The most impressive thing about Nutcracker man is the entire set of chewing apparatus. This upright walker would be able to chew pretty much anything and probably did, hence the name. This wonderful set of chompers all boils down to a simple set of genes.
By Photographed by User:Lillyundfreya - Photographed at Westfälisches Museum für Archäologie, Herne, CC BY-SA 3.0, Link
These are the genes that control how chewing muscles (most notably the cheek muscles) are attached to the skull and how strongly they develop.
In the robust Austrolopithicenes or the Paranthropus branch we see that they have retained the set of genes that are now poorly developed in chimps, hominins, humans and the gracile Australopithicenes.
These are the genes that cause the cheek muscles to wrap around over the sides and top of the scull and attach to a piece of bone that develops down the middle of the top of the skull. This bony ridge is called the sagittal crest and is prevalent in most species that have well developed jaws and a strong bite.
By Nrkpan , CC BY-SA 3.0, Link
The most significant drawback to this set of genes is that the strong muscles wrap around over the top of the scull and cramp the brain from getting significantly larger.
Consequently, this set of genes being knocked out allowed for the attachment of the jaw muscles to the side of the head and enabled the growing brain to expand freely as tool use, language and other cultural advancement stimulated the brain.
As a result of the lessening of the jaws muscular and attachment structure it was necessary to diminish the size of the jaws and teeth. This was not a problem from an eating perspective since by then hominins had tamed fire and figured out how to cook food etc.
In modern humans the jaws are now comparatively so small that we don't have enough room for all the teeth we have inherited from our ancestors. Consequently we suffer from wisdom teeth impaction and other space related dental issues.