you can find out how it all started from nextstrain.org, an open-source real-time tracking of the pathogen evolution. there's no doubt among researchers and scientists who rely on this site for genomic data and peer-reviewed analyses that it is derived and mutated from a specific genre of horseshoe bats which are native to the prc. due to molecular processing and replication within the host cell, the time delay or duration of incubation and each mutation can be understood and calculated - leading to a "time-stamping" of each strain. since the data and samples (from patients and those infected) are submitted to
gisaid (a central and global depository) from various locations by medical and research communities across the world, each time-stamped strain can be tracked to the precise location on when and where it occurs and originates. they all point back to wuhan as having the mother of all progenitor strain. in the genomics and epidemiology, there's no u.s.-originated strain-zero. if there is, it will be in the genome data and will have a time-stamp of mid-2019.
https://nextstrain.org