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| Source: National Geographic Genographic Project |
Human species is an African one: Africa is
where our species first evolved, and it is where we spent most of our time on
Earth.
The earliest fossils of recognizably
modern Homo sapiens appear in the paleontological record at Omo Kibish in
Ethiopia, around 200,000 years ago. Our evolution at that time ran parallel to
Neanderthals in Europe and in the Levant and to Homo erectus in Asia. Then
major climatic shifts began to happen — a sudden cooling in the Earth’s climate
triggered by the onset of one of the worst periods of the last Ice Age — that
would have made life difficult for our African ancestors. The genetic evidence
points to a sharp decline in population around this time. In fact, the human
population likely dropped to fewer than 10,000. We were holding on by a thread.
Once the climate started to improve, about
70,000 years ago, we came back from this near-extinction event. The population
expanded, and some intrepid explorers started to leave Africa between 60,000
and 70,000 years ago. As they ventured out of Africa, these explorers left
genetic footprints still visible today. By mapping the appearance and frequency
of genetic markers in modern people, scientists have created a picture of when
and where ancient humans moved around the world (see above).
The earliest people to colonize the
Eurasian landmass likely did so across the Bab-al-Mandab Strait separating
present-day Yemen from Djibouti. Once these people were out of Africa, a brief
period of interbreeding with Neanderthals occurred, helping to explain why
individuals of European and Asian heritage today retain traces of Neanderthal
DNA. These early beachcombers expanded rapidly along the coast to India and
reached Southeast Asia and Australia some 50,000 years ago. The first great
foray of our species beyond Africa had led us all the way across the globe.
Somewhat later, a little after 50,000
years ago, a second group appears to have set out on an inland trek, leaving
behind the certainties of life in the tropics to head out into the Middle East
and Southern Central Asia. From these base camps, they were poised to colonize
the northern latitudes of Asia, Europe, and beyond.
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Meadowcroft Rockshelter, near Pittsburgh,
is the oldest known
site of human habitation
in North America dating back 16,000 years ago.
Source: National Geographic
|
Around 20,000 years ago, a small group of these Asian hunters
headed into the face of the storm, entering the East Asian Arctic during the
Last Glacial Maximum. This era witnessed great ice sheets covering the far
north that had literally sucked up much of the Earth’s moisture in their vast
expanses of white wasteland, dropping sea levels by more than 300 feet. This
exposed a land bridge that connected the Old World to the New, joining Asia to
the Americas. In crossing it, the hunters had made the final great leap of the
human journey. By 15,000 years ago, they had penetrated the land south of the
ice, and within 1,000 years they had made it all the way to the tip of South
America. Some might have even made the journey by sea.
These great migrations over a 50,000-year
period eventually led the descendants of a small group of Africans to occupy
even the farthest reaches of the Earth.
The story doesn’t end there, of course.
The rise of agriculture around 10,000 years ago — and the population explosion
it created — has left a dramatic impact on the human gene pool. The rise of
empires, the astounding oceangoing voyages of the Polynesians, even the
extraordinary increase in global migration over the past 500 years could all
leave traces in our DNA. There are many human journey questions waiting to be
asked and answered.
Have you ever
wondered if you would want to test your DNA, perhaps after watching TV
commercials like 23andMe or Ancestry? What would a DNA test tell you
about your ancestors? What would it mean to you and how would it help your
genealogy? Human genomics has made significant progress in the recent
years to address our ancestral migration and in a way our own genealogical questions.
Population geneticists tend to focus on human
mitochondrial DNA (mtDNA) haplogroup — mitochondria, which is located outside
the nucleus, is the powerhouse of a cell — to seek answers to these questions. Mitochondrial
DNA not only tells us about people we match who are currently living, who share
ancestors with us at some point in time, but it also reaches back beyond the
range of what genealogy in the traditional sense can tell us — past the time
when surnames were adopted, peering into the misty veil of the past!
A haplogroup is a combination of alleles (a
variant form of a given gene) at different chromosome regions that are closely
linked and that tend to be inherited together. Think of a haplogroup as your
genetic clan. Haplogroups have a history and a pedigree chart, just like people
do. Those are commonly depicted as the mitochondrial phylogenetic tree. Haplogroups
and their branches can identify certain groups of people, such as people of
African descent, European, Asian, Jewish and Native American.
Haplogroups are labelled A through Z in
the order of their discovery. Mitochondrial DNA haplogroup is a haplogroup
defined by differences in human mtDNA. Haplogroups pertain to a single line of
descent and are used to represent the major branch points on the mitochondrial
phylogenetic tree. Every single one of your ancestors has their own individual
story to tell – and if you really want to know who you are and where each
ancestral line came from, mtDNA is the insider story on your mother’s
matrilineal line. Only females pass mtDNA on to their children, so all children
carry their mother’s mtDNA.
While the matrilineal DNA is passed intact
with no admixture from the father, occasionally mutations do happen — the rate
at which mtDNA mutates is known as the mitochondrial molecular clock, which is an
area of ongoing research with one study reporting one mutation per 8000 years —
and it’s those historical mutations that form clans and branches of clans as
generation after generation is born and continues to migrate to new areas. Understanding
the evolutionary path of the female lineage has helped population geneticists
trace the matrilineal inheritance of modern humans back to human origins in
Africa and the subsequent spread around the globe.
The hypothetical woman at the root of all
the mtDNA haplogroups on the mitochondrial phylogenetic tree is the matrilineal
most recent common ancestor (MRCA) for all currently living humans. This is the
most recent woman from whom all living humans descend in an unbroken line
purely through their mothers, and through the mothers of those mothers, back
until all lines converge on one woman. She is commonly called Mitochondrial Eve
(mtEve).
In terms of mitochondrial haplogroups, mtEve
is situated at the divergence of macro-haplogroup L into L0 and L1–6. As of
2013, estimates on the age of this split ranged at around 150,000 years ago in
East Africa. This analysis is consistent with a date later than the speciation
of Homo sapiens but earlier than the recent Out-of-Africa dispersal.
One common misconception surrounding mtEve
is that since all women alive today descended in a direct unbroken female line
from her, she must have been the only woman alive at the time. However, nuclear
DNA studies indicate that the size of the ancient human population never
dropped below tens of thousands. Other women living during Eve's time may have
descendants alive today but not in a direct female line.
The definition
of mtEve is fixed, but the woman in prehistory who fits this definition can
change. Because mtDNA mapping of humans is very incomplete, the
discovery of living mtDNA lines which predate our current concept of
"Mitochondrial Eve" could result in the title moving to an earlier
woman.
Of the lineages of macro-haplogroup L originating
in mtEve, haplogroup L3 represents the most common parent maternal lineage of
all people outside Africa, and for many individuals within the continent as
well. The time to MRCA for the L3 lineage has recently been estimated to date
to between 58,900 and 70,200 years ago. This is around the time of and
associated with the Out-of-Africa expansion of the ancestors of non-African
modern humans from Eastern Africa into Eurasia and with a similar expansion
within Africa also from the East of the continent.
The descendants of haplogroup L3 outside
Africa largely coalesce into macro-haplogroup M and its sibling macro-haplogroup
N. The latter is found in Eurasia and parts of Africa due to back-migration. Haplogroup
M, an enormous haplogroup spanning all the continents, represents more than 60%
of South Asian mtDNA. The origin of macro-haplogroup M is estimated to be
around 60,000 years ago. A sub-haplogroup M2, which is prevalent among the Bengalis
of India and Bangladesh (also in Andhra Pradesh, coastal Tamil Nadu and Sri
Lanka), is reported to be the oldest lineage of macro-haplogroup M on the
Indian sub-continent. Its age is between 30,200 and 42,400 years.
If you test your mtDNA using ancestry DNA
kit like National Geographic’s Geno 2.0, Family Tree DNA or AncestaryDNA,
you’ll receive a haplogroup assignment. If you opt for the entry level mtPlus
test, which only tests about 6% of the available mitochondrial markers, those
most likely to mutate, you will receive a macro-haplogroup like M, because
that’s all that can be determined by those markers. If you take the mtFull
Sequence test, which tests all the 16,569 mitochondrial locations, you will
receive a full haplogroup designation like M2a'b, plus a lot more.
What stories are waiting to be told in
your own DNA?
