The fossil record of the later Proterozoic through the Early Cambrian is marked by extraordinary change. This change indicates a fundamental reorganization of the biosphere from the exclusively single-celled prokaryotic and protistan ecosystems that prevailed during much of the Proterozoic, to ecosystems characterized by complex multicellular plants and animals of the latest Proterozoic and Early Cambrian. The first recorded events in this transition took place about 900 Ma and the last about 550 Ma, a period of time exceeding that since the end of the Paleozoic. But the final and most dramatic phase, the “Cambrian Explosion,” occurred over a few tens of Ma at the onset of the Cambrian.
The glaring contrast between the Precambrian and the Phanerozoic has long been recognized as a major problem in the history of life. Darwin (1859) attempted to explain the sudden appearance of the Cambrian fauna by inadequacies of the rock record, and Walcott (1910) used a similar concept in his “Lipalian Interval” at the base of the Cambrian. Certainly the abrupt appearance in some local areas (for example, in the contact between Yudomian dolomites and Tommotian limestones in the Aldan-Lena region of Yakutia; Rozanov et al. 1969; Khomentovskij and Karlova 1986) may still be explained by incompleteness of the record.