Some of the earliest known multicellular animals, members of the Ediacaran fauna, from about 570 million years ago: Charnodiscus (large, orange sea pen); Ediacaria (three jellyfish on left), Kimberella (tall, skinny jellyfish on right); Dickensonia (large, flat, segmented worm), Spriggina (small, slender, green segmented worm); Tribrachidium (pinwheel-shaped, possibly echinoderm); and Parvancorina (lavender arthropod). Image of a diorama from the National Museum of Natural History, courtesy of the Smithsonian Institution.

Just Breathe

580 million years ago, the ocean that once covered the present-day Sultanate of Oman was flooded with enough oxygen that the way in which life was constructed was completely changed. This moment, the birth of multi-cellular organisms, shortly preceded the burst of biological diversification called the Cambrian explosion. Recent evidence indicates that this was the last in a series of similar increases in oxygen availability.

“The presence of oxygen on Earth is the best indicator of life,” says John Grotzinger, the Jones Professor of Geology at Caltech who coauthored a recent paper on the subject in Nature. “But it wasn’t always that way,” he adds. “The history of oxygen begins about two and a half billion years ago and occurs in a series of steps. The last step is the subject of this paper.”

The study was led by Dave Fike, an MIT grad student who made the move to Caltech in 2006 to stay with his advisor, Grotzinger. Fike uncovered evidence for this final stage in oxygenation at three kilometers’ depth in the oil fields of Oman, where the oldest commercially viable oil on the planet is found. He analyzed carbon and sulfur isotope ratios from core samples and drillings to determine the oceanic conditions under which the deposits were originally laid down.

At the time, the ocean covering Oman resembled the modern-day Black Sea, which has a thin oxygen-rich layer on top underlain by an oxygen-starved (what chemists would call a “reduced”) environment. “The ocean today is pretty well mixed and thus oxidized at all layers, but the ocean before the Cambrian period must have been very different,” says Grotzinger. Different enough to be hostile to complex life—a deep ocean devoid of sufficient oxygen can’t sustain multicellular life forms. For this reason, life continued in its single-celled form from its first days, more than three and a half billion years ago, until just before the Cambrian explosion. At that time, according to the team’s geologic evidence, deep water began mixing with the shallow ocean, and the result was the first fully oxidized deep ocean. With enough oxygen in the deep ocean came the successful establishment of Earth’s first multicellular community, the Ediacara fauna, some of which looked a lot like upright leaves waving on the ocean floor.

Grotzinger says the clarity of the new evidence is persuasive. Geologists have long believed that the rise of oxygen was a key element of the Cambrian explosion, and this discovery certainly seems to confirm it.

The other authors of the paper, which appeared in the December 7, 2006, issue of Nature, are Lisa Pratt of Indiana University and Roger Summons of MIT. —RT