Our lives depend on the sea. As gifted science writer Deborah Cramer makes clear in this extraordinary volume, the ocean has been earth's lifeline for more than three and a half billion years. Life began in the scalding inferno of deep-sea hot springs. The first cell, the first plant, and the first animal were all born in the sea. Climate changes wrought by the sea created evolutionary pathways for mammals and gave rise to our human ancestors some 200,000 years ago. The one, interconnected sea still sustains us. Invisible plants in the ocean's sunlit surface give us air to breathe. Rushing currents supply water to the atmosphere's protective greenhouse and rain to dry land.