The greening of the pre-Cambrian
It's one of the biggest problems in the Earth's history: What prompted the Cambrian explosion of multi-cellular animal life? (Though calling it the 'Cambrian explosion' is a misnomer at this point; most geologists agree that life took off a bit earlier than the Cambrian 540 million years ago -- probably 50 million years or so before that boundary.)
Now, a new study in Nature [subscription] presents an intriguing explanation: It was photosynthetic life on land, beginning 850 million years ago, that allowed oxygen levels to rise in the atmsophere. Higher oxygen levels, in turn, allowed respiratory animals to get bigger. As I explain in an accompanying news story, the land life wasn't plants -- it was more likely a dense, mossy, worldwide matting.
The evidence is indirect: isotope records trapped in carbonate rocks formed in shallow seas. Some researchers are skeptical since fossil evidence is virtually non-existent. Moreover, the study will be controversial in geochemistry circles because the authors' interpretations of the isotope data is very different from another popular line of thinking, which sees radical changes in ocean chemistry closer to the Cambrian boundary as being the impetus for the animal explosion.
Image: Diego Cupolo
Eric Hand is a staff reporter in Nature's Washington Bureau.
