Equidae died out in the Western Hemisphere at the end of the last glacial period. A question raised is why and how horses avoided this fate on the Eurasian continent. It has been theorized that domestication saved the species. While the environmental conditions for equine survival in Europe were somewhat more favorable in Eurasia than in the Americas, the same stressors that led to extinction for the Mammoth had an effect upon horse populations. Thus, some time after 8000 BCE, the approximate date of extinction in the Americas, humans in Eurasia may have begun to keep horses as a livestock food source, and by keeping them in captivity, may have helped to preserve the species. Horses also fit the six core criteria for livestock domestication, and thus, it could be argued, "chose" to live in close proximity to humans.