 The Archean and earliest Proterozoic microfossils termed "acritarchs" may actually be prokaryotes.The earliest eukaryotic acritarchs known (as of 2020) are from between 1950 and 2150 million years ago.At about 1000 million years ago the organisms responsible for acritarchs started to increase in abundance, diversity, size, complexity of shape and especially size and number of spines.