Spillover: the powerful, prescient book that predicted the Covid-19 coronavirus pandemic.

by David Quammen

base-by-base comparisons between DRC60 and ZR59. It also involved broader comparisons, placing those two within a family tree of known sequences of HIV-1 group M. The point of such comparisons was to see how much evolutionary divergence had occurred. How far had these strains of virus grown apart? Evolutionary divergence accumulates by mutation at the base-by-base level (other ways too, but those aren’t relevant here), and among RNA viruses such as HIV, as I’ve explained, the mutation rate is relatively fast. Equally important, the average rate of HIV-1 mutation is known—or anyway, it can be carefully estimated from the study of many strains. That rate of mutation is considered the “molecular clock” for the virus. Every virus has its own rate, and therefore its own clock measuring the ticktock of change. The amount of difference between two viral strains can therefore reveal how much time has passed since they diverged from a common ancestor. Degree of difference factored against clock equals elapsed time. This is how molecular biologists calculate an important parameter they call TMRCA: time to most recent common ancestor.

Viral dating