Final answer:
Earth 4 billion years ago was cooling from a molten state with no atmosphere or oceans, whereas Earth 550 million years ago had established atmospheres, oceans, and burgeoning multicellular life following the Cambrian Explosion.
Step-by-step explanation:
Comparing Earth 4 billion years ago to Earth 550 million years ago involves looking at how our planet has developed over time. 4 billion years ago, Earth was just cooling down from its initial molten state. It lacked an atmosphere and oceans, but over time, these formed, allowing for the eventual emergence of life. By around 3.5 to 3.8 billion years ago, simple cells known as prokaryotes emerged, and after another half billion years, photosynthesis began to occur, drastically altering the atmosphere and setting the stage for more complex life.
By 550 million years ago, Earth had undergone significant changes. This period is at the tail end of the Precambrian era and the beginning of the Phanerozoic eon. It was also the time of the Cambrian Explosion, a major event in which a plethora of life forms, including many multicellular organisms, appeared within a relatively short geological period. At this point, Earth had established atmospheres, oceans, and continents teeming with life, albeit primitive compared to today.
In summary, over the course of nearly 3.5 billion years, Earth cooled from a molten state, developed an atmosphere and oceans, and went from hosting simple single-celled organisms to a planet bustling with complex multicellular life.