Final answer:
Life on Earth started very early in the planet's history, with single-celled organisms appearing around 4 billion years ago. These were the most complex life forms for about 1 billion years before more diverse life evolved.
Step-by-step explanation:
The question regarding when life appeared on Earth can be answered by looking at our planet's long history. The earliest living things on Earth, believed to have emerged around 4 billion years ago, were single-celled organisms. For about 1 billion years, these simple forms of life were the most complex living entities on Earth. The dominant forms were Bacteria and Archaea, and it is still debated which of these two groups was the first to appear. Over Earth's vast history, life continued to evolve, with multicellular organisms emerging about 610 million years ago during a period known as the Cambrian Explosion.
Comparing these timelines, humans are a recent addition to Earth's history, having appeared only within the last 2.5 million years. Therefore, the correct answer to the question 'Life appeared on Earth ________.' would be option 5) very early in Earth's history. This acknowledges the billions of years that microorganisms dominated before complex life forms gradually evolved.