Final answer:
The first forms of life on Earth were prokaryotic cells, specifically bacterial and archaean prokaryotic cells, which emerged around 3.5 billion years ago. They are characterized by the lack of a nucleus and membrane-bound organelles.
Step-by-step explanation:
The First Forms of Life on Earth
The first forms of life on Earth were thought to be prokaryotic cells. This includes both bacterial and archaean prokaryotic cells. The fossil record and genetic evidence suggest that these cells emerged approximately 3.5 billion years ago, about 1 billion years after the Earth formed. Prokaryotic cells were the only life forms on our planet until the appearance of eukaryotic cells some 2.1 billion years ago. It's important to note that prokaryotic cells do not have a nucleus or other membrane-bound organelles, which differentiates them from eukaryotic cells that contain these structures. The term 'prokaryotes' specifically refers to unicellular organisms that lack a nucleus.
During their time as the sole inhabitants, photosynthetic prokaryotes evolved, which were capable of using sunlight to synthesize organic materials, indicating a significant step in the history of life on Earth. The cell theory does reinforce that all living organisms are composed of one or more cells, and the first organisms would have been adapted to the harsh conditions of early Earth.
Earth's diverse life forms range from unicellular to multicellular organisms, falling under two basic cell plans: prokaryotic and eukaryotic. American microbiologist Carl Woese's 1977 research recognized three life domains based on cell types: Bacteria, Archaea, and Eukarya, where the first two comprise prokaryotic organisms.