Final answer:
The evolution of life on Earth started over 3.5 billion years ago with single-celled organisms. Crucial events include the Cambrian Explosion and mass extinctions that led to significant changes in biodiversity. Key developments such as photosynthesis, multicellularity, and sexual reproduction contributed to the complexity of life.
Step-by-step explanation:
Life on Earth began approximately 3.5 to 4 billion years ago with the emergence of single-celled organisms resembling bacteria. These primitive life forms were the foundation for the evolution of more complex life forms. A significant event in this evolution was the appearance of multicellular organisms around 610 million years ago. The diversification of life rapidly accelerated during the Cambrian Explosion, approximately 600 million years ago, which saw a plethora of new organisms emerge due to conducive environmental changes.
Throughout Earth's history, several mass extinctions have had a profound impact on the course of evolution. Occurring around 450, 360, 250, 205, and 65 million years ago, these extinctions were precipitated by drastic global conditions and sometimes asteroid impacts that completely reshaped the biodiversity of the planet. Notably, the extinction event 65 million years ago is widely credited with the demise of the dinosaurs, allowing mammals to become the dominant life forms.
A number of key developments mark the watersheds of evolutionary history, such as the advent of photosynthesis, the emergence of cells with nuclei, the rise of multicellularity, the onset of sexual reproduction, and the colonization of land by life. The path from simple single-celled organisms to a planet teeming with complex life, including humans, spans over 3 billion years, indicating the vast scale on which evolution operates.