Final answer:
Prokaryotes were the earliest forms of life on Earth, existing for billions of years before plants and animals appeared. Fossil evidence of microbial mats suggests their presence about 3.5 billion years ago.
Step-by-step explanation:
The earliest forms of life on Earth were prokaryotes. These single-celled organisms are the simplest form of life and existed for billions of years before plants and animals appeared.
Microbial mats are thought to represent the earliest forms of life on Earth, and there is fossil evidence of their presence about 3.5 billion years ago.
Prokaryotes like bacteria and archaea were the dominant life forms on Earth until eukaryotic cells emerged approximately 2.1 billion years ago.