Earth's mantle plays an important role in the evolution of the crust and provides the thermal and mechanical driving forces for plate tectonics. Heat liberated by the core is transferred into the mantle where most of it (> 90%) is convected through the mantle to the base of the lithosphere.
A community which is a ecosystem
They would not be able to survive in an environment without sunlight. For instance, they would not be able to survive in caves, because they need sunlight for food and if there is no sunlight then they can not create food and would starve.
A vestigial organ, something that has yet to fall prey to evolution, but no longer serves a purpose, or the purpose it used to serve.
