Getting a little tired of this Earth? Think you might want to get a shiny new one? Maybe one with less plastic bits floating in the ocean, more fresh air, and less annoying neighbors? If you have the right elements and enough free time on your hands, here's how you get one just like this one. This is a timeline of the biggest, most important periods in Earth's long history. To get a true sense of the scale of geologic time, note that the mammals that humans developed from didn't even show up till near the very end.NOTE: I found a lot of these images online through various museum sites, and I did not create them. If you see your artwork, let me know and I will either put your credit on it or take it down at your request.
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.
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 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.
The Permian is well 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. Most 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't 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 into the void left by all the lost species. The great forests of fern-like plants shifted to gymnosperms, plants with their offspring enclosed within seeds and ... here come the dinosaurs.
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.