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By Root 1993 0

susceptibility to infectious disease might have a second cause: helper-T cells, which like HLAs help the immune system recognize foreign objects. To simplify considerably, helper-T cells occur in two main types, one that targets microorganisms and one that targets parasites. The body cannot sustain large numbers of both, and hence adult immune systems tend to be skewed toward one or the other, usually depending on whether as children they were more often exposed to microorganisms or parasites. Indians have historically been burdened by flukes, tapeworms, and nematodes, so they have long had majorities of parasite-fighting helper-T cells. Europeans, who grew up in germ-filled environments, usually lean the other way. As a result, the three researchers suggested, adult Indians were—and possibly still are—more vulnerable to infectious diseases than adult Europeans. Conversely, Europeans would be comparatively more vulnerable to parasites. If further research supports this hypothesis, preventing childhood parasite infections might allow Indian immune systems to orientate themselves toward bacteria and viruses, possibly reducing future deaths.

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*13 Historians increasingly shy away from the term “Aztec,” because the nineteenth-century naturalist Alexander von Humboldt coined it in a misapprehension. Humboldt’s “Aztecs” were actually the people of three nations, the members of the Triple Alliance.

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*14 I use the hedge words “basically,” “almost,” and “in essence” because sperm actually have 50 to 100 mitochondria, just enough to power them through their short lives. By contrast, the egg has as many as 100,000 mitochondria. When the sperm joins the egg, the egg eliminates sperm mitochondria. Every now and then, though, a few escape destruction and end up in the embryo’s cells.

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*15 A puzzle to Europeans, anyway—Indians seem to have been, as a rule, satisfied with traditional explanations of their origins.

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*16 Hrdlička’s complaint about the lack of skeletal evidence was unfair for another reason: paleo-Indian skeletons are extremely rare. In Europe, archaeologists have discovered scores of skeletons ten thousand years old or more. By contrast, only nine reasonably complete skeletons of similar age have been found in North America (a few more exist in South America, although, as with the Lagoa Santa skeletons, their provenance is often unclear). “It’s a big mystery why we don’t find the burials,” the University of Vermont archaeologist James Petersen told me. “Some Indians will tell you that their dead all moved to a spiritual plane, and that’s about as good as any answer that we’ve got.”

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*17 Here and throughout I give the currently accepted dates, which are made with better techniques and more grasp of the vagaries of carbon dating than were then available to Haynes. Scientists discovered in the 1960 s that the rate of C14 formation and intake varied more than Libby had thought. As a result, raw C14 dates must be corrected (“calibrated,” in the jargon) to obtain calendar dates, something archaeologists do not always make clear. In addition, they often write dates not as years A.D. or B.C. but as years B.P. (Before Present), with the present set by convention at 1950 A.D. Thus 2000 B.P. is 50 B.C. In an attempt to reduce confusion, all dates in this book are ordinary calendar dates—that is, radiocarbon dates corrected by the most recent calibration. Scientists usually report C14 dates with their potential error, as in 3000 ± 150 B.P. (1050 ± 150 B.C.). To avoid typographical clutter, I do not include the error spread, believing that readers understand the unavoidable uncertainties in measuring minute levels of residual radioactivity.

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*18 I am not criticizing McNeill for failing to include the Americas on his list of civilizations; he was simply reflecting the beliefs of his time. I would criticize World History: Patterns of Change and Continuity, a high school text published two decades later, in time for my son to