PROTEROZOIC EON 2.5 Billion - 542 Million Years Ago Edit
The last in the line of Precambrian Eons, the Proterozoic Eon is a staggeringly long time in the Geological Record - the longest Eon of all time; lasting for over 1.96 Billion Years. During the Proterozoic Eon, Oxygen produced by photosynthesising bacteria, collected in the oceans, and then in the atmosphere. Iron, present in exposed rocks, reacted with this new oxygen, and so a rust coloured the planet surface red. Oxygen-tolerant organisms thrived and evolved further in these changing conditions, however, the older Oxygen-intolerant anaerobic organisms dwindled, surviving only in airless pockets.
At last, around 900 Million Years Ago, a substantial way into the Proterozoic Eon, soft-bodied organisms first evolved; which are the first lifeforms large enough to be seen by the naked eye. By 650 Million Years Ago, nearer to the end of the Proterozoic Eon, these soft-bodied organisms had multiplied and become numerous. By 550 Million Years Ago, very close to the end of the Proterozoic Eon (by 10 million years) however, they are much more scarce. As a result of there being so little fossil evidence of animals from this time having been discovered, scientists are not certain whether these earliest animals were wiped out in what was possibly a major extinction event, or whether they had actually progressively evolved into the relatively well geologically recorded hard-shelled life that soon follows.
The Hadean Eon The Archaean Eon The Proterozoic Eon The Phanerozoic Eon The Palaeozoic Era The Cambrian Period The Ordovician Period The Silurian Period The Devonian Period The Carboniferous Period The Permian Period The Mesozoic Era The Triassic Period The Jurassic Period The Cretaceous Period The Cenozoic Era The Tertiary Period The Palaeocene Epoch The Eocene Epoch The Oligocene Epoch The Miocene Epoch The Pliocene Epoch The Quaternary Period The Pleistocene Epoch The Holocene Epoch