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Establishing a Biological Origin

Debate about the origin of stromatolite-like structures is usually based on comparisons with undoubted living stromatolites (like those at Shark Bay and Lake Clifton in WA), and with modern non-biogenic structures (such as deposits found in limestone caves). Although organisms play a crucial role in forming modern stromatolites, an undisputed biological origin for ancient forms can only be demonstrated in rare instances where microbial remains are preserved as fossils within the stromatolites. There have been various attempts to develop alternative criteria for biogenicity, but most criteria are so exacting that even Proterozoic stromatolites (545 million to 2.5 billion years old), which few researchers doubt are biogenic, fail to qualify.

Archaean fossil discoveries older than about 3.0 billion years are usually highly controversial. Debate ranges around matters such as the age, confirmation of biogenicity, proof that microbial fossils are not contaminants, the overall significance of the specimens to the pathways by which life evolved, and the mechanisms by which the organisms lived and grew (palaeobiology).

PHOTO: Structures of undoubted biogenic origin: Living thrombolites (unlaminated stromatolites) at Lake Clifton, near Mandurah, Western Australia