Evolutionary History of Life
Hey all, I have been taking a course about the history of life this term and thought maybe people would like to know more about it. God knows I love reading about it! This information was collected in both parts from my course, and research online on my own time. Note that as with all paleontology, all of these findings are based only on the evidence we currently possess, and are subject to change. I hope you enjoy!
Living Rocks?!: Stromatolites
Possibly as early as 3.7 billion (!!!) years ago, bacteria and (green) algae became visible on the fossil record, due to this phenomena. Still existing today in some brackish lakes, colonies of bacteria come together and gradually form a “rock” structure. Only the outermost layers (centimeters) are composed of living organisms, with the rest being solid sediments from the surrounding area. The world was covered in the things, peaking 1.25 billion years ago, then falling as they were eaten by newly evolved “complex” life.
The Goopy Times: Edicarian
What we could call animals first evolved roughly 600 million years ago. With a few exceptions, nearly everything at this time had no hard body parts. Most animals at this time were only a few millimeters in length, however the large flesh pancake seen center stage in the picture below, Dickinsonia, could grow up to almost 5 feet long! Also growing to large sizes were Charnia, plant-like animals that could grow almost 7 feet tall from the seafloor. They highly resemble an unrelated modern animal, the sea pen.
Not-coral-reefs and Hard Body Parts (finally): Tommotian (Early Cambrian)
The change from Precambrian to Cambrian is tied to the appearance of trace fossils (fossils depicting the activities of organisms, such as footprints) about 541 million years ago, specifically trace fossils in the sediments. Before this, food was likely found through photosynthesis for those that lived in harmony with green algae, filter feeding, or just roaming the ocean floor for scraps. Once animals evolved to look under the floor though, a new feeding mechanism was introduced to life.
Hard body parts, and thus more easily fossilized organisms, were diversified at this time. One group of organisms in particular, the archaeocyathids (pictured, background), were almost entirely hard material, forming “cups” and filtering water carried through the tide, over time forming the first reefs of the world.
Life!!!!!Death!!Spines!?: Cambrian
From 541 to 485 million years ago, life began taking on many weird and wonderful forms. Trilobites evolved early (two are present slightly up-left of the center of the image), nature’s super successful bottom-feeders (until they died (spoilers)). Predation wasn’t seen in the fossil record until now, and nature came in swinging. Meter long predators like Anomalocaris appeared (two are visible in the image), weird weird stuff like Opabinia swam about (one is in the center of the image). The ocean became scary!
Also appearing in this period is a big step towards us: vertebrates! Pikaia was a small fishlike creature resembling a modern lancelet (down-left of the Opabinia). It had no spine, but it had a notochord, a long bundle of nerves, which developed into the spines most of us are using at this moment.
Where I start relying on the internet Ordovician
After a 42 million year long mass extinction that wiped out 60% of marine life, life bounced back in a big way. Trilobites survived the extinction event, and continued to flourish, diversifying into many odd and spiny forms to protect against a new predator, eurypterids and nautiloids.
Eurypterids were large arthropods (the largest ever known!), related to arachnids, and were very successful predators, achieving worldwide distribution throughout their time on the planet. Nautiloids we still have today to some extent, were once the main predators of the ocean for an era. Some could grow to lengths of 11 feet, and somewhat questionable evidence hints at much larger individuals up to 36 feet (paleontology is hard).
Land! Plants! Bigger Better Fish!: Silurian
After a particularly horrid series of extinction events in sequence lasting from 455 to 430 million years ago, killing 60% of all marine species, life recovered. Our lovable trilobites were hit hard, but again they survived and thrived once more. Their predators the eurypterids continued to grow and thrive as well, though now beginning to move their attention to less saline waters.
Dry land had up to this point been relatively uninteresting, however in this period it began to quickly be taken over by recently evolved vascular plants. Life on land at this point was present, but not yet diverse, consisting of various arachnids and millipede/centipede ancestors.
“Age of Fish”: Devonian
Back in the ocean 419-358 million years ago, life kept getting bigger, with fish diversifying into many new groups, particularly the armored fish, placoderms. The largest member, Dunkleosteus (second picture), and maybe my favorite animal, grew to sizes of 20 feet, and weighed up to 1 ton. In addition to it’s size, it was heavily armored, and had a complex jaw mechanism that let it open and close it’s maw in just 50-60 milliseconds. It’s theorized that this could create a suction to draw in prey, and then slice them with a 6000-7400 N bite focused along a sharp, scissor-like jaw. In other words, a pretty scary fish.
Plant life on land also diversified in a major way, with rooted plants and leaves evolving. These early “trees” were relatively very short compared to our modern variety, only several centimeters tall. However, giant fungal spires, Prototaxites, grew up to over 8 meters tall. However, all this photosynthesis led to lower carbon dioxide, possibly leading to some very unfortunate side affects.
Bugs, Sharks, and Frogs: Carboniferous
With another chain of extinction events out of the way, this one dealing a major blow to the trilobites they would never recover from (at this point the eurypterids were fairly content on land and in fresh water), life resumed it’s grind. Trees became the titans we know today, and as a result of a material that allowed the increased height, were not able to be broken down for a long long time, meaning that when a tree fell, it stayed there.
Insects radiated, with the massive 8 foot Arthropleura (pictured) crawling the forests, as well as giant cockroaches and scorpions, and dragonflies in the air. Amphibians, who had been present for a small while before, became common. Reptiles, evolving from amphibians due to a more dry climate, began spreading. Meanwhile in the oceans, sharks radiated, with the placoderm competition gone. 298 million years ago, the carboniferous ended, and a new period began, one that’s name is closely related to death, devastation, and extinction.
Permian
It is unfortunate that at least in my mind the permian is so exclusively thought of in a mournful tone. Many amazing animals evolved during this time, and the supercontinent Pangaea lives on in the public, unconnected to the wasteland it became. In the center of Pangaea was a massive desert stained red by the sun. Seeded plants and conifers replaced the previous swampy trees. The most successful insects at this time were cockroach-like.
Synapsids, a group that like reptiles branched off from amphibians early on, diversified in this period, leading to such species as the famous Dimetrodon, and would later evolve into mammals. Diapsids, which would one day become our beloved dinosaurs, also evolved at this time. With the first large terrestrial carnivores and herbivores, the land finally began to somewhat resemble the world we know today.
Then it almost all ended. Life itself on the surface of this planet, billions of years in the making, might have ended. 95% of ocean life died, 70% of all life on land. It took the land 30 million years to recover from the Permian extinction. Simply put, the world boiled. Lava poured from the earth’s mantle for thousands of years, possibly releasing so much carbon dioxide that the global temperature would have increased by 5 degrees Celsius. Another factor might have been oceanic venting of hydrogen sulfide, causing poisonous ocean in the deep, and then evaporating into the atmosphere to destroy ozone, allowing ultraviolet radiation to burn life. The five degree temperature increase could thaw methane deposits, further increasing the heat by another five degrees. It could have been caused by a supernova, there is also evidence of a giant meteorite striking the Earth at this time.
The worst catastrophe of life on our planet drew to a close, life recovered slowly, and soon after were the dinosaurs so that’s good. It is 3 am now, I started this at 11:30. To spare you all increasing plagiarism from wikipedia, I will take a rest, and begin anew later. I hope you enjoyed learning, if my style wasn’t interesting at least you can google key terms and find better more accurate material. Byeee
