I haven’t given it a lot of thought, but some clear transition where the two species are clearly transforming from one into another... but the only pictures I’ve seen show separate skulls which theorists cannot even clearly agree as to the species - let alone the relationship between them. (Hope I’m making myself understood and that my answer hasn’t muddied the water.)
I think I understand, but I still invite you to think about what exactly would constitute “some clear transition where the two species are clearly transforming from one into another.” Evolutionists would say that a bunch of skulls that we have a hard time saying were definitely ape or definitely human would be exactly what we would expect if apes and humans had a common ancestor. If you disagree, what else should we look for?