Has life on Earth been around longer than we thought?

The journal PNAS has published research by geochemists from the University of California, Los Angeles (UCLA) suggesting that life began 300 million years earlier than previously thought.  The significance of the results isn’t purely to do with the time difference: it’s about what was happening to the Earth during that period.

“We generally believe Earth was still frequently bombarded by huge meteorites that would have steam sterilised the oceans.”  Those are the words of Dr. Mark van der Giezen, Senior Lecturer in Evolutionary Biochemistry and laboratory group leader at the University of Exeter, who kindly gave me an insight into the implications of the research.

“This new evidence suggests that life might indeed have existed during this very early hostile period of our planet.  It could therefore be totally unrelated to life as we currently know it.”

Each element of the periodic table comes in a variety of sizes, or “isotopes”.  ¹²C is the most abundant carbon isotope: it has 6 neutrons and 6 protons.  ¹³C, or carbon-13, has an extra neutron.  Certain ratios of ¹²C to ¹³C suggest life may have been present.  As Dr. Mark van der Giezen explains, “life prefers lighter isotopes.”

The UCLA research team studied thousands of pieces of a particular mineral called zircon.  Contained in one zircon, they found one of those ratios that can indicate life.  The surprise?  The zircon was much older than life is presumed to be – about 4,100,000,000 years old. Or, in their words, the “δ¹³C_PDB of −24 ± 5‰ is consistent with a biogenic origin and may be evidence that a terrestrial biosphere had emerged by 4.1 Ga”.

Not only was the Earth still in that hostile period of asteroid bombardments, but the figure is surprisingly soon after the Earth was actually formed.  But how else could the carbon have got there?  Carbon isotopes can come from non-biological sources, such as meteorites, but the researchers say that this is unlikely due to the amounts found.

Additionally, as Dr. Mark van der Giezen points out, “previous studies using similar data have been shown to be incorrect… zircons were shown to be contaminated by ‘modern’ carbon.”  Those lighter isotopes can seep in through tiny cracks in the zircon.  However, the researchers used transmission X-ray microscopy to show that the carbon had “complete encasement in crack-free, undisturbed zircon”.

The evidence seems sound so far.  Professor Mark Harrison, who co-authored the paper, told the UCLA newsroom that he is “very confident” that the zircon does indeed show a 4.1 billion year old carbon ratio.  “With the right ingredients, life seems to form very quickly.”

Thanks go to Dr. Mark van der Giezen for providing us with amazing insights, and helping to decipher this powerful piece of research!