Kaplan Nursing Entrance Exam Practice 2025 – Comprehensive Prep

The first organisms believed to have existed on Earth are prokaryotes. These simple, unicellular organisms lack a defined nucleus and other membrane-bound organelles, making them some of the earliest forms of life. Fossil evidence indicates that prokaryotes, such as bacteria and archaea, emerged approximately 3.5 billion years ago, which aligns with the earliest conditions known on Earth.

While autotrophs and heterotrophs are classifications based on how organisms obtain their energy and nutrients, both of these categories stem from more complex forms of life than prokaryotes. Autotrophs can produce their own food through processes such as photosynthesis, while heterotrophs rely on consuming other organisms. However, the earliest life forms were unicellular prokaryotes that could utilize various chemical processes to generate energy.

Eukaryotes, characterized by their more complex cellular structure and the presence of a nucleus, evolved later in the Earth's history from prokaryotic ancestors. Therefore, prokaryotes are recognized as the first organisms, representing a crucial development in the evolution of life on Earth.