A new theory proposes that ancient worms maintained the planet in a manner that enabled more complex life forms to develop, breathing new life into the Gaia hypothesis in the process.
Multicellular life forms date back to 2.1 billion years ago, but they really got going in the Ediacaran era 575 million years ago. By the time the Cambrian started 541 million years ago the seas were fizzing with life, but unbeknownst to our ancestors their circumstances were highly delicate. It was only the rise in atmospheric (and oceanic) oxygen that had made it possible for these increasingly advanced animals to live. A fall in oxygen levels would have been fatal.
On the other hand, Tais Dahl, a geochemist at the University of Denmark, has pondered the effects if oxygen levels got too high. By this time plants had colonized the land, and an oxygen-rich environment would allow fires triggered by lightning strikes to run out of control. “How come oxygen levels didn’t crash or double?” he asks. “Something [regulated] oxygen in relatively narrow limits.”
Just how much of a problem high oxygen levels would have been is debatable, but the limits were never tested. From 530 to 500 million years ago oxygen levels steadily dropped, based on four independent paleochemical measurements.
We know that animals consume oxygen, but so do rocks as they weather. Dahl, in collaboration with Richard Boyle has proposed in Nature Geoscience that Cambrian worms burrowing through the seabed created tunnels that exposed extra material to sea water. The greater boundary area between floor and sea provided scope for bacteria to grow, and these in turn drew phosphate from the water to grow.
By storing so much phosphate on the sea bed the bacteria limited the growth of photosynthetic algae, and the oxygen they produce.
However, there was never a danger the worms would bring the oxygen levels down too much, since they themselves required oxygen to flourish – if oxygen levels fell too low the worms growth was stunted.
The worms, and the bacteria they promoted, formed the perfect negative feedback loop, lowering oxygen levels when they might get too high, but letting them rebound if they got too low. “We think these animals may have completely transformed geochemical cycles,” says Dahl.
“Hypothetically, bioturbation’s oxygen sensitivity alone is sufficient to regulate the size of the oxygen reservoir in a net negative feedback,” the paper argues Nevertheless, as elegant as the theory, and impressive as the modeling, may be, such worms didn’t leave much in the way of fossil record. It isn’t clear whether thy flourished worldwide, or were limited to specific regions by latitude and ocean depth.
2