Story was about a planet that lay between Mars and Jupiter. Some calamity happened, forming the asteroid belt. A (lone?) survivor reaches Earth when Neanderthals were the predominant Hominid form. Probably mated with a Neanderthal resulting in modern homo-sapiens. The conclusion of the story regards some modern archaeologists digging in an old river bed or such, , and the one doing the digging is distracted and misses the only remnant/evidence of the ancient alien's existence on Earth.
This is Inherit the Stars by James P. Hogan. As noted by user Danny3414 in a comment above, it's the first novel of Hogan's Giants series. The name of the destroyed planet was Minerva. At the start of the novel, members of a Moon mission discover a body in a spacesuit of unusual design.
The book has previously been asked about and answered here at Book about ancient Martian astronauts being the root of modern humanity and time travel
Per the blurb on the back of one paperback edition:
They called him Charlie. He had big eyes, abundant body hair and fairly long nostrils.
His skeletal body was found clad in a bright red spacesuit, hidden in a rocky grave..
They didn't know who he was, how he got there... or what had killed him.
All the did know was that his corpse was 50,000 years old... and that meant that this man had somehow lived long before he could have existed.
Here's some text from the final scene you remember:
It was about the size of a large cigarette pack, not including the wrist bracelet, and carried on its upper face four windows that could have been meant for miniature electronic displays. It suggested a chronometer or calculating aid, or maybe it was both and other things besides. The back and contents were missing, and all that was left was the metal casing, somewhat battered and dented, but still surprisingly unaffected very much by corrosion.
"There's a funny inscription on the bracelet," Magendorf said, rubbing his nose dubiously, "I've never seen characters like it before."
Zeiblemann sniffed and peered briefly at the lettering.
"Pah! Russian or something." His face had taken on a pinker shade than even that imparted by the Sudan sun. "Wasting valuable time with---with dime-store trinkets!" He drew back his arm and hurled the wrist set high out over the stream. It flashed momentarily in the sunlight before plummeting down into the mud by the water's edge...
And the final paragraph:
In the mud by the side of the stream below, the wrist unit rocked back and forth to the pulsing ripples that every few seconds rose to disturb the delicate equilibrium of the position into which it had fallen. After a while, a rib of sand beneath it was washed away and it tumbled over into a hollow, where it lodged among the swirling, muddy water. By nightfall, the lower half of the casing was already embedded in silt. By the following morning, the hollow had disappeared. Just one arm of the bracelet remained, standing up out of the sand below the rippling surface. The arm bore an inscription which, if translated, would have read: KORIEL.
This cover may be familiar: