* Biosphere: This is the layer of Earth where all living things exist. It encompasses the atmosphere, hydrosphere, and the upper portion of the lithosphere where life thrives.

* Lithosphere: This is the Earth's rigid outer layer, composed of the crust and the uppermost part of the mantle. It provides the solid foundation for life, supplies minerals and nutrients, and stores water that living organisms need.

While other layers like the hydrosphere (water) and atmosphere (air) are vital for life, the biosphere and lithosphere are the two most directly relied upon for sustenance and habitat.