The Horse, Equus caballus, is a large ungulate mammal, one of the seven modern species of the genus Equus. It has been important for transportation; to ride on, or for pulling a chariot, carriage, horse-drawn boat, stagecoach, tram, etc.; also as plough horse, etc. as well as food; see also domestication of the horse. Until the mid 20th century, the horse was used heavily in warfare. The evolution of the horse from the very early (around 55 million years ago) Hyracotherium or eohippus to the wild equids listed below, is well understood in comparison to our understanding of the evolutionary succession of most animals. By natural selection, the toes of early horse ancestors reduced to the single central toe which forms the hoof of the modern equine. The opposite being animals with 'cloven' hooves (2 toes), like cows, pigs and sheep. Vestiges of other toes remain as the splint bones, the callous-like "chestnuts" on the inner sides of all four legs, and the "ergots" hidden in the hair of the underside of the fetlock joint. Horses live in family groups in primarily grassland habitats. These normally consist of a mature stallion, his harem of mares, and the mares' offspring. Once young males reach breeding age and begin to attempt to breed mares or challenge the herd stallion, they are driven out of the herd and form "bachelor bands" with other young stallions. It's usually not until a stallion reaches 7 or 8 years old that he stands a real chance at acquiring mares, eventually becoming, if successful in the attempt, a "band stallion", i.e. having a harem of his own, having separated female equids from another stallion's band.