Hypothesis: Microorganisms, such as bacteria and viruses, cause disease.
How it was tested:
- In the 1850s, the Hungarian physician Ignaz Semmelweis observed that the death rate from puerperal fever (a type of infection that can occur after childbirth) was much higher in the maternity ward of the Vienna General Hospital than in the midwife-run clinic. He hypothesized that the fever was caused by something in the hospital environment and implemented a policy of requiring doctors and nurses to wash their hands with a chlorine solution before examining patients. The death rate from puerperal fever dropped dramatically as a result.
- In the 1860s, the French chemist Louis Pasteur conducted a series of experiments that showed that microorganisms can cause fermentation and spoilage of food. He also showed that microorganisms could be killed by heat, which led to the development of the pasteurization process.
- In the 1870s, the German physician Robert Koch developed a set of postulates that are used to establish the cause-and-effect relationship between a microorganism and a disease. Koch's postulates state that:
1) The microorganism must be present in every case of the disease.
2) The microorganism must be isolated from the host and grown in pure culture.
3) The pure culture of the microorganism must cause the disease when introduced into a healthy host.
2. The Theory of Evolution by Natural Selection
Hypothesis: Evolution occurs through the process of natural selection, where individuals that are better adapted to their environment are more likely to survive and reproduce.
How it was tested:
- In the 1830s, the British naturalist Charles Darwin began to develop the theory of evolution by natural selection after his voyage around the world on the HMS Beagle. He observed that different species of plants and animals were adapted to different environments, and he hypothesized that these adaptations had evolved over time through natural selection.
- In the 1850s, Darwin published his book On the Origin of Species, which laid out his theory in detail. He presented a wealth of evidence from a variety of sources, including:
- Fossil record
- Comparative anatomy
- Embryology
- Geographical distribution of organisms
- Since the publication of On the Origin of Species, the theory of evolution by natural selection has been extensively tested and is now considered one of the most important and well-supported theories in science.
3. The Big Bang Theory
Hypothesis: The universe began about 13.8 billion years ago with a hot, dense state and has been expanding ever since.
How it was tested:
- In the 1920s, the American astronomer Edwin Hubble discovered that the universe is expanding. He observed that the further away a galaxy is, the faster it is moving away from us. This led to the development of the Big Bang theory, which was proposed by the Belgian physicist Georges Lemaître in 1927.
- In the 1960s, the Big Bang theory was confirmed by the discovery of the cosmic microwave background radiation (CMB). The CMB is the faint afterglow of the Big Bang, and its existence provides strong support for the theory that the universe began in a hot, dense state and has been expanding ever since.
- Since the discovery of the CMB, the Big Bang theory has been extensively tested and is now considered the leading theory of how the universe began and evolved.
