Unknown Mother / African Eve, Part 2

Sex, Time, and Power: How Women's Sexuality Shaped Human Evolution
by Leonard Shlain, M.D.

(Page 6 of 6)

Scientists working in the field of evolutionary biology hypothesize a mechanism to explain how a new species often seems to appear all at once in the fossil record. Imagine an isolated local population of an existing species living in harmony with its ecosystem. Suddenly, some new, harmful environmental factor impinges upon the system; large numbers of the local population begin to die. At the eleventh hour, a beneficial random mutation (or mutations) that had previously occurred in the genes of one individual increases its owner's chances of surviving to the next generation. The offspring of this fortunate individual inherit the gene (or genes) and it quickly spreads. Within the span of several generations, the hard-pressed local population that was on the verge of extinction surges back by evolving an innovative suite of internal metabolic adjustments, physical changes, or modified behavioral responses that allows it to adapt to its new circumstances.

The animal to emerge sometimes differs so significantly from its predecessor that it can be categorized as an entirely new species. Scientists refer to this large dying off of the many so that the few (or even one) can evolve as "passing through a bottleneck." When there is a sudden discontinuity between a precursor species and a new one, some scientists propose that this evolutionary process is due to what they call "punctuated equilibrium."

Many conditions can precipitate bottlenecks. Geologic catastrophes, major volcanic eruptions, abrupt climatic changes (such as the sudden onset of ice ages), pluvials (periods of rains of Biblical proportions), and prolonged droughts can all position a species in the crosshairs of extinction. Epidemics of viruses, bacteria, or parasites can decimate food sources or attack the local population directly.

Approximately 150,000 years ago, in a small region of East Africa, around present-day Uganda, Kenya, and Tanzania, the current countries bordering Lake Victoria, one such bottleneck occurred. A local population of Homo erectus, a tool-making hominid, had been living there successfully for over a million years. Then some yet-to-be-identified event occurred that affected the survival of this particular group of hominids. From this stressed band, a single female known as Mitochondrial African Eve succeeded where Unknown Mother had failed, giving birth to the new species originally classified as Homo sapiens sapiens, the doubly wise human.

Though the exact birthdate of our species remains uncertain, the scenario that a single woman birthed the modern human species is on firmer scientific grounds thanks to the reliability of the new science of molecular biology. Laboratory tests performed on mitochondrial DNA can accurately measure the genetic variation that exists between members of a species and the differences existing among species. Scientists can then construct "molecular clocks" and calculate how long ago a particular species split away from its precursor.6 Molecular biology has proved to be the great Rosetta Stone of evolutionary changes. The existence of an African Eve is extremely likely, because the genetic material of all humans alive today is eerily similar.

The genes of chimpanzee communities inhabiting ranges only a few thousand yards apart have more genetic diversity than those of humans separated by oceans. Despite the dramatic differences in the skin pigmentation, eye color, body shapes, and hair types of people from disparate regions of the world, all humans are genetically homogeneous to an extraordinary degree. In fact, there is less than 0.1 percent difference between the gene structure of any one human and another. This suggests that each of us is a not-so-distant descendant of one fairly recent ancestral female. Since we have not had time to diverge very far genetically, our species' birthdate can be calculated backward in tens of thousands of years, instead of millions.

Some dire factor, condition, or event adversely affected the species that lived in the area around present-day Lake Victoria, leading to a population bottleneck. Let us call it Factor X. But what was X? What environmental challenge could have been the catalyst for the radiation of a new species? The geologic and archeological record is relatively silent. Variations in the local climate did occur, but none seems harsh enough to prompt our origin. Scientists have not identified sudden discontinuities in the area's flora or fauna. And yet some extreme condition must have occurred, for African Eve to burst forth like Athena fully formed from the brow of Zeus. Though there are many competing scientific theories, none has managed to gain sufficient support to explain the bottleneck fully.

I propose that the "bottleneck" through which our unfortunate immediate ancestors squeezed was actually a real bottleneck. Scientists scouring the landscape in search of an external Factor X may have been looking in the wrong place. The precipitating event that pushed a local population of hominids toward the edge of extinction was neither a climate change, a geological force, the arrival of predators or disappearance of prey, nor a shift in the availability of food resources. It was an internal, anatomical one.

The unyielding walls of the birth canal, like the alignment of Scylla and Charybdis, produced the bottleneck that shaped all subsequent hominid evolution. The death of the Unknown Mother and her unlucky baby, and the subsequent dying off of increasingly large numbers of hominid mothers and their newborns, was the stressful Factor X that precipitated the Homo sapiens line.

The muscles making up the human pelvic floor formerly served the genial function of wagging animals' tails. Natural Selection urgently pressed what was left of them into a new use in the bipedal hominid. They now served to buttress a potentially lethal defect. Some intermittently upright animals for example, penguins have evolved similar adaptations to defend against this problem, but in no other species is the gravitational hazard as serious as it is in humans.

The soft bones of a human baby's skull resemble tectonic plates. As the infant's head wends its way down the mother's tortuous birth canal, the plates slide and bend to conform to each twist and turn. To assist this molding process, the bony circle of the mother's birth canal relaxes. Under normal conditions, the iliac, pubic, ischial, and sacral bones are welded together by dense bridges of cartilage nearly as rigid as the bones they join. During delivery, however, this tissue undergoes a remarkable transformation, akin to concrete dissolving into Silly Putty. As the fetal head progresses, the pelvic circle, in a complementary maneuver, stretches imperceptibly, its new elasticity conveniently conforming to the mush-skull pushing through it.

* There have been sightings of dolphins and whales attempting to assist pregnant females of their kind with deliveries. In one spectacular example observed in captivity, three different species of dolphins were involved. A full-term female was in trouble: Her newborn's dorsal fin was caught in her pelvis. The second dolphin pulled out the baby and assisted the mother to raise it to the surface. While this was happening, the third female delivered the afterbirth, using her teeth. A few land mammals, such as rodents and primates, may also offer limited assistance to a female in labor. Despite these isolated reports, what is clear from observations of many animal births is that no other species' full-term females routinely signal their need for birth assistance. Contrary to popular myths, indigenous women do not simply go into the field and bear their babies alone. In a cross-cultural study of 296 peoples, only 24 reported that a woman on occasion has her baby without assistance. In none of the cultures studied was an unassisted first birth a routine event.

*A single gene's sequence of DNA contains the instructions for how to build a protein, which in turn can become an enzyme that further directs the building of an organism. Since there are many variations on the 3-D configuration of proteins and the timing of their entry into the building schedule, a single gene can have an enormous impact on the final form, metabolism, and responses of an organism.

To avoid a clutter of scientific terms, I will gloss over the subtle distinctions between the fossils referred to as Homo heidelbergensis, Homo ergaster, archaic Homo sapiens, and many other recent paleontological finds. Each new detail gleaned from studying these ancient bones adds incrementally to our understanding of the evolution of our species, but, unfortunately, to do the subject justice would, I believe, distract from my narrative. Also, a species can evolve in response to a positive development in its environment. A new untapped food source, for example, can prod a species into evolving novel adaptations to take advantage of the bounty. I conjecture that the dominant influence affecting our species was a negative one rather than a positive one, however.

* This term is now outmoded. With the recent identification of Neanderthal DNA, it is no longer necessary to call us Homo sapiens sapiens, except for occasional emphasis. For the rest of this book, I will use the current classification of our species, Homo sapiens.

* This is the "obstetrical dilemma" first described by Sherwood Washburn in 1960 and elaborated by others, particularly Wenda Trevathan and Karen Rosenberg.7 The major focus in the literature has been on the effect of difficult labor on child development. The narrowness of the human female pelvis caused infants to be born alitricial that is, extremely immature. According to estimates based on the size of other primate infants, the length of a human pregnancy should be eighteen months instead of nine. Bringing infants into the world long "before their time" created unique survival problems.

Helpless babies imposed immense child-rearing responsibilities on mothers, forcing a drastic division of labor between the human sexes. And it required women to enjoin men to assist them in raising their offspring, since failure in this endeavor would have fatal consequences for the entire species. It also created novel opportunities for children to have a longer period, called childhood, in which to learn. Much has been written about the consequences of the prolonged human childhood. I wish to redirect the focus away from immature infants to what I consider to be a relatively neglected aspect of the human species' obstetrical dilemma namely, maternal mortality.

* From a Greek myth, Scylla and Charybdis were two dangerous obstacles between which Odysseus' ship had to navigate during the odyssey.

© 2004 Penguin, a division of Penguin Putnam, used by permission.

About the Author

Leonard Shlain is the author of Art & Physics: Parallel Visions in Space, Time & Light, and The Alphabet Versus the Goddess: The Conflict Between Word and Image. He is the chief of laparoscopic surgery at California Medical Center in San Francisco.