Chapter 14, Problem 21A

Summary Introduction

Introduction:

Scientists did not know what the first cells were like as the first cells left no fossils. The earliest fossils are 3.5 billion years old. Chemical markings in rocks as old as 3.8 billion years suggest that life was present at that time even though no fossils remain. Many scientists think that modern prokaryotes called archaea are the closest relatives of Earth’s first cells.

Answer to Problem 21A

Correct answer:

The correct answer is option C. 3.5 billion years ago

Explanation of Solution

Explanation/justification for the correct answer:

Option C. 3.5 billion years ago − The organisms shown in the given photo are cyanobacteria. Dome shaped structures called stromatolites are formed from cyanobacteria. These are evidence of existence of photosynthetic organisms on Earth during the Precambrian period. The cyanobacteria are primitive prokaryotes that are found in rocks as old as 3.5 billion years. Hence this is the correct option.

Explanation for incorrect answer:

Option A. 1.0 million years ago − The organisms shown in the given photo are cyanobacteria. The cyanobacteria are primitive prokaryotes that are found in rocks as old as 3.5 billion years. Hence this is not the correct option.

Option B. 2.0 million years ago - The organisms shown in the given photo are cyanobacteria. The cyanobacteria are primitive prokaryotes that are found in rocks as old as 3.5 billion years. Hence this is not the correct option.

Option D. 4.5 billion years ago − The organisms shown in the given photo are cyanobacteria. The cyanobacteria are primitive prokaryotes that are found in rocks as old as 3.5 billion years. Hence this is not the correct option.