They had figured out farming well before your ancestors.
"The gardens of the New Guinea highlands are ancient, intensive permacultures, adapted to high population densities, very high rainfalls (as high as 10,000mm/yr (400in/yr)), earthquakes, hilly land, and occasional frost. Complex mulches, crop rotations and tillages are used in rotation on terraces with complex irrigation systems. Western agronomists still do not understand all practices, and native gardeners are notably more successful than most scientific farmers. Some authorities believe that New Guinea gardeners invented crop rotation well before western europeans. "
The diversity of languages, as you have said before, is not surprising as the cultures today survived the ancient flooding of the subcontinental shelf where most of the earliest cultures originated. The lack of early high cultures and then the "explosion" of highly developed Ban Chiang and Vietnamese Don Song Cultures between 3,500 and 5,000 years ago point to possible multiple centers of early civilization. Just now, the University of Pennsylvania is expanding their study of the Ban Chiang Culture to include Mekong sites going back 10,000 years. Along the great rivers and under the ocean seem the be the locations of the oldest peoples -- as one would expect.
Even the Chinese are -- reluctantly -- moving away from their dogmatic insistence on a Yellow River origin of all Eastern Cultures. They are pushed by the Southwest China discoveries -- Sichuan and Yunnan.