Selective breeding just before and during the Iron Age nearly 3,000 years ago is likely the reason for the lack of variability in modern domestic horses' paternally inherited DNA, a trait unique among livestock animals. This finding challenges the current assumption that high maternal DNA diversity and low paternal DNA diversity of modern horses is generally explained by unequal sex ratios (many mares, few stallions) at the beginning of horse domestication. The estimated 56 million horses around the world today are immensely diverse in their maternally-inherited mitochondrial DNA. Yet, modern horses show very little variation on their paternally-inherited Y chromosome, a curious finding since recent studies suggest that Y chromosomal diversity among horses was much larger in past. When and why this diversity disappeared remains controversial, as does any possible influence of the horse domestication process. Here, Saskia Wutke and colleagues genotyped 16 Y chromosomal single-nucleotide polymorphisms (SNPs) in 96 ancient Eurasian stallion remains spanning roughly the last 5,000 years (from the early domestication stages - Copper and Bronze Ages - to the Middle Ages). Their results showed that lack of paternal diversity appeared to begin in the Late Bronze Age about 3,500 years ago - a time when large-scale human migrations required horses for transportation - and became even more noticeable during the Iron Age. Through several additional analyses, the authors found that deliberate selection for or rejection of certain stallions imposed by humans in breeding processes can explain the loss of paternal diversity. Additionally, when the dominant stallion lineage that is present in modern horses took hold during the Iron Age, the authors say, the Roman Empire changed the mode of animal breeding by shifting the focus from female-based to male-based selective breeding, which likely increased the reproductive success of a few selected stallions, and thus their lineages.