Pushing Back The Origin Of Life

Rocks from the Pilbara region contain some of the earliest known fossils

As we go further and further back in time rocks become increasingly scarce. Correspondingly, the number of fossils also decreases. This poses a problem for palaeontologists looking to uncover the oldest fossils and glimpse the origins of life.

In recent decades they have turned to molecular techniques to estimate the origins of living organisms. They can be used to date the earliest divergences in the tree of life, which lie close to its origins.

Yet while dates are backed up by sophisticated analyses, ultimately they are hypothetical. It's fossils that confirm these dates. The oldest cellular fossils are around 3.4 billion years old, but the traces of life are older still. Previously the boundary lay at 3.8 billion years in the form of graphite from the Isua Greenstone belts. Now the date for the oldest traces of life has been pushed back by another 300 million years. Researchers, led by Elizabeth Bell from UCLA, studied more than 10,000 zircons from Western Australia. Some of the zircons contained grains of graphite whose isotopic signature also matched that produced by biological processes, specifically photosynthesis.

'There is no better case of a primary inclusion in a mineral ever documented, and nobody has offered a plausible alternative explanation for graphite of non-biological origin into a zircon,' said Mark Harrison, professor of geochemistry at UCLA. Zircons are physically tough and so the graphite grains within the crystals have remained unaltered since they formed.

One of the zircons and the graphite inclusions

Zircons have another desirable property, which is augmented by their durability; during their formation they can incorporate atoms of uranium into their crystal structure. This allows zircons to be dated radiometrically with incredible accuracy. The zircons were dated at 4.1 billion years old.

'The early Earth certainly wasn't a hellish, dry, boiling planet; we see absolutely no evidence for that,' said Harrison. 'The planet was probably much more like it is today than was previously thought.'

The zircons come from a time in the Earth's history known as the Hadean. Apart from a handful of zircons, there are only two terrestrial rock formations which come from this period. The insight gained into its conditions is therefore extremely limited.

Yet each new piece of data shows that we need to reconsider our conceptions of what the planet was like. A view fast changing from a volcanic hell to an environment which may have had liquid water, stable landmasses and even life.