Because they often survive at extreme temperatures, are thought you have been the first organisms on the early earth

Extremophiles

Explanation:

Extremophiles are organisms that have been discovered on Earth that survive in environments that were once thought not to be able to sustain life. These extreme environments include intense heat, highly acidic environments, extreme pressure and extreme cold. Different organisms have developed varying ways of adapting to these environments, but most scientists agree that it is unlikely that life on Earth originated under such extremes.

there really is life in the deepest part

as the gila monster consumes the eggs or young of the desert cottontail and gambel's quail, thus its main food supply is only accessible for a restricted time duration every year. however, for this, the gila monsters exhibit three adaptations, which elevate their survival rate in the desert condition.  

first, is that they possess huge bodies, and can consume greater concentrations of food during a small period. secondly, they require to feed infrequently as they can accumulate fat in their tails. ultimately, their rate of metabolism is very low, so that they can burn their reservoirs of fat gradually.