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See Inside September 2009

Legs, Feet and Toes

The essential parts for walking on land evolved in water

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.

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