Final answer:
Determining the sequence of hominin ancestors leading to Homo sapiens is complex due to an incomplete fossil record, the presence of a mixture of primitive and derived traits in fossil remains, and the existence of multiple hominin species with overlapping time frames.
Step-by-step explanation:
The question asks about the difficulties in determining the sequence of hominin ancestors that led to modern Homo sapiens. There are indeed significant challenges in this area of study because the evolutionary record is incomplete and the identification of direct lineage is often complicated by a mixture of traits in fossil specimens. For instance, Ramapithecus was once considered to be a close human ancestor due to its dentition that seemed to resemble that of humans. However, more complete fossils and further analyses led to the conclusion that it was more likely to be related to the orangutan lineage.
Similarly, while Ardipithecus ramidus exhibited traits such as reduced canines and bipedality suggesting a close relation to humans, the discovery of other early hominins like Sahelanthropus tchadensis and Orrorin tugenensis indicate a more complex evolutionary picture. Sahelanthropus, for example, lived around 7 million years ago and has been suggested to be a common ancestor to humans and chimpanzees, but its bipedality is questioned. Australopithecus afarensis shows a blend of ape-like traits and human-like bipedalism as well, which illustrates the complexity of the evolutionary paths.
Moreover, since various hominins express a combination of both primitive and derived characteristics, it's difficult to establish a clear lineage that conclusively connects them to Homo sapiens. Today, researchers rely on an array of techniques including morphological comparisons, dental patterns, and genetic analysis to piece together the puzzle of human evolution.