TL:DR – The infamous “March of Progress” diagram is not a scientific illustration of human evolution, although creationists haul it out repeatedly as some kind of evidence that evolution is wrong.
Creationists often tell their flock that there are millions of “chimps” and millions of people, and ask: “Where are the millions of in-between species?”. Trouble is, there is no “chimp” in the diagram, the creature depicted on the left represents an ancient, long-extinct human ancestor, as do all the species in between, from which modern humans evolved. That’s the basic flaw in the creationist’s critique.
Rudolph Zellinger who painted the picture to illustrate anthropologist F. Clark Howell’s 1965 book Early Man apparently never intended the diagram to be taken to show a linear progression, although of course, that is how it appears and how many people have take it in countless pastiches since.
Human evolution is the result of many a convoluted turn over millions of years illustrated in this “family tree” which shows the common ancestor, a forest ape, from which several species, chimps, modern humans and gorillas, ultimately evolved. There are numerous dead-ends, but there is a direct line back from today’s apes (including ourselves) to that common ancestor 8 million years ago.