Neoteny is the retention of features seen during early development in related species.
Humans exhibit neoteny from their descent from apes. While becoming sexually mature, humans retain many features of juvenile chimps. Neoteny in humans includes a larger ratio of brain to body size, lighter bones, a flattened and broader face, larger eyes, smaller nose, smaller teeth and jaw, and glabrousness (hairlessness). Human limbs and body posture are also indicative: limbs proportionately short compared to torso length, longer leg than arm length, foot structure, and upright stance.
Human neoteny in the genitals is seen by lack of a penis bone (baculum) in males, and in females a forward-facing vagina and the presence of a hymen.
Female faces that are more neotenized are most alluring to men, while less neoteny is the least attractive, regardless of age. As women choose a mate largely on concerns other than physicality, neoteny is not as looming an issue in male attractiveness.
Neoteny has been seen in several animal species. It typically results in larger individuals with longer lifespans, as it has with humans. There is a notable exception: birds. Avian evolution included downsizing, partly attained by sticking with the skeletal structure of the young dinosaurs birds descended from.
Neoteny in animals is often indicative of a change in the vector of evolutionary descent. This was the case with salamanders, birds, and humans. Chordates arose over a half-million years ago via neoteny.
Another association with neoteny is a transformation of sociality. Animal domestication, such as seen in dogs, cats, and pigs, has repeatedly resulted in neoteny. Hominin neoteny likely reflected a shift toward comity.
Ishi Nobu, The Elements of Evolution, BookBaby (2019).