11.1.4 11.15: The movement onto land required lungs, a rigid backbone, four legs, and eggs that resist drying.

Movement from water onto land required four major evolutionary adaptations: lungs, a backbone, four legs, and eggs that won’t dry out. All four of these characteristics are necessary for a land animal, and all four evolved in predatory fishes that lived in shallow water, the immediate ancestors of terrestrial vertebrates.

As we’ve noted, lungs probably evolved from the air sacs that were also evolutionary precursors to the swim bladder found in ray-finned fishes. All of the terrestrial vertebrates—amphibians, reptiles (including birds), and mammals—are descendants of the lobe-finned fishes that lived during the Devonian period, some 400 million years ago. At this time, the lobe-finned fishes had lungs and the rudiments of limbs, but they were still fully aquatic and living near the shore. The four sturdy fins on the underside of their body helped them move through shallow water, and lungs allowed them to breathe air when the oxygen concentration in the warm, stagnant water was low. Thus, these fishes already had some of the basic characteristics of terrestrial animals. The jointed bones in the fins of lobe-finned fishes don’t look much like your arms and legs, but they are homologous structures.

To move onto land, a vertebrate needed more than just legs and lungs. It needed structural support to resist the pull of gravity. Each vertebra of a terrestrial vertebrate has projections that interlock with projections from the vertebra ahead of it and the vertebra behind it. These interlocking projections prevent the backbone from sagging under the pull of gravity, and the body weight is transmitted through the limbs to the ground.

The last innovation necessary to move onto land was an egg that resists drying out (FIGURE 11-25). When eggs are deposited on land, they are exposed to air and lose water by evaporation. The eggs of terrestrial animals need a waterproof covering—a membrane and a shell—to prevent them from drying out before they hatch. The appearance of eggs with a shell (about 380 million years ago) further facilitated the evolution of entirely terrestrial vertebrates, the groups that evolved into mammals and reptiles (including birds). But these initial egg coverings were soft and likely still required a watery environment. The eggs of terrestrial vertebrates have a water-tight membrane that keeps the embryo surrounded by a bath of fluid called amniotic fluid.