The fossil record is incomplete, and there are periods of time for which we do not have any direct fossil evidence. This means that we have to rely on indirect evidence, such as genetic analysis and comparative anatomy, to infer the evolutionary relationships between different species.
Additionally, the evolution of humans is a complex process that involves many different factors, including natural selection, genetic drift, and environmental changes. This means that it is not always possible to draw a clear and direct line between different hominid species.
Despite these challenges, scientists have made significant progress in understanding human evolution. We have a good understanding of the major hominid species that existed during the past few million years, and we are constantly learning more about their biology and behavior. As more fossils are discovered and new research is conducted, our understanding of human evolution will continue to improve.