LUCA (Last Universal Common Ancestor).

The modern research of our origins is manly based on the etymology of modern languages so as to reveal fundamental things about their evolution.
It is commonly believed that there was an early RNA world, where early life on Earth was composed of ribonucleic acid (RNA), rather than deoxyribonucleic acid (DNA).
RNA is particularly sensitive to heat and is unlikely to be stable in the hot temperatures of the early Earth, so that it must have found a cooler micro-climate to develop.
Later they discovered the more thermostable DNA molecule, which they independently acquired (presumably from viruses), and used to replace the old and fragile RNA vehicle.
They could so leave the small cool microclimate, evolved and diversify into a variety of sophisticated organisms that could tolerate heat.