Final answer:
The earliest discovered fossils are of prokaryotes dating back to 3.5 billion years ago, including stromatolites and microbial mats with evidence of photosynthesis, setting the stage for later life forms. The correct option is C.
Step-by-step explanation:
The earliest discovered fossils are of prokaryotes dating back to 3.5 billion years ago. These prokaryotic life forms, which include ancient stromatolites and microbial mats, are some of the most primitive organisms found on Earth.
They indicate that life began on our planet at least a billion years after its formation. The fossil record demonstrates complex prokaryotic communities that were capable of photosynthesis, pointing to their significant role in shaping Earth's atmosphere and paving the way for later evolutionary developments.
The first fossils of eukaryotes, which are organisms containing complex cells with membrane-bound organelles, appear about 2.1 billion years ago. One of the oldest eukaryotic fossils is a red alga dating to approximately 1.2 billion years ago. However, eukaryotic cells are thought to have originated around 2 billion years ago. Prokaryotic and eukaryotic cells eventually evolved into more complex life forms, including plants, animals, and fungi.