It is still unclear as to how life on Earth began. There are many competing theories out there ranging from the Panspermia Theory to the proven incorrect Primordial Soup experiments. One of the newest theories is that life began in hydrothermal vents.
Hydrothermal vents are structures in the bottom of the ocean that have extreme conditions. Since sunlight cannot reach to the depths of these structures, there had to be another energy source for early life that may have formed there. The current form of the vents contain chemicals that lend themselves to chemosythesis -- a way for organisms to create their own energy similar to photosynthesis.
These types of organisms are extremophiles that can live in the severest of conditions. The hydrothermal vents are very hot, hence the word "thermal" in the name. They also tend to be acidic, which is usually harmful to life.
Archaea live and thrive in and near these vents. Since this Domain of life tends to be considered the most primitive of organisms, it is not a stretch to believe they were the first to populate the Earth. Conditions are just right in the hydrothermal vents to keep the Archaea alive and reproducing. With the amount of heat and pressure in these areas, along with the types of chemicals available, life can be created and changed relatively quickly. Scientists have also traced the DNA of all currently living organisms back to a common ancestor extremophile that would have been found in the hydrothermal vents.