Are all areas of the earth inhabitable?

Surprisingly yes. Snails can actually live at 50 degrees below zero. Some one-celled animals can live in water that is 120 degrees. People also live in Siberia where he temperature reaches negative 75 degrees as well as the desert where temperatures can exceed 125 degrees. Fish can live in the ocean deeps where people cannot travel. Llamas and goats can climb high on mountains and live where only mosses grow. Kangaroo rats and camels inhabit the hottest deserts. Seals and penguins can handle frozen polar regions and oceans. Almost each patch of soil in the world contains worms and insects. Every drop of water has microscopic plants and animals in it as well. 

With the exception of the frigid North and South poles and complete barren portions of mountains and deserts their are animals everywhere.