A 3.5-billion year old Pilbara find is not the oldest fossil: so what is it?

Less than 10km outside Marble Bar in the Pilbara region of Western Australia lies one of the more famous sites for scientific research in Australia. Around a quarter of a century ago, UCLA palaeontologist James William Schopf discovered tiny filaments preserved within a silica-rich rock, the so-called Apex chert.

These were interpreted as the fossilised remains of primitive filamentous bacteria and thus thought to constitute the earliest known evidence for life on Earth, dated at 3.46 billion years old.

Apex chert filaments, once interpreted as Earth’s oldest microfossils.Martin Brasier, Author provided

With the technology available to researchers at that time this was a reasonable interpretation. The sizes of the filaments (mostly 1-20 micrometers in diameter) were comparable to known filamentous bacteria and they had an internal structure that resembled multiple cells joined in chains.

During the following decade these filaments became embedded in both the textbook and popular science literature as Earth’s oldest microfossils.

They were also heralded as the standard against which other possible signs of ancient (or even extra-terrestrial) microbes should be judged.

The Apex microfossil debate

Everything changed in 2002 when a team led by Oxford palaeobiologist Martin Brasier questioned the authenticity of the microfossils .

Brasier and colleagues had re-interpreted the geological setting of the filaments, demonstrating that they were trapped in rocks that formed at high temperatures during volcanic activity. This brought into doubt the initial interpretation by Schopf.

Re-examination of the filaments under the microscope revealed that some appeared to branch and others followed the edges of mineral crystals.

These new findings led the Brasier group to propose that the filaments were not microfossils. Instead they were merely bits of carbon, arranged in roughly filamentous patterns around crystal boundaries, probably formed by hot fluids.

In the ensuing decade or so the Apex microfossil debate has been intense. Although it is now accepted that the geological setting is likely a hydrothermal one, this has not diminished the Schopf group’s belief in the authenticity of the microfossils.

They’re now suggesting the filaments are fossils of heat-loving (thermophilic) bacteria, similar to those found around deep-sea hydrothermal vents today.

On the other side of the debate, the Brasier group presented more detailed geological and microscopic analysis consistent with the filaments being non-biological artefacts.

A scientific stalemate had been reached.

Not everything that looks like biology is biology

In collaboration with the late Professor Brasier, we have now used high spatial resolution electron microscopy techniques to investigate the detailed structure and chemical composition of the filaments.

This research, published this week in the Proceedings of the National Academy of Sciences, has confirmed that the Apex filaments are not microfossils. Instead, they are mineral artefacts, comprising stacks of silicate grains onto which later carbon adsorbed.

Our data provided a picture of the morphology and chemistry of the filaments at a spatial scale up to one hundred times better than previous studies. At this scale it becomes apparent that the filaments are made of hundreds of plate-like grains of a potassium and barium rich silicate mineral.

Electron microscope image of part of an Apex filament showing stacks of silicate grains (green) and iron minerals (red) that have been coated by later carbon (yellow)Martin Saunders, Author provided

These grains are similar in appearance to common mica that you might see glistening in granite tables around Australia. Although carbon is present in the filaments, its distribution is incompatible with any known biological morphology.

Today mica-like minerals are used to clean up oil spills due to their very high capacity to adsorb (attract to their surface) hydrocarbons. We believe that the carbon in the Apex filaments was arranged by a similar process.

While the lower resolution techniques previously employed allow for a potential biological interpretation, our high resolution data shows that the arrangement and distribution of the carbon within the minerals does not support the biological hypothesis.

The supposed cellular compartments have very inconsistent lengths, plus length/width ratios that match crystal growth patterns but are unlike any known microbial cells. The carbon is found to have entered the filaments after the formation of the surrounding minerals, again inconsistent with it being the in situ remains of bacteria.

What does this all mean for the search for early life?

The field of early life research is fraught with difficulty. Data initially interpreted as biological in origin are often reinterpreted at a later date as having a (less exciting) geological explanation. As new analytical techniques become available, accepted paradigms may have to be questioned.

While our latest research does not really move the goalposts for when life first originated on Earth – there are robust microfossils only a few million years younger than the Apex material – it emphasises that not everything that looks like life really is life.

Perhaps most importantly it shows that microstructures that appear to tick all the boxes for biology when examined down to the micrometre scale, can fail some of these same criteria when examined at the sub-micrometre scale. This may usher in a new way of analysing possible signs of life in the future, on Earth or further afield.

David Wacey receives funding from the Australian Research Council and the European Commission. He is affiliated with the University of Bristol, UK.

Martin Saunders receives funding from the Australian Research Council.