Six key layers of the internet are illustrated, with indicated years for significant advancement and highlighting the contributions made by some of the instrumental people involved in inventing and developing the technology. Listed on the left side are the highlighted rudimentary advances. Listed on the right side are the more refined advances that helped to enable major breakthroughs. For example, while computers were around since the 1940s, it was not until the advent of inexpensive personal computers in the mid-1970s that the Internet revolution could be enabled.