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Proterozoic Period 2.5 billion to 545 million years ago.
| Although primordial RNA (the molecules of first life) may have appeared some 4 billion years ago, the Earth was still heavily volcanic, with low quantities of surface water and free oxygen. The oldest microfossils of bacteria date from around 3.5 billion years ago, however the earliest fossils of more complex organisms and animals do not appear until the Proterozoic period.
Protoctista (microorganisms with nuclei) have existed for over 1.2 billion years and include algae, slime molds, amoeba, plankton and other protozoans. During this period, certain protoctists would evolve into the other four kingdoms of Animals, Plants, Fungi and Bacteria.
By around 545 million years ago, the earliest Chordates (animals with a spinal chord) appeared. The Phylum Chordata would eventually include mammals, birds, reptiles, fish and the dinosaurs. 545 million years ago however, our ancestor was a small multicelled organism with a rudimentary group of nerve cells creating a nerve chord.
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Cambrian Period 545 million to 490 million years ago.
| Around 500 million years ago, Chordates, the ancestors of Vertebrates (backboned animals) further evolved in the oceans. The spinal chord of the Chordates that appeared in the Proterozoic period was a amazing control system that allowed for the eventual evolution of higher brain function. At the same time, it needed protection and these species developed cartilage based structures around this spinal chord that would eventually evolve into bone and other hard body components that would assist in the move onto land and allow for out-of-water mobility. During the Cambrian, most life and all animal life remained water bound.
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Ordovician Period 490 million to 443 million years ago.
| Invertebrate life diversified greatly during the Ordovician period, from around 160 families at the start to some 530 families at its close.
Although there is some arguable evidence of a move by plants on to land, in the form of mosses and liverworts, the evolution of life remained predominantly waterbound. The greatest variety of animal life then, as now, was invertebrate. No true vertebrate animals existed during the Ordovician. The Chordates found in fossils had no bony internal skeletons and structure is therefore assumed to have been cartilaginous. The fish were small jawless forms (Agnatha). It would take the initial move of plant species and the subsequent evolution of Vertebrates with stronger internal body structures to begin the animal migration to land.
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Silurian Period 443 million to 417 million years ago.
| The Silurian period saw a relative stabilisation of the Earth's climate. Coral reefs made their debut, as did freshwater fish and the first fish with jaws. Perhaps most importantly, both plant and animal life made significant appearance on land. Some animals of a form similar to spiders and centipedes have been found in land based fossils.
Most important for significant future moves to land, primitive plants such as Cooksonia and more complex lycopods began to find traction in the tough climate of largely lifeless and windblown land. The first Vertebrates with bone instead of cartilage most likely appeared in the late Silurian, although fossil structures only confirm them during the Devonian period to follow.
Numbers of Vertebrate species, in the form of Amphibians would increase rapidly after the Silurian for some hundred million years before declining during the rule of the Reptiles, from around 300 million years ago, until the extinction of the Dinosaurs some 65 million years ago, followed by the era of Mammals which continued through to modern times.
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Devonian Period 417 million to 354 million years ago.
| Amphibians were the first Vertebrates to make the transition from water to land. Some 360 million years ago, toward the end of the Devonian period, Amphibians would begin to spend longer on land. Whilst still laying soft eggs in water, mature Amphibians had bone structure and circulatory systems capable of sustaining them out of water. The tetrapods (four-legged animals) were the predecessors of all later Vertebrates (including mammals, birds and reptiles).
Also during the Devonian period, the world's land masses were drifting together to form the super-continent, Pangea. This aggregation of land would accompany the explosion of Vertebrates out of the sea and onto land, ensuring that Reptiles and other Vertebrates would be found in almost all quarters of the modern world.
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Carboniferous Period 354 million to 295 million years ago.
| All modern reptiles, with the exception of turtles, descended from the subclass Diapsida. Meaning 'two-arched reptiles', this classification refers to the presence of two openings in the temporal region of the skull (Ciofi, 1999). These holes enabled wide jaw opening as a result of longer and stronger jaw muscles.
For those of us who like to compare reptiles such as the Komodo Dragon to dinosaurs, we have to go back to this period, some 300 million years ago to make the ancestral connection. The subclass Diapsida was superseded after the Carboniferous period by the subclasses Archosauria (which included dinosaurs, birds and crocodilians) and Lepidosauria (from which most other modern reptiles, including snakes and lizards descended).
As much as many modern day reptiles remind us of the extinct dinosaurs of the Jurassic and Cretaceous periods, most reptiles evolved separately from the Carboniferous period onward.
It is also during the Carboniferous period that the mega-continents Laurentia and Gondwana were first brought together by the movement of tectonic plates to form the massive landmass Pangea. Eventually this landmass would separate back into Laurentia (now northern Europe and eastern North America) and Gondwana (now South America, Africa and Australia) eventually resulting in the significant divergence of flora and fauna from these regions (Dixon et. al., 2001).
The climate of this period seems similar to today, with arctic conditions at the poles, including central Gondwana, and rain forests close to the equator.
The Carboniferous period is also notable for the amazing proliferation of insect species and the arrival of winged insects, all forming an important part of the food chain for the evolution of many Reptile species.
The oldest reptile fossils are around 340 million years old and were found in the early Carboniferous formations of Scotland and Nova Scotia (Cogger and Zweifel, 1998).
The Tuatara (Sphenodontia) found today only in New Zealand is perhaps the closest remaining example of the Diapsids of the Cretaceous period. Fossils some 225 million years old are virtually identical to the Tuatara. All species of the order Sphenodontia became extinct some 60 million years ago, at the end of the Cretaceous period, leaving the Tuatara as the sole representative of the Order.
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