Defined Words: cyanobacteria, eukaryotic cells, Snowball Earth, Cambrian Explosion, plants, insects, lignin, trees, amphibian, Ichthyostega, reptiles, birds, flowers, mammals
So far, for the most part, we have been discussing either random or spontaneous processes based on physics and probability. Now we’re going to talk about the specific history of life on Earth.
In this chapter, I’ll be throwing out a lot of names that we have given to different parts of the history of life. If the names are helpful, great; if not, just ignore them. I will occasionally use the scientific unit Ma, meaning mega anum, in place of the phrase “million years ago.” Okay, here we go!
Life started approximately 3.7 billion years ago, depending on who you ask, and it evolved slowly as single cellular organisms for the first 2.8 billion years. Most of the life at this point existed in the oceans; nothing was yet living on land.
Around 2.7 billion years ago, a single cellular organism called cyanobacteria appeared. The cyanobacteria were really good at performing photosynthesis, consuming the huge amount of carbon dioxide in the atmosphere and replacing it with oxygen. These guys are a major reason the atmosphere contains any oxygen at all.
About 2 billion years ago, some single celled organisms develop adaptations that allow it to have an enclosed nucleus where the genetic material can be safely stored. We call these eukaryotic cells and they make up all of the multicellular life that has ever existed.
Continuing to fast forward to about 650 million years ago, the harsh conditions of Snowball Earth may have been a factor in the development of multicellular organisms. It makes sense; conditions that are hard to survive are made easier by symbiosis. Those individuals that could mutually benefit each other had a better chance to survive. A large release of carbon dioxide into the atmosphere allowed for the greenhouse effect to raise the temperature enough to melt the glaciers.
Multicellular organisms diversified and filled up the niches of the ocean ecosystems. Around 600 Ma, we start to see the first plants. Simple animals like worms, mollusks, and coral also appear during this time. Around 560 Ma, sex appeared and the resulting diversification likely caused what we call the Cambrian Explosion. More diverse species means more possible niches to occupy.
This is also around the time when the tardigrades appear. These guys don’t have much to do with us; I just like to mention them because they’re still living today, 530 million years later! I’m pretty sure if they could call dibs, Earth would be rightfully named after them.
The Cambrian oceans were crowded, and about 480 Ma photosynthesizing plants were forced onto land in order to find more resources. These plants looked mostly like mosses and ferns at the biggest and they were able to thrive in the carbon dioxide rich atmosphere. Meanwhile back in the oceans, the first fish were making their way toward center stage of the tense competition of life. A mass extinction around 445 Ma empties many niches, but life persists.
At this time, insects began to explore the land. The land plants had been photosynthesizing for a while now so the amount of oxygen in the air was very high. Because insects absorb gases through their exoskeleton, they could grow to very large sizes in the oxygen rich atmosphere and became the top predators. Imagine a world with insects as big as you wandering the land in search of prey.
A random mutation in a plant allowed it to produce a protein that is strong yet flexible, we call it lignin. With the ability to produce lignin, plants could now grow much taller and the race toward the sun began. This was the birth of the trees. It would be several million years before any organism developed the enzymes to break down lignin so dead trees just piled up. The gases released from digesting all those trees would eventually spell doom for many organisms.
Approximately 400 Ma, the first amphibian vertebrates dragged themselves onto the land. Ichthyostega, the name given to the organism that would be the ancestor of all land animals, had four limbs and five digits on each limb; we still carry that trait today. Another mass extinction wipes clean many niches; life still persists.
The competition from the shallow ocean spilled over onto land as amphibians continue to explore in search of food. Some of the amphibians with thicker skin and an adaptation to lay hard-shelled eggs allowed them to move further inland, away from the water. We call the line of these organisms reptiles. The Permian period (360 Ma) was ideal for reptiles as our planet heated up; this formed a landscape with many deserts. At this time, the poles of the planet had tropical climates!
250 million years ago, the Permian period ends with the largest mass extinction the planet has ever seen. Volcanic eruptions in present-day Siberia lasted for hundreds of thousands of years and filled the atmosphere with sulfur and carbon that blocked out the sun. This mass extinction killed 70% of all life on land and 95% of all aquatic life; life persists.
The fallout of the volcanoes left an incredibly warm planet with desert climates near the equator and tropical climates by the poles. It was at this time that the dinosaurs ruled the planet. The dinosaurs thrived in an oxygen rich environment, along with a lack of competition, allowing them to grow to huge sizes.
A mass extinction separates the Triassic and Jurassic periods, 200 Ma. It was during this time that our first mammal ancestor appeared. During the Cretaceous period, 150 Ma, some of the remaining dinosaurs adapt to take on features we know to be distinct to birds. In fact, birds are the only surviving line of the dinosaurs that were wiped out through the mass extinction 65 million years ago. Also during this period, 130 Ma, plants develop the adaptation to produce flowers!
Mammals adapted a tendency to allocate resources toward maintaining a constant body temperature. These early rodents were able to hide from predators and withstand the harsh conditions of the last major extinction. Eventually our mammal ancestors ventured out and diversified to fill up all the niches wiped clean from the last extinction. This is the current time we live in: the Cenozoic era, reign of the mammals.
When looking at the history of life from this zoomed back perspective, it is very easy to see the persistence of life, the surging progress, as well as the connectivity between all living things and our planet. The condition of the Earth’s atmosphere has had a significant effect on life over its history and has even caused a few mass extinctions. The delicate balance of the Earth allows for life as we know it. When the planet experiences drastic changes, many organisms will die but life will always persist.