If (hypothetical) Andromedan humans had left 2.5m years ago and managed to travel at very, very nearly the speed of light the entire way (tricky given acceleration, deceleration, turnover, and fuel), they would only just now be arriving on earth. Awkward. Alternately, they would have had to have left home 5m years ago to arrive on earth 2.5m years ago. Which is about the time the light from their departure would have arrived on earth too. You could go chasing that light, but you won't catch it, at least until you have FTL (and a telescope the size of a solar system to do anything useful with it like see the decaying remains of their hypothetical civilization).

Anyway, thanks to time dilation, whoever arrives from Andromeda under these circumstances could plausibly be the same people who left, which is fun.

Also, we evolved on this planet and are genetically all but identical to everything from chimpanzees to oak trees, on the scale of the universe.