I read a book about a man who was geneticall modified during prehistoric times by aliens, and lived forever. I keep thinking it was Ben Bova or Greg Bear story but I cannot think of it. I read it as a kid and now I cannot remember name or author. It seemed to follow his life through out history into modern times. I have googled this to no end. Anyone recall this book?
What you described fits a book called "In the Face of My Enemy" by Joseph H. Delaney from the early 1980s.
In it a prehistoric man, Kah-Sih-Omah, is modified by aliens which allows him to totally control his genetic makeup and organic structure. He never ages, he can regenerate his limbs and resist all disease. He can alter his structure to incorporate capabilities from other species (e.g. night vision from felines, improved sight from avians, etc).
The plot line continues into modern day and then into the future. The book cover and reader description can be viewed on Amazon.
Are you thinking of the Orion series by Ben Bova?