NOT FAR FROM THE Andalusian coast in southwestern Iberia, an unusual group of women was laid to rest around 4,800 years ago. Their communal tomb, known as the Montelirio tholos, was built into the east side of a natural hill. A 125-foot-long subterranean corridor with sandstone, granite, and slate roofing extended inward from the entrance. Along this passageway were vertical slate slabs coated with the blood-red pigment cinnabar, or mercury sulfide. At the corridor’s end was a circular chamber 15 feet in diameter, with a vaulted earthen ceiling rising at least 13 feet high, most likely supported during construction by wooden posts. Its sides, too, were lined with cinnabar-coated slate slabs, which served as a canvas for black, white, and red geometric motifs. One of these, a circle with short…