Out of Africa
Book Review of Before the Dawn
Recovering the Lost History of Our Ancestors
Penguin Books, 2007
Modern Humans Arrive
A profound change from the physically modern humans of 100,000 years ago to behaviorally modern humans in only 50,000 years was the result of a genetic revolution. Modern behavior appeared prominently in Upper Paleolithic Europe about 45,000 years ago. These people had a new set of carefully crafted tools made of stone, bone, antler and ivory; they adorned themselves with items made of ivory and shell, and fashioned flutes of bird bone. They were avid fishermen and hunters of large game, and had developed trade networks to obtain distant materials.
Paleoanthropology and archaeology cannot provide information about the ancestral population of 50,000 years ago because this population vanished without a trace. Thus until very recently, it was thought that no deep understanding could ever be gained about the 5 million years of evolution from primate to physically modern human and the 45,000 years of prehistory that followed this evolution. The physical evidence for this time is unearthed fossil bones, skulls and stone tools. Geneticists, however, following a genome trail that leads back in time can now fill in unexpected detail.
A good example of this is the genetic history of the louse and what its relationship with our ancestors has revealed. When our forebears were fully covered with hair, like any other primate, the louse was free to travel from head to toe. As our ancestors shed their body hair, the louse was left with only the hair on the head. In time the body louse branched off from the head louse and developed small claws that can grasp material rather than shafts of hair. Archaeologists couldn’t tell when the first clothes were worn because the materials were perishable. But since the body louse evolved from the head louse approximately 72,000 years ago, we now have a date when our ancestors began to cover up (see the PBS video Lice and Human Evolution). About this time the ancestral population developed language and began to move out of Africa, eventually colonizing the rest of the world.
Information in the genome allows human ancestries to be traced through the male Y chromosome, which is passed down essentially unchanged from one generation to the next with males assigned to different lineages based on the pattern of mutations carried on their Y chromosome. Surprisingly, human genes have continued to evolve until the present day and carry the marks of recent evolution in response to diseases, changes in diet and climate.
The Neanderthals were formidable with large brains and a brawny build. They had developed some heavy weapons, including stone-tipped thrusting spears. They occupied the southeast corner of the Mediterranean that included the area that is now Israel. The human lineages evolving in Africa appear to have tried many times to emigrate into this wider world but were unsuccessful because of the threat from these outside groups.
A new line of inquiry about the origin of language was opened with the discovery of a gene involved in the fine aspects of language. The gene, FOXP2, shows no signs of having changed in chimps but has changed significantly in humans (click here for more on FOXP2). The gene came to light when a geneticist discovered a London family of three generations in which 15 of its 37 members have a severe language deficit and a defective FOXP2 gene. Their speech is very difficult to understand; the problems include the inability to repeat the same word three times in a row or form the past tense of verbs and difficulties with writing.
FOXP2 swept through the population and became universal, and the latest update of this gene was fairly recent, sometime within the last 200,000 years. FOXP2 may have been the last gene added to the human language faculty, perhaps a final step in the development of modern human speech.
In tracing populations genetically, the two parts of the human genome that are particularly useful are the Y chromosome that only males possess and the mitochondrial DNA, both of which are not subject to the usual shuffling of genetic material. When an egg is fused with a sperm, all of the sperm’s mitochondria is destroyed. So the male’s mitochondria cannot be passed on to his offspring, only the female’s mitochondria passes unchanged to a child of either sex.
All males in the world today carry the same Y chromosome inherited from a male who belonged to the ancestral population, and both males and females carry a copy of the same mitochondrial DNA that belonged to a female of the ancestral population.
The physically modern humans of 100,000 years ago showed no signs of modern behavior, no apparent capacity for innovation, and may have lacked the faculty for speech. In the relatively brief span of 50,000 years that followed the evolution of physically modern humans, a form of click language developed and the pace of evolution picked up, especially in the brain. These developments gave rise to the ancestral modern human population that left its Eden in East Africa 50,000 years ago and entered the larger world beyond.
When the first humans left Africa, India was their first stop. In India also, there was an historic parting of the ways. Some continued along the coasts of Asia, eventually reaching Australia, China, and Japan. Others pushed inland through Iran and Turkey. Both paths tested the ability of these modern humans to innovate and survive in hostile surroundings, and the long contest with the Neanderthals for Europe began.
The modern humans expanded into new territories as the communities split. Year by year, the territory of the modern humans expanded and that of the Neanderthals shrank. The Neanderthals did not yield easily. But by 30,000 years ago, the Neanderthals had disappeared from their last holdouts in the Iberian peninsula.