The last ancestor shared by all living organisms was a microbe that lived 4.2 billion years ago, had a fairly large genome encoding some 2600 proteins, enjoyed a diet of hydrogen gas and carbon dioxide, and harbored a rudimentary immune system for fighting off viral invaders.

That’s the conclusion of a new study that compared the genomes of a diverse range of 700 modern microbes and looked for commonalities to identify which features arose first. Although the analysis doesn’t reveal how life got its start, it suggests a complex cellular organism somewhat similar to modern microbes evolved only a few hundred million years after Earth’s formation.