More details on "Primeval" C can be found here: https://www.bell-labs.com/usr/dmr/www/primevalC.html
Just recently, fragments of the earliest Unix OS source code and B interpreter were discovered [0]. Now, the earliest Unix for the PDP-7 and PDP-11 [1] can run under emulation thanks to the incredible efforts of Warren Toomey and others at the Unix Heritage Society [2].
[0] https://github.com/DoctorWkt/pdp7-unix