Inherit the Stars by James P. Hogan
This is the first book of the series (sometimes referred to as the Minerva series, sometimes called the Giants series).
Inherit The Stars was written in 1977. The Apollo missions to the Moon had come to an end without immediate plans for more manned voyages beyond Earth orbit. Inherit The Stars worked on the premise that a re-dedication to manned exploration of the solar system had begun in the 1980s. By 2028, the United Nations Space Agency (UNSA) has a large space station in orbit, colonies on the Moon and bases elsewhere.
A body in a spacesuit is found in an out of the way spot on the Moon. None of the UNSA bases are missing anyone. Examination finds the body in the suit is mummified from age. The corpse is dated as being about 50,000 years old. Yet, everything about it suggests it is human. Not just humanoid, but showing every quirk of evolution on Earth. The biologists conclude that he must have been from a space-going society on Earth 50,000 years ago. Some others counter that scientists have found plenty of bones and stone tools from humans 50,000 years ago, but never a single piece of evidence of a technological civilization.
Further investigations find written materials in a strange language, with maps of a place that is neither Earth nor the Moon. Scientists eventually piece together evidence indicating the map is of a planet that used to be where the asteroid belt is today. Remains of some Moon bases from 50,000 years ago are also found. In one of these there is a store of food including canned fish. The fish show no signs of being a part of Earth's evolutionary history. Meanwhile, a large spaceship is found buried in the ice on Ganymede. The ship was carrying a large selection of animals from Earth's biological past. The ship and animals are from 25 million years ago. The ship also has non-human skeletons - presumably the crew.
Scientists try to learn what they can and make sense of all these facts which seem to have some elements connecting them and some elements making them inconsistent with each other. It's a sort of scientific mystery. It's not designed like an SF mystery / detective story such as Asimov's Caves Of Steel. To the scientifically-inclined, it can be an interesting puzzle. But it didn't strike me as having the kind of flair I expect in a good detective story.