A journey's end. A bold plan. A final conflict. Skyra, Lincoln, and their companions have made their last time jump and are stuck 47,000 years in the past. For Skyra, it's home. For Lincoln and his team, it's fraught with menace-a land of claws, teeth, and stone weapons. Just surviving here is hard enough, but the group now must deal with the consequences of binary existence-two copies of the same person coexisting in the same timeline. For Skyra, coexisting with a copy of herself becomes a struggle to hold on to her own identity and personal worth. She has returned to her homeland to save her sister Veenah, only to find that her other self has already rescued Veenah and does not want Skyra around. As the group strives to establish a permanent home, Skyra's personal dilemma gives way to a much greater challenge. Is perpetual war or extinction unavoidable, or can this ragtag team of humans and Neanderthals somehow usher in a better future for an entire world? Binary Existence, the fourth and final book in the Across Horizons series, is for readers who love time travel adventures, mind-bending surprises, and unforgettable characters.