Tiny tubes point to ancient life
Tiny tubes thought to have been etched into South African rocks by microbes are at least 3.34 billion years old, scientists can confirm.
A new analysis of the material filling the structures shows they were created not long after the volcanic rock itself was spewed on to the seafloor.
The tubules could therefore represent the earliest "trace" evidence of activity by life on Earth.
The dating work is reported in Earth and Planetary Science Letters.
It is a follow-up study to the University of Bergen team's discovery of the microscopic tunnels and pits first published in 2004.
End Quote Dr Nicola McLoughlin University of Bergen
We're kind of looking at their 'footprints' - we're looking at the holes, the microborings, left by the bugs as they dissolved into, or chewed, into the rocks”
The structures are seen in rocks from the famous Barberton Greenstone Belt in the Mpumalanga Province of South Africa.
These rocks were originally erupted underwater but over the course of Earth history have been lifted on to dry land.
The basalt that forms the rock had previously been dated to 3.47-3.45 billion years old, but there was some doubt about when the tubules themselves were created.
By comparing the ratio of different types, or isotopes, of uranium and lead atoms in the material that now fills these tunnels, the team can show they must have been etched by about 3.34 billion years ago - in other words, very soon after the host rock itself was formed.
The issue of when life first appeared on our planet is a hotly debated topic.
The constant recycling of rock means there are very few locations like Barberton where a physical record of the ancient Earth can still be examined.
Some researchers argue that the peculiar chemistry of rocks at Isua in Greenland betrays the presence of bacteria some 3.8 billion years ago.
What is different about Barberton is that this geochemical signal is also supported by shapes and textures - so-called trace fossils - in the rock which could have been cut by the ancient microbes.
It is not the same as having the "body" fossils of the organism, but researchers can make a strong case that the shapes have a biological origin if they can point to similar tubules made by modern microbes. The Bergen team believes it can do this.
"We're kind of looking at their 'footprints' - we're looking at the holes, the microborings, left by the bugs as they dissolved into, or 'chewed', into the rocks," explained Dr Nicola McLoughlin from Bergen's Centre for Geobiology.
"So instead of looking at the microbe itself, you're looking at the cavity or hole that it makes. We're still working to convince people of the biogenicity of these things and we think we have really good constraints on the modern seafloor," she told BBC News.
"But things get more difficult in the ancient [setting] because the shapes are simpler and the chemistry has been modified. What this paper does show, however, is the progress we have made in dating these structures."
The Barberton rocks in which the tubules were first identified were found at the surface. The University of Bergen is now analysing rocks that have been drilled from deep underground.
At the very least, this type of investigation will researchers more about what conditions were like on Earth almost 3.5 billion years ago.