Out of Africa
Book Review of Before the Dawn
Recovering the Lost History of Our Ancestors
Penguin Books, 2007
About the Author: Nicholas Wade is a British-born scientific reporter, editor and author who currently writes for the Science Times section of The New York Times. Wade was born in Aylesbury, England and educated at Eton College and King’s College, Cambridge. He is the author of several books including The Nobel Duel, Betrayers of the Truth (co-authored with William J. Broad) and The Faith Instinct.
About 6.5 million to 5 million years ago, some 100,000 apes, the ancestors of today’s apes and humans, lived in East Africa in a forest shrinking due to climate cooling. There were two kinds of survivors of the climate change: one stayed with what was left of the forest and continued as before and it led to the chimpanzee; the other was the beginning of the human line, it survived by occupying the trees and the new spaces in between them. The human split from the chimpanzee occurred some 5-6 million years ago. These ancestors could survive in both terrains because of their critical new ability: to walk upright on two feet.
Australopithecenes appeared about 4.4 million years ago. A haunting trace of their presence was found in a trail of footprints left about 3.5 million years ago at Laetoli in Tanzania. Two individuals walked in the ash from a nearby volcano, leaving behind their human-looking footprints in tracks that extend for 165 feet. They retained the long arms of the chimpanzee and could move in the trees, but their brains were slightly larger than the chimp’s, about 400 to 500 cubic centimeters. As with the apes, males were about 50% larger than females, suggesting that australopithecene society was like that of chimpanzees with a separate male and female hierarchy.
From 3 to 2 million years ago another cooling shrank the forests and many species living in them went extinct. By this time the australopithecenes were adapted to open woodland and each evolved different solutions for survival. The robust australopithecenes evolved larger cheek teeth for eating coarse leaves.
The other, Homo habilis, evolved about 2.4 million years ago with smaller teeth, and as a scavenger of meat and bone marrow.
Meat-eating allowed for a smaller gut and jaw and also provided the nutrition that made possible a larger brain of 600 to 800 ccs. It’s fairly certain that habilis fashioned and used the primitive stone tools known as the Olduwan Industrial Complex, consisting of pebble cores and the rough flakes struck from them, that were first found with habilis remains in the Olduvai Gorge in eastern Africa.
Bipedalism and the beginning of a larger brain were two major steps in the process of changing the chimp-like ancestor into modern humans.
A third revolution occurred 1.7 million years ago in the form of a new species, Homo ergaster, whose skeleton shows most of the physical features of modern humans but whose brain measures about 800 ccs, which is below the modern capacity.
Ergaster’s arms were of human length, suggesting that it lived on the ground. The barrel-shaped chest and smaller stomach meant it ate a richer diet of meat and tubers. The sharp reduction in male-female size differences in ergaster is a transition from the apelike hierarchies to the start of a family structure in which males took some interest in protecting and feeding the mother of their children.
Although ergaster’s brain was only a bit larger than habilis’s, it was capable of sophisticated toolmaking. This Acheulian Industrial Tradition included the lozenge-shaped stones thought to be hand axes as well as cleavers and other large tools that may have been used for butchering, slitting hides, breaking bones, and woodworking.
Ergaster may have been the first of the human line to shed body hair. Human hairlessness may have come about when ergaster left the forest for the hot, dry savanna and evolved a way to cool the body and its larger brain. Sweating is an efficient way to achieve cooling but requires a naked skin.
After a vital gene has become universal in a population, it accumulates “silent mutations” that do not significantly alter the gene and are not eliminated by natural selection. Silent mutations accumulate at a known rate, so that the number of them is a measure of the time elapsed since the last version of the gene swept through a population. Using this, geneticists calculate the latest version of the African receptor gene (the gene involved in synthesizing vitamin D) swept through the ancestral population 1.2 million years ago. The advantage of light skin at high latitudes is that it allows more sun absorption, leading to increased production of vitamin D3, necessary for calcium absorption and bone growth. The lighter skin of women at higher latitudes most likely results from the higher calcium needs of women during pregnancy and lactation.This development was an advantage for emigrants settling in northern climates who needed lighter skin for exposure to the weaker sunlight to synthesize enough vitamin D. Dark skin can lead to vitamin D deficiency which can cause fatal cancers affecting the colon, lung and prostate. Dark-skinned people are also at higher risk for rickets, cardiovascular disease, diabetes and multiple sclerosis.
A close relative of ergaster, Homo erectus, reached Asia one million years or so ago, at approximately the same time ergaster reached both southern and northern Africa. By at least 500,000 years ago, our ancestors reached Europe, and these were perhaps descendents of ergaster. In Europe under glacial conditions, Homo heidelbergensis may have evolved into Homo neanderthalensis, a heavily boned, thickset people who adapted to the cold. Neanderthals are known as “archaic” humans to distinguish them from the lineage that remained in Africa and ultimately became modern. When the archaics left Africa, the human gene pool that remained eventually split into three main areas: Africa, Asia, and Europe. In Africa, our ancestors began to attain the skull size and skeleton of contemporary humans some 200,000 years ago, but as millennia passed, their behavior hardly altered. Their populations and social networks were small, and they seem neither to have hunted nor fished very much.