Hybrids

From back cover Tor paperback November 2004:

In Hominids Robert J. Sawyer introduced a character readers will never forget Ponter Boddit, a Neanderthal physicist from a parallel Earth who was whisked from his Reality into ours by a quantum-computing experiment gone awry -- making him the ultimate stranger in a strange land. In that book and its sequel, Humans, Sawyer showed us the Neanderthal version of Earth in loving detail -- a tour de force of world building; a masterpiece of alternate history. Now, in Hybrids, Ponter Boddit and his Homo sapiens lover, geneticist Mary Vaughan, are torn between two worlds, struggling to find a way to make their star-crossed relationship work. Aided by banned Neanderthal technology, they plan to conceive the first hybrid child, a symbol of hope for the joining of their two versions of reality. Meanwhile, as Mary's Earth is dealing with a collapse of its planetary magnetic field, her boss, the enigmatic Jack Krieger, has turned envious eyes on the unspoiled Eden that is the Neanderthal world...