In Assassin’s Creed, the First Civilization’s precursor race is called the Isu.
The Isu are an ancient, highly advanced species who existed on Earth before modern humans. They created powerful Pieces of Eden, including the Apple of Eden, and engineered humans for labor and warfare. Their civilization was devastated by a solar catastrophe, after which human populations survived and eventually became dominant.
The modern Assassin–Templar conflict is partly shaped by attempts to find and control Isu technology. Across the games, the Isu are also referred to as the First Civilization or Those Who Came Before. These names describe the same broad civilization, while individual Isu such as Juno, Minerva, and Aita have distinct roles in the lore.
“Precursor race” is the clearest answer because “First Civilization” is a title for the group, not a separate species. The word Isu became the standard modern name in later series lore. Their story links the historical adventures to the contemporary plot involving Abstergo and genetic memories.