If I recall it correctly, this book has two main characters, both male, one of them big and well muscled. During the novel, they found evidence that the current human race descends from ancient Martian astronauts who traveled to Earth in prehistory, causing the extinction of prehistoric Earth humans, (which explains the Missing Link in human evolution).
Later in the novel the big guy travels back in time (no idea of how or why) to Prehistory.
And near the end of the book, the other main character, during an archaeological field research, finds remains of a human being who lived thousand of years ago, a human with large bones, and who holds an item that could not be made in that era (a gold plaque? A metal rod?)
That's all I can remember. I know it is not from the ABC, Asimov, Bradbury or Clarke.
Thanks in advance.