Run your family tree backward until the names run out. Keep going. Your line joins those of other animals, then plants and microbes, until all living cells share a distant ancestor.
Scientists call it LUCA (loo-kuh): the last universal common ancestor. Think of a crowd of cells, not a lone first cell. Life was already here when that crowd lived.
The clue is in living cells. They share a code for genes, the instructions passed from parent to offspring, and tiny machines that read it. Shared instructions let scientists trace the family back.
One study picked out 355 families of proteins, the working parts cells build. Deal a card for each family and you have nearly 7 decks. The cards hint at cells that used hydrogen fuel and could manage without sunlight or oxygen. A warm seafloor vent could fit that picture.
Here is where we are unsure: the cards cannot prove LUCA's home or age. One gene-based estimate puts it around 4.2 billion years ago; widely accepted rock signs of life are younger. The chapter keeps that gap open. What we can still find is the code, copied from cell to cell all the way to you.
