Describe how each of these features contributed to being able to live on land.
Tetrapods made the move from water to land. Their move was not water-free: The first tetrapods could spend time on land but were still tied to water. Reptiles were the first group of vertebrates to adapt successfully to a completely terrestrial lifestyle. Four characteristics of reptiles that allow for a completely terrestrial lifestyle are the amniotic egg, scales, a double-circuit circulation pathway, and limbs.
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The key component for animal life to survive on land was (and is!) the ability to retain moisture. Animal life in water had a much easier time, relatively speaking, in carrying out life processes in an aquatic environment, as it provided an easy medium for biochemical reactions to occur. Not so on the harsh ground, where the sunlight, air, and large temperature swings could quickly desiccate living things. Rather than giving birth to live young that could float in the ocean, land-based young were produced in an embryonic state, the egg essentially providing an encased liquid medium analogous to the ocean for the young to develop. Scales, in a sense, do the same thing -- keeping an aquatic environment inside the animal (blood) and providing a barrier to protect it from drying out. In order to keep that blood efficiently circulating, a "send and return" system and a more advanced "pump" evolved. Finally, In order to be mobile, animals couldn't just float along or use appendages to push through the water medium; fins evolved into legs. The development of the amniotic egg was the critical function to allow creatures to not have to return to the water to reproduce; all their life processes could be on land.
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