"Ur-organism" is the term loosely given to the hypothetical "first life" species, from which all other life presumably evolved. The term was used first by Charles Darwin, and has been picked up by various subsequent thinkers addressing the same questions of origin of species. Not to be confused with the Last universal ancestor (LUA), the term "ur-organism" refers to the first universal ancestor common to all life existing on earth today. It is therefore the ancestor of the LUA.
The ur-organism is not necessarily the first instance of life arising abiogenically on Earth. As with the concept of the "Mitochondrial Eve, "the existence of the ur-organism does not imply the existence of a population bottleneck or a first organism. It is estimated to have lived some 3.9 to 4.1 billion years ago.