Although nothing answers everything completely, I have never questioned the validity of evolution. It surprises me, however, how many people still today, some politicians included unfortunately, do not think that it is a rational explanation of our world. Especially because there is so much evidence that supports it.
As to our knowledge of early Hominin species, while we know quite a bit, there is still a lot to learn. Findings like the Neander Skull, Java Man and especially Lucy have done a lot to further our understanding of where we may or may not have come from. With what we know, we have ascertained that early hominids did, in fact, emerge from Africa and not Asia. By looking at wear patterns on stone and bone tools from the different paleolithic era periods, we have been able to see how the making and use of such tools evolved, which can be connected with cognitive development as well. By looking at animal bones and early Hominin teeth, we are also able to get a sense of what their diets consisted of.
The Hominid diagram demonstrates that we know these beings existed; that they are somehow related and connected to us. It also shows what we do not know in the fading lines with no specific origin. Unlike a modern family tree, not all the lines are concretely connected. While we have a lot of knowledge, more is still needed to fill in the blanks and help explain connections further. I think there is good support to link us to early hominids through both cultural and physical ways, from social aspects like group hunting to bipedalism.
The Piltdown Man was, and still somewhat is, a big blemish for the scientific community. While the discovery got a good bit of attention and put archaeology in the spotlight, it was ultimately more harmful when the truth, that it was a hoax, came out. There was really no follow through of the scientific method or real examination of the specimen(s) or demand for it. People were so caught up in just finding something noteworthy to be put on the map, they put aside principles and made bad choices. While things such as national pride or personal fame are easily understood as possible motives for this event, it does not excuse what was fabricated. “Discoveries” like that make people second guess things and wonder what is or is not valid.