When did life first emerge on Earth? Mark Harrison and colleagues from the University of California Los Angeles believe slivers of carbon entombed in an ancient gem suggest life existed here 4.1 billion years ago. The controversial findings were published in the Proceedings of the National Academy of Sciences in October.
Before now, the earliest evidence of life was carbon trapped in 3.8-billion-year-old apatite minerals in Greenland. Harrison’s finding would mean life began a mere 300 million years after Earth itself formed, 4.54 billion years ago.
Carbon is found in both living and non-living things. The element naturally occurs in two stable forms: as the far more abundant C-12, and as C-13, which is slightly heavier thanks to an extra neutron in its nucleus. Life prefers C-12: enzymes, the workhorse molecules that carry out biological reactions inside our cells, favour the lighter, easier-to-manipulate carbon C-12.
Geochemists take note of this preference when searching for evidence of ancient life. The carbon from a living organism has around 2.5% more C-12 than carbon taken from a non-biological object, such as a rock. “If you today saw a signature like this, you’d interpret it as biological activity,” Harrison explains.
“It’s quite remarkable that we can go that far back.”
To look for ancient examples of this signature of life, Harrison went to the Jack Hills, a geological site around 700 kilometres north of Perth renowned for its ancient rock. The area is especially known for crystals of the mineral zircon that formed from molten magma when the Earth was a few million years old. These little time capsules have yielded snapshots of Earth’s magnetic field from 4.2 billion years ago and its liquid water from 4.4 billion years ago. Could they also contain evidence of life – tiny nuggets of carbon, trapped inside the crystal, that have elevated C-12 levels?
Harrison and his team sifted though 10,000 of these tiny zircon crystals, each about the width of a hair, and found a single crystal, 4.1 billion years old, that contained two specks of carbon. They carefully sliced the zircon using an ultra-fine beam of ions and measured the ratio of heavy and light carbon in the specks. Sure enough, they contained 2.4% more C-12 than would be expected from a non-biological carbon source.
“It’s quite remarkable that we can go that far back,” says Louis Moresi, a geoscientist at the University of Melbourne. But he’s cautious. Finding light carbon indicates a “primitive chemical reactor”, but doesn’t tell you anything about its form. “It could be ‘pre-biological’,” he says – enzyme-like chemicals floating free in a pool of water, the precursors to life.
Harrison admits there are possible non-biological explanations for the carbon ratio he detected. The extra light carbon could even have been carried to Earth on meteorites, he adds.
David Wacey, a geochemist at the University of Western Australia, is even less convinced Harrison’s results show life on Earth had emerged 4.1 billion years ago. “Such a remarkable claim would require watertight evidence, and this is far from that,” he says. Even though scientists can tell the difference between non-biological and biological carbon today by how much C-12 they contain, he says, they don’t know if the same rule of thumb held up more than four billion years ago. Wacey believes far more samples will have to be analysed to confirm the results.
Harrison agrees. “The next thing would be to generate a thousand times more data,” he says. But he’s confident the zircon will not be the only one found to contain these carbon time capsules. He’s eager to keep looking – “now it’s just a matter of time and money” – to one day fill the gaps of life’s family tree.