Humans have always had to deal with disease. Skeletons more than 12,000 years old show evidence of tuberculosis and other diseases. The 9400-year-old mummified remains of Spirit Cave man, found in Nevada in 1940, indicate that he suffered from back problems and tooth abscesses. The remains of Ramses V, ruler of Egypt around 1150 bc, show that his face was disfigured by smallpox scars. Disease has had a dramatic impact on human history. For most of the 250,000 years that humans have been on the earth, disease has played a central role in limiting population growth. As ways to combat disease were discovered, people lived longer and had more children, who lived long enough to have children of their own. The human population slowly increased and then exploded. By 1804 the human population reached 1 billion. Just over 100 years later, in 1927, after the advent of the first vaccines and the recognition of the importance of sanitation and safe water supplies, the population had doubled to 2 billion. By 1974 it had doubled again to 4 billion. Since then, recognition that the earth's environment has a limited capacity to support an ever-increasing population has led to concerted efforts to limit population growth. Nevertheless, as the 20th century neared its end, the population had reached 6 billion. It is expected to rise to more than 8 billion by 2021.