The dilemma of not being able to breathe and move rapidly at the same time was a huge obstacle to colonizing land. The first land tetrapods would have been at a great disadvantage to even the land arthropods, such as the scorpions, for the vertebrates would have been slow and would have needed to stop constantly to take a breath. This is why oxygen levels would have been critical. Only under high-oxygen conditions would the first land vertebrates have had any chance of making a successful living on land.

One consequence of limited respiration while moving was that the early amphibians and reptiles evolved a three-chambered heart. This kind of heart is found in most modern amphibians and reptiles and is adaptive for creatures that have the problem of inferior respiration while moving. While a lizard is chasing prey it is not breathing, and thus the fourth chamber of the heart, which would be pumping blood to the lungs, is superfluous. The three chambers are used to pump blood throughout the body, but the price that must be paid is that it takes the lizard longer to reoxygenate the blood when activity ceases.

One group of reptiles, the mammal-like reptiles or synapsids, either partially or totally solved the reptilian problem of not being able to breathe while running by changing their stance. The synapsids show an evolutionary trend of moving their legs into a position so that they were increasingly under the trunk of the body, rather than splayed out to the side as in modern lizards. This created a more upright posture and removed, or at least greatly decreased, the lung compression that accompanies the sinuous gait of lizards and salamanders. While there was still some splay of the limbs to the sides of the trunk, it was certainly less than in the first tetrapods. With the evolution of the therapsids in the middle Permian, the stance became even more upright.

Another important variable is the nature of thermoregulation—the possibilities of endothermy (warm-bloodedness), ectothermy (cold-bloodedness), and a third category that is essentially neither of the others and is associated with very large size. Warm-bloodedness