Bootstrapped

Zack invents a time machine and uses it to take a romantic vacation in Pleistocene California with his new girlfriend, Helen. They are alone in paradise, at a time before humans arrive in the Americas. The large, now-extinct predators don't identify humans as normal prey, and mostly leave them alone. The other animals don't recognize humans as predators, and so walk among them unafraid. It's a romantic idyll for Zack and Helen, at least for a time. Helen, however, has a scary encounter with a saber-toothed cat, and is almost eaten by dire wolves. Zack encounters what appears to be a murdered version of himself. How is that possible? And of more immediate concern to Zack, who murdered him, and why?
Their time machine can only go into the past, but people from their future can bring Helen and Zack to their own time. Helen and Zack are bootstrapped to the future. In this future time, humans have genetically modified themselves, creating different human species. Zack and Helen must learn their place in this strange future society and navigate the challenges of surviving in Pleistocene California. Solving the mystery of Zack's murder is central to both endeavors, and will determine the fate of all humanity.