Neanderthal introgression reintroduced functional ancestral alleles lost in Eurasian populations

Neanderthal ancestry remains across modern Eurasian genomes, and introgressed sequences influence diverse phenotypes, including immune, skin, and neuropsychiatric diseases. Interpretation of introgressed sequences has focused on alleles derived in the Neanderthal lineage. Here, we demonstrate that Neanderthal introgression also reintroduced thousands of ancestral hominin alleles lost in the Eurasian out of Africa bottleneck. Combining evolutionary simulations, expression quantitative trait loci (eQTL), massively parallel reporter assay (MPRA) data, and in vitro validation, we show that reintroduced alleles (RAs) have different fitness effects than Neanderthal-derived alleles (NDAs) and that some RAs regulate gene expression independent of NDAs. Illustrating the broad potential influence of RAs, we find that over 70% of known phenotype associations with NDAs are equally associated with RAs. Finally, we discover enrichment for RA eQTL activity in several tissues, with strongest enrichment in the brain. In summary, our study reveals that Neanderthal introgression supplied Eurasians with many lost functional variants and demonstrates that RAs must be considered when evaluating the effects of introgression. ONE SENTENCE SUMMARYNeanderthal interbreeding with modern humans restored to Eurasians, hundreds of thousands of ancient alleles that were lost in the out of Africa bottleneck.