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The most ancient evidence of life on Earth?

Ancient bacteria in Pilbara rock are merely volcanic minerals, scientists say. Belinda Smith reports on a long-running controversy.

The worm-like shapes in the rock are described in text books as the oldest form of life ever found on Earth. But not everyone is convinced. – University of Bristol

To some scientists, the Apex chert microfossils found in the Pilbara region of Western Australia are the oldest evidence of life on Earth. To others, the "fossils" are nothing more than geological anomalies in the rock. This latter group now claims to have delivered the knockout blow in this decades-long debate.

David Wacey and colleagues from the University of Bristol and University of Western Australia have carefully sliced open some of the 3.46-billion-year-old "microfossils" and found they are merely unusually shaped minerals. Their work has been published in the Proceedings of the National Academy of Sciences.

“Our work solves a major controversy and removes this particular piece of evidence for life on the early Earth,” Wacey says.

But not everyone is convinced.

The story begins in the 1980s in the Pilbara – half a million square kilometres of dry rocky landscape across Western Australia – which contains some of the most ancient rock formations on Earth. University of California palaeobiologist J. William Schopf collected samples of fine-grained sedimentary rock called chert near the town of Marble Bar. Under a microscope he saw filament-like structures that look a bit like segmented worms, which he claimed to be microfossils – the preserved remnants of ancient bacterial clusters.

Writing in Science in 1987, Schopf compared them to cyanobacteria – “dark-brown, carbonaceous, filamentous microfossils” of “about three micrometres in diameter and 30 micrometres to 40 micrometres in length”. They looked like cells and he thought the fossils’ carbon-based nature indicated their biological origins. The microfossils were dated at 3.46 billion years old and became enshrined in textbooks as the earliest known life.

Enter Oxford University palaeobiologist Martin Brasier – the main challenger to Schopf’s claim. In 2002 he pointed out that the Pilbara’s violent volcanic past provided perfect conditions for the filaments to form from silicate minerals, and that the “cells” were merely carbon fortuitously arranged around crystal boundaries.

Schopf’s group has since countered, claiming the bacteria were thermophiles – organisms capable of living in hot, usually inhospitable environments such as hydrothermal vents. But Brasier’s team wasn’t convinced. They decided to slice the filaments to examine their internal structure.

“I think it’s a very interesting study,” Schopf says. “But it’s flawed.”

Schopf’s specimens are housed at the Natural History Museum in London, which doesn’t take kindly to its specimens being cut apart. Luckily Wacey – Brasier’s long-time collaborator – had already returned to the Pilbara to collect more rocks. “Fortunately we know the field area very well and we were able to recollect material identical to the Schopf-type material,” Wacey says. “We were then free to apply any technique we wanted to it!”

Wacey used a focused ion beam to slice open the fossils, a cutting technique common in the semiconductor industry but relatively new to palaeontology. The instrument shoots out a focused stream of ions that can etch a line narrower than 10 nanometres – precise enough to delicately slice the filaments like a scalpel.

Those thin slivers showed the carbon-rich “walls” were actually made of sheet-like silicates – including mica, the mineral that gives some rocks a shimmery sparkle, and another silicate called vermiculite. The silicates contained large amounts of barium, which is not of biological origin but is typical of minerals formed in hot, wet conditions.

The silicates also contain layers of carbon, but Wacey has an explanation. Vermiculite is a sticky type of silicate, and would gradually have accumulated any nearby carbon. He hypothesises the filaments formed when wet mica grains in a volcanic environment heated and expanded rapidly. As they pushed water out, they left tiny cell-like segments that joined together. Any hydrocarbons floating around would stick to the silicate surface and then fossilise over time, leaving what look like segmented “worms”.

Under an electron microscope a slice of Pilbara rock reveals it is comprised of spiky filaments that do not have the rounded form of fossilised bacteria. – University of Bristol

“I think it’s a very interesting study,” Schopf says. “But it’s flawed.”

He’s not convinced that the specimens Wacey collected and examined were, in fact, the same as his original Apex chert. “No one asked me exactly where I got my specimens from,” he says – information he says he would happily share.

It’s easy to mistake mineralised filaments for his microfossils, Schopf says. In a study that appeared in Nature in 2002 he examined 120 or so original Apex chert microfossils – “virtually every doggone specimen” – with a technique called laser-Raman spectroscopy, which maps a chemical’s distribution through a sample. Schopf saw no vermiculite-like material in the filaments.

But with Wacey’s work, “we don’t know how many specimens were analysed or how many didn’t have that vermiculite-like material”, he says.

And even if we did remove the Schopf microfossils from the fossil record, would that change what we know about life on a young Earth? Not really, Wacey says: “There have been lots of recent discoveries of microfossils in rocks not much younger” – only 20 million years or so – “than the Apex chert.”

Brasier died last December. Before his death he pointed out that the close scrutiny given to the Apex chert microfossils has had one definite advantage: “Such discussions have encouraged us to refine both the questions and techniques needed to search for life remote in time and space, including signals from Mars or beyond.”

Belinda smith 2016 2.jpg?ixlib=rails 2.1
Belinda Smith is a science and technology journalist in Melbourne, Australia.
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