animal and vegetable food sources. More than 60 species of animals, including Australian megafauna, became extinct by 10,000 ago. Researchers have variously attributed these extinctions to human hunting and firing practices, climate change or a combination of these factors. The inhabitants developed other technologies to better exploit diverse environments. Fibre and nets for use in watercraft and fishing developed before 40,000 BP. More complex tools, such as edge-ground axes hafted to wooden handles, appeared by 35,000 BP. Elaborate trade networks also developed. Ochre was transported 250 kilometres from the Barrier Range to Lake Mungo 40,000 years ago. Shells (for decorative beads) were transported 500 kilometres by 30,000 BP. More extensive trade networks developed in later times. The earliest Aboriginal rock art consists of hand-prints, hand-stencils, and engravings of circles, tracks, lines and cupules, and has been dated to 35,000 years ago. Around 20,000 year ago Aboriginal artists were depicting humans and animals. However, the dating is contentious and some researchers believe that known examples of Aboriginal rock art are possibly more recent. The Aboriginal population was confronted with significant changes to climate and environment. About 30,000 years ago, sea levels began to fall, temperatures in the south-east of the continent dropped by as much as 9 degrees Celsius, and the interior of Australia became more arid. About 20,000 years ago, New Guinea and Tasmania were connected to the Australian continent, which was more than a quarter larger than today. About 19,000 years ago temperatures and sea levels began to rise. Tasmania became separated from the mainland some 14,000 years ago, and between 8,000 and 6,000 years ago thousands of islands in the Torres Strait and around the coast of Australia were formed. Josephine Flood writes that the flooding and loss of land as coastlines receded might have led to greater emphasis on territorial boundaries separating groups, stronger clan identity, and the development of the Rainbow Serpent and other mythologies. The warmer climate was associated with new technologies. Small back-bladed stone tools appeared 15–19 thousand years ago. Wooden javelins and boomerangs have been found dating from 10,000 years ago. Stone points for spears have been found dating from 5–7 thousand years ago. Spear throwers were probably developed more recently than 6,500 years ago. Sea levels stabilised at around their current level about 6,500 years ago. Warmer weather, wetter conditions and the new coastlines led to significant changes in Aboriginal social and