The phrase "single ancestor" means that all living beings or a specific group of organisms in a family are descended or come from a common individual in the past. Full definition
The great apes (man, ape, baboon, chimpanzee, etc.) are all descended form a single ancestor. (religion.blogs.cnn.com)
In fact, each human gene can be traced back to a single ancestor at some point over the last few million years, but different individuals at different times, and this is entirely to be expected statistically. (faith.org.uk)
While researchers had previously thought that ST258 K. pneumoniae strains spread from a single ancestor, the NIH team showed that the strains arose from at least two different lineages. (sciencedaily.com)