Shadows of Forgotten Ancestors

By the 1870s and 1880s, the evidence amassed by Darwin was changing many minds. Reviews were acknowledging “the certainty of the action of natural selection,” and even the possibility that humans evolved from some lower animal.¹⁵ However, some of the conclusions of Darwin’s 1871 book, The Descent of Man, stuck in the craws of even the most sympathetic reviewers. The debate, we find, had moved into a new arena:We deny [animals] … the power of reflecting of their own existences, or of inquiring into the nature of objects and their causes. We deny that they know that they know, or know themselves in knowing In other words, we deny them reason.

We return to this new level of debate later, and here note only how quickly many of the theological reservations about evolution had dissipated as Darwin’s argument became better understood. “Nothing is more remarkable,” he wrote in his Autobiography, “than the spread of scepticism or rationalism during the latter half of my life.”¹⁶

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Of innumerable modern examples of natural selection in the real world, we select one—of interest because it involves humans and because it is the outcome of an experiment, although one performed inadvertently and under tragic circumstances. Malaria is endemic among nearly half the people of the world (just before World War II, the number was two thirds of all humans). It is a serious illness associated, in the absence of appropriate medicine or natural immunity, with high mortality. Even today several million people die from it each year. When the Plasmodium parasite causing malaria is injected (usually by mosquito bite) into the bloodstream, it eventually invades the red blood cells that carry oxygen from the lungs to every cell of the body. The red blood cells are rendered sticky, adhere to the walls of very small blood vessels, and are prevented from being circulated to the spleen—which destroys Plasmodium parasites. This is good for the parasites and bad for the humans.

People in malarial zones of tropical Africa, as elsewhere, have an adaptation to malaria, the sickle-cell trait. Under the microscope some of the red blood cells do look a little bit like sickles or croissants. But in someone with the sickle-cell trait, the altered red blood cells are surrounded by needle-like microscopic filaments that work, it is suggested, a little like a porcupine’s quills. The parasites are impaled or otherwise damaged, and the red blood cells—protected from the parasites’ sticky proteins—are then carried to the “untender mercies” of the spleen. With the parasites dead, many of the red blood cells return to their normal state, “unruffled” by the experience.¹⁷ However, when the genes for this trait are inherited from both parents, serious anemia, obstruction of the small blood vessels, and other infirmities often result. The trade-off, it is natural to think, is that it’s better for a part of the population to be seriously anemic than for most of the population to be dead of malaria.

In the seventeenth century slave traders from Holland arrived in the Gold Coast of West Africa (present-day Ghana). They bought or captured slaves in large numbers and transported them to two Dutch colonies—Curaçao in the Caribbean and Surinam in South America. There is no malaria in Curaçao, so the sickle-cell trait conferred anemia but no compensating advantage to the slaves brought there. But malaria is endemic in Surinam, and the sickle-cell trait was often the difference between life and death.

If now, some three centuries later, we examine the descendants of these slaves, we find that those in Curaçao show hardly any incidence of the trait, while it remains prevalent in Surinam. In Curaçao the sickle-cell trait was “selected against”; in Surinam, as in West Africa, it was “selected for.” We see natural selection operating on very short time scales, even for such slowly reproducing beings as humans,¹⁸ As always, there is a range of hereditary predispositions in a given population; the environment elicits some but not others. Evolution is the product of a hand-in-hand interplay between heredity and environment.

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At the end of his life, Darwin called himself a theist, a believer in a First Cause. He had doubts, though:[C]an the mind of man, which has, as I fully believe, been developed from a mind as low as that possessed by the lowest animal, be trusted when it draws such grand conclusions?¹⁹