The assertion that the human genome consists largely of "junk" DNA was made by scientists beginning in the early 1970's when they began to realize that a lot of DNA was not coding for proteins, about the only thing scientists thought DNA was good for at that time. Why there would be so much DNA (up to 95% or more of DNA) that was not making up the protein coding portion of the genome was perplexing to some, and to cover their ignorance, scientists coined the word "Junk DNA" to refer to the mass of DNA that was not coding for proteins. For example Stephen Gould remarked:
Our 30,000 genes make up only 1 percent or so of our total genome. The rest - including bacterial immigrants and other pieces that can replicate and move - originate more as accidents of history than as predictable necessities of physical laws. Moreover, these noncoding regions, disrespectfully called "junk DNA," also build a pool of potential for future use that, more than any other factor, may establish any lineage's capacity for further evolutionary increase in complexity. S. Gould, New York Times, Feb 19, 2001
More recently, Wojciech Makalowski summarizes the current thinking about "Junk DNA":
2010 Arthur V. Chadwick, Ph.D.