As the Archean Eon ended, cyanobacteria had begun producing oxygen as a result of photosynthesis. At first, the oxygen produced bonded to iron to form hematite, a reddish mineral. However, over time, the cyanobacteria produced so much oxygen that is began entering the atmosphere.
During the Archean Eon, the atmosphere had been full of gases such as carbon dioxide and methane. These chemicals made the sky red. As oxygen was added to the atmosphere and methane disappeared, the sky turned blue. Where did the methane go? Several places. Bacteria found on the surface of the Earth eat methane, so it is possible that bacteria from the Proterozoic Eon were eating methane as well. Methane also reacts with hydroxyl ions found in the troposphere and stratosphere to form water vapor and carbon dioxide. Volcanoes released methane into the atmosphere when they erupted. There were fewer eruptions during the Proterozoic Eon, so less methane was released into the atmosphere.
The increase in oxygen in the environment caused one of the greatest mass extinction events in Earth’s history. The anaerobic prokaryotes that had dominated Earth during the Archean Eon couldn’t survive with oxygen in the atmosphere. Anaerobic means without oxygen. Oxygen was toxic to these early creatures, and most of them died. However, the new atmosphere was necessary for more complicated life to evolve.
During the Proterozoic Eon, life was concentrated in the shallow seas around the continents. The water was shallow enough for sunlight to reach photosynthetic organisms, and nutrients floated through the water. The single-celled organisms of the Proterozoic Eon couldn’t move to get to food, so they had to wait for food to come to them.
Around 2.3 billion years ago, the first eukaryotes formed. Eukaryotes are more complicated than prokaryotes. Prokaryotic cells only have an outer membrane with DNA floating around inside. Eukaryotes have an outer membrane too, but inside the membrane are organelles. Organelles are different parts of the cells that each have their own membranes and a specific job within the cell. Eukaryotes are more complex than prokaryotes.
Scientists believe that the first eukaryotes formed when one larger prokaryote absorbed a smaller prokaryote. The smaller prokaryote kept making energy inside the larger prokaryote. This gave the larger prokaryote extra energy that allowed it to develop organelles. These changes happened over hundreds of millions of years.
Prokaryotes reproduce by making exact copies of themselves. So, when the larger prokaryote made a copy of itself, the copy also had the extra prokaryote inside of it. Over time, the cell developed organelles and became a eukaryotic cell.
We think that the smaller prokaryotes inside the larger prokaryotes became the mitochondria inside each cell. The mitochondria’s job is to make energy for the cell. Plus, the DNA in the mitochondria is different than the DNA inside the rest of the cell. The DNA from the smaller prokaryote would have been different than the DNA from the larger prokaryote. We believe that chloroplasts in plant cells formed the same way.
At first, all eukaryotes were single-celled, just like the prokaryotes. Over hundreds of millions of years, the single-celled eukaryotes began to live together in groups. Then, the differentorganisms in each group began doing different jobs for the group. Eventually, the single-cells organisms merged to become one organism made up of many cells.
Scientists believe that eukaryotes were able to evolve more quickly than prokaryotes because eukaryotes reproduce using meiosis instead of mitosis. Mitosis is making an exact copy. Only one parent is needed for mitosis. Meiosis requires two parents. DNA from both parents is combined to form a new organism. The new organism has different DNA than the parents. This diversity led to faster changes in the organisms.
The first multi-cellular organisms were probably sponges. Sponges have many cells, but they don’t have a mouth, a heart, muscles, a skeleton or a brain. These first multi-cellular creatures appeared about 600 million years ago. Other multi-cellular organisms developed during the Proterozoic Eon including jellyfish, flatworms, soft corals, sea anemones and seaweed.
Interestingly, as multicellular organisms appeared, stromatolites disappeared. By 540 million years ago, stromatolites were almost totally gone. This is most likely because eukaryotes began eating the cyanobacteria that formed the stromatolites. Today, the only stromatolites that exist are those that live in a place that other animals can’t live. The stromatolites of Shark Bay in Australia are protected by the extremely salty water around them. The disappearance of the stromatolites suggests the appearance of new life on Earth.
Earth’s crust is not smooth. It is broken up into pieces, like the pieces of a puzzle. These pieces are constantly moving around the Earth. The pieces are called plates. The plates are being pushed by convection currents in the liquid magma of the mantle. This movement of the plates is called plate tectonics. Plate tectonics, as we know it today, began during the Proterozoic Era.
The Earth’s plates are covered by both water and land. Large masses of land are called continents. The continents on Earth looked very different during the Proterozoic Era than they do today, but the core of the continents is the same. These are called cratons. The cratons of today’s continents are over one billion years old.
As Earth’s plates move, the continents move too. Sometimes the continents come together to form a supercontinent, and other times they move apart to form separate continents, like we have today. The first supercontinent occurred during the Archean Eon. It was called Kenorland. Over millions of years Kenorland broke apart. During the Proterozoic Eon, two supercontinents formed and broke apart. Columbia formed around 2 billion years ago, and Rodinia formed around 1 billion years ago.
The movement of the continents is very slow. Our continents are moving right now! North America and Asia are moving closer to each other at a rate of about 3 inches per year. Someday, North America and Asia will come together to form a larger continent.
Before the Proterozoic Eon, the Earth was much hotter than it is today. Heat from the formation of the Earth was trapped by the carbon dioxide and methane in the atmosphere. However, during the Proterozoic Eon, oxygen replaced methane in the atmosphere and the planet was cooling.
There were several ice ages during the Proterozoic Eon. During an ice age, water on Earth is frozen into ice in the form of glaciers. These glaciers can reach the warmest area on Earth, the equator. Around 700 million years ago, scientists believe an ice age was so intense that the entire planet became a huge ice ball.
The End of the Eon
The line between the Proterozoic Eon and the Cambrian Eon is based on the development of life on Earth. None of the Proterozoic Eon organisms had a skeleton. The Proterozoic Eon ends, and the Cambrian Eon begins about 542 million years ago as the organisms on Earth develop skeletons.