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versions less common._It may take many generations, however, for the shift in frequencies to become significant or for one version of the gene to supplant the alternative versions altogether. The belief that human evolution is essentially complete rests on the conjecture that it moves too slowly for significant changes to have occurred recently, for instance within the last 50,000 years.

But the human genome, just like those of all other species, can adapt quite quickly to changes in the environment. Without this capacity, humans would long ago have lapsed into extinction. At least four types of recent evolutionary change have already become evident and many more will doubtless come to light. The recent genetic changes already discovered include defenses against disease; increases in fertility; responses to cultural changes that affect the human environment; and changes in cognitive behavior.

Disease and other parasites are among the most serious threats to the welfare of large animals, and few diseases have presented a greater threat to human existence than malaria. Though the malaria parasite is very ancient, malaria is thought to have become a common disease among people only within the last 10,000 years, and perhaps within the last 5,000 years or so when slash-and-burn agriculture was introduced into West Africa. The sun-lit pools in the clearings would have provided an ideal breeding place for the mosquitoes that carry the parasite. Confronted with a severe and sudden threat like the Plasmodium falci parum form of malaria, natural selection will favor any helpful mutation that crops up. Several blood diseases, such as sickle cell anemia, have arisen because of changes in hemoglobin that protect against malaria. Another natural defense against the parasite is the impairment of an enzyme known as G6PD (for glucose-6-phosphate dehydrogenase) which kicks off the train of reactions leading to the metabolism of glucose. Genetic variations that sharply reduce the efficiency of the enzyme work wonders against the malaria parasite, although they also cause a serious blood disorder. There are two principal variants of the G6PD gene, which seem to have arisen independently of each other. One is found in African populations, the other in peoples of the Mediterranean. Sarah Tishkoff of the University of Maryland has dated the age of the two G6PD variants. The statistical method she used gives a broad range of possible ages, but all are consistent with the idea that the human genome has evolved its resistance to malaria only recently. The variation of the G6PD gene that is common in Africa arose sometime between 4,000 and 12,000 years ago, according to Tishkoffs calculations. The Mediterranean variant

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