Vital to the idea of Darwinism is the concept of a common ancestor. According to evolutionary biology, all life on earth descended, over billions of years, from a single life form, a molecule with the capacity to replicate itself. Thus, all life on earth has one common ancestor from which each tributary of the genetic river sprung.
If we imagine the progression of genes through time, with each separate path of the river representing a species, then the branching point between two paths can be called the common ancestor. Basically, a group of organisms belonging to one species (defined as a group capable of reproducing to produce fertile young) is divided into two subgroups, usually by geographic seperation. No longer kept genetically equivalent, the two groups will diverge to the point that they can no longer interbreed. Each will have become a new species and the species from which both are descended can be referred to as their common ancestor.
Thus, looking back through time, we can find the point of convergence between all genus', families, orders, classes, phyla, and kingdoms of plants, animals, and monerans (bacteria). This process of branching is fascinating when contemplated as a massive network of genes, cooperating for survival, but concerned only with their own selfish needs.