古太古代是太古宙的第二个代，前一个是始太古代、后一个是中太古代，时间介于36~32亿年前。

此时出现第一批蓝绿藻，也是最古老的化石年代。

The Paleoarchean (pronunciation: /ˌpeɪlɪ.oʊ.ɑːrˈkiːən/; also spelled Palaeoarchaean (formerly known as early Archean)) is a geologic era within the Archaean. It spans the period of time 3,600 to 3,200 million years ago—the era being defined chronometrically and not referenced to a specific level in a rock section on Earth. The name derives from Greek "Palaios" ancient. The oldest ascertained life form (well-preserved bacteria older than 3,460 million years found in Western Australia) is from this era. 3,480 Ma: Fossils of microbial mat found in 3.48 billion-year-old sandstone discovered in Western Australia. The first supercontinent Vaalbara formed during this period.