In the evolution of life, cooperation among organisms is probably at least as important as competition. The first complex single-celled creatures-eukaryotes-probably appeared when a host bacterium engulfed a smaller bacterium, perhaps one that could use photosynthesis to turn sunlight into energy. But to make such a symbiotic relationship permanent-to make one new organism out of two-is no easy task. "If you get more nutrients into the environment, from upwelling or pollution, the little photosymbiotic plants living in the host will swim away," says geologist Martin Brasier of Oxford. When nutrients aren't scare, there is no reason for them to maintain the symbiosis.
Even 10 million years probably isn't long enough to cement the relationship, says Brasier. The organisms must be subjected to hundreds of millions of years or more of unvarying conditions. Was there ever such a time on Earth? Brasier and his colleague John Lindsay of the Australian Geological Survey Organization say there was-a billion-year period in the Precambrian. Not coincidentally, say the researchers, the first eukaryotes appeared near the end of that billion years.
Brasier and Lindsay studied cores drilled from ancient ocean sediments in north central Australia. They looked specifically at the ratios of two carbon isotopes, carbon 13 and carbon 12. Oceanic plants tend to absorb more carbon 12 than 13, leaving more of the heavier isotope behind in seawater, which eventually becomes incorporated into rocks. So the amount of carbon 13 in the ocean core samples indicates how much plant life was once present. Geologists expect to see big fluctuations in the carbon-isotope record, especially over a billion-year period, because plant populations would be expected to rise and fall in response to events like mountain building. When mountains rise, erosion accelerates, increasing the amount of mineral nutrients washed into the sea. The nutrients fuel plant growth. During quieter geologic times, erosion rates fall off, and ocean plant populations plunge.
Yet from about 2 billion until 1 billion years ago, the Australian cores showed almost no isotope variation. Plant populations were remarkably stable. The stability would have provided enough time for plants to team up symbiotically into more complex organisms.
"The wider significance of this," says Brasier, "is that the biosphere's history is strongly coupled to Earth's crustal history. Higher organisms would not have arisen had the Earth not gone through this phase."