When did life on Earth first arise? Scientists are narrowing in on the answer with perhaps the oldest fossils known to date – a staggering 3.7 billion years old – uncovered in Greenland.
The discovery, unveiled in Nature by Australian and UK researchers, suggests the planet teemed with life during Earth’s violent youth. If confirmed, it beats the previous record-holding oldest fossils by around 220 million years.
But the implications of this discovery reach further than understanding the origin of life on Earth – the possibility that life thrived on Mars too is also given a boost.
So what kind of life survives asteroid collisions to leave a footprint lasting billions of years?
The fossils appear to be remnants of “living” rock formations called stromatolites, which are considered one of the oldest life forms on Earth. (A few are still alive and kicking in Western Australia.)
Stromatolites are built layer-by-layer by a mat of photosynthesising bacteria in shallow water. Over time, they create structures of varying size and shape, from tall domed towers to small, pointed cones.
They’ve dotted the Earth for at least the past 3.48 billion years, as established by an ancient reef uncovered by Australian astrobiologist Abigail Allwood in the Pilbara region of Western Australia in 2006.
This latest discovery, by Australia’s University of Wollongong’s Allen Nutman and colleagues in 2012, was not made down under. Rather, the team travelled to the other side of the globe to Greenland’s Isua Greenstone Belt, a prime source of ancient rock material.
There, encased in 3.7-billion-year-old volcanic rock, were stromatolite-looking objects: layered triangles between one to four centimetres high.
Craig O’Neill, a planetary scientist at Macquarie University in Sydney, Australia and who was not involved in the study, says the morphology and geology of the fossils is certainly compelling evidence.
But some ancient fossil finds have come under fire in recent years. A long-running controversy has surrounded Pilbara microfossils unearthed by American paleaobiologist William Schopf in the 1980s. Other groups claim Schopf’s strings of what look like cells actually formed volcanically – not biologically.
Writing in a News and Views article, Allwood – now heading a team on the 2020 Mars mission at NASA in California – says Nutman’s find “will no doubt spark controversy”, but the composition and texture of the fossils and rock “are fairly credible hallmarks of microbial activity”.
So if these conical patterns are what they seem, what does it mean?
For microorganisms complex enough to construct stromatolites, Nutman and colleagues write, life must have had a significant pre-history. This puts the origin of life far further back in time than the age of the stromatolite fossils – right in the midst of the Late Heavy Bombardment – and this has implications for life on Mars.
Some four billion years ago or so, the solar system was a maelstrom of asteroid, comets and planetoids, whizzing around and crashing into planets and each other.
Earth wasn’t spared – it was pelted with asteroids, leaving rivers of molten rock on the surface.
If life could survive in these inhospitable conditions, the researchers write, it could easily have evolved on the friendlier Martian surface.
Back then, O’Neill says, the red planet probably looked a little bit like Earth today, with stable bodies of water such as lakes and oceans.
“A lot of the prerequisites for life were there on Mars, and given life on Earth got going so quickly after the global sterilising event calmed down, it possibly could have on Mars as well,” O’Neill says.
But finding rocks from this era, let alone fossils, isn’t easy. Few rocks from those early years are still around and those geologists can find must be sliced open to expose any fossils trapped within.
Nutman’s team probably wouldn’t have made their discovery if they’d gone hunting a decade before. The rocks containing the fossils were recently exposed for the first time as the normally permanent overlaying snow and ice melted.
“It has been said the only people who see a positive in climate change are geologists because suddenly they’ve got a whole lot more rocks to explore as the glaciers and snow melts,” O’Neill says.
“It’s sad Greenland’s losing its ice sheet and permanent snow cover, but to make the most of a bad situation, they have new rock exposed there.”
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