Consider the following statements:

1. The Earth’s magnetic field has reversed every few hundred thousand years.

2. When the Earth was created more than 4000 million years ago, there was 54% oxygen and no carbon dioxide.

3. When living organisms originated, they modified the early atmosphere of the Earth.

Which of the statements given above is/are correct?

The correct option is 1 and 3 only.

Let us analyze each statement step-by-step to understand the logic behind the solution:

Statement 1: The Earth’s magnetic field has reversed every few hundred thousand years.

This statement is correct. Earth's magnetic field undergoes periodic geomagnetic reversals, where the magnetic north and south poles swap places. Paleomagnetic studies of oceanic crust and rocks show that these reversals happen irregularly, averaging every few hundred thousand years (typically between 200,000 to 300,000 years, though the last major reversal occurred about 780,000 years ago).

Statement 2: When the Earth was created more than 4000 million years ago, there was 54% oxygen and no carbon dioxide.

This statement is incorrect. The primordial atmosphere of the Earth (formed around 4.5 billion years ago) was devoid of free oxygen (O₂). It primarily consisted of gases like water vapor, carbon dioxide, methane, ammonia, nitrogen, and hydrogen. Free oxygen appeared in significant quantities much later through photosynthesis, particularly during the Great Oxidation Event around 2.4 to 2.1 billion years ago.

Statement 3: When living organisms originated, they modified the early atmosphere of the Earth.

This statement is correct. Early photosynthetic organisms, such as cyanobacteria (blue-green algae), began taking in carbon dioxide and releasing oxygen gas as a byproduct. Over hundreds of millions of years, this biological activity fundamentally altered the composition of Earth's atmosphere, converting it from a reducing atmosphere to an oxygen-rich atmosphere capable of supporting complex aerobic life.

Conclusion:

Since statements 1 and 3 are correct while statement 2 is incorrect, the correct choice is 1 and 3 only.