Wild horses have lived in Britain for hundreds of thousands of years. Some remains found date as early as 700,000 BC, while others are as recent as 3,500 BC. However, no genetic studies to date have correlated these prehistoric remains to any modern breed. What has been studied are Y-chromosomes (Y-DNA) and mitochondrial DNA (mtDNA) obtained from Exmoor ponies. The Y-chromosome is passed on through the male line, and worldwide shows very little genetic variation in horses, except for a second Y-chromosome haplotype found in China, suggesting that a very limited number of stallions contributed to the original genome of the domestic horse. The Exmoor pony shares this general Y-chromosome haplotype. In contrast, mitochondrial DNA is passed on though the female line, and shows far more variation than Y-DNA, indicating that a large number of wild mares from several regions have contributed to modern domestic breeds. Some mtDNA-haplotypes have been found in DNA samples obtained from wild horses in prehistoric deposits, while other mtDNA-haplotypes have only been found in domesticated horses, from both living individuals and archeological finds. The Exmoor pony has a high frequency of pre-domestication mtDNA-haplotypes, but these are also found in different breeds all around the world. Currently, for the British Isles, there are only three DNA archeological samples available, all from Ireland.