The obvious paradox which makes the thesis of man’s animal origin thus understood actually serve idealism has already been pointed out by A. Leroi-Gourhan. Haunted by the idea of the ape-man, the paleontology and prehistory of the nineteenth century and the beginning of the twentieth century have persisted in the mistake for a long time.
This is a rather convoluted sentence (you can read the entire article here), but I think it breaks down like this:
- The obvious paradox which makes [the thesis] ... actually serve idealism ...
- the thesis of man’s animal origin
- the thesis thus understood
- [the paradox] has already been pointed out by A. Leroi-Gourhan.
So the part you've picked out is qualifying the thesis - it is not just any thesis of man's animal origins, it is the particular thesis that is understood because of something in a preceding sentence.
A more direct phrasing might be something like this:
An obvious paradox in this thesis of man's animal origins makes it actually serve idealism, as already pointed out by A. Leroi-Gourhan.