Final answer:
Determining the sequence of hominin ancestors is difficult due to the lack of complete fossil evidence and the complex interbreeding patterns revealed by genetic research, which show that multiple species existed simultaneously and not all contributed to modern human lineage.
Correct option is a) Due to the lack of available fossil evidence.
Step-by-step explanation:
It is so difficult to determine the sequence of hominin ancestors that have led to modern Homo sapiens due primarily to the lack of available fossil evidence. The evolutionary history of humans is complex, with multiple hominin species often existing at the same time and not all species contributing to the lineage of modern humans. This makes it challenging to trace a direct line of descent. Additionally, the fragmentary fossil evidence requires paleoanthropologists to construct hypotheses around the incomplete information they have, often revising the human family tree as new discoveries are made.
Furthermore, genetic research has complicated the picture by revealing that modern Homo sapiens have interbred with other hominin species, such as Neanderthals and Denisovans. This interbreeding has left traces in the human genome that provide insights into our past but also make it harder to define clear lines of ancestry. The field continues to evolve as new fossils are discovered and genetic technologies advance, allowing scientists to piece together more of humanity's complex evolutionary puzzle.