[cmiller1]

There's a whole wikipedia page answering exactly this question: https://en.wikipedia.org/wiki/Evidence_of_common_descent

From biochemical similarities, the chirality of certain sugars and amino acids, dna similarities, etc. the evidence for a single common ancestor of all life on Earth is fairly strong; however we don't really have evidence that there never were other lifes that simply were outcompeted at their very early stages.

[vlovich123]

Or the environment of Earth is hospitable to only one structure for life. Or all life we’d find anywhere has the same structure. We don’t yet know enough about the phase change transition from inorganic chemistry to biology.