There is a few different potential reasons as well as sexual preference.
Genetic incompatibility - the interspecific cross could only occur one way.
Genetic bottlenecks in the neandertal lineage. A high inbred coefficient could have decreased the neandertal females overall fertility (high deleterious alleles load). This could also cause a rapid reduction in the percentage of neandertal DNA in a mixed population.
Maternal behavior - Neandertals females might not have cared for hybrid offspring appropriately. This could be for anything from milk production requirements to differences in physiological developmental rates.
I think the idea of there being health issues in certain types of mixed families is super interesting because that almost certainly would have been noticed and lead to certain cultural practices or taboos within both species’ societies.
If I had to guess the successful crosses were potentially much healthier than either parent line. Heterosis (hybrid vigor) would likely be pretty extreme in genetic lines that has been isolated by 300,000+ years of time. Of course the degree of fertility was likely lowered due to genetic distance. Once the initial cross was made however, back-crossing to either species by the hybrid would likely be much easier.
Many of those ancient stories about individuals with super strength and size etc could have likely been based upon these crosses.
The evidence is showing neadertals never truly died out. Their smaller population bred back into the modern humans who came later.
Keep in mind heterosis isn’t always the result of hybridization and even then the magnitude of isolation doesn’t always positively correlate. Outbreeding depression can also be the result, increasingly so when two groups are more genetically distant or when one group is already subject to heavy inbreeding depression, as the neanderthals were thought to be.
a paper came out recently. it’s also possible that male humans and female neanderthals made non viable babies.
There is a few different potential reasons as well as sexual preference.
Genetic incompatibility - the interspecific cross could only occur one way.
Genetic bottlenecks in the neandertal lineage. A high inbred coefficient could have decreased the neandertal females overall fertility (high deleterious alleles load). This could also cause a rapid reduction in the percentage of neandertal DNA in a mixed population.
Maternal behavior - Neandertals females might not have cared for hybrid offspring appropriately. This could be for anything from milk production requirements to differences in physiological developmental rates.
I think the idea of there being health issues in certain types of mixed families is super interesting because that almost certainly would have been noticed and lead to certain cultural practices or taboos within both species’ societies.
If I had to guess the successful crosses were potentially much healthier than either parent line. Heterosis (hybrid vigor) would likely be pretty extreme in genetic lines that has been isolated by 300,000+ years of time. Of course the degree of fertility was likely lowered due to genetic distance. Once the initial cross was made however, back-crossing to either species by the hybrid would likely be much easier.
Many of those ancient stories about individuals with super strength and size etc could have likely been based upon these crosses.
The evidence is showing neadertals never truly died out. Their smaller population bred back into the modern humans who came later.
Keep in mind heterosis isn’t always the result of hybridization and even then the magnitude of isolation doesn’t always positively correlate. Outbreeding depression can also be the result, increasingly so when two groups are more genetically distant or when one group is already subject to heavy inbreeding depression, as the neanderthals were thought to be.