Even if this were a small ancestral group instead of one woman, wouldn't that rule out fossil forms that existed much earlier than 200,000 years ago as our direct ancestors?
Not quite, this age is a theoretical date for the genetic population that became
Homo sapiens when it first diverged from those older direct ancestors, based on the theory of a common ancestor and fairly uniform gradual change in the DNA since then.
Note that the earliest fossil now known for
Homo sapiens comes from Ethiopia and is almost 200,000 years old:
From
160,000-year-old fossilized skulls uncovered in Ethiopia are oldest anatomically modern humans(click):
The fossilized skulls of two adults and one child discovered in the Afar region of eastern Ethiopia have been dated at 160,000 years, making them the oldest known fossils of modern humans, or Homo sapiens.
The skulls, dug up near a village called Herto, fill a major gap in the human fossil record, an era at the dawn of modern humans when the facial features and brain cases we recognize today as human first appeared.
The fossils date precisely from the time when biologists using genes to chart human evolution predicted that a genetic "Eve" lived somewhere in Africa and gave rise to all modern humans.
"We've lacked intermediate fossils between pre-humans and modern humans, between 100,000 and 300,000 years ago, and that's where the Herto fossils fit," said paleoanthropologist Tim White ....
The most complete of the three new fossil skulls, probably that of a male, is slightly larger than the extremes seen in modern Homo sapiens, yet it bears other characteristics within the range of modern humans - in particular, less prominent brow ridges than pre-Homo sapiens and a higher cranial vault. Because of these similarities, the researchers placed the fossils in the same genus and species as modern humans but appended a subspecies name - Homo sapiens idaltu -to differentiate them from contemporary humans, Homo sapiens sapiens.
Also see
Ethiopia is top choice for cradle of Homo sapiens -- if you have a log-in subscription to Nature.
This would be the youngest that
Homo sapiens could be, but it is still possible that older fossils will be found.
Thus this is pretty good confirmation of the theoretical genetic age being in the right ball-park.
There is also evidence that before spreading out to cover the globe that the founding population went through a "bottleneck" event
From
The new batch - 150,000 years ago (click):
Clues from genetics, archaeology and geology suggest our ancestors were nearly wiped out by one or more environmental catastrophes in the Late Pleistocene period. At one point, the numbers of modern humans living in the world may have dwindled to as few as 10,000 people.
By a strange twist of fate, the harsh conditions that caused this near extinction may also have allowed the cultural explosion that gave rise to human behaviour as we know it today.
Professor David Goldstein, a molecular biologist at University College in London, has uncovered evidence of a very ancient population bottleneck. A bottleneck is an event that reduces the genetic difference, or diversity, in a population of animals.
This puts the genetic information in a little different light: the "eve" may date from the bottleneck and not from the beginning of
Homo sapiens. OR this could explain the difference in dates for the mtDNA "eve" and the Y choromosome "adam" (which is younger, but based on the same genetic theory).
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