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June 29, 2003

What was M.E.?

My post about Mitochondrial Eve attracted a bunch of interest - thank you! - and several people asked a key question: what species was ME?

There is no direct evidence about ME at all - we have not found a fossil record of this particular individual.  We can infer logically that she must have existed, and we can deduce approximately how long ago she lived from the amount of variation in Mitochondrial DNA (mtDNA) among present humans - about 200,000 years ago.  This analysis also indicates the probable region in which she lived - East Africa.

From the age and region, we can deduce the species, see the following timeline:

modern human family tree

As you can see, homo heidelbergensis was the direct ancestor species for homo sapiens.  This species has been further divided into homo sapiens archaic and home sapiens modern; a distinction made based on recent fossil finds.  The earliest human skulls were recently found in Ethiopia, dated about 160,000 years ago.  ME was most likely homo sapiens archaic, although based on this date and region, it is possible she was an early homo sapiens modern.  [ thanks to Dennis O'Neil for the diagram ]

One fascinating point to make in this connection is that there is never a "first organism" for any species.  Much like ME itself, the crown of "earliest organism" for a species is retrospective, since speciation takes place over thousands of years and can only be discerned gradually.  The classic definition of species - organisms which can interbreed - is not as cut and dried as one would like; there are often cases where A can breed with B, and B can breed with C, but A cannot breed with C.

It is interesting to speculate on the early history of ME and her ancestors.  Was there a catastrophic event which eliminated many of ME's competitors, funneling the genetic ancestry through a single line?  Perhaps ME migrated into a region which was spared from a climatic or other environmental event.  Or perhaps ME embodied a mutation which conferred immunity from a particular disease.  ME's daughters and granddaughters might have followed a single evolutionary path, living together in the same region and contributing to a common gene pool.  Or perhaps one or more daughters split off, forming subspecies which ultimately died out.

The transition period from H. sapiens archaic to H. sapiens modern is about 50,000 years, or about 250,000 human generations.  Although that seems like a lot, this is actually a short timescale from an evolutionary standpoint.  The genetic changes over this period would be slight.

It is suggestive that ME apparently lived right at the earliest time where the fossil record indicates the transition from H. sapiens archaic to H. sapiens modern.  The family tree for ME must have contained thousands of branches which did not successfully make it to the present day, although we know from the very definition of ME that it does contain at least two which did!

Posted by ole at 12:08 PM

I thought a human generation was considered to be about 20 years. 50,000 years would convert to 2500 generations, not 250,000.

Posted by: Matt Harris at July 5, 2003 10:32 PM