Final answer:
The first organisms on Earth were simple, unicellular organisms supported by evidence from fossil records, genetic evidence, and the study of modern prokaryotes.
Step-by-step explanation:
The first organisms on Earth were simple, unicellular organisms. Scientists know this through various lines of evidence, including fossil records and genetic evidence. The fossil record shows that prokaryotic cells, which are single-celled organisms without nuclei, were the first organisms on Earth, dating back approximately 3.5 billion years. Genetic evidence further supports this by indicating that eukaryotic cells, which are more complex and have nuclei, emerged approximately 2.1 billion years ago.
Additionally, advancements in technology have allowed scientists to identify and study small, relatively simple single-celled organisms known as prokaryotes that still exist today. These organisms resemble the first life forms and provide further evidence for the initial simplicity of early organisms.
Overall, the combination of fossil records, genetic evidence, and the study of modern prokaryotes supports the notion that the first organisms were simple, unicellular organisms.