noun
Proterozoic era
Geological era spanning from about 2.5 billion to 541 million years ago, preceding the Paleozoic.
原生代は、酸素濃度が上昇し始めた時代です。
The Proterozoic era is when oxygen levels began to rise.
From 原生 (primitive, original life) + 代 (era). The term is a direct geological classification.