From Single Cells to Humans
The emergence of life began 4 billion years ago in the form of single-cell organisms. It proceeded in three different directions called domains, which are the top classification levels. They included different types of algae that became the plant kingdom; various molds or the fungus kingdom; and amoeba-like organisms that evolved into the animal kingdom. Because the mechanisms involved, such as mutation and natural selection, had long periods to work with, each new development along the evolutionary cycle took tens of millions of years. Matt Baker of Useful Charts explains the process.
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