Recent research shows the mutation rate in mitochondria much higher than
previously thought (Loewe and Scherer 1997; Gibbons 1998). The date of
"Mitochondrial Eve," the common maternal ancestor of all humankind, was
based on that mutation rate. The revised molecular clock indicates that
she lived about 6500 years ago, not about 200,000 years ago as previously
claimed.
The claim is founded primarily on the work of Parsons et al. (1997),
who found that the substitution rate was about 25 times higher in the
mitochondria control region, which is less than 7% of the mitochondrial
genome (mtDNA). Revised studies of all of the mtDNA find that the
control region varies greatly in substitution rates in different
populations, but that the rest of the mtDNA shows no such variation
(Ingman et al. 2000). Using mtDNA excluding the control region, they
placed the age of the most recent common mitochondrial ancestor at
171,500 +/- 50,000 years ago.
Gibbons (1998) refers to mutations that cause heteroplasmy (inheritance
of two or more mtDNA sequences). This does not apply to mitochondrial
Eve research, which is based only on substitution mutation rates.
A study similar to the mtEve research was done on a region of the X
chromosome which does not recombine with the smaller Y chromosome; it
placed the most recent common ancestor 535,000 +/- 119,000 years ago
(Kaessmann et al. 1999). Since the population size of X chromosomes is
effectively three times larger than mitochondria (two X chromosomes
from women and one from men can get inherited), the most recent common
ancestor should be about three times older than that of the
Mitochondrial Eve, and it is.
Gibbons, A. 1998. Calibrating the mitochondrial clock. Science 279:
28-29.
Ingman, M., H. Kaessmann, S. Pääbo and U. Gyllensten. 2000.
Mitochondrial genome variation and the origin of modern humans.
Nature 408: 708-713.
Kaessmann, H., F. Heissig, A. von Haeseler and S. Pääbo.
1999. DNA sequence variation in a non-coding region of low
recombination on the human X chromosome. Nature Genetics 22: 78-81.
Loewe, L. and S. Scherer. 1997. Mitochondrial Eve: the plot thickens.
Trends in Ecology and Evolution 12(11): 422-423
Parsons, T. J. et al. 1997. A high observed substitution rate in the
human mitochondrial control region. Nature Genetics 15: 363-368.
