Humankind in the Cthulhu Mythos differs from its real-world counterpart in a number of ways, perhaps most importantly it’s origins, and it’s ability to interbreed with vastly different beings (usually referred to as "miscegenation" in Lovecraft’s writings). This page aims to cover these variations.
According to H. P. Lovecraft, humankind arose - like most life on Earth - as a result of natural evolution acting on the distant descendants of fauna engineered as servants and sustenance by Elder Things at around the time their empire was coming to its end. In these far-distant days, proto-human simians were regarded as sources of amusement and food by the Elder Things (At the Mountains of Madness).
At some unspecified point in their history, humans mastered the art of magic, although this was limited to a very few learned individuals, and is widely believed to be a myth in the modern age. A possibly-related trait is the ability of certain humans to travel to the alternate dimension known as the Dreamlands in their sleep (The Dream-Quest of Unknown Kadath).
Interbreeding and Hybridisation
Lovecraft established that humans are capable of mating and producing sterile offspring with a number of beings without even the remotest of biological relation. These beings include creatures such as the Deep Ones (The Shadow Over Innsmouth) and White Apes ("Facts Concerning the Late Arthur Jermyn and His Family").
In addition, it has been shown that entirely alien or even interdimensional entities such as Yog-Sothoth and Tsathoggua are capable of breeding with humans, and may in fact use their offspring to gain access to our own reality ("The Dunwich Horror", "The Scroll of Morloc").
As well as evolving, humankind has on occasion been seen to devolve into a more feral, bestial state, such as with the creatures portrayed in "The Beast in the Cave" and "The Rats in the Walls". The causes for this may vary, but in the examples given appears to be due to an adaptation to subterranean life.
Unconventional genius Herbert West demonstrated on multiple occasions that with the proper application of science, human bodies - and even individual parts thereof - may be returned to life, although the time elapsed between the death of the subject and it’s reanimation has a drastic effect on its mental faculties ("Herbert West--Reanimator").
In at least one instance, it has been seen that it is possible for a human being to actually change species through an unknown means, apparently triggered by nothing more than extended proximity to that other species ("Pickman's Model"). Whether this is an inherent ability possessed by all ghouls or some other effect was at play remains unclear.