 Another theory about the origin of the Indo-European language centres around a hypothetical Proto-Indo-European people, who, according to the Kurgan hypothesis, can be traced to north of the Black and Caspian Seas at about 4500 BCE.They domesticated the horse and possibly invented the wooden disk wheel, and are considered to have spread their culture and genes across Europe.The Y haplogroup R1a is a proposed marker of these "Kurgan" genes, as is the Y Haplogroup R1b, although these haplogroups as a whole may be much older than the language family.