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New research suggests that everything alive today descended from a cell that lived 4.2 billion years ago, just a few hundred million years after the Earth formed.

That last universal common ancestor, which biologists affectionately called LUCA, wasn’t all that different from the fairly complex bacteria alive today—and it lived in an ecosystem teeming with other kind of life and viruses.

“What’s really interesting is that it’s clear that it had an early immune system, showing that our ancestor was already engaged in an arms race with viruses even 4.2 billion years ago,” Davide Pisania genomics researcher at the University of Bristol in the United Kingdom and co-author of the new study, said in a statement.

All cellular life forms on Earth share certain key features: they use the same protein building blocks, everything uses the same energy currency to power its cells (ATP), and all cells use DNA to store information. These similarities are probably not a coincidence; they all point to the fact that the life we ​​know today emerged from a single origin.

Related: What is the tree of life?

Before this study, scientists estimated that LUCA was alive 3.9 billion years ago. However, accurately dating genetic events that occurred so long ago is challenging.

In the new study, published July 12 in the journal Nature Ecology & Evolutionresearchers wanted to pinpoint the origins of LUCA more precisely. The team compared all the genes in 700 living species of bacteria and archaea (microbes that resemble bacteria and often live in extreme environments). They chose organisms in these domains because they are considered the oldest forms of life, where eukaryotes evolved from a union between both cell typesThe researchers then counted the mutations that had occurred over time in the genome and within 57 genes shared by all 700 organisms, using estimated mutation rates to back-calculate when LUCA was alive.

They based their age estimate on fossils that contain traces of ancient life, such as the remains of 3.48 billion year old microbial mats from AustraliaAncient fossils gave them insight into the atmospheric conditions of early Earth and provided a lower estimate for when LUCA might have survived.

This suggests that LUCA lived about 4.2 billion years ago.

“We didn’t expect LUCA to be so old, within just hundreds of millions of years of Earth’s formation,” said co-author Sandra Álvarez-Carreteroa researcher at UCL in the United Kingdom At that time, during the Hadean (4.6 billion to 4 billion years ago), the Earth was an inhospitable place, with hot oceans and very little atmospheric oxygen.

Furthermore, by sorting genes based on their cellular function, the researchers could say something about how and where LUCA lived and what it ate. Their analyses did not identify LUCA’s exact habitat, but suggest it likely lived in an ocean environment, a shallow hydrothermal vent, or a hot spring. Furthermore, they found that LUCA was likely able to tolerate extreme temperatures and “breathed” without oxygen, instead relying on the waste products of others sharing its ecosystem.

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Evidence that LUCA was not alone comes from the reconstruction of LUCA’s metabolic pathways. It suggests that LUCA may have used organic material already broken down by other microbes for energy. Other supporting evidence comes from the surprising result that LUCA was already equipped with genes that could help defend against infectious viruses.

The fact that LUCA was living in a thriving ecosystem even then has interesting implications for life on other planets, said the study’s lead author. Philip Donoghueprofessor of paleobiology at the University of Bristol, in the statement.

“This suggests that life could flourish elsewhere in the universe in biospheres similar to Earth’s,” Donoghue said.