THERE'S NO SUCH THING AS SPECIES:

Low-cost, high-throughput DNA sequencing--a technique in which millions of DNA base-pairs are automatically read in parallel--appeared on the scene less than a decade ago. It has already transformed our ability to see just how the genes of human beings, their domestic animals and their diseases have changed over thousands or tens of thousands of years.

The result is a crop of new insights into precisely what happened to our ancestors: when and where they migrated, how much they intermarried with those they met along the way and how their natures changed as a result of evolutionary pressures. DNA from living people has already shed some light on these questions. Ancient DNA has now dramatically deepened--and sometimes contradicted--those answers, providing a much more dynamic view of the past.

It turns out that, in the prehistory of our species, almost all of us were invaders and usurpers and miscegenators. This scientific revelation is interesting in its own right, but it may have the added benefit of encouraging people today to worry a bit less about cultural change, racial mixing and immigration.

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