Oxford paleontologist Martin Brasier asserts that his fossilized cells are the remains of primitive anaerobic bacteria that lived 3.4 billion years ago. Paleobiologist J.W. “Bill” Schopf’s samples, he believes, are just ancient, patterned rock, with no fossils at all. Settling the debate matters a great deal. At its heart is one of the biggest questions in science: When and where did life begin?