Now, having seen that fish did not come out of the water before they developed legs and lungs, one can return to the original question about why vertebrates left the water to begin with. A likely explanation is that for animals that could breathe and move outside of the water, the land represented a rich, new, uncharted domain. By the Late Devonian Epoch, both saltwater and freshwater habitats were populated by an abundance of fishes. Competition in the water was predicated by size: No matter how big a fish was, there was probably a bigger fish that could eat it. Early tetrapods were still primarily aquatic, and their main food source was certainly fish. The ability of tetrapods to maneuver effectively in shallow water systems or to cross land to reach other pools was critical to their success because in these habitats they had virtually no competition for fish prey. As tetrapods became increasingly terrestrial, they found themselves to be the largest land predators. They benefited from the greening of the world by living in the safe shadows of plants, breathing the oxygen-rich air enriched by plants, and profiting from the lush, moist habitat the enabled them to lurk effectively for prey.
Arthropods, including insects, may also have been a source of tetrapod food. By the Middle Carboniferous Period, there is fossil evidence that insects had begun to eat plants, a development that not only expanded the diversity of insects, but also expanded the primary food source for tetrapods. By the Late Carboniferous, some vertebrates developed a taste for plants, further extending their domain in the direction of an alternative food source that was available in virtually unlimited quantities. Tetrapods left the water, therefore, to occupy a largely untapped habitat that they could easily dominate.
Chapter 5 continues this discussion by presenting fossil evidence for the successful transition of tetrapods from the water to the land.
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