FreeBSD is likely the most directly-derived modern UNIX-based OS. In June of 2017, Diomidis
Spinellis of the Athens University of Economics and Business
published a research paper which documents the evolution of the Bell Labs UNIX codebase from its early beginnings through to the modern-day FreeBSD 12. Using a variety of sources and methods, he essentially reconstructed a Git repository timeline which traces current-day FreeBSD back to epochal origin of UNIX.
The project has achieved its major goal with the establishment of a
continuous timeline from 1970 until today. The repository contains:
- snapshots of PDP-7, V1, V2, V3, V4, V5, V6, and V7 Research Edition,
- all available BSD releases,
- the CSRG SCCS history,
- two releases of 386BSD,
- the 386BSD patchkit,
- the FreeBSD 1.0 to 1.1.5 CVS history,
- an import of the FreeBSD repository starting from its initial imports that led to FreeBSD 2.0, and
- the current FreeBSD repository.
The files appear to be added in the repository in chronological order
according to their modification time, and large parts of the source
code have been attributed to their actual authors. Commands like
git blame and
git log produce the expected results.
In particular, Figure 5 of the paper documents the proportional provenances of
the codebases comprising several Berkeley UNIX milestone releases.
I recommend his paper highly to anyone interested in the historical evolution
of UNIX and BSD. The GitHub page for the project is also interesting reading.