Why do some animals lay eggs and some have live births?
In extremely simplified terms, the birds, turtles, crocodilians, amphibians and mammals are all derived from a common ancestor that was an egg layer. That ancestor can be traced back to the fishes, also egg layers. The sexually-reproducing invertebrates (from which the vertebrates descended) also produced eggs. External fertilization can be considered the ancestral characteristic and results in the formation of an egg that develops outside the body. From there, variations on the common theme have arisen, including animals that fertilise internally but still develop their eggs externally. Only in the mammals (and some of the sharks) has the foetus being retained within the mother’s body and nurtured there until birth. When the first aquatic tetrapod (a salamander-like animal) ventured onto land to become our terrestrial ancestor, a number of terrestrial adaptations were required for it to survive and reproduce successfully. One, as mentioned, was that to internal fertilisation (because
