The field of paleoanthropology is a young science. There have been many paleoanthropologists over the years who have found many important fossils. But, even by the early 1970s, we didn't know everything about human evolution—and we still don't!
Back then, reserachers knew a few key events in human evolution and how they make us different from our primate cousins. But, they did not know which events happened first, which is very important to understanding why humans evolved the way we did.
Our species, Homo sapiens, are incredibly unique among the species on Earth today.
There are many thousands of species across the world, from microscopic bacteria to massive whales. And many more species have become extinct since life arose on our planet. But humans are the only upright walking, large-brained, tool-using species alive today.