Asked by: Ben Arthur, Hull
All land vertebrates today are descended from a common ancestor that had four legs, with five toes on each foot. This arrangement is known as the pentadactyl limb. Some species have subsequently fused these fingers into hooves or lost them altogether, but every mammal, bird, reptile and amphibian traces its family tree back to a pentadactyl ancestor that lived around 340 million years ago.
Before, there were animals with six, seven and even eight toes on each foot, but they all went extinct at the end of the Devonian period, 360 million years ago. These other creatures were more aquatic than the pentadactyl animals. Evidence in the fossil record suggests that their ribs weren’t strong enough to support their lungs out of water and their shoulder and hip joints didn’t allow them to walk effectively on land. We don’t know the exact reasons behind the late Devonian extinction, but it seems to have killed off the coral reefs and reduced the oxygen levels in shallow coastal waters and rivers. This environment would have favoured fish species that were able to survive in the open sea, and animals that were able to climb out of the water.
There’s nothing particularly special about having five toes that allowed the pentadactyls to do this; they just happened to be the ones with the other important adaptations for life on land. All the other groups went extinct before they evolved them.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.