Jake Ransom and the Skull King's Shadow by James Rollins

Action adventure story about Jake and Kady, who somehow get pulled into a world full of neanderthals and the Lost Tribes. As Jake struggles to figure out where they are and how to get home, there is a bigger threat lurking: the Skull King, who desperately wants Jake and Kady.

This is a great YA action adventure but a little confusing at times. I kept trying to figure out if they were in the past or the future and how all the Lost Tribes (Romans, Neanderthals, Vikings, etc) ended up on this strange world. Even though it was a tad confusing, it was still a good read and I look forward to the second book.