A 2014 study compared DNA from ancient horse bones that predated domestication and compared them to DNA of modern horses, discovering 125 genes that correlated to domestication. Some were physical, affecting muscle and limb development, cardiac strength and balance. Others were linked to cognitive function and most likely were critical to the taming of the horse, including social behavior, learning capabilities, fear response, and agreeableness. The DNA used in this study came from horse bones 16,000 to 43,000 years ago, and therefore the precise changes that occurred at the time of domestication have yet to be sequenced.