Well, can we not tell, through genes, the relatedness between two people?
And don't we expect the genes to be more similar the more closely related two people are, and less similar the less related two people are? Children, IOW, will share more of their parent's genes than they will of their grandparent's, or their great-grandparents, etc.
Why wouldn't this work with all life, for as far back as we wanted to go, if genes work exactly the same in all organisms (which they seem to)?