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The authors investigated viral adaptation’s necessity prior to successful zoonosis to sustain human-to-human transmission by examining pandemic causing viruses (Ebola virus, Marburg virus, influenza A virus, SARS-CoV, and SARS-CoV-2). Viral adaptation was measured as selection rate (ω) – purifying selection(ω < 1), diversifying selection(ω > 1) and neural selection (ω ~ 1). Genome-wide selection was then compared on three periods on the phylogenetic tree for the analysis: 1) viruses in their natural animal hosts (background); 2) during the transmission from animals to humans (stem); and 3) in the early stages of outbreaks in humans (outbreak). Authors found that by comparing with selection rates during background period, viruses don’t have significant adaptation (selection rate) change during stem period but outbreak periods. 

Overall, the manuscript is clear on their findings. However, it would improve the manuscript if the authors address the following points: 

1. “we have extended the RELAX framework to account for multiple evolutionary histories in different genomic segments” An explanation on the implementation of such extension would be useful to facilitate understanding of the analysis results in the manuscript. 

1. Three time periods for each virus were chosen differently rather uniformly. Although each outbreak could be different due to the virus's unique transmissibility, it would be useful to test out whether varying the length of each period would change the finding. This would verify the robustness of analysis conducted in the manuscript.

Competing interests

The author declares that they have no competing interests.