Life appeared on Earth almost as soon as it could, but it took more than 3 billion years before complex multicellular organisms evolved. Analysis of some very old diamonds has challenged one of the favorite theories about what took so long.
Life, particularly complex life, requires a suitable atmosphere, so a chief strategy in the search for it on other planets is to look for the gases thought to be necessary for, or at least compatible with, living things. It would be helpful then to know what Earth's atmosphere was like at the times when life was appearing, and when major advances occurred.
Most of the gases in the atmosphere, known as volatiles, are thought to have once been trapped in the Earth's mantle, so the timing of their escape is a crucial part of the story of atmospheric evolution. Dr Michael Broadley of the Centre de Recherches Pétrographiques et Géochimiques has presented evidence at the Goldschmidt Conference that the mantel gas composition has been unchanged for a long time.
“Studying the composition of the Earth's modern mantle is relatively simple...We can collect samples thrown up by volcanoes and study the fluids and gases trapped inside. However, the constant churning of the Earth's crust via plate tectonics means that older samples have mostly been destroyed,” Broadley said in a statement.
While not quite going so far as to say diamonds are forever, Broadly noted they are so robust they make “ideal time capsules”, and trap gasses inside them, indicating the conditions in which they formed.
Diamonds themselves are hard to date, but at Wawa on the Canadian shore of Lake Superior, Broadley found diamonds in 2.7 billion-year-old rocks, indicating the diamonds are at least as old and possibly much older. The diamonds don't look like gemstones and have no value as jewelry, but that just means there is less competition for scientists wanting to analyze them. Broadley turned them to graphite above 2,000º C (3,700º F), releasing the noble gasses trapped inside.
The concentration of helium, neon, and argon escaping were similar in these ancient diamonds to the upper mantle today, so the major releases from the mantle must have been before the diamonds formed, certainly, before the Earth was half its current age. Although the noble gases are not important for life, Broadley and colleagues consider them proxies for gasses that are, such as nitrogen, hydrogen, and small carbon-based molecules.
"This was a surprising result. It means the volatile-rich environment we see around us today is not a recent development, so providing the right conditions for life to develop,” Broadley said.
Mantle degassing was not the only prerequisite for the expansion of life beyond its simplest forms. The release of oxygen by photosynthesizing organisms was also necessary, and something whose timing has been much debated. However, mantle gas release has been thought to be an essential prerequisite that might have served to delay other events.