* Oxygen is essential for cellular respiration: Our cells need oxygen to produce energy. This process, called cellular respiration, converts glucose (sugar) into ATP, the molecule our bodies use as fuel. Without oxygen, our cells would quickly die.

* Earth's atmosphere contains oxygen: Around 21% of the Earth's atmosphere is composed of oxygen, which is a byproduct of photosynthesis by plants.

* Humans have evolved to breathe air: Over millions of years, humans have developed respiratory systems (lungs, diaphragm, etc.) perfectly adapted to take in oxygen from the air and expel carbon dioxide, a waste product of cellular respiration.

To put it simply, humans breathe on Earth because:

1. We need oxygen to survive.

2. Earth's atmosphere has oxygen.

3. We are built to breathe air.