The bowerbirds show the evolutionary continuity between body ornamentation and art. They happen to construct their courtship displays out of twigs and orchids instead of growing them from feathers like their cousins, the birds-of-paradise. We happen to apply colored patterns to rock or canvas. Biologists no longer draw a boundary around the body and assume that anything beyond the body is beyond the reach of evolution. In The Extended Phenotype, Richard Dawkins argued that genes are often selected for effects that spread outside the body into the environment. It is meaningful to talk about genes for a spider's web, a termite's mound, and a beaver's dam. Some genes even reach into the brains of other individuals to influence their behavior for the genes' own benefit. All sexual ornaments do that, by reaching into the mate choice systems of other individuals. At the biochemical level, genes only make proteins, but at the level of evolutionary functions they can construct eyes, organize brains, activate behaviors, build bowers, and create status hierarchies. Whereas an organism's "phenotype" is just its body, its "extended phenotype" is the total reach of its genes into the environment.
In this extended-phenotype view, bipedalism freed our hands for making not just tools, but sexual ornaments and works of art. Some of our ornaments are worn on the body, while others may be quite distant, connected to us only by memory and reputation. We ornament the skin directly with ocher, other pigments, tattoos, or scars. We apply makeup to the face. We braid, dye, or cut our hair. We drape the body with jewelry and clothing. We even borrow the sexual ornamentation of other species, killing birds for their feathers, mammals for their hides, and plants for their flowers. At a greater distance, we ornament our residences, be they caves, huts, or palaces. We make our useful objects with as much style and ornament as we can afford, and make useless objects with purely aesthetic appeal.
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