this post was submitted on 03 Mar 2026
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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.
This could be human male-neanderthal female (HMNF) coupling didn't result in fertile offspring right? Could it also be that HMNF (coupling) didn't result in fertile female offspring, but did have fertile male offspring?
Usually these issues are caused by mitochondrial DNA not nuclear DNA. Mitochondrial DNA is only passed on from the female. So if there is an incompatibility, it's usually completely lethal to any offspring.
So a HMNF coupling could not have been possible because of the neanderthal's female mitochondrial DNA.
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.
Out-breeding depression primarily results in a decrease of fertility and infant mortality. So although it is occasionally observed in surviving offspring, in general it is much lower probability.
Also the neandertal crossing was deleterious it would be much lower percentage in modern humans. It also would not have come from multiple crossing events.
Just to preface, I'm a scientist: micro- and molecular biology. I'm not saying to take what I say as gospel, just giving context that I might know things. Sometimes.
Outbreeding depression has more possible implications than fertility decrease and infant mortality increase, entirely dependent on the heritable traits responsible for the depression effects. While the probability of persistent outbreeding depression seen in subsequent generationa would be lower due to traits subject to higher selective pressure, like increases in early infant mortality, the overall probability of outbreeding depression itself isn't influenced post facto by its results, just its persistence.
Given we don't know the original extent of neanderthal/human interbreeding, what we're seeing now COULD be the "much lower percentage" you mention and still could come from multiple events. In fact, if these crosses resulted in stronger depression effects, I'd argue a greater number of crossings would be one factor behind the persistence of some genes today.
Just to preface, I am a plant breeder/geneticist with extensive experience in interspecific crosses in multiple species - all practical. This discussion should definitely be finished over some a few drinks.