羊膜の獲得は陸上における繁殖を可能にした

古生代デボン紀に海から陸へと進出した脊椎動物は、乾燥にさらされる陸上生活には十分適応できず、水辺での半水棲生活を余儀なくされていた。しかし、石炭紀に「羊膜」をもつ有羊膜類と呼ばれるグループが進化した。

羊膜は卵の中の赤ちゃん（胚）を乾燥から守り、呼吸を可能にすることで、水辺から離れた環境への進出を促した。羊膜は現生の爬虫類や鳥類、そして親の体の中で育てた子どもを産む私たち哺乳類にも受け継がれている。

Vertebrates, which have expanded from sea to land during the Devonian period, were exposed to dry out and were not enough adapted to the terrestrial life. This lack of adaptation forced them to live a semi-aquatic life.

However a clade called Amniota developed an amnion during the Carboniferous. This amniotic membrane protected babies in the eggs (embryo) from drying out and enabled them to breath through the membrane, encouraging the first Amniotes to explore environments farer from the waterside. The amniotic membrane has been passed on to modern reptiles, birds but also to us, mammals, who are raising fetus in our own body before giving birth.