Have you ever been to the Natural Museum of Natural History? I, Jeong Keun, went to the Museum of Natural History and I learned about the Universe and the history of living things. I shall now tell you about the four Eras of living things.

 The first era was the Precambrian Era. It was very important because there were cyanobacteria. They made oxygen by photosynthesis using the sun. The oxygen allowed for new things to exist and they breathed this oxygen. I am interested in 'EXTREMELY' important cyanobacteria. The reason why I think like this is two reasons: there would be no people without cyanobacteria, and there were living things which make oxygen such as plants, in the Precambrian Era.

 The next Era is the Paleozoic Era. In this Era, there were plants that made oxygen like cyanobacteria. An example of one of these plants is bracken, which is known for relieving constipation. Also, there were animals living in the sea such as ostracoderms and trilobites.

 In the Mesozoic Era, as you may know, dinosaurs lived. You should know that dinosaurs are divided into three types-carnivores, herbivores, and omnivores. Carnivores are meat-eating and have sharp teeth. An example of a carnivore is the acrocanthosaurus, which had saw-like protrusions. Herbivores are plant-eating animals. Stegosauruses had small heads and round chins. Omnivores eat both meat and plants. I hope you'd know what the most well-known omnivore is, yes, people!

 The Cenozoic Era is the most modern of all four Eras; Precambrian Era, Paleozoic Era, Mesozoic Era, and Cenozoic Era. This era was the time of the mammoth at first, but because of the Ice Age with super-cold days, all of the mammoths died and then came the time of humans.

 To sum up, there was the Precambrian Era for oxygen-making cyanobacteria. Second Era, the Paleozoic Era was for bracken, ostracoderms, and trilobites. Next was the Mesozoic Era, the era for dinosaurs-carnivores, herbivores, and omnivores. Cenozoic Era was for mammoths until the Ice Age, but it became humans'. I learned lots of things by traveling to the Museum of Natural History, I even want to go again. Thank you for reading my essay, and I hope you it helped you understand more about the four eras!

Great work, I fixed some sentences so that they sounded more natural, but overall so much great information :)