By far the strongest, but by no means the only evidence for this is the fact that they _all_ encode DNA into proteins using the same language.
DNA is a double helix of a polymer of the sugar ribose, except each link has an acidic nature. These are balanced by a long string of paired organic bases, one for each helix, taken from a quartet of purines and pyrimidines known by their initials G,T,A,C.
These four bases are the elements of a code, and for protein synthesis the code spells out the amino acids by triplets of bases, as if we had an alphabet of four letters, and each word is three letters long. A whole sentence (one of the codes isn’t for an amino acid, it simply means “stop”) corresponds to a protein, and some of them are very long.
This is a language!

When biochemists analyse and compare and perform the necessary experiments, it turns out that the words in the language have exactly the same meaning for every organism on Earth! This is like all human beings writing in Sanskrit, and knowing how to speak it!
Therefore, we must all be related by a common ancestor.

Darwin added the immensely important concept of how it came about by Natural Selection. The idea was so brilliant that Thomas Henry Huxley, himself a genius and a brilliant writer, said that he asked himself “How could I be so stupid as not to have thought of that!”