Chickens (Gallus gallus domesticus) were domesticated from the red junglefowl (Gallus gallus) in Asia around some 5,000 years ago. Chickens are undoubtedly the most ubiquitous and abundant species of domestic livestock and occur more or less everywhere that humans do. In intensive systems they tend to be raised either for eggs (layers) or meat (broilers) but in small-scale systems they tend to be dual purpose. With short lifecycles and potentially high feed conversion ratios chicken production can be rapidly increased to meet rising demand for animal source foods. They are consequently the fastest-growing livestock sub-sector.