Water is considered to have formed on Earth earlier than oxygen in its free form.

Water is considered to have formed on Earth earlier than oxygen in its free form.

### Here's why:

1. **Water Formation**:

   - Water is believed to have appeared on Earth as early as **4.4 billion years ago**, shortly after the planet's formation. It likely came from two sources: **volcanic outgassing** from Earth's mantle (which released water vapor into the atmosphere) and **comets/asteroids** that contained ice. Once the planet cooled enough, the water vapor condensed to form Earth's first oceans.

   - Thus, water existed on Earth before the atmosphere had significant amounts of oxygen.

2. **Oxygen Formation**:

   - Free oxygen (O₂) did not appear in Earth's atmosphere in significant quantities until the **Great Oxidation Event**, which occurred around **2.4 billion years ago**. Oxygen was initially produced by **photosynthetic cyanobacteria**, but for hundreds of millions of years, most of this oxygen was absorbed by minerals like iron in the oceans. Only when these "oxygen sinks" were saturated did oxygen begin to accumulate in the atmosphere.

### Timeline Summary:

- **Water**: Appeared approximately **4.4 billion years ago**, soon after Earth's formation.

- **Oxygen**: Appeared in significant amounts around **2.4 billion years ago**, after the Great Oxidation Event.

Therefore, **water** is the "senior" substance that formed earlier on Earth compared to **free oxygen**.