Chapter 14, Problem 23A

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.

Answer to Problem 23A

Correct answer:

The correct answer is option C. first cells.

Explanation of Solution

Explanation/justification for the correct answer:

Option C. first cells−Scientists have fossil evidence for first cells. They believed that the first cells were prokaryotes. Many scientists think that modern prokaryotes called archaea are the closest relatives of Earth’s first cells. Fossil evidence of primitive prokaryotes called cyanobacteria has been found in rocks as old as 3.5 billion years. Hence, this is the correct option.

Explanation for incorrect answer:

Option A. first amino acids−The cyanobacteria are primitive prokaryotes that are found in rocks as old as 3.5 billion years. The fossil evidence of these organisms indicates that the first cells were prokaryotes. There is no fossil evidence for the first amino acids. Hence this is not the correct option.

Option B. first RNA-The cyanobacteria are primitive prokaryotes that are found in rocks as old as 3.5 billion years. The fossil evidence of these organisms indicates that the first cells were prokaryotes. There is no fossil evidence for the first RNA. Hence this is not the correct option.

Option D. first autotrophs −The cyanobacteria are primitive prokaryotes that are found in rocks as old as 3.5 billion years. The fossil evidence of these organisms indicates that the first cells were prokaryotes. Hence, this is not the correct option.