This is a second novel for Byrne, whose first novel, The Girl in the Road, won the Tiptree (now Otherwise) Award in 2015. It is a dense, disturbing work, spanning two millennia and three human civilizations: a well-researched Mayan realm called Toyna in 1012, modern Belize in 2012, and a world-spanning future in 3012. The civilizations in all three eras are poised on the brink of change and full of unacknowledged cruelty. The author posts a trigger warning for self-cutting, but not for human sacrifice, euthanasia/murder, or the individual attainment of another world, called Xibalba, which looked to me very much like suicide.
The protagonists in all three eras live with the philosophy that they, personally, if not all of humanity, do not belong in what we would call the real world. They center their lives around the possibility of reaching Xibalba, a place of fear and wonder, the true realm. In 1012, this seems to be only for the Mayan nobility, but by 3012 everyone lives for this goal, traveling constantly, mostly on foot, trying to find their own, personal entrance to that place where they truly belong. In a nice interweaving of stories, each civilization has been inspired by the ideas and accomplishments, slowly adopted over time, of the protagonists of the previous eras.
The world of 3012 is richly visualized and, to me, the most interesting part of the story, though the characters exist mostly to describe how they live. The civilization seems almost a utopia: with complete freedom to change gender, race, culture, and sexual orientation; worldwide wireless communication; and equal sharing of resources. But there are far fewer people, and they must use advanced technology and ongoing genetic engineering to survive the harsh climate. They have given up all personal possessions and also, sadly, all permanent ties with others. Like many well-imagined utopias, it raises questions about which parts of being human are essential and explores the place of humanity in the universe.