Speculations about its nature have been going on for literal

Speculations about its nature have been going on for literally thousands of years, and one odd findingvthat makes the problem puzzling is thay it looks very much ad if sleeping is not simply a matter of giving the body a rest. 请问这篇新概念四册第19课 的句子中，为什么literally放在thousands of years 之前?谢谢:D