There is a fair amount of research to indicate that Neanderthals and modern humans were indeed distinct species. A few of these areas are highlighted in this article (which has links to papers published on the subject): https://geneticliteracyproject.org/2014/02/04/were-neanderthals-a-different-species/
Homo erectus, Neanderthals and other hominins are well outside of the morphological and genetic variability of modern humans. Over and above genetic “percentage differences”, there are specific stretches of the genome which are extremely dissimilar. The Y-chromosome is one of these - when the Neanderthal Y-chromosome was finally published, it was found that a modern human mother would not have been able to carry a male hybrid to term due to the severe incompatibilities on the Y-chromosome. “Neanderthal deserts” (stretches of the genome unique to modern humans and never yet found in any Neanderthal genome) are another area of difference, and these have mainly to do with brain and bone function, which is important. Areas of extreme similarity, on the other hand, are typically in ‘adaptive genes’. Mitochondrial DNA is another area which would suggest that Neanderthals and modern humans are separate species._
Other issues that I have known RTB scholars to examine (carefully - and by reading the original papers, not the popular press interpretations of these, or just the abstracts) are as follows:
- differences in developmental milestones: modern humans are quite unique in having extended childhoods; Neanderthal dental eruption patterns follow the same pattern as other hominins and great apes. Brain development, similarly, continues long into childhood in modern humans but is quicker in Neanderthals. Brain arrangement is very different in key areas between Neanderthals and modern humans.
- evidence of symbolism. Everyone seems to “know” that Neanderthals buried their dead, often with grave goods. What everyone doesn’t seem to know is that the evidence from actual sites don’t ratify that claim - interpretation is always key. Shanidar, probably the “poster child” of Neanderthal burials, has been confirmed as being wrongly interpreted. Pollens around the skeletal remains do not represent flowers buried tens of thousands of years ago with the corpse - instead, these are today’s pollens, taken into the soil by insects and burrowing animals. Similarly with Teshik-Tash (where a skeleton was found with so-called “grave goods”. The same applies to other sites, each and every one. IA good start would be to watch “Did Neanderthals bury their dead?” by renowned palaeontologist, Harold Dibble (available on YouTube). Other researchers who have systematically studied “burial” sites include Rob Gargett. The sad point is that a claim is made by one team and then another, and after three or four such claims (most will openly admit that their interpretation may be speculative or inconclusive), it seems that the narrative is fixed, and woe betide anyone who challenges it. RTB’s position, from what I understand it to be, is not to “challenge”, per se, and certainly not to be dogmatic. It just seeks to make sure that what is reported in the press is indeed what the evidence indicates. The same applies to claims of Neanderthals making fire (the evidence seems to suggest that Neanderthals were used to harnessing naturally-caused fire, but could not make fire); jewellery; and now, in the news, art. The key to this latest claim is a controversial study using U-Th dating on “plaques”. The dates from one single piece of cave art in the study range from a couple of thousand of years ago to 80-odd thousand years ago. only the most ancient date makes the headlines. Here it is the dating method which is invalid: you simply shouldn’t be dating thin deposits of calcites and other flowstone substances using U-Th, if your deposit is in an open system, because Th can be (and easily is) leached out of your deposit due to the water flow down the walls or along the cave floor, or on top of artefacts, providing extremely variable dates and in most cases, artefactually early dates. This is a well-established problem - see, for instance: https://www.sciencedirect.com/science/article/pii/S1040618215002530 The assumptions of this methodology are not easily satisfied at all, in cave situations, except right at the heart of massive speleothems.
There is ample evidence of undisputed, clear evidence of symbolic behaviour (art, music, even evidence of an understanding of spiritual realms and the afterlife) that can be securely associated with the arrival of modern humans around the world as they migrated globally. There is clear evidence of an immediate innovation in the stone “toolkits” associated (without dispute) with modern humans, and a continued advance/acceleration in the technology of such, which is unique to modern humans.
On balance, there is clear evidence of developmental, morphological, genetic, behavioral and cognitive differences between Neanderthals and modern humans. This is according to our current knowledge. For the moment, then, it is unnecessary to “push back” the Image of God into Neanderthals and Homo erectus. If securely interpreted data comes to light that changes that, I’m sure that RTB will investigate the data, as usual, carefully, and if necessary, change its model. There would be no danger of “pushing people into an abyss”.