The following quote is attributed to Renato Davila Riquelme, who the story states works for the Privado Ritos Andinos museum in Cusco:
"It has a non-human appearance because the head is triangular and big, almost the same size as the body. At first we believed it to be a child's body until Spanish and Russian doctors came and confirmed that, yes, it's an extraterrestrial being."
Here, have some photos (from io9):
Okay, so two things right off: 1) I am not an osteologist, human skeletal anatomy is not my specialty. Like most archaeologists, though, I do have some training in osteology and have handled a wide rnage of human skeletons both with and without pathologies, so I have a good baseline idea of what I am talking about, though it should be kept in mind that I am not an expert. 2) All I have to go on is these photographs, and not the original bone.
So, that being said, when I first saw these photos, the first thought that went through my head was not "GAH! What the HELL is THAT! ALIEN!" It was, in fact "oh look, human bones exhibiting signs of pathology and possibly some intentional cranial deformation." In other words, something unusual, but definitely well within the range of known and well-understood human variation.
The assurance of how it was determined that these are alien bones is pretty damn comical. Leave aside the fact that it's a group of Spanish and Russian physicians who are never named who say that it's an alien (really, this is the sort of thing that would end up in a journal, with the names of the researchers highlighted in order to ensure their impending flood of grant money), it's that they "confirmed" that it's an extra-terrestrial. As the website io9 puts it:
"BOOM. There ya go. Four out of five faceless scientists agree that what you're looking at are mummified alien remains. Case closed."
Now, let's assume, for a moment, that it was definitiely shown to not be exaclty what it looks like (a human skeleton exhibiting bone pathology), how would you go about confirming that it was an alien?
I have done faunal analysis, and when I have a bone that I can confirm does not belong to any of the animals with which I am familiar, I don't confirm that it is, therefore, from another planet. I conclude that it belongs to an animal for which I don't have a sample for comparison. So, let's say that these bones were shown clearly to not be human. That would imply that they were from another animal, true, but why assume that this animal is from another planet? Why not an undiscovered primate from Earth? If you don't have an extra-terrestrial body to which to compare it (and this article says nothing about the bones being taken to Nevada for comparison with the Roswell...oh, I've said too much!), then you have no reason to think that it's an alien.
I suppose that if the bone contained some element or compound not found on Earth, you could conclude that it came from elsewhere, but then why would a group of doctors and not a group of chemists and physicists be making that announcement (again, with their names in bold to help catch the flood of grant money coming their way)?
Anyway, I suspect this is a hoax. If it's not a flat-out hoax, then it's a case of someone being very, very stupid.