What does the term "missing link" refer to in human evolution?

The term "missing link" refers to transitional fossils that demonstrate evolutionary connections between different species or groups, particularly in the context of human evolution. These fossils provide critical evidence of how one species may have evolved into another, highlighting the gradual changes that occur over time in the fossil record.

Transitional fossils often exhibit characteristics that are intermediate between ancestral forms and their more derived descendants, thus illustrating the evolutionary process. For example, fossils like Ardipithecus, Australopithecus, and various species of early Homo show a mixture of traits from both Australopithecus and modern humans, helping to fill gaps in the understanding of how humans evolved from earlier primates.

The other terms suggest different concepts that don't align with the traditional use of "missing link." For instance, an extinct species with no known descendants does not provide evidence of a transitional form. Similarly, while the last common ancestor of all primates is an important concept in understanding primate evolution, it does not specifically represent the transitional fossils that connect ancestors to modern humans. A genetic defect associated with hindering evolution is unrelated to the concept of transitional forms or the evidence they provide for evolutionary change.