What it was
Run your family tree backward. Past the last written name, your line keeps joining those of other living things. At the bottom sits LUCA (loo-kuh), the last universal common ancestor of all living cells today.
Picture a crowd of cells, not a single first cell. Life had begun before this crowd lived. Other cells shared its world, but their family lines did not all reach the present. LUCA is the shared ancestor whose line is still going.
When and where
One study puts LUCA around 4.2 billion years ago by comparing genes, the instructions cells pass down. Changes in those instructions can act as a rough clock. Like counting spelling changes in a copied book, you try to work back toward an older version.
Some reconstructed genes hint at life without oxygen, using hydrogen gas for fuel. A warm seafloor vent could fit parts of that picture. But a list of genes does not tell you which patch of rock a cell lived on.
How we know
Living cells share a code and tiny machines that read it. Scientists compare the machines across distant branches of life. One study traced 355 families of proteins, the parts cells build, back toward LUCA. Deal out a card for each family and you have nearly 7 decks.
Still a mystery
Scientists still argue about LUCA's age and home. Cells trade genes sideways as well as pass them to offspring, so a shared gene need not have come down the same family branch. Rocks give another check on early life, but none carries a label saying which cells were LUCA.