In a Genealogical Adam scenario, I don’t think the argument would be about not all being in the image of God, but about what the image means - which changes nothing if all of us bear it/are it, because all share in Adam’s calling.
In an evolutionary scheme, or any where there are questions to ask about other human “varieties”, the question of who were image bearers and who not will always be there, probably unanaswerable, and irrelevant to human rights now, which I would argue are firmly based on being created in the image of God. I think only theological outliers would disagree, and it’s a risky place to be, given the history of excluding certain groups from full humanity.
But as I recently stated, my own view has come round to treating Gen 1 and 2 fully sequentially, so that mankind outside the garden is, phenomenologically speaking, assumed to be created in the image and likeness of God. The when and how of that is irrelevant, unless someone invents a time machine to go and enslave or evangelise Neanderthals.
Note that it is no shame not to be in that image if you’re not one of the creatures designated scripturally as mankind. Australopithecus no doubt had the dignity of an intelligent ape of whatever capacities it had, just as orang utans are worthy of respect for what they are - more respect than sinful man gives them, in fact. But they are not bothered about how closely they resemble God, or not - and maybe that’s one of the factors involved in the image.