Changes in the genome of all living beings occur constantly. This happens as a result of mutations. Mutations may not affect the function of cells and organs in any way, may worsen or improve some functions, which is revealed in the process of natural selection, or be directly fatal for a given organism.
The set of genes in the descendants of any organism is similar to its set of genes, but it never completely repeats, therefore, the more generations are replaced, the less it is possible to find some kind of sequence of genes, which can be said to be inherited from this particular organism. Therefore, the assertions that some kind of scientists found in some population of modern people, for example, 4 percent of the genes of Neanderthals or Denisovans, do not correspond to reality. In reality, all the genes of any population of people are similar in individuals belonging to this population, and have real differences from the genes of other populations, and, undoubtedly, have a commonality with their ancestors, but they cannot exactly repeat the genes of any ancestors: Cro-Magnons, Neanderthals, Denisovans.
People of the modern type, and their ancestors Cro-Magnons, Neanderthals, Denisovans, nevertheless, have something in common in their gene set that distinguishes them from all other predecessors, something that allowed them to interbreed among themselves and did not allow it to do with predecessors — different races of pithecanthropes: Javanese pithecanthropes, Sinanthropes, Heidelberg pithecanthropes, African erectus. This genetic commonality of all people is a set of chromosomes: 23 pairs of chromosomes in humans, as opposed to 24 pairs of chromosomes in their predecessors monkeys. In addition, in humans, as in all mammals, in the presence of only a strictly defined number of “zinc fingers” in each chromosome, it is possible to produce offspring from any partners of their species with only the same number of “zinc fingers”. Very rare mutations that remove or add «zinc fingers» in any chromosome do nothing to prevent such an individual from living, but usually do not allow him to have offspring.
German scientists have identified a gene that makes the human brain unique: ARHGAP11B dramatically increases the number of neurons in the neocortex, a region of the brain that plays a critical role in thought processes, language and perception. This gene is present only in humans, Neanderthals and Denisovans. The discovery is featured in the journal Science and briefly reported by The Guardian.
The fact that Cro-Magnons, Neanderthals, Denisovans could have mestizo offspring shows that they belong to the same species of Homo sapiens, but to its different races.
The species Homo sapiens appeared as a result of a serious mutation in which one of the ancestors of humans formed 23 pairs of chromosomes, out of 24 pairs that monkeys had. (The second human chromosome combines the twelfth and thirteenth chromosomes of the hominid. This was discovered in 1991.) Such mutants among hominids, like the first people, could not generally have been obtained under conditions of extensive contact with representatives of their own species of monkeys. In order for a mutation that changed the number of chromosomes and created a new species of Homo sapiens to be preserved, complete isolation of the very small group in which such a mutation occurred, and prolonged inbreeding within this group, were absolutely necessary. This could have happened to some family of synanthropes, lost somewhere in the gorges of Tibet, but not in Africa, not in India or China.
Sinanthropes could be the ancestors of people, since people have preserved some features of the structure of their teeth. No other races of Pithecanthropes have something like that directly passed on to people.
The time of the appearance of the first representatives of the species Homo sapiens can be estimated approximately. This happened, apparently, about 450 thousand years ago. Since that time, the races of Pithecanthropes begin to disappear, apparently unable to withstand competition with people, and a little later the postures of Neanderthals and Denisovans are already known.