Lost cities / archaeology / ancient history

Brain case size was a metric/shorthand for intelligence and there is a correlation between cognitive abilities and size/ complexity so early hominids ( think Lucy and the Australopithecines ) did not have the same level of complex reasoning ability that Sapiens, Neanderthals and all the other undiscovered relatives undoubtedly did.

I wouldn't doubt that, but I'm much less concerned with those remote ancestors, much more so with the later groups. As you say, I doubt very much that there was a lot of difference between them.

They were however different species albeit very closely related whereas obviously all living human beings are members of the same species.

But what is this tied to? At what point do you establish a different category, e.g. a species barrier, if the fertile offspring explanation doesn't work?

There is evidence of relatively low interbreeding ( I’m going to stick with that as it’s the scientific term, although I share your distaste of its appropriation by those trying to ‘scientifically’ justify their racist nonsense) between Sapiens & Neanderthals shown by the genetic transmission of Neanderthal genes - less than 2% . ‘ We find that observed low levels of Neanderthal ancestry in Eurasians are compatible with a very low rate of interbreeding (<2%), potentially attributable to a very strong avoidance of interspecific matings, a low fitness of hybrids, or both‘.

https://doi.org/10.1073/pnas.1107450108

Well, this kind of genome analysis work hasn't been going on for very long, and consequently shows a wide variation. I'd give it some time before it can settle on solid conclusions. As I've said before, I strongly suspect that Neanderthal populations were always small, certainly much smaller than the populations stemming from the sub-tropics of northern Africa, where I suspect populations exploded in size at some point (conditions there probably being ideal for humans until the environment began to deteriorate), or at least caused sufficient overpopulation to cause waves of migration--the main reason why there was such a long gap between the migrations of groups who made up the Neanderthal population being the Mediterranean. Without better estimates of population sizes in ancient Europe as well as the size and impact of migration, I think calculating any 'rate of admixture' isn't going to be very reliable.

I think establishing beyond doubt that there have never been any really significant differences between people would help to some extent with the fight against racism.

Yes absolutely agree and in fact would qualify this to extend the notion of personhood to other forms of life and outwards from Homo Sapiens who as you point out are significantly the same but that is of course another debate

Yes, I've been following that debate for many years. People like Peter Singer have been involved in that drive. My own take on it is that it absolutely shouldn't require a designation of 'personhood' to stop animal abuse, although I'm very sympathetic to extending our 'circles of meaningful responsibility' based on similarity, especially to Great Apes. It just strikes me that anchoring it in personhood is a pragmatic idea based on the legal systems that exist, and that even were we to extend it to Great Apes, that would implicitly legitimise the slaughter of animals denied the designation. It might well eventually be extended further outwards, but I'd rather we first understood why animals are worthy of a protected status, and I don't just mean mammals, but also birds, fish, and insects. Still, it's a debate worth having. I don't know what would bring the greatest benefits most quickly, either. Obviously, to fight racism you don't need to extend to any of this.