The first Linux distributions were created from scratch, having only the Linux kernel and a bunch of dispersed tools (like the GNU Project ones) with which to work. It was necessary to create a series of new tools and scripts to compose what we consider a Linux distro. Over time they would become more sophisticated, advanced, and useful, but the very first distros were created in this way, from zero.
But once there were some distros that were good enough, and because some of them were Free Software, it began to make sense to create new distros based on these early ones, rather ...
