Final answer:
Orrorin tugenensis has features indicative of bipedalism, such as a large femoral head, which could classify it as a hominin. The diversity of traits and the overlap among early hominin fossils make the classification and determination of the human evolutionary sequence complex.
Step-by-step explanation:
The question about whether or not Orrorin tugenensis should be classified as a hominin revolves around the traits that define this group. The definition of hominins typically includes characteristics such as bipedalism and certain dental and cranial features. However, there are reasons to be skeptical about classifying Orrorin as a hominin. The most compelling evidence that Orrorin might be a hominin includes anatomical traits like its large femoral head and thick dental enamel. These features could indicate that Orrorin was bipedal like modern humans and, accordingly, part of the hominin family.
However, some scientists point to the need for a broader range of traits to classify a genus under Homo. This includes brain size, skull shape, and evidence of tool use, as proposed by paleoanthropologists such as Mary Leakey, Louis Leakey, and John Napier. Despite its ancient lineage and human-like traits, some features of Orrorin, such as its teeth, are more similar to that of australopithecines. Thus, the direct ancestral role of Orrorin in the human lineage is a topic of ongoing research and debate among scientists.
Determining the sequence of hominin ancestors leading to modern Homo sapiens is difficult due to the complexity and diversity of early hominin fossils. Many of these ancient species, such as Homo habilis and H. floresiensis, exhibit a mixture of primitive and derived features, which complicates their classification. For instance, while H. habilis had a larger brain size than australopithecines, its postcranial features are not as well established. Similarly, H. floresiensis displayed a unique form of bipedalism and larger brain size relative to its body size but did not exhibit other defining human-like traits such as a chin.