Final answer:
Language is the derived homology that expands a cladogram to include humans with gorillas, orangutans, and chimpanzees. While other traits are shared earlier in evolutionary history, the complex language ability is unique to humans and indicative of our lineage within Hominidae.
Step-by-step explanation:
To include humans in a cladogram with gorillas, orangutans, and chimpanzees, the derived homology that would expand the cladogram is language. While traits such as hair, mammary glands, skull, and backbone are also present in humans and relate to earlier evolutionary adaptations shared with other animals, language is a derived trait that distinguishes the human lineage within the Hominidae.
It is an aspect of human evolution that, while other members of Hominidae can develop forms of communication which may resemble and are precursors to language, the sophisticated and complex language skills are unique to Homo sapiens and represent a significant aspect of human evolution.
The family Hominidae includes the great apes and humans, and cladistics is based on shared derived characteristics. Evidence from both the fossil record and DNA comparisons suggest that humans and chimpanzees diverged from a common ancestor about six million years ago. The hominin line, which are species more closely related to humans than to chimpanzees, exhibits certain features such as increased brain size, bipedalism, and an opposable thumb.