BioEd Online

Researchers have pinned down a way to identify signs of ancient life in stromatolites — strange, lumpy sedimentary rock formations. The method has been used to show the unambiguous remains of cells in some 2.7-billion-year-old Australian stromatolites, and is lined up to help those hunting for early signs of life on Earth.

No one knows how or when life began on Earth, although there are hints it may have been around some 3.7 billion years ago. Stromatolites, some of which are 3.5 billion years old, are a good place to look to learn more about some of these early organisms.

Bacteria that secrete calcium carbonate, or lime, along with mucus, can build up layers of deposits to form the strange-shaped rocks we see today. But not all stromatolite rocks are made by microbes. They can form by simple precipitation of minerals in a similar way to how stalagmites or stalactites are usually formed in caves.

It is hard to pin down which mechanism is at work in the very oldest examples. Tell-tale microbial signs, such as organic matter mixed up with calcium minerals that formed as the microbes underwent photosynthesis, are hard to spot.

Shine a light on me

Kevin Lepot, of CNRS (the French national centre for scientific research) and the Denis Diderot University in Paris, and his colleagues used a powerful synchrotron light source to take a very close look at the 2.7-billion-year-old stromatolites from the Tumbiana Formation in Western Australia. To do this they used a microscopy technique known as scanning transmission X-ray microscopy. In this way, Lepot and his colleagues saw globules of organic material shaped like cells inside the rock.

They also used the powerful near-edge X-ray absorption fine structure (NEXAFS) spectroscopic technique to find out which molecules were present. In the mix, they found carboxylic acids and other carbon-based molecules that are signatures of microbial life.

Mixed in with the organic globules were tiny crystals of the calcium carbonate mineral aragonite. Aragonite is a fairly unstable mineral, unlikely to be preserved unless protected by the remains of microbes. Seeing aragonite is another strong indication that microbes were present, says Lepot.

This is by far the oldest aragonite found; the previous oldest sample has the youthful age of 350 million years. The results are published in Nature Geoscience.¹

Older yet

The oldest known stromatolites are the 3.5-billion-year-old formations in the nearby Dresser Formation in Pilbara. Those stromatolites are badly preserved, but another, slightly younger formation in the same Warrawoona Group, at Strelley Pool, is better off. “These are the oldest ‘visible’ remains that are potentially biogenic,” says Roger Summons, at the Massachusetts Institute of Technology in Cambridge.

"The real impact of this paper will be in showing the way to a new methodology applicable to studying the older and more difficult terrains of the Warrawoona Group," says Summons. Lepot’s PhD supervisor, Pascal Philippot, says they have taken a sample from the Dresser Formation and are planning to use the same technique on that.

There is some, even more controversial evidence for early life in graphite from rocks found near Greenland, Summons says. Pinning down that evidence is a hard task, he adds. “The older you go, the more feeble the evidence.”