When early North Americans first domesticated crops<\/strong><\/p>\n

The region of eastern North America covered by the study includes most of Missouri, Illinois, Indiana, Ohio, West Virginia, Kentucky, Tennessee and Arkansas, and portions of Oklahoma, Kansas, Iowa, Virginia, North Carolina, South Carolina, Georgia, Mississippi and Louisiana.<\/p>\n

\u201cThis is the region where these plant foods were domesticated from their wild variants,\u201d Weitzel says. \u201cEverywhere else in North America, crops were imported from elsewhere,\u201d particularly Mexico and Central America.<\/p>\n

Four indigenous plant species constitute what scientists call the Eastern Agricultural Complex, which people began to domesticate about 5,000 years ago.<\/p>\n

Previous research shows specific domestication dates were 5,025 years ago for squash at an archaeological site named Phillips Spring in Missouri, 4,840 years ago for sunflower seeds domesticated at Hayes in Tennessee, 4,400 years ago for marshelder at the Napoleon Hollow site in Illinois, and 3,800 years ago for pitseed goosefoot found in large quantities at Riverton, Illinois, along with squash, sunflower and marshelder.<\/p>\n

Three more recent sites also have been found to contain evidence of domestication of all four species: Kentucky\u2019s Cloudsplitter and Newt Kindigenash rockshelters, dated to 3,700 and 3,640 years ago, respectively, and the 3,400-year-old Marble Bluff site in Arkansas.<\/p>\n

Sunflower and squash \u2013 including acorn and green and yellow summer squashes \u2013 remain important crops today, while marshelder and pitseed goosefoot are not (although the related quinoa is popular).<\/p>\n

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Deducing population swings from radiocarbon dates<\/strong><\/p>\n

\u201cIt\u2019s really difficult to arrive at measures of prehistoric populations. So archaeologists have struggled for a long time coming up with some way of quantifying population levels when we don\u2019t have historical records,\u201d Weitzel says.<\/p>\n

\u201cPeople have looked at the number of sites through time, the number of artifacts through time and some of the best work has looked at the effects of population growth,\u201d such as in the switch from a diet of tortoises to rabbits as population grew in the eastern Mediterranean during the past 50,000 years, he adds.<\/p>\n