.  the greater the differences, the less likely common ancestry it.  i wouldn't put it on a definitive "yes or no" basis ... but i think there are degrees of reasonable belief in common ancestry.  somewhat similar to phylogenetic trees, i would think.  there's no definitive "ABSOLUTE UNDENIABLE TREE"  there are just most-parsimonious and maximum likelihood tress that are the most reasonable indicators of common ancestry.  i think this is an area of inherent ambiguity, but i do think we can come to some very reasonable hypotheses.  i think cats are a SOLID bet for common ancestry.  whales and pigs?  i just don't see it