May 2016 - By two and a half billion years ago, when the Archaean Eon ended and the Proterozoic Eon began, trillions of prokaryote cells lived in Earth's oceans. Some of these cells could photosynthesize their energy from sunlight, water, and carbon dioxide.
These photosynthesizing cells pooped out a lot of oxygen, and by about 2.3 billion years ago, the iron and sulphur rocks of Earth had soaked up all the oxygen they could use. Now there began to be more and more left-over oxygen floating around in Earth's atmosphere.
With all this oxygen in the atmosphere, by two billion years ago a few cells had evolved that could use oxygen to get energy. Eukaryote cells, while not evolving to be able to use oxygen or photosynthesize themselves, captured smaller cells that could and worked with them. Gradually these smaller cells lost the ability to live independently and turned into mitochondria and chloroplasts. The eukaryotes got more and more complicated. They also evolved to be able to reproduce through meiosis, with both a father and a mother, instead of only by cell division (mitosis).
A billion years later, one billion years ago, plate tectonics brought the continents together into one big supercontinent we call Rodinia, which then broke apart, so that the pieces soon began to float away from each other again. During this time, also, there were several Ice Ages, when all of Earth was much cooler than it is today, and a lot of the water in the oceans turned to ice. Possibly for a while about 700 million years ago the whole Earth may have been one big ball of ice, but then it soon melted again.
Meiosis, or sexual reproduction, gave cells a lot more diversity in their DNA, and the diversity let evolution happen more quickly. So it was only a little more than a billion years after meiosis began that the first creatures developed that had more than one cell - they were something like hydras. That was about 600 million years ago. Around the same time, the first divisions between animals, plants, and funguses (like mushrooms) happened. Soon there were sponges and jellyfish and flatworms in addition to hydras, and the first multi-cellular plants like seaweed, as well.
The end of the Proterozoic, about 542 million years ago, is roughly the time when the first segmented worms and arthropods - insects like beetles - appear on Earth. The next period is the Cambrian Era.
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