Prehistory

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For our purposes Prehistory refers to the history of the Earth from its accretion to the emergence of Humanity. Over the last few centuries and in particular over the last few decades, we've gradually managed to piece together a consistent image of the past from geological, paleontological and astronomical evidence.

Early Days

Hadean Eon

• 4.54-4 billion years ago.

The Newly formed Earth was a hellscape of volcanoes, lava flows, meteor showers and toxic gases. Another planet called Theia crashed into the earth, with the ejecta kicked up by the impact forming our Moon.

Archaean Eon

• 4 to 2.5 billion years ago

Things calm down and cool off, with vulcanism slowly declining. The soup of toxic volcanic gasses cools off, resulting in massive rainstorms which fill up the oceans. From this churning chemical soup heated by geothermal vents emerge organic chemicals, some of which form into complex structures and some of which begin to replicate. Eventually these basic molecular replicators give rise to the earliest life-forms, simple single celled creatures floating about in the ocean that concentrated around geothermal vents and floated about consuming whatever bits of useful chemicals they stumbled across.

Proterozoic Period

• 2.5 billion to 540 million years ago

At this point, some single cell develops a capacity to use sunlight to make sugar from carbon dioxide and water. This gives them an edge over those cells which just float around for whatever bit of odd chemical energy they can scrounge up and they fill the seas with these Cyanobacteria. They change the atmosphere, filling it with oxygen and eventually cause a massive ice age that nearly wipes out life on Earth before it could really get started. This was a set back, but life slowly continued to refine itself in this period. Things such as sex evolves at this time as well as the first multicellular life emerges. Though these are simple things such as slime-molds and sponges.

Ediarcaran Period

• 638 to 540 million years ago

Earth gradually gets more hospitable to life and basic multicellular life more complicated than a sponge begins to emerge bit by bit. Still fairly simple at this point, but they are there. Also includes some early molluscs.

Phanerozoic Eon

The Period in Earth's History in which life really takes off and (unless you are really into micro-organisms or the really strange things of the Ediarcaran) gets interesting.

Cambrian Period

Anomolocaris, it was not named "Strange Shrimp" for nothing

• 540 to 485 million years ago

There's an explosion in diversity in multicellular life in the seas. Large numbers of invertebrates and the ancestors of every major branch of the tree of life emerge along with a bunch of others which would eventually fade away or be wiped out. A lot of really strange body plans like Anomalocaris and Opabinia get tried out. Hell, one Cambrian critter was named Hallucigenia because paleontologists could literally not make heads or tails of it for decades.

Ordovician Period

Megaloratpus, not very cuddly

• 485 to 444 million years ago

Still a lot of weird critters swimming about in the seas, but the biggest thing at this time were Sea-Bugs. Arthropoids were doing very well at that time, most notably Eurypterids. Imagine a meter long meter long giant sea scorpion. There were also (among other things) a lot of early molluscs such as Orthocones and some rather derpy primitive fish. Fumbling tadpole shaped things with fleshy tails, a lot of boney armor and jawless mouths.

Ends with the Late Ordovician Extinction Event. The first of five such mass extinctions in the paleotological record.

Silurian Period

Your noble fore-bearer: a jawless Derp-Fish!(Sacabambaspis janvieri)

• 444 to 420 million years ago

A bit of an interim period. The first plants begin to show up on land in swamps and gradually begin to turn muddy swamps and eventually dry deserts green. Atmospheric oxygen levels rise significantly, briefly triggering another ice age. Arthropods begin to crawl onto land, combing the beaches and latter nibbling on early land plants.

In the sea, various forms of invertibrates would continue to dominate. That said fish diversify and some of them begin to develop jaws to bite with.

Devonian Period

Dunkleosteus: apex predator of the late Devonian Sea

• 420 to 359 million years ago

The Age of Fish. Jawed fish take over the seas and end up filling a lot of niches. In particular, there are the Placoderms. A group of armored fish which filled a lot of niches in the sea and which grew to massive sizes. But there were also sharks and their relatives, some families that died out and bony fish. One strain of lobed finned bony fish makes its home in rivers and swamps, develop lungs and then legs and begin to crawl out of the sea into new steamy forests away from predators and with access to bugs to eat. These were the first Stegocephalians, or "Fishapods". The first land vertebrates.

Ends with the Late Devonian Extinction Event.

Carboniferous Period

• 359 to 299 million years ago

At this time, the Earth's a hot house. Lots of CO2 in the air and with warm and damp weather. The Result was muggy rainforests of Scale-Trees from the equator to the arctic and antarctic. Most of the world's Coal was made during this period as this was before fungii which could break down cellulose had developed. The result was that logs generally did not rot away and unless they were burned got buried. It was also a time when you got some fucking big bugs, like Dragonflies the size of Falcons and two meter long millipedes.

For the most part, vertebrates at this time were basically Salamanders of various sizes which would eventually give rise to lizard like things with scales and amniotic eggs that could be laid on land. First there were Anapsids, which would eventually give rise to Synapsids (the ancestors of mammals) and Diapsids (the ancestors of Reptiles, Birds and Dinosaurs). Even so, it was slow going. It was only at the end of the Caboniferous that a few land vertebrates began eating plants.

All the continents of the earth scrunch up into one big continent called Pangaea (literally "The Entire Land"), which will remain the case for the next few geologic periods.

Permian Period

A Gorgonopsid: kinda lizard-like, kinda dog-like, totally not someone you'd want to come across in the late Permian

• 299 to 252 million years ago

The world cooled down to something similar to what it's like today. Also tetrapods begins to rapidly diversify. Anapsids, Synapsids and Diapsids move into fill a wide variety of ecological niches, from large herd dwelling herbivores to large predators to small burrowing critters. These were still pretty unrefined and clunky, but it was not just a world of Newts and Lizards. Synapsids end up on top, probably because they developed endothermy and as such could better live in places that got snowfall and would not go into a torpor when the temperature dropped. That said they were not totally dominant.

The Permian Period ends with the End Permian Extinction Event, also known as The Great Dying. Caused by massive volcanic eruptions in what is now Siberia it was the single worst mass extinction in Earth's history.

Triassic Period

• 252 to 201 million years ago

Aka when Dinosaurs show up. Not that it was the only thing that happened. After the Great Dying a few species left standing filled the empty niches, and in praticular one species of vaguely pig synapsid called Lystrosaurus swarmed across Pangaea and made up to 95% of all terrestrial vertebrate life by mass and it took some time for biodiversity to really spring back. The Triassic was on the whole a hot and fairly dry time and a wide range of new Reptiles and Synapsids, with Reptiles gradually gaining the upper hand. This was also when the first vertebrates (pterosaurs) took to the sky, as well as reptiles returning to the sea, most notably Ichthyosaurs. Dinosaurs were at this point just one family of reasonably successful reptiles. Our ancestors at the time were Cynodonts, which would eventually give rise to Mammals.

Ends with the End Triassic Extinction event, pretty much a less extreme version of the Great Dying as Pangaea began to come apart.

Jurassic Period

• 201 to 145 million yeas ago

The Dinosaurs take over. The End Triassic Extinction Event takes out most of the competition leaving the fast moving warm blooded dinosaurs to fill the vacated niches. This is also when Dinosaurs get BIG. This is the time of the Sauropods, with vast herds of titanic herbivores stomping across the forests. Other dinosaurs refine pictofibers into feathers and some of them begin to glide.

Mammals meanwhile have established themselves in the nooks an crannies as small shrew-like critters eeking a life out under the dinosaur's feet. Gradually Eutherian mammals emerge, which give birth to live young.

Pangaea would split apart at this time.

Cretaceous Period

• 145 to 66 million years ago

The last period of the Dinosaurs, but by no means a low point for them. Indeed they reached new heights. Small agile Dromaeosaurid Raptors show up along herds of Hadrosaurs and dead 'ard ankylosaurs. And who could forget the motherfucking T-Rex. Sauropods were a bit less common, but a few huge examples like Alamosaurus were still large and (in their niche) in charge.

This is also when flowering plants begin to emerge, which would come to dominate the Cretaceous landscape. Also when fruit started to show up.

Famously ends with the Cretaceous–Paleogene extinction event when a 10km long asteroid crashed into what's now the Yucatan Peninsula, causing a perfect storm of hitting the wrong place at the wrong time (as the impact coincided with yet another flood basalt and hit an area rich in sulfur, the climate damage got a lot worse than if it was just the impact debris alone). Needless to say, the Dinosaurs did not survive, except for the birds.

Cenozoic Period

• 66 million years ago to now

Mammals crawl out from the ashes and repopulate the earth fairly quickly. They don't get as big as the dinosaurs, but they fill most niches rapidly from herd dwelling herbivores to predators to flying bats and whales to replace the lost giant marine reptiles. This has been a time of considerable change, with several cycles happening in which new clades of mammals rise to prominence and die off, replaced by newer groups which fill the same niches. It used to be the case that a major group of predators were hooved ungulates with bone crushing jaws.

In particular one strand of Arboreal mammals develops unusually keen eyesight, large brains and dexterous hands well suited to navigating their canopy homes and picking fruit, but also using tools. Eventually they loose their tails, come down from the trees, walk upright and begin to speak and use fire, but their story continues elsewhere.

The Appeal of Prehistory

In short, it's quite amazing what life-forms that we've discovered in the geological record. Dinosaurs may be the most famous, but they are just the tip of the iceberg. In the last few decades our understanding of this time period has exploded and we've developed a remarkably complete picture of the history of life on earth. From the strange early animals of the Ediacaran and Cambrian to monster sea creatures to early tetrapods, swift land crocodiles and huge hairy beasts there is an amazing diversity of creatures to be inspired by.

Similarly, having an understanding of life's history from simple replicators to Us is a useful asset in crafting the biology of your own world. The relationships between predators and prey, mass extinctions leading to a sudden contraction of biodiversity to be repopulated elsewhere and the pressures which lead simple tree shrews to develop big brains, dexterous hands, walk the earth and eventually tame it is a good tool if you want to design an alien race or biosphere which is both exotic and believable.

Another source of inspiration is that of speculative evolution. Imagine if the Asteroid that wiped out the Dinosaurs never happened, What would the Earth look like today? A wilder speculation would be about lost civilizations of Dinosaurs and similar in earth's geological past.

Prehistory inspired Games, Factions and Settings

• Lizardmen from Warhammer Fantasy Battle in pretty much everything: Sauruses are anthropomorphic Pachycephalosauruses, while their mounts are all various Mezozoic reptiles.
• On the other hand, you have Ogre Kingdoms, which battle monsters are directly lifted from the Ice Age with some minor changes.

Historical Time Periods
Deep Time:	Prehistory
Premodern:	Stone Age - Bronze Age - Classical Period - Dark Age - High Middle Ages - Renaissance
Modern:	Age of Enlightenment - Industrial Revolution - The World Wars - The Cold War - Post-Cold War