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I read this 15-20 years ago but no idea how old it actually is. I read it in french but no clue if that was a translation or not. Looks like '50s or '60s science fiction to me but can't know for sure.
An advanced alien race has been discovered but they're extinct. A bunch of scientists are on the planet to try and figure out what happened to them and more are sent every once in a while but as time goes on less and less money is sent toward the project.
Story start with our main character, a promising young scientist in the shuttle to the planet. Situation is tense on Earth and a war is imminent and scientist might be recalled away from the planet. They have a statue of the aliens so they know how they look like. They have huge buildings full of crystals that they think were used for data storage but they all look too similar with only differences due to piezoelectric effect. MC decides to try and live like an alien to understand how they thought because he did it to understand some earth tribes. They build him a suit so he looks like one, hypnotise him, leave him alone for a couple months in the alien ghost town. He nearly dies but figures out what happened, but can't remember (yeah, that part is weird) No shuttle ever come back from Earth again so they assume Earth died. They continue working on the project while trying to ensure human race survival. Finally MC understands (again) what happened. The aliens had 3 stages of life, they started neutral then became male, then female (or the opposite, female then male, don't remember). But the middle stage was the lowest status in society and they could control their change, so as time went on, less and less alien spent a long time in that second stage, and they stopped reproducing, until their civilisation disappeared. The crystals were not actually data storage but a bank (i remember bank but not if it was money bank or genetic material bank, I'm pretty sure it was linked to their sexuality somehow. It just wasn't data storage as they were sure it was.). The mystery resolved, the whole team move to a "more hospitable" spot to start their new life. They all die. MC has a kid that dies after a few months, and he cut her open to try and figure out what happens, turns out there are spores, in the air in their new site, that clogged her lungs and they're all infected. MC is the last to die and he buries everyone while writing their story with acid on metal tablets in case Earth ever comes back.
Yes i remember basically the whole story except like, ANY name of character/alien/author. But i have wanted to reread this for so long and my father too and we can't for the life of us find the book again. Thanks in advance if anyone has a clue and merry christmas !