Tiny shapes in 3.4-billion-year-old rocks from Strelley Pool, Western Australia, are indeed of biological origin, new evidence confirms. Although substantially older candidates exist, this makes the Strelley Pool fossils the oldest specimens that can be proven to have once been alive. The surviving traces of their original chemical composition makes these fossils a precious window into life's origins.
Early life forms were small and soft-bodied, neither of which make for good fossilization, and the passage of time has not been kind to many traces. Consequently, we’ve found a lot of rocks that might either contain fossils from near the dawn of life or simply represent much less interesting geologically formed shapes. The oldest of these are 4.1 billion years old, but only a handful of sites predate Strelley Pool.
Dr Julien Alleon of MIT examined the Strelley Pool rocks using X-ray absorption spectroscopy and compared the results with both modern bacteria and similar-looking microfossils from the Gunflint Formation, Ontario. The Gunflint fossils are definitely bacterial remains, and at 1.9 billion years old, provide a good point of comparison.
All three samples produce similar absorption patterns, Alleon reports in Geochemical Perspectives Letters indicating the presence of the same chemicals. The spectroscopy also proved the biological molecules were encased within the rocks at the time of formation, rather than having combined with them later.
Amazingly, despite having been exposed to temperatures of 300ºC (570ºF) for long periods, the paper describes organic molecules from the fossils as “only slightly degraded”, which is attributed to the early encasement in silica crystals. Indeed, the Strelley Pool molecules are better preserved than those from Gunflint.
Strelley Pool is thought to have been a shallow marine environment, heated by hydrothermal activity, at the time the fossils were formed.
The question of when life began on Earth is important for many reasons. For one thing, if it arose fairly soon after conditions became suitable – a little over 4 billion years ago – it would suggest an easy process likely to occur often throughout the universe. A later date for life's appearance would support the opposite conclusion.
Molecular clocks date the last common ancestor of all life to 3.5 billion years ago, suggesting life is probably older still. Wavy lines in Greenland rocks and graphite from northern Canada is depleted of carbon-13 isotopes, something usually associated with biological activity, but these examples have yet to convince all palaeobiologists they represent the real thing.
Alleon argued in a statement; “This analytical strategy needs to be applied to other ancient samples to help settle the controversy [of which fossils are the oldest].” Until that is done, however, Alleon regards Strelley Pool as the oldest confirmed claimant.