The first living thing to emerge on our planet appeared at least 100 million years earlier than previously thought, according to a new study.
Around 3.9 billion years ago – shortly after the Earth had been struck by the planet Theia and while it still faced a barrage of meteors – the ancestor of all living things sprang into being.
Scientists have traditionally used the fossil record to trace the origins of life on Earth, but the further back in the planet’s history they travel the more difficult this becomes.
“The problem with the early fossil record of life is that it is so limited and difficult to interpret – careful reanalysis of the some of the very oldest fossils has shown them to be crystals, not fossils at all,” explained PhD student Holly Betts from the University of Bristol, the lead author of the study.
In their research, published in the journal Nature Ecology and Evolution, Ms Betts and her colleagues used a combination of fossil and genetic data to trace back the so-called “Luca” – the last universal common ancestor.
“Fossils do not represent the only line of evidence to understand the past,” said co-author Professor Philip Donoghue.
“A second record of life exists, preserved in the genomes of all living creatures.”
By combining data from all available sources scientists are able to construct “molecular clocks”, based on the idea that the number of differences in genetic code between different species is proportional to the time since they shared a common ancestor.
Using information on 29 genes from a total of 102 living organisms, the team assembled a timeline with dates for the appearance of all the major groups of life forms, such as bacteria.
The scientists concluded that the hypothetical Luca existed before the “late heavy bombardment” when multiple meteors smashed into Earth.
This is far earlier than the oldest fossil evidence of life, which is no more than 3.8 billion years old.
While there are still uncertainties, the scientists knew that life on Earth could not stretch back further than 4.5 billion years, when Theia crashed into the fledgling planet.
This devastating event not only broke off pieces of the Earth that would ultimately form the Moon, it would have effectively sterilised the planet and killed any life already existing there.
As the first life consisted of tiny microscopic cells, surviving fossils are seldom found, and those that are have been the source of much debate.
Some have suggested the carbon found in a 4.1 billion-year-old mineral called zircon could be evidence of ancient life, but this has not been confirmed.
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