Question

Describe It is thought that life could not have existed on land until the ozone layer formed about 2 billion years ago. Why does life on land require an ozone layer?

Short answer

Life requires the ozone layer to be shielded from harmful UV radiation from the sun.

Step-by-step solution

Understanding the Role of the Ozone Layer

The ozone layer plays a critical role in protecting life on Earth by blocking harmful ultraviolet (UV) radiation from the sun. Without this layer, the surface of the Earth would be exposed to excessive UV radiation.

Impact of Ultraviolet (UV) Radiation

UV radiation from the sun can be harmful to living organisms as it can cause damage to DNA and other critical cellular structures. High levels of UV radiation inhibit cellular function and can prevent life from thriving.

Formation of the Ozone Layer

The ozone layer is believed to have formed from oxygen molecules (O2) which were photodissociated by solar UV radiation, leading to the formation of ozone (O3). This process increased the atmospheric concentration of ozone, creating a protective shield.

Protection Offered by Ozone Layer

The presence of the ozone layer means that only a reduced amount of UV radiation reaches the Earth's surface. This reduction makes the environment more hospitable for life by allowing biological processes to occur without being disrupted by harmful radiation.

Evolution of Life on Land

The decreased levels of UV radiation after the formation of the ozone layer allowed life, which was until then restricted to underwater or protected areas, to colonize land. Organisms were able to evolve mechanisms for surviving and thriving in these less hostile environments.

Key concepts

Ultraviolet (UV) Radiation

Ultraviolet (UV) radiation is a type of electromagnetic radiation emitted by the sun. It is invisible to the naked eye and is more energetic than visible light, making it potentially harmful to living organisms. When UV radiation reaches Earth, it has the power to damage cellular structures, including DNA, which can lead to mutations or cellular death. Such damage can hinder the ability of life to thrive, as essential biological processes can be interrupted or impaired. As a result, high levels of UV radiation make it tough for organisms to survive on land without protection. A natural shield against this radiation is crucial for sustaining life.

Formation of the Ozone Layer

The formation of the ozone layer was a pivotal event in Earth's history, transforming the planet into a more life-friendly place. This layer is made up of ozone (O3) molecules, formed from oxygen (O2) molecules in the atmosphere. The formation process begins when solar UV radiation dissociates oxygen molecules, splitting them into individual oxygen atoms. These free atoms can then bond with other oxygen molecules to form ozone. This transformation significantly increased the concentration of ozone in the atmosphere, resulting in the creation of the ozone layer. This protective layer absorbs and reduces the amount of harmful UV radiation reaching Earth's surface, thus providing a safer environment for life to develop and evolve on land.

Evolution of Life on Land

The evolution of life on land became possible due to the protection offered by the ozone layer from excessive UV radiation. Prior to the formation of the ozone layer, life was likely limited to aquatic environments where water acted as a natural UV filter. With the ozone layer in place, the level of UV radiation reaching the surface decreased significantly. This reduction in UV exposure allowed life forms to inhabit terrestrial environments. Organisms began evolving various adaptations to thrive on land, such as developing protective structures and mechanisms to conserve water and shield against residual UV radiation. The availability of this more welcoming environment paved the way for a broader diversity of life forms and the evolution of complex ecosystems on land.