The first cells appeared about ___ billion years ago.

Question

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The first cells appeared about 3.5 billion years ago, as evidenced by fossilized stromatolites, which are layered microbial mats found in ancient rocks. Option A (5 billion years ago) is incorrect, as this is around the time when the Earth was formed, and life had not yet been established. Option B (4.5 billion years ago) is incorrect, as this is the estimated age of the Earth's crust, and once again, life had not yet emerged. Option D (2 billion years ago) is too recent, as there is evidence of prokaryotic microorganisms that are older than this. Finally, option E (1 billion years ago) is also too recent, as this is around the time that the first complex multicellular organisms evolved.

The First Cells Appeared About ___ Billion Years Ago