Researchers report that the ancient deep ocean was not only devoid of oxygen but also rich in iron, a key biological nutrient, for nearly a billion years longer than previously thought -- right through a key evolutionary interval that culminated in the first rise of animals. "We will need to rethink all of our models for how life-essential nutrients were distributed in the ocean through time and space," the authors say.

Source: Evidence for a persistently iron-rich ocean changes views on Earth's early history