Depends on what you mean by "valid." If you mean whether or not humans definitely evolved from the primordial soup, you're right: we don't "know" that as an indisputable fact.
However, if by "valid" you mean "known to be possible," then you could not be more wrong. Genetic algorithms are an undeniable and incontrovertable existence proof that humans could have evolved from non-humans. Failure to acknowledge that fact removes one from the communuty of rational and reasonable discourse.
Making an argument of necessity and an argument to possibility are two very different things.
Since you can use the qualifier "could have"--like someone who expects and perhaps even welcomes a challenge--maybe you can entertain a discussion of my ultimate problem with the theory of the evolution/progression of the species. (I've provided a simple definition in posts above)
I've named it, even--"the problem of fortuitous coherence." You may know about what is called "coherent light"--for those who don't, and it's great fun, since it's where lasers and holographs come from--most light is made up of different waves flipping about in different directions at differing speeds. If you can stack the light waves like you'd stack corrugated aluminum roofing--all the waves the same size going the same way--you'd get a powerful beam of zap.
Evolutionists not only expect fortuitous accidents to happen in fortuitous order (and millions of them)--they suggest "fortuitous coherence"--stacks of accidents on top of each other at the same time, and the same size and speed.