Original architectural decisions can constrain and push development in a certain direction. An analogy might be made to evolution: one's phylogeny constrains th

Original architectural decisions can constrain and push development in a certain direction. An analogy might be made to evolution: one's phylogeny constrains the space of available organisms available in a given timespan. 5% might not sound like much, but I'd want something comparative. How much code in the 3.8 kernel is shared with what Linus released in 1991? I suspect it's similarly low, but we still recognize them as the same project.