Physiological Transition from Fetal to Neonatal Life in Pregnancy

More than 3 billion years after the formation of the earth, vertebrate life first originated in the sea. Jawless eel-like vertebrates and placoderms (primitive fish, now extinct) evolved initially, and 100 million years later in the Devonian period of the Paleozoic era, cartilaginous fish (the shark families) and boney fishes first appeared in the sea. By the end of the 40 million year Devonian period, boney fishes similar in many ways to the lungfish of today first crawled onto land. These evolved into the crossopterygian fishes, which had some capacity to breath air and use their fins to move across the remaining one third of the world not previously inhabited by vertebrates—dry land (1, 2).