Some fossil stromatolites, of undoubted biological origin, are 3.5 billion years old: the layers that they form are unmistakable, and absolutely similar to the layers of the modern, life-containing domes.
Photo copyright 2006 b y Andrew Alden
The(proposed ) 3.5 bya date makes them 100 million years older than the earliest true fossils of individual microbes (remember, fossil stromtolites are traces of bacteria)
However
Biological origin on the earliest stromatolites (3.5 bya) is still dubious.
http://www.astrobio.net/pressrelease/3194/evidence-of-earths-earliest-life".....But when you look back 3.45 billion years, to the early Archean period of geologic history, things aren't quite so simple.
"Because stromatolites from this period of time have been around longer, more geologic processing has happened," Grotzinger says.
Pushed deeper toward the center of Earth as time went by, these stromatolites were exposed to increasing, unrelenting heat. This is a problem when it comes to examining the stromatolites' potential biological beginnings, he explains, because heat degrades organic matter. "The hydrocarbons are driven off," he says. "What's left behind is a residue of nothing but carbon."
This is why there has been an ongoing debate among geologists as to whether or not the carbon found in these ancient rocks is diagnostic of life or not....
....."When the rocks are old and have been heated up and beaten up," says Grotzinger, "all you have to look at is their texture and morphology."
Which is exactly what Allwood and Grotzinger did with samples gathered at the Strelley Pool stromatolite formation in Western Australia.
The samples, says Grotzinger, were "incredibly well preserved." Dark lines of what was potentially organic matter were "clearly associated with the lamination, just like we see in younger rocks. That sort of relationship would be hard to explain without a biological mechanism." "
