Four reassortment events led to H7N9 emergence.

With thanks to Prof. Ross Barnard, University of Queensland for helpful contributions.

A new article just published at the Lancet describes a detailed genetic analysis of four H7N9 virus sequences, in the context of all likely contributing influenza virus sequence. The resultant virus' HA and NA genes may have been contributed by migratory birds a year prior to acquisition of internal genes from poultry influenza viruses. 

Since its emergence "several months ago", H7N9 has divided into at least two different groups (lineages). One strain (Shanghai/1) has signs of oseltamivir resistance.