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The evolution of terrestrial creatures from aquatic fish with fins may have begun with the need for a breath of fresh air. Animals with limbs, feet and toes—a group known as the tetrapods (literally, “four-footed”)—arose between 380 million and 375 million years ago. Scientists long believed that limbs evolved as an adaptation to life on terra firma. But recent discoveries have revealed that some of the key changes involved in the fin-to-limb transition occurred while the ancestors of tetrapods were still living in the water.
Tetrapod evolution experts such as Jennifer Clack of the University of Cambridge hypothesize that these early modifications to the bones and joint surfaces of the pectoral fins might have benefited tetrapod ancestors in two key ways. First, they could have allowed the creatures, which lived in the plant-choked shallows, to perform a push-up that raised their heads out of the oxygen-poor water for a breather. (Changes in other parts of the skeleton, such as the skull and neck, also facilitated air breathing.) The protolimbs could have also helped these animals to propel themselves along the bottom and to steady themselves against the current while waiting to ambush prey.
Researchers once thought that the bones making up feet and toes were an evolutionary innovation unique to the tetrapods. But over the past few years analyses of tetrapod forerunners, such as the Tiktaalik fossil unveiled in 2006, have revealed that these bones derive directly from bones in the fish fin. Curiously, the earliest tetrapods and tetrapodlike fish had feet with between six and eight digits, rather than the five of most modern tetrapods. Why tetrapods ultimately evolved a five-digit foot is uncertain, but this arrangement may have provided the ankle joint with the stability and flexibility needed for walking.