Final answer:
The oldest known fossils are approximately 3.5 billion years old and belong to ancient life-forms like bacteria and algae.
Step-by-step explanation:
The oldest known fossils are approximately 3.5 billion years old. These fossils belong to ancient life-forms such as bacteria and algae. These organisms are classified as prokaryotes, which means they lack a nucleus and membrane-bound organelles. The discovery of these fossils provides important evidence for the earliest forms of life on Earth and the processes of evolution.