But surely the Athenians had it backward; it’s the land that rests in the lap of the sea. Thalassa, not Gaia, is the guardian of life on the blue planet. A simple, albeit apocalyptic, experiment suggests Thalassa’s power. Destroy all life on land; the ocean creatures will survive just fine. Given time, they’ll even repopulate the land. But wipe out the organisms that inhabit the oceans and all life on land is doomed. “Dust to dust,” says the Bible, but “water to water” is more like it, for all life comes fromand returns to the sea. Our ocean origins abid within us, our secret marine history. The chemical makeup of our blood is strikingly similar to seawater. Every carbon atom in our body has cycled through the ocean many times. Even the human embryo reveals our watery past. Tiny gill slits form and then fade during our development in the womb. The ocean is the cradle of life on our planet, and it remains the axis of existence, the locus of planetary biodiversity, and the engine of the chemical and hydrological cycles that create and maintain our atmosphere and climate. The astonishing biodiversity is most evident on coral reefs, often called the “rain forests of the sea.” Occupying less than one-quarter of 1 percent of the global ocean, coral reefs are home to nearly a third of all marine fish species and to as many as nine million species in all. But life exists in profusion in every corner of the ocean, right down to the hydrothermal vents on the seafloor (discovered only in 1977), where more than a hundred newly described species thrive around superheated plumes of sulfurous gasses. The abundance of organisms in the ocean isn’t surprising given that the sea was, as already mentioned, the crucible of life on Earth. It is the original ecosystem, the environment in which the “primordial soup” of nucleic acids (which can self-replicate, but are not alive) and other molecules made the inexplicable and miraculous leap into life, probably as simple bacteria, close to 3.9 billion years ago. A spectacular burst of new life forms called the Cambrian explosion took place in the oceans some 500 million years ago, an evolutionary experiment that produced countless body forms, the prototypes of virtually all organisms alive today. It wasn’t until 100 million years later that the first primitive plants took up residence on terra firma. Another 30 million years passed before the first amphibians climbed out of the ocean. After this head start, it’s not surprising that evolution on that newcomer-dry land-has never caught up with the diversity of the sea. Of the thirty-three higher-level groupings of animals (called phyla), thirty-two are found in the oceans and just twelve on land.