About 4.2 billion years ago

LUCA, the last universal common ancestor

Every living thing shares one distant ancestor. It was not the first life, just the last one we all have in common.

One family

Bacteria in the soil, the mushrooms on a log and the cells in your body all use the same genetic code and build proteins with the same kind of machinery. That shared inheritance points back to a single population of ancestors: the last universal common ancestor, or LUCA.

LUCA is often misunderstood as the first living thing. It was not. Life had already been around for some time, and LUCA lived among other microbes whose lines have since died out.

What the evidence shows

By comparing genes that were duplicated before LUCA lived, one 2024 study estimated that LUCA lived about 4.2 billion years ago, with a range of roughly 4.09 to 4.33 billion years.
The same study reconstructs LUCA as a microbe without a nucleus, with a genome comparable in size to many modern bacteria.

What is still uncertain

Both the date and the reconstruction are model-based inferences, and earlier estimates have varied widely.
Nobody knows what LUCA looked like. There are no fossils of it.

Sources

Moody et al. 2024. The nature of the last universal common ancestor and its impact on the early Earth system. Nature Ecology & Evolution. (opens the publisher’s site)
What it supports: An estimate that LUCA lived about 4.2 billion years ago, and a reconstruction of it as a microbe living within an existing ecosystem. Both are model-based inferences.

Summaries here are our own. Some publishers limit access to full papers; learning on this site is never paywalled.