UNIX was written in assembly at first. By the Fourth Edition, the kernel was rewritten to be in a version of C (that won't go through a modern C compiler anymore), both 1973. The C compiler itself was introduced in the Third Edition. Parts of userland would still be in assembly even until the 7th Edition (1979), such as roff(1) (nroff/troff were in C), parts of as(1) or chess(6).