The Many-Colored Land, by Julian May (1981). The cover at left is the cover I have. All other covers are wrong. This one is so much a favorite that I am on my second copy, having read the first to death; I think my mother might be on her second copy as well, which just goes to show. Even my husband enjoyed it [edited to add: the even is because he has very high standards]. It was marketed to adults, but I think it has lots of teen appeal (I was a teen when I first read it), and as well as being a darn good story, there's a generous sprinkling of paranormal romances (lots of people get to have romances. Some happy, some less so).
So. Imagine that in the future, various alien races with psionic powers have made contact with Earth, Earth having reached a critical mass of psionic inhabitants of its own. Earth is now part of a galactic milieu of calm order; more and more humans are being born with mental gifts, war is over, all is happy. Except that there are some people who still aren't--the deviants, mystics, misfits, eccentrics, criminals, those whose souls are out of step somehow with a galaxy of good feeling.
Then imagine that a French professor invented a machine that allows one way travel back to the Pliocene (six million or so years ago). He dismissed it as worthless, and his widow was just about to dismantle it, when the first would-be time traveller begged to pass through, wanting the chance to explore an unpeopled world. And more and more travellers came...some willing travellers, some pushed back in time because they were too troublesome to be allowed to stay.
Time travel becomes organized; the travellers equipping themselves with what they need for life they'll imagine they'll have (I love reading all the lists of what people are taking back to the past!). They are sent back in groups, after a brief period of bonding. One such group (our main characters in this first book--men and women, old and young) is about to pass through....a group whose members are going to change the past, and in so doing, make the future what it's going to be.
It's not a walk in the park, back in the Pliocene. There are surprises (you know that paranormal romance thing? that's a hint). What the time travellers find will blow their minds (some to the point of insanity). And the reader (if the reader is at all like me) will be riveted.
I don't generally like books with multiple main characters, and story lines of great complexity and fantastical-ness going of hither and thither. My first time through, lo these many years ago, I might have found myself uncertain during the introductory period--there are a lot of characters, and we meet them all individually, and there's a lot to keep track of. But May makes it all work in a masterpiece of plotting and characterization and exuberant imagination. For those who like the mental powers and the paranormal, there's that. For those that like the survival in a strange land, there's that. For those that like their characters put through various emotional ringers, and/or their characters finding love and friendship, there's that too. Magic. Sex. Death. Flying on the wings of the mind. Extinct mammals (so few fantasy books do as nice a job with extinct mammals). Crafting of beautiful things. Generous splashes of humor. Tragedy.
In short, I really cannot recommend this too highly to anyone who wants a sci fi/fantasy adventure of epic proportions, set on a very different earth. But I've read it so many times I can't be dispassionate about it...this first book, and the three that follow it, are and integral and much loved part of my mental map. However, since my mother and my husband, both of whom are less emotional thinkers than me, and both of whom read grown-up books, enjoyed the series as well, I feel pretty confident in my recommending.
(I also don't feel like writing a thoughtful review, because that would be full of spoilers. I hope I haven't spoiled it too much as it is!)