Here's why:

* Early Earth's atmosphere was primarily composed of:

* Carbon dioxide (CO2)

* Methane (CH4)

* Ammonia (NH3)

* Nitrogen (N2)

* Photosynthesis evolved: Around 3.5 billion years ago, the process of photosynthesis emerged, allowing certain organisms to harness energy from sunlight and release oxygen as a byproduct.

* The Great Oxidation Event: The gradual increase in oxygen levels over millions of years, known as the Great Oxidation Event, began around 2.5 billion years ago.

* Consequences of oxygen: The rise of oxygen had a profound impact on Earth, leading to the extinction of many anaerobic organisms and paving the way for the evolution of more complex life forms that could use oxygen for respiration.

So, while oxygen is essential for life as we know it today, the Precambrian era was largely oxygen-free.