Final answer:
The appearance of multicellular organisms around 600 to 700 million years ago is believed to be the most important event in the evolution of complex multicellular life on Earth.
Step-by-step explanation:
The most important event in the evolution of complex multicellular life on Earth, according to Nick Lane, is the appearance of multicellular organisms around 600 to 700 million years ago. These organisms, known as Ediacarans, were flat and soft-bodied creatures that marked a significant milestone in the history of life on our planet. This event paved the way for the development of more complex and diverse life forms.