Did Eve exist?
by Aaron Tockstein
In 1987, a group of scientists working with Alan Wilson from the University
of California at Berkeley claimed that by analyzing DNA from mitochondria,
they had traced the maternal lineage of all humans back to a single woman
who lived in Africa about 200,000 years ago. This study laid the groundwork
for the "Eve Hypothesis". Skeptics immediately raised the question:
could this really be true? Did Eve really exist?
The Wilson team used a computer program written to find the most "parsimonious"
family tree. Parsimony analysis is a method originally designed for deducing
the evolutionary relationships between species. Every tree's "length"
is equal to the minimum number of evolutionary changes (mutations) required
to explain the observed differences. The criterion for finding Eve says
that the tree requiring the fewest changes is preferred. Using the DNA
from the mitochondria from about 100 people, the Wilson group concluded
that the mother of all of us lived in Africa about 200,000 years ago. Mitochondrial
DNA (mtDNA) was chosen for the study because it accumulates mutations rapidly,
and because it is passed on intact from mother to offspring. Therefore,
it can be used to trace maternal ancestry without the complications of recombination.
Since Africans have greater diversity in their mtDNA than the inhabitants
of any other continent, it stands to reason that in order to accumulate
the largest number of mutations, humans must have lived longer in Africa
than anywhere else. This diversity is the strongest piece of evidence for
an African origin and was suggested before the Eve hypothesis was published.
Further evidence from the ZFY gene in males supports the Eve hypothesis.
ZFY, or zinc finger Y, is a gene on the Y-chromosome that helps with sperm
or testes maturation. It is handed down exclusively from father to son
and sits on a part of the Y-chromosome that doesn't recombine. Scientists
compared the DNA sequence of humans to chimps, orangutans, and gorillas.
They then divided the number of DNA mutations by the number of years since
the two shared a common ancestor. The results from this study show that
humans had to have originated roughly 270,000 years ago to have such similar
Y chromosomes. Most importantly, results from this experiment support the
Eve hypothesis, but from a different part of the genome. (Adler, Science
Harvard University molecular anthropologist Maryellen Ruvolo says that
by using sequence data and the use of a molecular clock instead of a simulation
program, she has come up with a more concrete time for the origin of Eve.
She says that Eve was around about 222,000 to 370,000 years ago. Though
her findings say nothing of the geographic origin of Eve, she was able to
come up with a time which supports that conclusion found by the Wilson group.
Most scientists have little trouble believing that one African woman was
the mother of us all. Even though the fossil record disagrees, paleontologists
agree that it is still very sparse. "The surprise was not that all
sequences traced back to one woman. Rather, because it claimed to locate
mitochondrial Eve in time and space, and to give information about the size
and movements of the human population as a whole." (Goldman, Nature)
The biggest question that arises from all this is shared by palaeoanthropologists
across the world. Why are the time estimates from the Eve hypothesis and
those from the fossil record so different? The Eve hypothesis and it's
backing evidence says that the origin of man was anywhere between 130,000
to 370,000 years ago, while the fossil record places the dawn of man at
over 1 MYA.
The question of whether or not Eve existed will remain challenged until
technology can come up with a better method for finding her. Using the
best of our technology and knowledge, great strides have been achieved towards
answering that question. At this point, most studies show that there was
a woman who lived 130,000 to 350,000 years ago that was the mother of mankind.
The greatest advancements of this subject will be in the near future, when
better methods for making decisions between alternative hypotheses, and
for estimating population sizes and gene flow from DNA sequence data will
enable scientists to better pinpoint the origin of humans.
Marcia Barinaga, 2/7/92, "'African Eve' Backers Beat a Retreat",
Science, Vol. 255, Pg. 686-687.
T. Adler, 5/27/95, "Lineage of Y chromosome boosts Eve theory",
Science News, Vol. 147, Pg. 326.
Ann Gibbons, 2/26/93, "Mitochondrial Eve Refuses to Die", Science,
Vol. 259, Pg. 1249-1250.
N. Goldman and N. H. Barton, 6/11/92, "Genetics and Geography",
Nature, Vol. 357, Pg. 440-441.