Blame the Neanderthals!

It seems that, on average, around 2% of the DNA in Europeans, Asians and native Americans comes from interbreeding with our Neanderthal ancestors. One such gene produces an enzyme dipeptidyl peptidase 4 (DPP4) that is a good match for the spike protein in Sars-CoV-2 (https://www.sciencemag.org/news/2020/12/neanderthal-gene-found-many-people-may-open-cells-coronavirus-and-increase-covid-19). DPP4 has already been shown to facilitate the entry of the closely-related MERS virus into human cells. The most common entry for Covid-19 into human cells is via angiotensin-converting enzyme 2 (ACE2), so DPP4 essentially provides a 2nd 'door' for this process. There is some very tentative evidence that, having the DPP4 gene, increases the severity of a Covid-19 infection.