(Written with the Judeo-Christian story in mind, but scientific information is valuable) Eve and Her Tree

Eve and Her Tree


By
Stephen Jay Gould
Discover Magazine July, 1992

The Eve of biblical legend was a temptress, thus initiating a lamentable theme in the history of sexism. Unfortunately, her latest incarnation--as the so-called Eve theory of human origins--has equal power to mislead, even while embodying (as did Eve herself) fruits of great merit and consequence.

No subject has won more popular attention in press reports on human evolution during the past five years. In 1987, in the leading British journal Nature, Rebecca Cann, Mark Stoneking, and Allan Wilson published data suggesting that the mitochondrial DNA of all modern humans had a common ancestry in Africa some 200,000 years ago. (Wilson, who initiated this research at the University of California at Berkeley, died prematurely a year ago, and we mourn and deeply miss one of our favorite and most brilliant colleagues.) Their argument, if correct, is enormously exciting in its implications. (Wilson and his co-workers have responded to critics of their original work and have extended their conclusions in a more recent article, published in Science last September.) Unfortunately, they also sowed unnecessary confusion by giving a misleading, if snappy, name to their work--the “Eve hypothesis.” I would not fret unduly if the actual content of the theory were not so important, and if the ill-chosen name did not so effectively mask what should be featured. Two false impressions need correction.

First, contrary to what some people have imagined, the Eve theory has nothing to do with feminism, or with a reversal of androcentric biases that have permeated the history of anthropology. Our mitochondrial ancestor is Eve, rather than Adam, for a technical reason only. Although most DNA resides in chromosomes within the nuclei of our cells, the mitochondria (our cells’ energy factories) also include a relatively small amount of DNA. Both egg and sperm cells contain mitochondria, of course; without a power supply, sperm could not move. But the business end of a sperm, the part that unites with the egg in fertilization, includes no mitochondria and is, effectively, only a nucleus. Consequently, all our mitochondria, for both men and women, are inherited from our mothers alone. For a variety of reasons, including its rapid rate of change (important when calibrating a chronology for young species like Homo sapiens), mitochondrial DNA is especially well suited for studying evolution.

Wilson and colleagues proceeded by measuring mitochondrial differences among modern humans of all major racial groups. Assuming a constant rate of evolutionary change, they then extrapolated back to a most recent common ancestor with a common mitochondrial sequence. This ancestor is “mitochondrial Eve.” She is Eve because mitochondria are inherited in maternal lines alone. When we do a similar analysis--and several are now under way--using exclusively male genes on the Y chromosome, we will be able to speak of Adam.

Second, the singular nature of Eve is not a quasi-creationist argument for a pinpoint origin of humanity in a unique woman. All evolutionary reconstructions, using Wilson’s methodology, work back to a common ancestral state. You take current diversity, map it on the copiously branching tree of life, and try to work back to a common stem. That stem is a coherent ancestral population--a group of proto-people. In any evolving population, most people leave no ultimate offspring, and one or a few members produce all the descendants. (We know this well from studies of human family names and lineages; evolution, a massively chancy and basically destructive process, must lead to a lot of dead ends, as does any branching mechanism in our largely random world.) The Eve hypothesis does not feature a single mother, divorced from the evolutionary reality of life in ordinary populations, but rather claims that we can trace mitochondrial diversity to a common ancestor (or to several closely related women bearing the same mitochondrial gene sequence) within a population in Africa some 200,000 years ago.

The true excitement of the theory arises from the phrase “in Africa some 200,000 years ago,” not in any implication drawn from the misleading name Eve. And the main controversy also emerges from this claim. In fact, just recently, several publications have challenged the way Wilson and his colleagues used computer routines to generate their evolutionary trees in support of an African origin of Homo sapiens (though in my opinion, their out-of-Africa scenario 200,000 years ago remains the best hypothesis).

For reasons of cultural bias, rather than compelling data, conventional views had envisaged the brain power of Homo sapiens as arising simultaneously yet independently among populations spread all over the Old World. (Homo erectus, our ancestral species, did move from Africa to Europe and Asia more than a million years ago. So powerful are the evolutionary advantages of bigger brains, or so the argument went, that natural selection drove Homo erectus populations toward our exalted brainy state on all continents.) This idea reinforces our psychic desire to consider our species a predictable phenomenon now ruling by right and necessity. I have labeled such views as “tendency theories” of human origin.

But if we shared common ancestry in Africa only 200,000 years ago, then these older Homo erectus populations in Europe and Asia are not ancestral to Homo sapiens, and we evolved from a later branching event in Africa. Our spread throughout the world was therefore more recent, and our rise to dominance more tenuous and less predictable. In short, we become the results of a fortunate, singular historical event in Africa--the branching of an ancestral population into a twig that made us all. We are a thing, a singular event, an item of history--not the predictable result of necessary improvement. I have labeled these more humbling views as “entity theories” of human origin.

There is nothing surprising, in the slightest way, about such theories. They represent the way that evolution works all the time. Evolution is a copiously branching bush, not a ladder of progress; and although certain broad features are repeated from time to time (all flying vertebrates have wings of similar aerodynamic form, despite the separate evolution of wings in birds, bats, and pterosaurs), when and where individual twigs appear is quite unpredictable in our highly contingent world. We simply have to get one item of hubris-breaking, arrogance- smashing truth into our big heads: Homo sapiens is one of the little twigs, not one of the grand, overarching predictabilities.

Here Eve could perhaps help us. She plucked the fruit from the right object to secure our proper understanding of evolution--a tree. Perhaps the search for knowledge she thus initiated might also help us to obtain that even greater goal, wisdom, which Eve’s author rendered by the same image: “She is a tree of life to them that lay hold upon her; and happy is every one that retaineth her” (Proverbs 3:18).


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