Earliest

Fossilized remains of plants and animals have been discovered in many areas of Thailand, particularly in the Korat Plateau in northeastern Thailand. Most of the animal fossils found are of dinosaurs, which date primarily to the Jurassic era though some are from the late-Triassic, the oldest such evidence of dinosaurs in Southeast Asia.
The dinosaur bones encased in sandstone in the Phu Wiang hills of Khon Kaen province included Phuwiangosaurus sirindhornae, a gigantic plant-eating dinosaur that had a long neck and tail and a small head.
Four other species of dinosaur unearthed in Phu Wiang include Siamotyrannus isanensis, a smaller version of Tyrannosaurus rex, Siamosauraus suteethorni, a crocodile-like creature, Compsognathus, the world’s smallest dinosaur, and Ornithomimosaur, an ostrich-like dinosaur.
In nearby Chaiyaphum province two other new dinosaur species were discovered: Psittacosaurus sattayaraki, a parrot-billed dinosaur, and Isanosaurus attavipachi, which is similar to Phuwiangosaurus.
Homo erectus fossils have also been discovered in Thailand. Known as the Lampang man for its discovery in Lampang province, the remains have been dated to roughly 1,000,000 - 500,000 years ago in the Pleistocene Era.The first evidence of humans living in modern-day Thailand was discovered at Ban Chiang, near Udon Thani in northeastern Thailand; grave sites and artifacts including bronze tools and pottery provide evidence of a society that is thought to have had knowledge of rice cultivation and occupied the area continuously from 2100 to 200 BCE, spanning the Neolithic to the Iron Age.