Precambrian - From the beginning to 650Ma
The Precambrian isn't really a single unified time period... it makes up roughly seven-eighths of the Earth's history. In other words, most of what Earth did, it's done during the Precambrian. Hence, the most important events in biological history took place during this time. The Earth formed, the first tectonic plates began to move, eukaryotic cells evolved, the atmosphere became enriched in oxygen -- and just before the end of this period, complex multicellular organisms, including the first animals, evolved. So, a lot.
During the first part of the Precambrian - the Archaean - The atmosphere was pretty markedly different from what we know today; it was a soup of methane, ammonia, and other gases which would be toxic to pretty much everything that lives here now. Our oldest fossils date to to the Archean, approx. 3.5 billion years ago, and consist of bacteria microfossils.
The second half of the Precambrian was the Proterozoic and this was a particularly hoppin' time. Stable continents began to form over the next billion years, and the first abundant fossils of living organisms ( mostly bacteria) appeared, but by about 1.8 billion years ago eukaryotic cells began to enter the record, which is the first evidence of oxygen in the atmosphere. Oxygen meant life for some, but disaster for the existing inhabitants who happened to like their toxic soup just fine, thank you very much. Sorry, dudes.
Cambrian - From 543 - 490Ma
As you may have already figured out, 'pre' usually comes before something, and in this case, the Precambrian came before the Cambrian.
This was a time when most of the major groups of animals first appear in the fossil record. This event is sometimes called the "Cambrian Explosion", because of the relatively short time over which this diversity of life forms 'exploded'. Once the Earth hit on oxygen as a concept and a party theme, the ball really got rolling.
At least, it was rolling in the oceans... the land was completely barren, which makes sense if you think about the fact that dirt as we know it is the product of billions of years of erosion, and Earth hadn't had that kind of time to make any yet. It basically had a microbial 'crud' that acted as a soil crust covering the land. Apart from some minor evidence that a few animals might have made land, the new continents (which had just formed during the breakup of the supercontinent Pannotia) resembled deserts with shallow seas at the margins. The seas were relatively warm, and for most of this era, there was no polar ice at all.
Ordovician - From 490 to 443Ma
Hey everyone, a new supercontinent!
During the Ordovician, most of the world's land was collected into the southern super-continent Gondwana. Throughout this period, Gondwana shifted towards the South Pole and much of it was submerged underwater. Because of this, the Ordovician is known for its diverse marine invertebrates, including graptolites, trilobites, brachiopods, and the conodonts (early vertebrates). A typical marine community consisted of these animals, plus red and green algae, primitive fish, cephalopods, corals, crinoids, and gastropods. More recently, there has been found evidence of tetrahedral spores that are similar to those of primitive land plants, suggesting that plants may have invaded the land at this time.
From the early to mid part of this period, the weather was pleasantly warm and muggy (if you like that sort of thing). However, when Gondwana finally finished moving in on the South Pole at the end of this time, massive glaciers formed causing shallow seas to drain and sea levels to drop. That's bad news for all the things that really liked warm, shallow seas to live in. This is very likely what caused the mass extinctions that characterize the end of the Ordovician, in which 60% of all marine invertebrate genera and 25% of all families went extinct.
Silurian - From 443 - 417Ma
Thanks to the stabilization of the earth's general climate, the Silurian period ended the previous pattern of erratic climatic craziness. Because of this, the glaciers started to melt, which contributed to a pretty major rise in the levels of the biggest seas. Coral reefs entered the world stage for the first time, and fish evolution and diversity really turned on the gas. The Silurian saw the rapid spread of jawless fish as well as the incredibly important appearances of both freshwater fish and first fish with jaws. Jaws are a big deal when you think about how many creatures on earth have them.
The Silurian is also the first period in Earth's history where we have good, solid evidence of life on land, including relatives of spiders and centipedes, and also the earliest fossils of vascular plants.
Devonian - From 417 to 354Ma
Plants, baby. Let's get the hell out of the ocean and onto dry land... finally. The oldest known vascular plants in the Northern Hemisphere are Devonian. It was pretty much all small plants at that point, the tallest being only a meter tall, but while small, diversity was on the rise. By the end of this period, ferns, horsetails and seed plants had appeared. Devonian plants did not have roots or leaves like the plants most common today, and many had no vascular tissue at all. There were so many new plants at this time that it's called the "Devonian Explosion".
And on the animal front, two major animal groups were starting to really develop. The first land-living vertebrates appeared as did the first arthropods, including wingless insects and the earliest arachnids (yeah, bugs are OLD).
As for what the Earth itself looked like on its face, there were now three major continental masses: North America and Europe sat together near the equator, with a lot of their current land underwater. A mob continent of South America, Africa, Antarctica, India, and Australia dominated the southern hemisphere.
Carboniferous - From 354 - 290Ma
One of the most important things that came out of the Carboniferous Period was the amniote egg, which allowed for the further exploitation of the land by certain verebrates. The amniote egg let the ancestors of birds, mammals, and reptiles to reproduce on land by protecting the embryo in fluid, preventing it from drying out. This was also a time of larger plants and the tree-fern. Milder temperatures caused a decrease in lycopods and large insects and an increase in the number of these tree ferns. The plants from this time resemble the plants that live in tropical and mildly temperate areas today. Many of them lack growth rings, suggesting a uniform climate.
The presence of two large ice sheets at the southern pole was sucking up a huge amount of Earth's water, and because of this, the sea levels all over the world fluctuated. This led to another mass extinction, this time for shallow marine invertebrates. It also caused the gradual decline of swamps and the increase in dry land habitat.
Meanwhile, the continents were busy colliding. Laurussia (present-day Europe and North America) smashed into Godwanaland (present-day Africa and South America) and produced the Appalachian mountain belt ( I bet you never realized how much older the Appalachians were than the Rockies or the Sierras). In North America the environment was heavily marine, seas covering large parts of the continents.
Permian - From 290 to 248Ma
The Permian is mostly known for the extinction event that ended it, but we'll get to that. By the beginning of this period, the movement of the plates had brought much of the total land together, fused into the somewhat familiar supercontinent known as Pangea that many of us learned about in school. Many of the continents of today in somewhat intact form were mashed together (only Asia was broken up at the time), and stretched from the northern to the southern pole. Most of the rest of the surface area of the Earth was occupied by a corresponding single ocean, known as Panthalassa, with a smaller sea to the east of Pangea known as Tethys.
It's speculated that the interior regions of this massive continent were probably dry, with enormous seasonal fluctuations. When you don'd have a nearby body of water to moderate the effects of the weather, you get little rainfall. We know little about the ocean itself at that time, but there are indications that the climate of the Earth shifted, and that glaciation decreased, as the interiors of continents became drier. Because of the aridity, the swamp forests of the Carboniferous were gradually replaced by conifers, seed ferns, and other drought-resistant plants.
Aaand, we have reptiles! These early reptiles were in a good position for this new climate. Thick, moisture-retaining skin allowed them to move in where amphibians had previously been king. They became ideally suited to the desert-type habitats in which they still thrive today. Therapsids found an internal solution to keeping warm, they became warm-blooded, conserving heat generated through the breakdown of food. These more metabolically active reptiles could survive the harsh interior regions and they became the dominant land animals of the late Permian, rapidly evolving many different forms, ranging from dinosaur-like fanged flesh-eaters to plodding herbivores.
In the latter part of the Permian, smaller varieties emerged, likely warm-blooded and covered in insulating hair. From them, mammals would arise.
And then came the extinction event. The Permian (and the entire Paleozoic Era) came to a calamitous close, marking a biological dividing line in the Earth's history that few animals crossed. This extinction is estimated to have wiped out more than 90 percent of all marine species and 70 percent of land animals. There are several theories as to the cause of this, ranging from a series of cataclysmic volcanic eruptions to global climate change that the life forms could not adapt to to a release of methane gas from beneath the sea to the old standby of asteroid impact.
Perhaps a combination of factors was to blame. But whatever it was, whatever happened... it paved the way for new animals and plants to evolve. The great forests of fern-like plants shifted to gymnosperms, plants with their offspring enclosed within seeds and ... here come the dinosaurs.
Triassic - From 248 to 206Ma
This period marks the end of the largest extinction event in the history of life, and is the part of the story where the shell-shocked surviving species staggered onwards, regrouped and picked back up in the new landscape of the Triassic.
Pangaea began to break apart in the mid-Triassic, forming Gondwana (South America, Africa, India, Antarctica, and Australia) in the south and Laurasia (North America and Eurasia) in the north. The continents were well above sea level, and the sea level did not change drastically during the period. Due to this relationship, flooding of the continents to form shallow seas did not occur. Much of the inland area was isolated from the cooling and moist effects of the ocean. The result was a globally arid and dry climate (much like the Permian), though regions near the coast most likely experienced seasonal monsoons.
The ocean hosted reptiles such as the dolphin-shaped ichthyosaurs and the long-necked and paddle-finned plesiosaurs preyed on fish and ancient squid. The bottom rung of the food chain was filled with microscopic plants called phytoplankton; two of the major groups still in the oceans today first appeared.
Animal life diversified and exploded into a wide variety of creatures. Frogs, salamanders, crocodiles, turtles, and snakes emerged. Pterosaurs, a group of flying reptiles, appeared in the skies. On land you could find moss, liverwort, and ferns carpeting the floors of forests of conifers, ginkgoes, and palm-like cycads. Spiders, scorpions, millipedes, and centipedes thrived. Grasshoppers appeared.
But perhaps the biggest changes came with the evolution of dinosaurs and the first mammals in the late Triassic, starting around 230 million years ago. Another extinction event at the end of the Triassic took out a large number of these new life forms, but the Dinosaurs survived, and moved into the the most well-known period in Earth's history besides the one we occupy now.
Jurassic - From 206 to 144Ma
Yay, Dinosaurs! Haven't we all had enough ammonites by now? I mean, tentacles and squid faces are great and all... but I want something that can stomp other things into dust.
Land plants abounded in the Jurassic, but floras were different from what we see today. Although Jurassic dinosaurs are often drawn with palm trees, there were no palms, or any other flowering plants, at least as we know them today, in the Jurassic. Instead, ferns, ginkgoes and cycads flourished in the Jurassic. Conifers were around, including close relatives of living redwoods, cypresses, pines, and yews.
In the seas, ichthyosaurs shared the waters with the plesiosaurs and with modern-looking sharks and rays. Also prominent in the seas were cephalopods -- relatives of the squids, nautilus, and octopi of today. Jurassic cephalopods included the ammonites (more ammonites!), and the belemnites, close relatives of modern squid but with heavy, calcified, bullet-shaped, partially internal shells.
And look, scurrying in the undergrowth, no bigger than rats -- early mammals. Go mammals!
In the middle of the Jurassic, Pangea began to break apart, and by Late Jurassic the Central Atlantic Ocean was a narrow ocean separating Africa from eastern North America. Eastern Gondwana had begun to separate form Western Gondwana. Much of the Jurassic world was warm and moist, with a greenhouse climate. Although some arid regions remained, much of the rest of Pangea was lush and green. Northern (Laurasian) and southern (Gondwanan) climates were still distinct in many ways, but thanks to the connected land masses, animals had acquired a more intercontinental character. Some animals and plants were now found nearly worldwide, instead of being restricted to particular regions.
Cretaceous - From 144 to 65Ma
During this period, we find the first flowering plants - which is a big deal when you think about how plants now reproduce and how many insects depend on that process. We also find the first fossils of many insect groups (including the oldest known ants and butterflies), modern mammals and birds. We also have the first of the social bees, a very important parter in the development of flowering angiospores. Pangea had started to break apart during the Jurassic and it continued to do so, leading to increased regional difference in plants and animals.
The Cretaceous is also another period marked by an important extinction event that is, because of the fact that it killed the beloved dinosaurs, somehow thought of by most people as being the biggest and only extinction event in the earth's history. It's known as the K/T Extinction, and while it did kill the dinosaurs, it also allowed the emergence of the small, scuttling mammals to fill the void as the most successful animal group on the planet.
The Cretaceous was thus the time in which life as it now exists on Earth came together. Many species of foraminiferans went extinct at the end of the Cretaceous, as did the ammonites (aww, bye, guys). But many groups of organisms, such as flowering plants, gastropods and pelecypods (snails and clams), amphibians, lizards and snakes, crocodilians, and mammals "sailed through" the K/T with few or no apparent extinctions at all.
Miocene - From 24 to 5Ma
While the Miocene does not technically go all the way up to where we are at now, it does, with some small differences, look just like the Earth we know today.
Early in the Miocene, temperatures begin to rise. Extensive mountain building in the Americas and Asia had altered air circulation and weather patterns and created drier overall conditions. Antarctica had frozen and the global climate had cooled again. Land bridges formed as the oceans pulled back and as inland waters dried out, new waves of animal migration between continents ocurred. Grasslands expanded across the Americas and eastern Eurasia which offered plenty of food for evolving grazers like deer and antelope.
The great diversification of land mammals during the Miocene is mostly due to the formation of land bridges. These routes, which emerged as sea levels dropped and inland seas dried out, connected continents previously separated by water. The once-enormous Tethys Ocean no longer separated Africa and Eurasia. To the north, a dry corridor, the Bering land bridge, connected what are now Siberia and Alaska. At the close of the Miocene, North America and South America are close enough for some species to cross the narrow dividing waters. Australia took in animals from southeast Asia, like rodents.
And here, at last, at the very end... we have the first evidence of an upright ape with the forward-facing eyes of a predator, carrying what looks like a pointy stick who may or may not manage to find a way create its very own extinction level event.
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