THE widely accepted idea that animal life evolved in the oceans before moving onto land is being challenged by a geologist who says the oceans were too salty and hot for that to have happened. He suggests that animals evolved in freshwater pools or lakes, and that the huge increase in diversity during the Cambrian explosion was triggered when they moved into the sea.
Paul Knauth of Arizona State University in Tempe bases this idea on new calculations that until about 600 million years ago the oceans would have been nearly twice as saline as they are today. It was then that large salt beds were laid down, after parts of the ocean were cut off as inland seas, which then evaporated. This, Knauth reasons, would have removed large amounts of salt from the remaining oceans.
Knauth has previously estimated ocean temperatures as far back as 2.5 billion years ago by comparing the concentrations of two isotopes of oxygen in sediments and in the carbonates contained in material such as seashells. These estimates show that oceans were warmer than 55 掳C until about 2.5 billion years ago, and then started cooling before they almost completely froze over some 700 million years ago.
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The combination of salinity and high temperature would have kept oxygen levels too low to support marine animals, Knauth says. He points out that the warmer and more saline water is the less oxygen dissolves in it. Given their high salinity and temperature at the time, the oceans could not have dissolved sufficient oxygen until about 600 million years ago (Palaeogeography, Palaeoclimatology, Palaeoecology, DOI: 10.1016/j.palaeo.2004.10.014).
This raises the possibility that the Cambrian explosion of shelled organisms, which started 570 million years ago, occurred when animals which had evolved in fresh water migrated to the oceans. This idea 鈥渋s going to get me into a lot of trouble鈥�, Knauth says. He is now looking for fossils of freshwater animals that would support his claim. He hasn鈥檛 found any yet, but he told New Scientist, 鈥淚 haven鈥檛 given up yet鈥�.
