Ivan T. Sanderson was a Scottish explorer who became an American citizen and one of bigfoot's biggest boosters, even though he wasn't always taken seriously. His big idea was to turn the study of bigfoot into a science called ABSMery. He didn't like the term abominable snowman, so he took letters from it and married it to a suffix that gave the study a bit more respectability.
He was a follower of an early twentieth-century writer named Charles Fort, who collected articles and other tidbits about things that science couldn't explain, and in 1965, he started the Society for the Investigation of the Unexplained, which published its own Pursuit Magazine. In addition to many articles, he wrote Abominable Snowmen: Legend Come to Life, which was published in 1961. Below is a video review by Bigfoot fan boy, Henry May.
When Sir Edmund Hillary dismissed the yeti as mere legend, Sanderson was one of the many who objected. Later, Tom Slick paid him to hire John Green, Rene Dahinden, and other serious bigfoot seekers. In addition to bigfoot, Sanderson was also interested in UFOs and wrote a book called Invisible Residents: The Reality of Underwater UFOs, which speculated, among other things, whether long-ago alien visitors had built themselves underwater docking bases. For some people, this sort of interest and theory discredits his interest in sasquatch.
He passed away February 19th, 1973
Portion of Ivan Sanderson's bio came from: Finding Bigfoot: Everything You Need to Know