Half the world's population speaks a language that has evolved from a single, prehistoric mother tongue. A mother tongue first spoken in Stone Age times, on the steppes of central Eurasia 6,500 years ago. It was so effective that it flourished for two thousand years. It was a language that spread from the shores of the Black Sea across almost all of Europe and much of Asia. It is the genetic basis of everything we speak and write today--the DNA of...