Chenopodium quinoa Willd., an ancient Andean pseudocereal almost exclusively consumed in South America, jumped onto the global stage when Western cultures noted quinoa's advantageous nutritional profile. Quinoa seed's high protein content, nutritionally balanced amino acid profile, low glycemic index, and high fiber, vitamin, and mineral content, make it a highly sought-after 'superfood'. Pitseed goosefoot (C. berlandieri Moq.), a closely related North American species sharing quinoa's genome composition (AABB), grows across the North American continent, inhabiting diverse environments including the saline coastal soils of the Gulf of Texas and the drought-prone regions of the Southwest. Quinoa and pitseed goosefoot, along with South American avian goosefoot (C. hircinum Schrad.), make up the Allotetraploid Goosefoot Complex (ATGC). We hypothesize that an ancient hybridization event between A- and B-genome diploids, with a subsequent whole-genome duplication, gave rise to the common ancestor of the ATGC. Prior data indicate that allopolyploidization most likely occurred within North America, with long-range dispersal of the ATGC to South America. We have sequenced the genome of the North American AA-genome diploid C. watsonii and identified via DNA marker analyses the closest extant species to the AA-genome diploid ancestor of the ATGC from among a panel of 41 AA-genome diploid resequenced accessions, encompassing 30 putative AA-genome diploid species, from North and South America. We also present evidence for reciprocal long-range dispersal of Chenopodium diploids between North and South America.