Rock humans (Homo lithicus) are not necessarily a cryptic species for Homo sapiens, due to the fact that they can hybridize with them. A male H. sapiens may mate with a female H. lithicus to produce viable offspring, but the reciprocal cross cannot happen. Gestation period is 6 months, and delivery is during the spring or summer season. Maternal care is nonexistent; the mother will abandon the infant within a forest, against the roots of a dead tree.
This may seem counterintuitive for evolutionary fitness, but it is in fact advantageous. Infants of H. lithicus are altricial and are obligate parasites. Thus, they require a host rather than direct parental care. Infants possess a Fixed Action Pattern (FAP) that activates when a predatory eusocial insect (e.g. The Japanese Yellow Hornet) approaches them. During the FAP, the infant will clutch on the insect until the insect returns to its nest. During translocation, the wind pressure stimulates the infant to begin manifesting its lithic epidermis over half its own body.
Once at the nest, the infant will roll towards the queen, fuse with her body, and begin parasitizing all of her resources. This occurs over a 17-year critical period of development, during which the infant will learn concepts such as the laws of nature, time, numbers, and how to blend within society. Upon maturation, the adult Homo lithicus emerges from the host nest in full size, which completely destroys the nest in the process. The adult is then ready to assimilate within societies of Homo sapiens until death, whether it be by natural or unnatural causes. Unfortunately, despite individuals maturing within an altruistic environment, the concepts of empathy and love tend to evade the mindset of H. lithicus.
Thus spoke Hirohiko Araki.