Galliformes, the order of bird that chickens belong to, is directly linked to the survival of birds when all other dinosaurs went extinct. Water or ground-dwelling fowl, similar to modern partridges, survived the Cretaceous–Paleogene extinction event that killed all tree-dwelling birds and dinosaurs. Some of these evolved into the modern galliformes, of which domesticated chickens are a main model. They are descended primarily from the red junglefowl ("Gallus gallus") and are scientifically classified as the same species. As such, domesticated chickens can and do freely interbreed with populations of red junglefowl. Subsequent hybridization of the domestic chicken with grey junglefowl, Sri Lankan junglefowl and green junglefowl occurred; a gene for yellow skin, for instance, was incorporated into domestic birds through hybridization with the grey junglefowl ("G. sonneratii"). In a study published in 2020, it was found that chickens shared between 71% - 79% of their genome with red junglefowl, with the period of domestication dated to 8,000 years ago.