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Tests on the red lines that make up the Blombos drawing show that the pigment contains haematite, a common reddish-black iron ore.

The authors used a thin, flat sample of single-crystal haematite, α-Fe2O3 — an electrical insulator that is the most common antiferromagnetic iron oxide.

Some of the filaments resemble loose coils, some are branched, and others appear to be joined to knobs of haematite.

Westall said it remains possible that the haematite structures were formed as a result of the high temperatures and pressures experienced by metamorphic rocks.

The scientists' putative microbes from Quebec are one-tenth the width of a human hair and contain significant quantities of haematite - a form of iron oxide or "rust".