In this work, we reconstruct the evolutionary events that have accompanied the emergence of SARS-CoV-2, with a special emphasis on the RBD and its adaptation for binding to its receptor, human ACE2 (hACE2). In particular, we identify a specific recombination event which involved ancestral lineages to both SARS-CoV and SARS-CoV-2. By reconstruction of ancestral states of the Spike gene, we find that this ancestral recombination event is associated with an increased binding affinity towards hACE2. Through structure-based binding affinity predictions, we infer that affinity further increased in the lineages leading to SARS-CoV and SARS-CoV-2. These observations suggest that RBD recombination provides the backbone for CoV adaptation to humans, with subsequent point mutations further permitting high affinity RBD-hACE2 binding
