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what is evolution? What is the first living organism?
Evolution can refer to a variety of changes that occur over time—the uplifting of mountains, the wandering of riverbeds, or the creation of new species. To understand the history of life on Earth though, we need to be more specific about what kinds of changes over time we're talking about. That's where the term "biological evolution" comes in.

Biological evolution refers to the changes over time that occur in living organisms. An understanding of biological evolution—how and why living organisms change over time—enables us to understand the history of life on Earth.


 Scientists estimate that the Earth is some 4.5 billion years old. For nearly the first billion years after the Earth formed, the planet was inhospitable to life. But by about 3.8 billion years ago, the Earth's crust had cooled and the oceans had formed and conditions were more suitable for the formation of life. The first living organism formed from simple molecules present in the Earth's vast oceans between 3.8 and 3.5 billion years ago. This primitive lifeform is know as the common ancestor. The common ancestor is the organism from which all life on Earth, living and extinct, descended.

Photosynthesis arose and oxygen began accumulating in the atmosphere about 3 billion years ago. A type of organism known as cyanobacteria evolved some 3 billion years ago. Cyanobacteria are capable of photosynthesis, a process by which energy from the sun is used to convert carbon dioxide into organic compounds—they could make their own food. A byproduct of photosynthesis is oxygen and as cyanobacteria persisted, oxygen accumulated in the atmosphere.


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