Somebody else asked "when were highlighters invented?" and this is the answer 'Anonymous' gave them:

The first fiber tip pen was invented by Yukio Horie in 1962 in Japan. A highlighter is a fiber tip pen filled with transparent brightly colored ink. The Avery Dennison Corporation trademarked the name Hi-Liter®, they invented the name highlighter, but they did not invent the pen.

king kong invented the highlighters he needed something to highlight on the caves to warn the cave men to stay away or he was going to eat him this was around 2020

Highlighters and fine-line markers were first seen in the 1970's. Permanent markers also became available around this time. Superfine-points and dry erase markers gained popularity in the 1990's.

The modern fiber tip pen was invented by Yukio Horie of the Tokyo Stationery Company, Japan in 1962. The Avery Dennison Corporation trademarked Hi-Liter® and Marks-A-Lot® in the early '90s. The Hi-Liter® pen, commonly known as a highlighter, is a marking pen which overlays a printed word with a transparent color leaving it legible and emphasized.