The modern supermarket holds a bewildering variety of oranges, lemons, limes, grapefruit, tangerines and more. But they are all the product of 4,000 years of selective breeding by humans to tease out tastier, larger, sweeter and juicier variations and hybrids from four ancestral Asian fruits. Dr. Curt Stager and Martha Foley climb back down the citrus family tree to look at the Mandarin orange, the pomelo, the citron and the papeda.