The oldest known two-legged walker lived 290 million years ago
Long before dinosaurs, a small reptile called Eudibamus ran on its hind legs. Walking upright has evolved again and again since, in birds, kangaroos, springhares, pangolins and humans, even though ostriches and red kangaroos top out near 70 km/h while a cheetah passes 100 km/h.
A biped is any animal or machine that normally gets around on two rear limbs; the word joins Latin bis, double, and pes, foot, in contrast to quadruped. Two-legged movement covers walking, running, hopping and skipping. Walking keeps at least one foot on the ground at all times, running includes moments with both feet airborne, and even standing still is an active job that demands constant balance corrections in most bipeds.
The earliest known biped is the bolosaurid Eudibamus, whose 290-million-year-old fossils show long hind legs, short forelegs and telltale joints; it died out in the early Permian. Among archosaurs, the group containing crocodilians and dinosaurs, upright gaits arose more than once. Dinosaurs split from their archosaur ancestors around 230 million years ago, and all of them probably descend from a small two-legged predator resembling Eoraptor. Birds, as theropods, are all bipeds, iguanodonts re-evolved the habit, and the Triassic Effigia okeeffeae, from the crocodile side of the family, probably walked upright too. Fresh trackways show pterosaurs went on all fours.
Mammals have evolved habitual bipedalism several times independently: in humans and other hominins, ground pangolins, extinct giant ground sloths, many jumping rodents and macropods, which took up hopping no later than 45 million years ago. Humans, gibbons and large birds step one foot at a time, while most macropods, small birds, lemurs and bipedal rodents hop with both feet together; tree kangaroos alternate feet in branches but hop on the ground. Many more species stand up briefly, as bears and primates do to reach food, and lizards such as the spiny-tailed iguana, the fastest lizard, sprint on two legs.
A raised head sees farther, lets waders reach deeper water and grazers reach higher food, and it frees the forelimbs for handling objects, flying, digging in the giant pangolin, or fighting in bears and great apes. The trade-off is top speed, yet over long distances the endurance running hypothesis holds that bipedalism lets humans outlast most other animals. Zoologists separate facultative bipeds, which choose the gait, from obligate ones, though even people can crawl when needed.
Source: Bipedalism
