What is really meant by 'common ancestors'?

have read on , on again, explanation of parents being mine , sister's common ancestor in family tree. when apply situation say, domestic cat (felis catus) , sand cat (felis margarita), confused. know both belong same genus, felis, mean 'felis' once single individual sand cat, domestic cat , every other animal of genus, evolved from?

i have read on , on again, explanation of parents being mine , sister's common ancestor in family tree. when apply situation say, domestic cat (felis catus) , sand cat (felis margarita), confused. know both belong same genus, felis, does...

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