Final answer:
The human-Neanderthal interbreeding hypothesis explains why many humans share 1-4% of genes with Neandertals, suggesting mating between Homo sapiens and Neandertals was common as modern humans emerged out of Africa.
Step-by-step explanation:
Human Emergence and Neanderthal Genes
The model that explains why many humans share 1-4% of genes with Neandertals is the human-Neanderthal interbreeding hypothesis. Recently, genetic evidence has suggested that there was interbreeding between modern humans (Homo sapiens) and Neandertals as our ancestors moved out of Africa. The sequencing of the Neanderthal genome revealed that present-day humans living outside of Africa have about 2 to 3 percent greater similarity to Neandertal DNA compared to those in Africa. This finding supports the idea that mating between Homo sapiens and Neandertals was quite common during the time of their coexistence. An example of this interbreeding is the specimen known as Denisova 11, or “Denny”, who was a first-generation hybrid with a Denisovan father and a Neanderthal mother found in Denisova Cave in Russia.
Further studies have indicated that the incorporation of Neanderthal DNA into Homo sapiens genomes likely had both positive and negative effects on human health. Investigations into the genomes of diverse human populations reveal variations in the percentage of Neanderthal inheritance, which could explain certain hereditary traits and disease susceptibilities. Genes inherited from Neandertals even appear to have affected our cranial structure and metabolism, as differences in a gene called RUNX2 have been noted between humans and Neandertals.