5. The circulation of blood. Each person has a fixed amount of blood circulating throughout his or her system in one fixed direction. This fact, first discovered in the 12th century by an Arab doctor named Ibn al-Nafīs*, was rediscovered--for good, this time--by the 17th-century English doctor William Harvey. Harvey's work opened the floodgates to research a full understanding of the physiology of living bodies, human and animal. See Circulatory System.
6. Electrical currents. Ancient people knew about static electricity--rub something and it gives off a spark. They knew about lightning bolts--get struck by one and you're dead meat. But not till 19th-century scientists (such as Alessandro Volta*) got electricity to flow did people become aware of this as a distinct force. Today, electricity powers everything from light bulbs to computers, of course. But the discovery of electricity is bigger than its practical applications. Once scientists knew about this force, they couldn't stop wondering what it was. That's when they discovered that electricity, magnetism, radio waves, and light are all different versions of one underlying force, a glue that holds the universe together.
7. The Evolution of Species. People used to think that every life form now on Earth was here from the start--that no new species had been born and none had ever changed. Charles Darwin's theory of evolution, developed in the 19th century, revealed the dynamic nature of life on Earth. The word "theory" leads some to think that evolution itself remains controversial among scientists, but actually, no mainstream scientist doubts that old species die out and new ones come into being. It's only the exact mechanism of evolution that remains in play, and modified versions of Darwin's idea of evolution by random natural selection still dominate biological thought.
8. Genes. Gregor Mendel never described a gene, saw one, or used the word, yet this shy Austrian monk uncovered the principals of heredity simply by breeding snow peas, charting his results, and drawing brilliant conclusions. Mendel found that parents pass distinct traits to their offspring in combinations governed by predictable laws. Scientists soon decided some actual thing must carry these traits and coined the word "gene." Only in 1953, did Francis Crick and James Watson really figure out what genes are. That year, they discovered the structure of DNA, a molecule shaped like a twisted ladder and contained in every cell. Genes, it turns out, are the combinations of chemicals that form the "rungs" of this ladder. See Genetics.
9. The four laws of thermodynamics. In the 18th century, a series of scientists from Nicolas Carnot* to Baron Kelvin, Rudolf Clausius*, and others found four laws, just four, that governed the transformation of energy into work in any system--a locomotive, a body, a bonfire, a solar system, the universe--you name it. Engineering and inventions, especially of heat-engines, could not have moved forward without knowledge of these laws, for anything that runs on fuel is bound by them. But the laws of thermodynamics* have vast implications for the universe has a whole, not the least of which is this: The total amount of disorder is always increasing.
10. The dual nature of light. Newton learned that light behaves like a wave. Later, other scientists learned that light behaves like a stream of particles. So which is it--wave or particle? It can't be both--or can it? Early in the 20th century, Neils Bohr, Max Planck, Albert Einstein, and others discovered that yes, light is both wave and particle.This paradox gave rise to quantum mechanics, the dominant achievement of 20th-century physics and our deepest current description of "what the universe is really made of." But the quantum picture of reality can't be "pictured." It goes against intuition and laughs at all our senses. The only way to understand the sub-subatomic world of quantum mechanics is mathematically--which brings us right back to Pythagoras.
Source : Encarta