Final answer:
The sequence of hominin ancestors leading to modern humans is difficult to determine due to a complex, branching evolutionary tree, simultaneous existence of multiple hominin species, and misleading appearances. Molecular evidence, such as mtDNA and Y chromosome data, provides some clarity, although it still requires interpretation amidst gaps in the fossil record and historical accounts.
Step-by-step explanation:
Difficulties in Determining Hominin Ancestry
Understanding the direct evolutionary line of hominins leading to modern Homo sapiens is challenging because multiple hominin species often existed simultaneously, and many fossils represent extinct lineages not directly ancestral to us. The discovery of new fossils has necessitated discarding old theories and acknowledging a bushier, less linear evolutionary tree.
Researchers employ phylogenetic trees to map evolutionary history, though appearances can be misleading, and two closely related species may look very different due to diverse environmental pressures. Moreover, molecular evidence such as mtDNA and the Y chromosome corroborates that all modern humans share common ancestors from Africa, dating back 160,000 and 140,000 years ago, respectively.
Despite advances in research, gaps in the fossil record and uncertainties in historical timelines represent limitations, and thus our understanding is sometimes built on educated speculation and extrapolation regarding the speciation and migration of early humans.
For a deeper understanding of how appearances may lead to incorrect assumptions about phylogenetic relationships, one can reference resources like the OpenStax College page on relationships between organisms, though specific external links cannot be provided here.