Evidence of Life 3.7B Years Ago

W A S H I N G T O N,   Jan. 28 -- Danish researchers said today they had found what they think may be evidence of the oldest life on Earth — a signature left by plankton 3.7 billion years ago.

There is no standard fossil evidence of the plankton, but Minik Rosing and colleagues at the Geologisk Museum in Copenhagen, Denmark, looked for a chemical signature in ancient rocks in west Greenland.

“The oldest known fossils have an age of 3,500 million years (3.5 billion years) ago and may represent photosynthetic cyanobacteria,” they wrote in a report in the journal Science. These are bacteria that, like plants, use the sun’s energy to feed themselves.

What Came Before Fossils?

But Rosing’s group noted that such bacteria would be fairly complex creatures, and would have had to have evolved from something else. “It can be assumed that a long chain of evolutionary steps preceded the development of these complex organisms,” they wrote.

The trouble is, any rocks that would have been old enough to carry fossil evidence of this most ancient of life have gone through geologic changes. They have either been ground up, sucked back under the crust and remelted, or buried and compressed — a process known as metamorphism.

But at least metamorphic rocks would still contain some remains of any fossils, even if the rocks, which would once have been mud, did not themselves carry the physical imprints.

So they looked for chemical evidence.

Specific Carbon Isotopes

All life on Earth is based on the element carbon, and living things make chemical changes to this carbon. Rosing’s team looked at two particular variants, or isotopes, known as carbon-12 and carbon-13.

Modern plankton has a lot of carbon-12 and not much carbon-13.

Rosing’s group examined microscopic globules of graphite — which is pure carbon — from some metamorphic rocks known to be 3.7 billion years old and known to have once been seafloor sediment. These include shale and schist.