Based on the information provided, fill in the blank to correctly describe the evolutionary relationship between human beings and chimpanzees.

Human beings and chimpanzees had a [[1]] ancestor a long time ago.

The evolutionary relationship between human beings and chimpanzees is that we share a common ancestor from the past. This common ancestor diverged into two different evolutionary paths that led to the emergence of humans and chimpanzees as separate species. The similarities between humans and chimpanzees, including the genetic makeup and physical characteristics, are due to their shared ancestry. Therefore, the correct word to fill in the blank is "common."