it turned out to be much older than believed earlier: more than 4 billion years, rather than the 20 million years suggested by Lord Kelvin in 1862. The planets of the Solar System and their moons were closely observed via numerous space probes. Pluto was discovered in 1930 on the edge of the solar system, although in the early 21st century, it was reclassified as a dwarf planet instead of a planet proper, leaving eight planets. No trace of life was discovered on any of the other planets orbiting the Sun (or elsewhere in the universe), although it remained undetermined whether some forms of primitive life might exist, or might have existed, somewhere in the Solar System. Extrasolar planets were observed for the first time. Agriculture Norman Borlaug fathered the Green Revolution, the set of research technology transfer initiatives occurring between 1950 and the late 1960s that increased agricultural production in parts of the world, beginning most markedly in the late 1960s, and is often credited with saving over a billion people worldwide from starvation. Biology Genetics was unanimously accepted and significantly developed. The structure of DNA was determined in 1953 by James Watson, Francis Crick, Rosalind Franklin and Maurice Wilkins, following by developing techniques which allow to read DNA sequences and culminating in starting the Human Genome Project (not finished in the 20th century) and cloning the first mammal in 1996. The role of sexual reproduction in evolution was understood, and bacterial conjugation was discovered. The convergence of various sciences for the formulation of the modern evolutionary synthesis (produced between 1936 and 1947), providing a widely accepted account of evolution. Medicine Placebo-controlled, randomized, blinded clinical trials became a powerful tool for testing new medicines. Antibiotics drastically reduced mortality from bacterial diseases. A vaccine was developed for polio, ending a worldwide epidemic. Effective vaccines were also developed for a number of other serious infectious diseases, including influenza, diphtheria, pertussis (whooping cough), tetanus, measles, mumps, rubella (German measles), chickenpox, hepatitis A, and hepatitis B. Epidemiology and vaccination led to the eradication of the smallpox virus in humans. X-rays became a powerful diagnostic tool for a wide spectrum of diseases, from bone fractures to cancer. In the 1960s, computerized tomography was invented. Other important diagnostic tools developed were sonography and magnetic resonance imaging. Development of vitamins virtually eliminated scurvy and other vitamin-deficiency diseases from industrialized societies. New psychiatric drugs were developed. These