The fossil record is biased—what we see there must be preserved, and humans, as a rule do not live well in environments which are conducive to preservation.
As for that record, preservation is a problem. Create two of every species. In a few hours, there will be millions of bacteria, millions of single cell organisms, reproducing at a phenomenal rate compared to more complex organisms, and generally in environments more conducive to their preservation.
More complex critters have longer gestation periods, spells between those, and, by virtue of their complexity, are more difficult to preserve intact or even as recognizable parts in even a marine environment. Again, preservation is key.
There is additional bias against finding, say, a Teleost (”bony Fish”) bone in Cambrian Strata, partly because of denial, and partly because the strata, if the discovery was confirmed, would be re-dated to a different era or some other explanation for how that bone got there concocted—such would be the bias against the discovery. Of course, claims of misidentification and fraud would be rampant because the find would fly in the face of accepted theory.
Move on to land creatures, like humans, with a generational gap of fifteen years or more, and the proliferation rates compared to the less complex forms drop precipitously, making fewer individuals to be preserved, living in environments far less likely to preserve them than aquatic species.
For that reason, the time of a species inception becomes more difficult to ascertain than the time of their extinction—and even that has proven problematic on occasion.
So, the question becomes one of not whether or not all species existed at one time, or even when they existed so much as one of have we found all the fossils yet>
Fewer people are doing actual, get out and bang on rocks fieldwork, the laws and regulations pretty much discourage that on Federal Land, which is almost always tied to a University to get the permit. I have been places where I was literally surrounded by Eocene Turtle fossils eroding away, but to pick up a bone, much less the entire turtle, would be to invite prosecution, a Felony conviction, and a minimum of a $10,000 fine. Find a human bone there? No thanks.
The entire setup, though would discourage any paleontologist who wanted to keep their university job from finding a 40 million year old human fossil.
Circular reasoning will win, especially when tenure could be at stake. It can’t be human, because there were no humans then. OR The age of the beds is incorrect.
What’s a mutha to do?
Right, I "get" all that.
If I were working in the field and found a human looking skeleton in a stratum said to be 40 million years old, before I'd accept my first impressions, I'd examine every assumption -- is it really human? Is the stratum really 40 million years old? Was the fossil reburied at a later date? Etc., Etc..
I would not immediately believe anything that was totally unexpected -- you remember the old saying: "extraordinary claims require extraordinary proof."
On the other hand, if a scientist is ethical and the evidence after careful reconsiderations truly does stand up, then he/she has an obligation to publish it.
Wasn't there a dinosaur in recent years with amazingly preserved collagen, which had anti-evolutionists going berserk -- "see, see, see! No way can DNA survive millions of years, therefore must be Young Earth!!"
Well... there was no dino-DNA, but there were bits of collagen which showed that dinosaurs taste like chicken.
And I suppose that's good to know, right?
Bottom line: no evidence falsifying basic evolution theory has ever been confirmed.
Of course, evolution as understood today is vastly different from what Darwin published in 1859.
But his basic idea remains unimpeached -- evolution results from 1) descent with modifications and 2) natural selection.