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The Precambrian is divided into three eons: the Hadean (4567–4031 Ma), Archean (4031-2500 Ma) and Proterozoic (2500-538.8 Ma). See Timetable of the Precambrian. Proterozoic: this eon refers to the time from the lower Cambrian boundary, 538.8 Ma, back through 2500 Ma. As originally used, it was a synonym for "Precambrian" and hence included ...
These are thought to be the precursors for all animals. They are highly related to sponges (Porifera), which are the simplest multicellular animals. In order to understand the transition to multicellularity during the Precambrian, it is important to look at the requirements for multicellularity—both biological and environmental.
Several classifications have been used to accommodate the Ediacaran biota at some point, [115] from algae, [116] to protozoans, [117] to fungi [118] to bacterial or microbial colonies, [58] to hypothetical intermediates between plants and animals. [11]
The time span of the Phanerozoic starts with the sudden appearance of fossilised evidence of a number of animal phyla; the evolution of those phyla into diverse forms; the evolution of plants; the evolution of fish, arthropods and molluscs; the terrestrial colonization and evolution of insects, chelicerates, myriapods and tetrapods; and the ...
Prior to 1958, the Precambrian was thought to be completely devoid of fossils and consequently possibly devoid of macroscopic life. Similar fossils had been found during the 1930s (in Namibia) and the 1940s (in Australia) but these forms were assumed to be of Cambrian age and were therefore considered unremarkable at the time.
They were photosynthetic and multicellular, indicating that plants evolved much earlier than originally thought. [53] 750 Ma Beginning of animal evolution. [54] [55] 720–630 Ma Possible global glaciation [56] [57] which increased the atmospheric oxygen and decreased carbon dioxide, and was either caused by land plant evolution [58] or ...
The Avalon explosion, named from the Precambrian faunal trace fossils discovered on the Avalon Peninsula in Newfoundland, eastern Canada, is a proposed evolutionary radiation of prehistoric animals about 575 million years ago in the Ediacaran period, with the Avalon explosion being one of three eras grouped in this time period. [3]
The decision to base the Precambrian time scale on radiometric dating reflects the sparse nature of the fossil record, and Precambrian subdivisions of geologic time roughly reflect major tectonic cycles. It is possible that future revisions to the time scale will reflect more "natural" boundaries based on correlative geologic events. [16]