That wasn't so in the early days of photography,when taking

That wasn't so in the early days of photography,when taking pictures ...这里的SO是作什么成份?
That wasn't so in the early days of photography,when taking pictures required expensive cumbersome equipment,knowledge of chemistry,and lots of patience.
上问的 so 在句子里是充当什么成分?so 可以作代词吗?请高手指教,