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THE shared ancestor of every organism now on the planet lived at least 3.9 billion years ago 鈥� adding weight to theories that life started 100 million years earlier than we thought, at a time when Earth was still being pummelled by meteorites.

That鈥檚 according to a study that has combined genetic and fossil evidence to build a timeline of crucial shifts in the early evolution of life. The study also sheds new light on the birth of complex cells, which today make up all animals and plants.

For the first few billion years of Earth鈥檚 history, the only life present was single-celled microorganisms. Unlike the large plants and animals that arose in the past 600 million years, such as trilobites and dinosaurs, these microbes left few fossils 鈥� so understanding life鈥檚 early history has been tricky.

To find out more, Holly Betts at the University of Bristol, UK, and her colleagues combined two sources of evidence. They compared the sequences of 29 genes across 102 species to build a family tree that showed how they were all related, and the order in which new groups split away from their relatives.

The researchers then added some dates to this tree, taken from the geological record. This enabled them to estimate when the various groups evolved and parted company. For instance, they knew that life could be no older than 4.52 billion years because that is when a rock the size of Mars slammed into Earth, forming the moon. The impact was so severe that the planet鈥檚 entire surface melted. 鈥淣othing could have survived it,鈥� says team member Davide Pisani, also at the University of Bristol.

The result is a timeline of the first 3 billion years of life, from the ancestor of all modern life to the first complex animals (Nature Ecology & Evolution, ). 鈥淲e can go very deep in time, which we never thought was possible,鈥� says Pisani.

鈥淚 think they鈥檝e really made the most sincere and honest effort yet to actually get a coherent picture of the history of life,鈥� says Bill Martin at the University of D眉sseldorf in Germany.

鈥淭his is the best attempt yet to actually get a coherent picture of the history of life on Earth鈥�

One major finding is that the most recent organism that all existing life is related to 鈥� known as the last universal common ancestor, or LUCA 鈥� lived at least 3.9 billion years ago. There is tentative support for life to have existed this early in our planet鈥檚 history. Some have suggested that traces of carbon in 4.1-billion-year-old rocks are . Similar traces were found in 3.95-billion-year-old rocks in 2017. And another 2017 study claimed to have found fossilised single-celled organisms from 3.77 billion years ago. But all these findings are disputed.

Nevertheless, Greg Fournier at the Massachusetts Institute of Technology says the new date for LUCA is reasonable. This means life probably formed relatively quickly, within 600 million years of the moon-creating impact, when big meteorites were still battering the planet.

Supercharged life

LUCA gave rise to two groups of single-celled microorganisms called bacteria and archaea. This split happened at least 3.4 billion years ago, the study finds.

Later on, more-complex cells evolved 鈥� one of the most crucial evolutionary events ever. Compared with bacteria and archaea, these 鈥�eukaryote鈥� cells are larger, with membrane-bound compartments and tiny, energy-supplying mitochondria.

All animals, plants and fungi are eukaryotes, and if these cells had not evolved, we could never have existed. The timeline suggests these cells first appeared聽between 1.21 and 1.84 billion years ago.

It was thought that eukaryotes evolved in response to rising oxygen levels, but this finding suggests this might not have been the case, as the first atmospheric oxygen turned up much earlier 鈥� 2.4 billion years ago. However, it does not rule out a link, as there was little oxygen at first and levels later rose in fits and starts.

The timing of the eukaryotes鈥� origin is curious because it took place almost exactly when the study found that microbes called alphaproteobacteria evolved. The mitochondria inside eukaryotic cells were once free-living alphaproteobacteria, which were somehow swallowed by an archaean that then gave rise to the first eukaryotes.

The implication is that acquiring mitochondria was the key step in eukaryote evolution, says Pisani. This then spurred a massive evolution of diversity among eukaryotic cells, he says.

Martin and his colleagues have made a similar argument on physical grounds, suggesting that the first eukaryotes could only have evolved large, complex cells once they had mitochondria to supercharge them.

However, Fournier suspects the timings might be off. He says mitochondria are quite different from their alphaproteobacteria ancestors, 鈥渟o they should be substantially younger鈥�.

This article appeared in print under the headline 鈥淏orn on a battered Earth鈥�