Evolutionary biology hypothesizes the existence of a single form of life that is the ancestor of all life currently on Earth: the last universal common ancestor (LUCA). This is supported by the fact that all modern life shares the same amino acids (from a much wider possible range). In fact, by finding genetic commonalities and patterns across a wide range of lifeforms we can hypothesize that this ancestor probably lived on a hydrothermal vent.
Note, though, that it was not the first life, nor was it the only life at the time; it’s just the last lifeform that we’re all descended from. Note also that we can guess at what this lifeform may have been, but it’s all based on inference rather than (for example) a specific fossil.