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The study of human genetic variation— a kind of historical Global Positioning System — goes back to World War I, when two physicians work- © 20 08 SCIENTIFIC AMERIC AN, INC. July 2008 Illustration by Sean m ccabe; Sources: Daniel Sambraus (human skull), Pascal Goetgheluck (Neandertal skull), Biophoto Associates (chromosomes) Photo Researchers, Inc. Traces of a DNA furnishes an ever clearer picture of the multimillennial trek from Africa all the way to the tip of South America © 20 08 SCIENTIFIC AMERIC AN, INC.

They present whatever they encounter to the T cells that will eventually home in on and attempt to destroy cancerous or infected cells. It turns out that antigen-presenting cells carry receptors on their surface for the peptidebinding chaperones. The first such receptor was identified by Robert J. Binder, then a graduate student in my laboratory and currently an assistant professor at the University of Pittsburgh, as CD91. When the cells encounter an HSP-peptide complex, they internalize it through the CD91 doorway and present the HSP-chaperoned peptides to the T cells, which can then multiply and fight off the cancer or pathogen.

The genes of Helicobacter pylori (above), the ulcercausing bacterium endemic to humans, indicate that the microbe left Africa about 55,000 years ago, just when humans did. One lineage of the microbe appears both in East Asian and in native South American populations, supporting the notion that South Americans originally came from Asia. The answer to the absence of stones and bones: more DNA, from wherever. To bolster the case for genetics, researchers have looked to microbes that have hitched a ride on humans, inspecting their genes to look for similar patterns of migration.

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