Anatomical studies of humans indicate that the emissary veins belong to a dense web of microscopic veins draining blood from the brain to the vertebral plexus.
Emissary veins apparently served double duty in the gracile-Homo lineage, says Falk, by helping to drain blood out of the cranium and by cooling the brain when vigorous exercise or heat exposure warnmed up blood flowing into cerebral tissue.
Cabanac and Brinnel placed the tips of ultrasonic probes on the heads of bald male volunteers at sites where emissary veins poke thorugh their tiny cranial conduits.
Since the few emissary veins with impressions remaining on fossil skulls link up to an extensive network of microscopic veins in the cranium, Cabanac and Brinnel's data support her theory that a venous "radiator" cools the human brain, Falk says.
Sustained exercise on the savanna helped to promote the evolution of brain-cooling emissary veins, as well as copious sweat glands in the face and scalp, dark skin and reduced body hair, she notes.