Ancestors: The Humankind Odyssey Save Game Location
Windows
Ancestors: The Humankind Odyssey save games can be found on Windows here:
C:\Users\[user_name]\AppData\Local\Ancestors\Saved\SaveGames\
Linux
Ancestors: The Humankind Odyssey save games can be found on Linux (Proton) here:
~/.steam/steam/userdata/[user_id]/536270
Notes
Search for files with names like "Savegame0.sav," where 0 could be replaced by a higher number depending on how many saves you have. This is the only file you need to back up - you can store it in a secure location on your hard drive or in a cloud service like Dropbox. If your save file for Ancestors: The Humankind Odyssey ever becomes corrupted, simply copy this file back to the directory listed above.