The intriguing theory of a land bridge periodically linking Siberia and Alaska during the coldest pulsations of the Ice Ages had been much debated since Jos? de Acosta, a Spanish missionary working in Mexico and Peru, first proposed the idea of a connection between the continents in 1589. But proof of the land bridge - now named Beringia after eighteenth-century Danish explorer Vitus Bering - eluded scientists until an inquiring geologist named Dave...