Re: why human cortical bone layer is thiner then other animal?

Large fragments of human bone are often easily identified on the basis of 
gross anatomical features-- I am assuming, though, that you are speaking of 
small bone fragments.

It's usually pretty easy to tell mammal bones from bird, reptile and fish 
bones.  However, distinguishing fragments of deer bone, for example, from 
fragments of human bone can be trickier.  Your observation that the 
cortical layer of human bone tends to be thinner than the bones of other 
mammals (of similar size) is generally correct-- but not the rule, of 
course. There is variation in average thickness for many of the bones of 
the mammalian skeleton.

Which brings me to the point: Bone tends to become thicker under stress.  I 
can't remember the citation for this article, but I remember a study of 
oxen metatarsals (or cannon bones-- the bone above the hoof) that showed 
that the mass (thickness) of the metatarsal increased with the amount of 
agricultural plowing performed by the animal. There are other pathological 
reasons for increased bone thickness, but since bones are the 
superstructure of the body, the load they carry is probably the best 
determiner of their strength.

Hope this helps!

Daniel R. Pratt
Archaeologist