Sex born from hard rock and heavy metal

The origin of sexual reproduction was one of the most important events in the history of life ? and it would not have been possible without granite. The igneous rock began forming in vast quantities around 2 billion years ago, bringing to the surface the metals that fuel complex life.

Life relies on a number of heavy metals to form proteins. Zinc, molybdenum and copper are particularly important for the eukaryotes ? a group that includes animals, plants and fungi, all of which may reproduce sexually.

These complex organisms probably did not become widespread until the oceans became rich in the metals about 800 million years ago, but last year fossil evidence emerged showing that eukaryotes were living in freshwater lakes on land at least 1 billion years ago (New Scientist, 16 April 2011, p 20). So where did they get their heavy metal fix?

John Parnell at the University of Aberdeen, UK, and colleagues studied the rock record and say that an important event was the formation of an ancient supercontinent called Nuna, or Columbia, about 1.9 billion years ago, during the Palaeoproterozoic.

"This was a peak time of mantle-derived magmatism and hydrothermal activity perhaps unique in Earth's history," says Parnell.

When Nuna formed, large bodies of magma were injected into the continental crust where they cooled to form granite. This igneous rock is rich in metals including zinc, molybdenum and copper. Over time, natural erosion brought the granite to the surface where it, too, eroded, releasing the metals into the soil and water. Sedimentary records show that they were present in near-surface reservoirs and shallow bodies of water by 1 to 1.5 billion years ago, says Parnell.

"These metals were used by early cells to develop enzymes that enabled them to carry out a greater diversity of functions and begin to sexually reproduce," says Parnell. "They gave early life the added dimension of natural selection and variability."

"This paper is different in that it is looking at the biology first," says Paul Strother at Boston College in Massachusetts, who was a member of the team that found the billion-year-old freshwater fossils last year. "It identifies the requirement of particular metals for evolutionary processes and then shows these metals were in fact present."

The findings offer further evidence that evolution on land may have commenced far earlier than previously thought, Strother adds.

Journal reference: Geology, DOI: 10.1130/G33116.1

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