Simplify the following text for a Grade 7 native English-speaking person.
Original: Many theories have been advanced to account for the occurrence of Olmec influence far outside the heartland , including long-range trade by Olmec merchants , Olmec colonization of other regions , Olmec artisans traveling to other cities , conscious imitation of Olmec artistic styles by developing towns â `` some even suggest the prospect of Olmec military domination or that the Olmec iconography was actually developed outside the heartland .
Simplified: The Olmec were a people who lived about 3000 years ago in what is today south-central Mexico . The Olmec were the first civilization in the ancient area known as Mesoamerica .