They are tiny. They consist of filaments and tubes up to half a millimetre in length and around half the width of a human hair. They’re made of haematite, a type of iron oxide (better known as rust). Some of the filaments resemble loose coils, some are branched, and others appear to be joined to knobs of haematite. The tubes and filaments are thought to be the remains of bacteria that lived on iron and dwelt around hydrothermal vent systems – mineral-rich hot springs – on the seafloor. Similar systems have been proposed as a likely location for where life first arose.
Telltale chemical signatures, and distinctive structures such as pillow-shaped masses, point to an underwater formation for the rocks. The fossils themselves were discovered in a type of iron-rich quartz known as jasper. This jasper was found between volcanic rocks, and probably formed as material ejected from hydrothermal vents settled.
That’s the tricky bit. The authors of the study say they are confident for several reasons. They argue that the haematite structures are similar to those produced by iron-oxidising bacteria today, as well as to microfossils found in younger rocks, hundreds of millions – rather than billions – of years old. What’s more, the structures were found to contain graphite as well as the minerals apatite and carbonate – which are associated with biological matter. Finally, the team found iron-oxide granules and, in other sections of the rocks, structures such as carbonate rosettes (also associated with apatite, graphite and carbonate), which, they say, could have formed as biological matter broke down.
No. While the researchers say their investigation ruled out the chance that the structures were formed by geological processes, others are not convinced. The rocks in which the fossils were found are metamorphic, meaning that they have experienced high temperatures and pressures since they formed – some argue that this could have produced the structures instead. The size and arrangement of the haematite structures has also raised concerns, as has the fact that the microbes would have relied on oxygen at a time oxygen is thought to have been scarce. .
The previous “oldest fossils” date to about 3.5bn years ago and were found in Western Australia. However, they have been the subject of hot debate, with many experts unconvinced they are the remains of microbes. Another finding was reported last year, with researchers claiming that they had found fossil traces of microbial activity, known as stromatolites, in rocks from Greenland dating to 3.7bn years ago – 220m years older than the previous oldest known stromatolites. Again, experts were split on whether the evidence was strong enough to suggest the structures were made by organisms, rather than through geological processes. Other younger and better-preserved finds are less contentious.
If the structures are microfossils, the discovery shows that fairly sophisticated microbes might have been around as early as 4.3bn years ago, pushing back the origins of life on Earth to, potentially, 4.4bn years ago. With planet Earth only 4.5bn years old, that suggests life got going and diversified pretty quickly. That raises the possibility that life can crop up rapidly when the right conditions are present.
We know that, in its early beginnings, Mars, like Earth, had oceans. If these newly discovered structures are fossils, it raises the possibility that microbes were also thriving on Mars – and offers clues as to what scientists should be looking for in rocks on the red planet. The find also supports the idea that other bodies in the solar system, such as Jupiter’s moon Europa, might be good places to look for life, since it has been proposed that Europa might have hydrothermal vents under its icy shell.