LIFE × GENETICS

The Ancestor of Everything Alive

~4.2 billion years ago (estimates 4.3 to 3.5 Ga)

Hadean

Every living thing on Earth, from microbes to people, descends from one ancestral population of cells. No fossil preserves it, but its portrait can be computed all the same: compare millions of genes across the living world, keep what they hold in common, and a cell takes shape. That cell ran on hydrogen and carbon dioxide, lived without oxygen, and already carried the genes for some 2,600 proteins. Nor did it live alone: what it gave off could feed a modest ecosystem around it.

Why it matters

This is not the origin of life, but the furthest back that reading genomes can take us. Before this cell the evidence is chemical; from it onward, genetic, and every branch of the tree of life leads back to it.

Dating & uncertaintyDisputed

There is no fossil, only a calculation. One method uses genes that had already duplicated before this ancestor, so each copy can date the other; it arrives at 4.09 to 4.33 billion years. Earlier analyses, calibrated on the fossil record and on the assumption that life could not have survived the early bombardments, put it several hundred million years later. No rock settles it, and the gap matters: at the older date, life was already running when the first oceans had barely settled.

Sources

1. Moody et al. (2024), Nature Ecology & Evolution · The nature of the last universal common ancestor and its impact on the early Earth system
2. Weiss et al. (2016), Nature Microbiology · The physiology and habitat of the last universal common ancestor

Earth history as one day

01:47:5022 h 12 min before midnight

if Earth’s 4.54 billion years were compressed into a single day