What was the only type of life on the planet for hundreds of millions of years?

For hundreds of millions of years, the only type of life on Earth was unicellular organisms, primarily bacteria and archaea. These simple single-celled organisms dominated the planet during the Precambrian era, thriving in various environments and playing crucial roles in biogeochemical cycles. Multicellular life did not emerge until around 600 million years ago, marking a significant shift in the complexity of life on Earth.