The weird and wonderful of the Pre-Cambrian (Part 2): Strelley Pool stromatolites – among the oldest signs of life
Jon Trevelyan (UK)
This is the second of my series of short articles on fossils of the Pre-Cambrian. The Strelley Pool stromatolites, preserved in the ~3.45-billion-year-old Strelley Pool Formation of Western Australia, rank among the oldest widely accepted macroscopic evidence of life. Formed on shallow marine carbonate platforms during the early Archean, these laminated domes offer a rare glimpse into Earth’s earliest ecosystems, when microbial life had only recently emerged.
Discovery and appearance
First documented in detail in the 1980s and 1990s, the Strelley Pool stromatolites occur in beautifully preserved carbonate packages interbedded with volcanic rocks of the Pilbara Craton. These stromatolites form conical and domical structures up to tens of centimetres high, with fine, wavy laminations stacked in repeated sequences.
Some structures include:
- smoothly domed buildups;
- steep, conical forms; and
- stratified columns and ridges.
The laminations are composed of alternating carbonate layers, some interpreted as mineral precipitates mediated by microbial mats.
Interpretation and classification
Interpreting stromatolites at 3.45 billion years old inevitably invites caution. However, the Strelley Pool structures show several features strongly suggestive of biological influence:
- upward-directed lamination consistent with microbial mat growth;
- distinct domical and conical geometries;
- sediment trapping and binding textures; and
- association with shallow-water carbonate sand bodies.
These formations resemble younger, unambiguously biological stromatolites, making a microbial origin the most parsimonious explanation.
The microbes responsible were simple, perhaps like modern cyanobacteria. Scientists are still unsure whether these early microbes were already producing oxygen, or used a different kind of chemistry to live.
Significance
Strelley Pool stromatolites are globally significant because they demonstrate that large-scale microbial ecosystems were already established more than three billion years ago. They help constrain the timeline of early life, indicating that microbial mats had colonised shallow marine environments early in Earth’s history.
Their presence challenges assumptions about the environmental conditions of the early Archean. The formation records episodes of stable carbonate deposition, suggesting that at least some coastal environments supported long-term microbial colonisation.
Conclusion
The Strelley Pool stromatolites remain one of the most compelling examples of early macroscopic life on Earth. Their domical and conical forms, preserved in ancient Archean carbonates, reveal microbial communities thriving in shallow seas long before the rise of complex organisms. They stand as enduring traces of Earth’s earliest biological architects.