The recovery of DNA from ancient bone has rewritten prehistory faster than archaeology could absorb. Population movements once inferred cautiously from pottery styles can now be traced directly, and several long-running debates were settled within a decade. The technique's authority has, however, outrun its representativeness. Genomes are recovered where preservation is good — cold, dry, alkaline conditions — which means northern Eurasia is densely sampled while much of Africa, South Asia and the tropics remains nearly blank. Conclusions about human history as a whole therefore rest on an archive shaped by climate rather than by significance. Specialists are alert to this; the summaries that reach a wider audience often are not, and a map of migrations tends to be read as a map of what happened.