Hypothesis
Introgressed Neandertal DNA has been purged by negative selection, which shows up as a steady decline in Neandertal ancestry in Europeans over the past 45,000 years.
What failed
Using a statistic built on two high-coverage Neandertal genomes that does not rely on assumptions about modern human demography, the reported monotonic decline disappeared: there was no evidence of any change in European Neandertal ancestry over 45,000 years. Whole-genome simulations also showed negative selection should not produce a large long-term genome-wide decline.
Broken assumption
The ancestry statistics used earlier were insensitive to gene flow among modern human populations.
Why
The authors attribute the earlier decline to gene flow between modern human populations, which the previously used statistics did not account for.
From the published paper: Petr et al. Proceedings of the National Academy of Sciences, 2019. ↗CC BY