Humans are thought to have arrived in Australia about 30,000 years ago. The original inhabitants, who have descendants to this day, are known as aborigines. In the eighteenth century, the aboriginal population was about 300,000. The aborigines, who have been described alternately as nomadic hunter-gatherers and fire-stick farmers (known for using fire to clear the brush and attract grass-eating animals instead of cultivating the land), settled primarily in the well-watered coastal areas. Some observers believe that poor treatment of the environment by aborigines over many centuries may have led to the barren nature of much of the Australian interior. Higher forms of mammals never reached Australia because the land bridge from Asia ceased to exist about 50 million years ago.