Daughters of Neanderthal Men - Haldane's Law, Culture, or Both?
Oddly enough, however, neither Neanderthal mitochondrial DNA (passed down from mothers to their children) nor Neanderthal Y chromosomes (passed down from fathers to sons) have survived in the human population. This implies that our Neanderthal ancestry doesn't come equally from all of the possible types of hybrid children. Instead, we appear to be specifically descended from women with Neanderthal fathers, who inherited human mitochondrial DNA and no Y chromosome. Why?
Two research articles I've seen have put forward two possible explanations. This article suggests Haldane's Law as an explanation. Haldane's Law is a pattern often seen in interspecies hybridization, where the heterogametic offspring (males in mammals) have poorer fertility than the homogametic offspring (females in mammals). This would predict that among Neanderthal-human hybrids, the daughters would be fertile, but the sons would be sterile. However, this doesn't explain why daughters of Neanderthal women (with Neanderthal mitochondrial DNA) didn't contribute to our genetic pool.
Other accounts have suggested cultural explanations, rather than biological ones. Perhaps, for whatever reason, daughters of Neanderthal men were the only hybrids who actually mated with humans. The others, rather than being sterile, either remained celibate or mated only with Neanderthals. But what would cause this pattern of behavior?
Both chimpanzees and bonobos, despite their behavioral differences, show female-biased dispersal - males stay with their home troup, while their sisters leave and find new troups to join. This study suggests that Neanderthals showed a similar pattern of sex-biased dispersal, with the men in a Neanderthal tribe being more related to each other than the women were. Modern human cultures vary quite a bit in this practice, but given our relatives, patrilocality was probably the ancestral pattern for us as well. So, for the sake of argument, let's assume both humans and Neanderthals were patrilocal.
It's also important to note that humans show two different mating strategies. As far as I know, every culture has a normative expectation that men will live with and support the mother (or mothers) of their children. (If someone knows of a culture where this is not the norm, let me know.) However, in most cultures, a subset of men buck this pattern, impregnating women (through rape, voluntary flings, or feigned commitment) and then playing no part in the support of the resulting children. Let's assume, for the sake of argument, that Paleolithic human men showed the same two patterns of behavior, fathering children both in and out of committed relationships. Maybe Neanderthal men did so as well.
The big question is - which strategies resulted in hybrids?
If men of both species - or just human men - took wives of the other species occasionally, then we'd expect their wives to come live in their tribe, and the hybrid children would grow up with their father's people. Patrilocality would predict that the sons would stay in the tribe, while the daughters went off to marry men of one or both species. Sons of Neanderthal women would have a human Y chromosome, and their Neanderthal mitochondrial DNA would not be passed on to their children. However, their sisters would pass on Neanderthal mitochondrial DNA. So unless human men were willing to marry Neanderthal women but not hybrid women (which seems unlikely), intermarriage should have led to Neanderthal mitochondrial DNA surviving in human populations.
Conversely, if men had sex with the other species but didn't marry them, then the hybrids would be raised by their mother, either as a single mother or with a stepfather of the same species as her (who may or may not have realized the kids weren't his - though hybrids would undoubtedly look pretty unusual). Children with Neanderthal mothers would have grown up among Neanderthals, and both genders would most likely have taken Neanderthal partners, with their descendants dying out along with the other Neanderthals. However, children with human mothers would have lived with humans and intermixed with humans.
This account neatly explains why no Neanderthal mitochondrial DNA survived in human populations. But why didn't Y chromosome sequences survive? After all, hybrid men with Neanderthal fathers are expected to have married human women. But if we combine this account with Haldane's Law, then these men would have been sterile, and left no descendants. Only their sisters succeeded in passing on their genes, mingling down the generations until everyone in their group had a little bit of Neanderthal ancestry.