Final answer:
The status of Orrorin tugenensis and Ardipithecus kadabba as hominins is uncertain because their skeletal features suggest bipedalism and human-like traits, but conclusive evidence is lacking. Orrorin is nicknamed "Millennium Man," and its discovery suggests possible revisions in the human evolutionary tree. More research is needed to determine their exact relation to modern humans.
Step-by-step explanation:
We are not certain that Orrorin tugenensis and Ardipithecus kadabba were hominins because the evidence is not yet conclusive. Skeletal features of Orrorin, such as a larger femoral head and thicker dental enamel, suggest bipedalism and human-like traits, which are characteristics used to identify hominins. Similarly, features of Ardipithecus, including bipedalism, small canines, and an opposable big toe, indicate a potential place in the human lineage. However, the exact relationship of both species to modern humans is still under investigation due to limited fossil specimens and the presence of both primitive and derived characteristics within each species.
Orrorin tugenensis, which is approximately 6 million years old, has been nicknamed "Millennium Man" due to its age. This discovery raised the possibility that the australopithecines might not be direct ancestors of modern humans if Orrorin is indeed an earlier ancestor. More specimens and further research are necessary to clarify their place in human evolution.
Ardipithecus ramidus, dated to about 4.4 million years ago, is the first discovered hominin species from the Pliocene era. Ardipithecus' bipedalism and other traits such as thin dental enamel and reduced canine size contribute to the debate about the sequence of hominin ancestors leading to Homo sapiens.