We have good reason to believe life first appeared on Earth soon after the planet cooled enough to support it. However, whether we can find traces of these early organisms has been highly controversial. Two papers from one team of scientists support the theory that bands of graphite in ancient rocks come from some of the earliest life forms. The work may expand our capacity to find life on other planets.
Greenland hosts some of the world's oldest surviving rocks. Among these, particles of graphitic carbon can be found, not dissimilar to the contents of a pencil (google it, kids). The discovery of this carbon created plenty of enthusiasm, as it might be life's earliest remains. However, two other, much less exciting, theories have also been proposed – either the carbon had a non-biological source, or it was from more recent living things that worked their way into the rocks long after they formed.
In Earth and Planetary Science Letters, a team including University College London's Dr Dominic Papineau make the case that the hoped-for explanation is right. “We now have multiple strands of evidence that these mineral associations are biological in banded iron formations,” Papineau said in a statement.
The graphite is trapped in banded iron rocks, some of which are more than 3.8 billion years old. In 10 cases it has been found associating with apatite, a component of teeth and bones that can also have non-biological origins. They've been transformed under heat and pressure many geologists expected would destroy any distinctively biological chemicals trapped within them.
However, the paper reports the graphite is depleted in carbon-13 isotopes, like material laid down by more recent life forms. The distribution through the rocks is also consistent with the remains of organisms being deposited by hot fluids, such as from volcanically heated water.
In a second paper, published in the Journal of the Geological Society, Papineau demonstrates life's traces can survive similar metamorphosis. He used banded iron formations from Michigan, less than half the age of their Greenland counterparts.
The shapes of animals within the Michigan rocks reveal they once held life, but they have experienced temperatures of at least 550ºC (1,022ºF). Yet graphitic carbon in these rocks has formed expanded lattices containing chemical and isotopic signatures of life. If these could survive such temperatures and pressures, there is no reason to think they wouldn't make it through another 2 billion years.
If Mars once hosted life, the traces it left behind would probably resemble these, leading Papineau to claim: "This has huge implications for how we determine the origin of carbon in samples of extra-terrestrial rocks returned from elsewhere in the Solar System."
[H/T: Geological Society of London Blog]