Every cell on Earth is kin

All known terrestrial life shares a common ancestor. The evidence is a nested hierarchy of traits plus a shared genetic code.

Living things fall into a nested hierarchy: vertebrates among animals, mammals among vertebrates, primates among mammals. That pattern is what you get from descent with modification, not from a catalogue of independent types.

The molecular evidence closed the case. A nearly universal genetic code, shared ribosomal machinery, and sequence homologies that recover the same tree from independent genes. Horizontal gene transfer blurs some bacterial branches; it does not invent a second origin that we have found.

Fossils, biogeography, and vestigial structures were already enough for Darwin. Genomes made the kinship quantitative. LUCA — the last universal common ancestor — was a cellular organism with DNA, proteins, and a membrane, living more than 3.5 billion years ago.