
estimated that all human beings may be descendants
of a ‘Mitochondrial Eve’ who must have
lived with a “Y Chromosome Adam’ in
Africa, some 171.5 thousand years ago (TYA). There
were other African hunter-gatherers ancestors, but
the hereditary chain of mothers going back to that
‘Eve’ seems unbroken. Ancestral DNA
markers seem to be present most often in some African
tribes: San (southern Africa), the Biaka Pygmies
(central Africa) and in a few East African tribes.
The oldest modern human bones were found in Omo
Kibish, Ethiopia (estimated age 195 thousand years
old (TYO). Another relic found in Klasies River
Mouth in southern Africa is estimated to be 120
TYO.
What seems clearer now is that between 50 to 70
TYA, a small wave of these Africans (about 1,000)
may have been first to migrate out of Africa. These
emigrants carried a genetic marker (M168) on their
Y chromosome, which is shared by all non-Africans.
They followed three generalized paths out of Africa.
They didn’t have to cover long distances in
each generation, but over the millennia (and gradual
acclimatization, development of tools, other skills,
etc) they can spread far.
One path, up the Nile Valley, across the Sinai Peninsula,
north on to the Levant: Artifacts have been found
in Qafzeh cave, Israel (several human remains, 100
TYO, discovered in the 1930s), and Pestera cu Oase
(human mandible discovered in 2002 in southwestern
Romania, about 40 TYO) and farther into Europe.
There was some overlap in Europe with Neanderthals,
but there was no interbreeding, and the modern humans
had genetically diverged from Neanderthals 500 to
600 TYA.
Another route took them to north of Asia through
Siberia to the Bering Strait and across to Alaska.
Some of the notable finds on this route have been
Zhoukoudian (Shandingdong) ‘Peking Man’
skull discovered in 1929, some 11-18 TYO, and human
tools and artifacts, 30-40 TYO, in Yana River, north
in the Arctic Siberia. These people in Northern
Asia were the ones that migrated into the Americas.
When did this all occur is still being unclear,
but possibly between 15-20 TYA, when the sea-levels
were low (during the last Ice Age) and Siberia was
connected by land to Alaska. People took the West
coast routes, because at that time most of North
America was covered with ice. The sites of human
artifacts, from north to south America, include:
Kennewick, WA (Kennewick Man, discovered in 1996
(9.5 TYO), Spirit Cave, a burial ground in Nevada
(9.4 -9. 5 TYO), Clovis, a spear-point found in
New Mexico (13.5 TYO), Meadowcroft Rockshelter,
on the Ohio River in PA (12-19 TYO) and Monte Verde
in southern Chile (14.8 TYO). This migration occurred
in not just one but two to three distinct waves
(15-20 TYA).
An old genetic marker (M9) is found common in the
Y chromosomes of Eurasian populations of Central
Asia and Middle East. The genetic data also shows
similarities between the western Eurasian groups
and people of India; it is possible that there was
some migration between Asia and Europe, 30-40 TYA.
For the path through Asia to Australia, migrant
Africans probably walked across the mouth of Red
Sea (between the Horn of Africa and Arabia), which
was probably frozen 70 TYA when the last Ice Age
began. Perhaps they took a coastal route along the
Arabia Peninsula, to India and then (with more tools
and skills) to Australia. Their path is also strewn
with some fossils and other relics: in Malayasia
(human skull and others in Niah Cave; 40 TYO), and
two in Australia (Malakunanja near the northern
tip of Australia; foot prints and other relics;
50 TYO) and in Lake Mungo in southeastern Australia
(human skeletons cremated remains in Lake Mungo,
found in 1969 of a man and a woman in western NSW,
45 TYO, with underlying layers of soil bearing older
artifacts). This may be the earliest evidence of
modern humans in Australia. Except for genetic traces
in the Australian aborigines, and in some indigenous
groups (along the way in the Andaman Islands, Malaysia,
Papua New Guinea), no other signs are yet found
of their 8,000-mile journey from Africa to Australia.
The 5-year ‘Genographic Project’ launched
by the National Geographic magazine and other sponsors
is designed to pursue the population genomics, with
the help of 11 centers worldwide where the geneticists
will draw DNA from the blood and cheek swabs of
the people from at least a thousand indigenous populations
(as well as from fossils found). By following mutations
in populations and piecing together a line age,
they plan to reconstruct a molecular phylogenetic
tree and show human migrations around the planet.
National Geographic (March, 2006) has a cover story
on this project and TIME magazine (March 13) has
an exclusive on Kennewick Man.
References:
1. National Geographics: ‘The Greatest Journey
Ever Told: The Trail of our DNA. March 2006
2. TIME Magazine: The Untold Saga of Early Man in
America, 13 March 2006
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