Precambrian and Palaeozoic (4600 - 252 Ma)

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The Precambrian is an informal stratigraphic unit spanning from 4600 Ma to 542 Ma, since Earth formed by accretion from solar nebulae right up until the origin of organisms with hard body parts. Precambrian is divided into 3 eons - Hadean, Archean and Proterozoic.

 

In the Hadean (4600-4000 Ma), the Moon formed from a massive collision between young Earth and a Mars-sized planet called Theia. Ancient oceans may have began to form 4.4 billion years ago and life might have originated in the Hadean during the “Great Cometary Bombardment” of the Earth. Geological processes like sedimentation, metamorphism, erosion and plate tectonics had already started functioning during this time.

 

In the Archaean (4000-2500 Ma), we see the earliest fossil evidence of life – stromatolites at around 3.5 billion years old. The only forms of life at this time were prokaryotes, such as cyanobacteria (the first photosynthesing organisms) among other types of bacteria. The first stable continental blocks (cratons) had developed and consolidated to form the first supercontinent Vaalbara over 3 billion years ago.

 

The Proterozoic Eon (2500-542 Ma) is distinguished by the evolution of eukaryotic organisms, where the activity of photosynthetic microbes lead to an increase in oxygen levels in Earth’s atmosphere and hydrosphere. Late in this eon, glaciers may have covered the majority of the Earth's surface, extending near the equator. Near the end of the Proterozoic, the first multicellular organisms arose, undergoing rapid evolution at the start of the Paleozoic era (542 to 252 mya), which commenced with the "Cambrian explosion" (when the major diversification of most modern metazoan phyla occurred) and ended with the largest mass-extinction ever at the end of the Permian.

 

This was the era when life started to emerge from the water and onto dry land, including plants such as mossy growth and liverworts that began encroaching coastal environments during the Silurian (which is also the time when both cartilaginous and ray-finned fish first appeared). This was preceded by the earliest terrestrial animals, which were arthropods from the Ordovician. The development of tetrapods was the most important evolutionary step in the Devonian, when green plants of increasing size spread and swampy deltas/river estuaries provided important habitats for the emergence of vertebrate life onto land. The Devonian was known as the “Age of Fishes”, when the extinct placoderms and acanthodians peaked in diversity. During this period, insects were the first animals to evolve powered flight.

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The fragmentation of the Proterozoic supercontinent Rodinia led to the assembly of Gondwana, located in the southern hemisphere, with lots of smaller continental blocks in the north. Near the end of the Permian, the majority of continental blocks assembled again to form a new supercontinent called Pangea that straddled the equator. The Carboniferous was characterised by wide-ranging forests which would later become the coal beds of North America and Europe; oxygen levels were also at their highest in Earth’s history at ~ 35%, enabling terrestrial invertebrates to reach gargantuan sizes.

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The climate constantly changed, being hotter than today's most of the time, but the Paleozoic was also a time of several ice ages – during the late Ordovician, late Carboniferous and early Permian glacial phases, when ice sheets extended across most of the southern supercontinent Gondwana.

Amniotes, the most diverse clade of tetrapod vertebrates, originated in the late Carboniferous. By the Permian, the two major groups of amniotes had appeared- the diapsids (the group including lizards, crocodiles, snakes, turtles, tuatara and birds) and the synapsids (the group which led to mammals). By the end- Permian mass extinction, around 90% of all animal and plant life had become extinct, brought about by continental upheavals, further extreme changes in climate, massive volcanic eruptions and oxygen depletion in the oceans.