Then almost no humans should be alive anywhere. Almost all river valleys are former swamps, laid dry somewhere between antiquity and now to be habitable and ara

Then almost no humans should be alive anywhere. Almost all river valleys are former swamps, laid dry somewhere between antiquity and now to be habitable and arable. Rome was a swamp dried out by the cloaca maxima, among other reasons to get rid of the mosquito plague and associated diseases. We have made land habitable for millenia, you are arguing for a return to stoneage levels of settlement.