I was astonished to see that a Wikipedia article claims that Neanderthals had 23 pairs of chromosomes, because no one has ever found an intact Neanderthal cell, stained it, and then looked under the microscope to find out exactly how many pairs they had. In fact, the number of human chromosomes was miscounted for decades when it was first published that we had 24 pairs. It was a famous mistake that shows people, even scientists, sometimes see what they expect to see, instead of what is reality.
The claim that Neanderthals have 23 pairs of chromosomes is based on the speculation that Neanderthals interbred with humans. Since horses and donkeys have different chromosome numbers, and their hybrids (mules) are sterile, then Neanderthals must have had 23 pairs for them to interbreed successfully with humans. There is more than one problem with that reasoning. The first one is that some species are known to have different populations with different numbers of chromosomes, and they can successfully interbreed. For example, the domestic horse has 64 chromosomes and the Przewalski's horse has 66 chromosomes. Yet they can hybridize and their hybrids are fertile, with 65 chromosomes. Many scientists classify both as the same species but different subspecies. Others classify them as different species. Therefore, the ability to interbreed and produce fertile hybrids does not automatically guarantee that 2 species or populations of the same species must have the same number of chromosomes. Hence, even if Neanderthals and modern humans did indeed hybridize, it is not irrefutable evidence that they both had 23 pairs of chromosomes.
Further, there is also no irrefutable data to show that Neanderthals and modern humans interbred. Such data would be in the form of either Neanderthal Y chromosome found in modern human males or Neanderthal mitochondrial DNA found in female modern humans. So far no human tested has either. Some West Africans and their relatives living in the USA do have Y chromosomes that are much older than those of modern humans, and some scientists suggest that these West Africans might have interbred with Neanderthals. However, the Max Planck Institute scientists who propose that Neanderthals interbred with modern humans specifically claim that Neanderthals interbred with non-Africans when they migrated out of Africa about 60,000-70,000 years ago. They found no alleged Neanderthal DNA in any African. Therefore I am very skeptical of the claim of Neanderthals interbreeding with modern humans. Since the claim that Neanderthals had 23 pairs of chromosomes is based entirely on the claim that some modern humans have a tiny amount of supposedly Neanderthal DNA in them, the claim that Neanderthals had 23 pairs would be falsified, if there was no interbreeding.
Finally, the fact that chromsome 2 of humans is a fusion of 2 ape chromosomes is supported by the presence of a second centromere on chromosome 2. Since a chromosome normally has 1 centromere, the second one is evidence that this chromosome was formed by fusion. Other clues include the identical sequential arrangement of genes on this chromsome and 2 others on the apes. Since modern humans are not ancestral to Neanderthals, or vice versa, but they evolved independently from different populations of Homo erectus, they either evolved the same number of chromosomes independently and convergently (high unlikely because the exact 2 chromosomes would fuse independently in 2 different chance events in 2 different lineages) or they inherited it from their common ancestor, Homo erectus. If so, then Homo erectus must have had 23 pairs as well. But since Homo erectus first appeared in the fossil record 1.8 million years ago, That would mean that the 2 ape chromosomes must have merged 1.8 million years ago or even earlier. Yet, despite having merged for so long, the second centromere has not disappeared from chromosome 2. That seems to be rather hard to believe. To me, a more likely scenario is that modern humans evolved 23 pairs of chromosomes in Africa when Homo sapiens first evolved 150,000 years ago. It is far more likely that the extra centromere has not disappeared without a trace after 150,000 years instead of 1.8 million years.
For these reasons, I am very skeptical of the claim that Neanderthals or Homo erectus had 23 pairs of chromosomes. As some scientists suggest, the supposed Neanderthal genes found in some modern non-Africans but not in Africans may be the result of incomplete lineage sorting. It is well known that orangutans share some genes with humans but those genes are not found in chimpanzees, because of incomplete lineage sorting. These genes were present in the common ancestor of humans, chimps and orangs, but they have since been lost in chimps through evolution, but retained in humans and orangs. Another possibility is that the supposedly Neanderthal DNA shared by modern humans and found in Neanderthal fossils are simply contamination. They are simply genes first evolved in non-Africans after they had migrated out of Africa, and therefore they are not found in Africans. Since researchers who study Neanderthal fossils are almost always either European, and since Europeans and Asians share a more recent common ancestor with one another than they do with Africans, contamination from a European who come in contact with these remains will likely include some genes shared by Europeans and Asians after they left Africa. If we use African researchers to study Neanderthal remains in the future, we may see results that show that they too "interbred" with Neanderthals. It is a huge leap of faith to claim that Neanderthals and Homo erectus had 23 pairs of chromosomes, based entirely on the disputed claim that Neanderthals interbred with modern humans, and also on the erroneous conclusion that the ability to interbreed is solid proof of identical chromosome numbers. I refuse to make that leap.