The Precambrian covers 88 percent of Earth's history, and we know little about it
Nearly nine-tenths of our planet's story happened before the Cambrian, from Earth's birth about 4.6 billion years ago until hard-shelled animals appeared around 538.8 million years ago. Yet that vast stretch is poorly understood. Its rocks have been cooked, eroded or buried, and most of what we know was only uncovered after the 1960s.
The name simply means before the Cambrian, the period named after Cambria, the Latin name for Wales, where rocks of that age were first examined. It is an informal label: the International Commission on Stratigraphy and the US Geological Survey both treat it that way, even though it bundles together three formal eons, the Hadean, Archean and Proterozoic. Some call it a supereon, a term that is equally unofficial.
Earth is thought to have gathered from material orbiting the Sun about 4,543 million years ago, and may soon afterwards have been struck by a planet called Theia, flinging off debris that became the Moon. Zircon crystals from Western Australia dated to about 4,404 million years show that a stable crust existed very early.
When life began remains unknown. Carbon in 3.8-billion-year-old rocks from islands off western Greenland may be biological, and well-preserved bacterial fossils more than 3.46 billion years old come from Western Australia, with probable fossils 100 million years older nearby. Some evidence points to life before 4.28 billion years ago. One idea, the RNA world hypothesis, proposes that RNA preceded DNA and proteins, and that hot, porous rocks of the Hadean could have supported its replication.
Complex multicellular life may date back as far as 2,100 million years, though ancient fossils are notoriously hard to read; one researcher quipped that some definitions of multicellularity stretch from bacterial colonies to badgers. The first widely accepted complex organisms are the soft-bodied Ediacaran biota, dated between 635 and 542 million years ago. Tracks left by a creature with leg-like limbs have been found in 551-million-year-old mud. On land, there were no plants or animals, only microbial mats of cyanobacteria. Stromatolites are among the typical fossils, but they are of limited use for dating rocks.
Source: Precambrian
