Final answer:
Life on Earth began with simple single-celled organisms that have evolved over millions of years through natural selection, resulting in the diversity of life forms we see today.
Step-by-step explanation:
Life on Earth began about 3.5 to 4 billion years ago with simple single-celled organisms. Over time, these organisms evolved and diversified, leading to the emergence of more complex life forms. The process of evolution occurs through natural selection, where individuals with advantageous traits are more likely to survive and reproduce, passing on their traits to future generations. Evolution happens over a long timescale, with major changes taking place over millions of years. Small changes, known as microevolution, can accumulate over time to result in large changes, known as macroevolution.