Hominids
From back cover Tor paperback February 2003:

*Hominids* examines two unique species of people.  *We* are one of those species; the other is the Neanderthals of a parallel world where *they* became the dominant intelligence.  The Neanderthal civilization has reached heights of culture and science comparable to our own, but with radically different history, society and philosophy.

Ponter Boddit, a Neanderthal physicist, accidentally pierces the barrier between worlds and is transferred to our universe.  Almost immediately recognized as a Neanderthal, but only much later as a scientist, he is quarantined and studied, alone and bewildered, a stranger in a strange land.  But Ponter is also befriended -- by a doctor and a physicist who share his questing intelligence, and especially by Canadian geneticist Mary Vaughan, a woman with whom he develops a special rapport.

Ponter's partner, Adikor Huld, finds himself with a messy lab, a missing body, suspicious people all around and an explosive murder trial.  How can he possibly prove his innocence when he has no idea what actually happened to Ponter?
Fiction, Neanderthals, Prehistoric peoples, Hugo Award Winner, award:hugo_award=2003
Robert J. Sawyer