All terrestrial life came from the sea, but how did life develop in the ocean? Professor Tim Lenton of the University of Exeter, who led a new study, said: “There had been enough oxygen in ocean surface waters for over 1.5 billion years before the first animals evolved, but the dark depths of the ocean remained devoid of oxygen. We argue that the evolution of the first animals could have played a key role in the widespread oxygenation of the deep oceans. This in turn may have facilitated the evolution of more complex, mobile animals.”
And what were these animals that oxygenated the oceans? Sponges, according to the the new research. Sponges may lack the brains and other organs of more complex animals, but they are animals nonetheless, and around 700 million years ago, sponges began to flood the oceans with oxygen. The oceans had had relatively little oxygen and could not support more complex life.