Final answer:
Life on Earth originated in water around 3.5 to 4 billion years ago, with single-celled organisms that gave way to multicellular life and then the Cambrian Explosion.
Step-by-step explanation:
Life on Earth began in water and is thought to have evolved there for over 3 billion years before it began spreading onto land. The evolution of life started with the formation of the first simple, single-celled organisms approximately 3.5 to 4 billion years ago. These organisms were probably similar to bacteria and existed in warm nutrient-rich seas, utilizing the organic chemicals available. Over time, these simple forms of life began the long evolutionary path that led to the diverse range of organisms presently on Earth.
Eventually, life began to significantly alter the planet's chemistry, particularly when cyanobacteria evolved to perform oxygenic photosynthesis. This process was critical for the accumulation of atmospheric oxygen and paved the way for more complex, aerobic life forms. Around 610 million years ago, the first multicellular organisms emerged, and during the Cambrian Explosion, which occurred approximately 540 million years ago, a rapid diversification of life forms took place, leading to the emergence of a variety of complex organisms.