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Cyanobacteria became widespread in Earth’s oceans and used photosynthesis to make food. During this process, they released oxygen into the atmosphere. This led to the Great Oxygenation Event, which changed Earth’s atmosphere and allowed oxygen-using organisms to evolve.
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The oldest known fossils are stromatolites, which are layered rock structures formed by colonies of microorganisms. These fossils show that microscopic life lived in shallow ocean waters. Stromatolites provide the first physical proof of life on Earth. -
After Earth formed, it was extremely hot and covered in molten rock. As the planet slowly cooled, a solid crust formed on the surface. Water vapor in the atmosphere condensed into rain, which filled low areas and created the first oceans. These oceans were important because they provided the environment where life could eventually begin. -
During the Cambrian Explosion, a large number of new animal species appeared in a relatively short amount of time. Many organisms developed hard body parts like shells and exoskeletons. Most major animal groups that exist today first appeared during this period. -
Scientists have found chemical evidence in ancient rocks that suggests life existed at this time. Certain carbon patterns found in rocks are usually produced by living organisms. Even though fossils were not preserved, this evidence strongly suggests that life had already begun. -
The first living organisms on Earth were prokaryotic cells, such as bacteria. These cells were very simple and did not have a nucleus or complex structures. They lived in the oceans and were able to survive in harsh conditions, including environments without oxygen. -
Eukaryotic cells evolved and were more complex than earlier cells. They had a nucleus and membrane-bound organelles like mitochondria. Scientists believe these cells evolved through a process called endosymbiosis, where one cell lived inside another. This development allowed for larger and more complex organisms. -
Some eukaryotic cells began living together and cooperating, forming multicellular organisms. This allowed cells to specialize and perform different functions. Multicellular life led to greater complexity and the eventual evolution of plants and animals. -
Plants evolved adaptations that allowed them to live on land, such as roots and waxy coatings to prevent water loss. These early land plants helped form soil and increased oxygen levels. Their presence made it possible for animals to eventually move onto land. -
Dinosaurs were the dominant land animals for millions of years. They lived in a wide variety of environments and came in many different sizes and shapes. Their extinction was caused by a mass extinction event, likely triggered by an asteroid impact, which allowed mammals to rise afterward.