Mammoth is a 2005 novel by author John Varley. The book centers around the concept of Time Travel, and gives quite a bit of discussion to the concept that there may be limits to science.
Mammoth opens with the billionaire scientist Howard Christian, and his attempts to clone a mammoth, largely for the sake of the countless circuses he owns. His team finds a mammoth frozen in ice, along with two human beings, one of whom appears to be wearing a wristwatch. His assistant, Warburton, is initially convinced that this is a joke, but Howard takes the notion of time travel seriously, and hires physicist Matt Wright to study a metal briefcase which is presumably a time machine.