Earth is some 4.5 billion years old. When it formed from colliding rocks around a dim, young sun, it was presumably lifeless, and geologists long thought that life didn’t emerge for a billion years or more. This idea came from analysis of moon rocks brought back from the Apollo landings, which indicated Earth was pummelled by space rocks between 4 billion and 3.8 billion years ago – an event called the Late Heavy Bombardment. The implication was that the origin of life as we know it must have begun after that, since any earlier organisms would have been blitzed.
“There’s two issues with that,” says at the University of Bristol, UK. First, models suggest that some life could have survived deep in the oceans. More damningly, it now seems that the Late Heavy Bombardment didn’t actually happen. The Apollo missions only created the impression of a huge bombardment over a brief period because they all collected rocks of a similar age.
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We now know that, early in Earth’s history, large impacts occurred sporadically over hundreds of millions of years. However, we also know that a body the size of Mars collided with Earth just after it was formed, vaporising the planet’s surface. “If life originated before then, it would have been wiped out,” says Donoghue.
Earth’s oldest rocks
Life began when inert matter self-organised into living systems, but, despite decades of research, how that happened remains a mystery. Figuring out when it happened is also a big challenge because the fossil record gets worse the further back in time we go. As a result of the slow churning of rocks by plate tectonics, many of the oldest rocks have been dunked into Earth’s hot interior, where they were melted and crushed, distorting or destroying any fossils. For the first 500 million years of Earth’s existence, a period dubbed the Hadean, there are no known rocks: just a few tiny crystals.
Nevertheless, there are some truly ancient fossils. The oldest confirmed life comprises single-celled organisms, perhaps like modern bacteria, living in shallow water. Fossils of these were found in the Pilbara region of Western Australia and dated at 3.5 billion years old. But that is a billion years after the moon-forming impact. Two strands of evidence suggest life got started much earlier. First, there are many claims of older fossils, including apparent microorganisms from 3.7 billion years ago and traces of seemingly biological carbon in crystals from 4.1 billion years ago. These finds have been questioned, however, and none is as convincing as the Pilbara fossils. But Donoghue believes we shouldn’t outright reject them. “There’s a non-zero probability that they’re fossils,” he says.
Second, genetics points to an early origin. , Donoghue and his colleagues attempted to date the last universal common ancestor (LUCA): the organism that is the ancestor of all life today. They did so by identifying genes found in all living organisms, which probably date back to LUCA. Their best estimate was that LUCA lived 4.2 billion years ago. That’s just 300 million years after Earth formed. And things would have got started far earlier than that. “LUCA isn’t the origin of life by any stretch of the imagination,” says Donoghue. It seems to have been a fairly advanced microorganism, the product of a long period of evolution and growing complexity.
Donoghue’s LUCA date might turn out to be off – something he emphasises. Still, the fact that life appears as soon as there is a rock record speaks volumes. “I feel very comfortable saying that it probably originated at some point during the Hadean,” says at Harvard University. In other words, it seems that life on Earth began early and quickly became complex. If so, our planet must be even more perfectly suited for life than we thought.
This article is part of a special series exploring seven of the biggest chronological conundrums of all time.
The science of deep time: Wales
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