The high-grade rocks come from volcanic island arcs, and the low-grade metamorphic rocks come from deep-sea sediments. These sediments eroded from the island arcs and ended in a forearc basin. In short, greenstone belts show where protocontinents were stuck together.[15]p302-3 Archean life The processes that gave rise to life on Earth are not completely understood, but there is substantial evidence that life came into existence either near the end of the Hadean Eon or early in the Archean Eon. The earliest evidence for life on Earth is graphite of biogenic origin found in 3.7 billion–year-old metasedimentary rocks discovered in Western Greenland. Fossils of cyanobacterial mats (stromatolites) are found throughout the Archean—becoming especially common late in the eon—while a few other probable bacterial fossils are known from chert beds.[15]p307 In addition to bacteria, microfossils of the extremophilic archaea have also been identified. Life in the Archean was limited to simple single-celled organisms (lacking nuclei), called prokaryotes. In addition to the domain Bacteria, microfossils of the domain Archaea have also been identified.