John Ray was the first scientist to use empirical studies to explain the natural world. This meant acquiring knowledge based on observation and experiment, and using that knowledge to form hypotheses. Using his data, he created a classification system for animals and plants, based on their anatomy and physiology.

Until the 1600s, almost all answers to the origin of life were based on religious beliefs, which were not questioned. Ray was the first of many to begin using empirical studies to explain the world with observable data.

Based on Ray's work, Linnaeus expanded and developed upon Ray's classification system. This system is still in use today, and is the basis of taxonomy.

His system broke down organisms into a hierachy, and helped scientists back then — and now — recognize and think about the similarities and differences between living things.

Due to religious beliefs, it was believed that life was unchanging, and that the Earth was only 6000 years old.

One of the first to present evidence opposing this idea was Buffon, who published a 44-volume encyclopaedia describing the similarities between humans and apes. He suggested that we had a common ancestor, which changed over time. He also challenged that the Earth was older 6000 years, giving more time for this change to take place.

An important proof of evolution came from the study of fossils (Paleontology).

Mary Anning was a fossil collector notable for finding the first plesiosaur, an aquatic reptile. She was later recognized by Georges Cuvier, which let her become respectable. This was especially difficult for her, a young-woman in a male-dominated field.

Responsible for developing the science of paleontology, Cuvier analyzed fossils in each stratum (layer of rock).

He found that the species between each stratum was unique, and the deeper (older) he went, the more distant the species were. As well, species came and went as he moved from one stratum to another.

This showed that species could become extict, which Cuvier explained through destructive events such as floods and volcanic eruptions Cuvier called revolutions, which are now called catastrophism. He suggested that each stratum was seperated by these catastrophes.

Charles Lyell disagreed with Cuvier's theory of catastrophism, arguing that such catastrophes occured at the same rate in the past, as they do now. This is called uniformitarianism, based off work of James Hutton, a geologist.

Lyell proposed that if these changes were slow and continuous, the Earth had to be older than 6000 years old (just like the Comte de Buffon suggested). In addition, he theorized that these slow processes could build something big, like giant canyons forming after years of erosion.

These ideas, although only focusing on geology, inspired other naturalists, such as Charles Darwin, to wonder if these slow changes could occur in populations as well.

Lamarck was a French naturalist who compared current species of animals with different fossil forms. He noticed that there was a progression or "line of descent" that could be followed through from older fossils to more recent fossils, and eventually a modern species.

Based on this observation, he suggested that organisms became progressively better adapted for their surroundings, becoming more complex until "perfection". He believed that body parts that were used would become stronger and larger. If a giraffe wanted to have a longer neck, it could stretch it and pass on its long neck to its offspring. He called this idea inheritance of acquired characteristics. He also theorized that body parts that were not used would eventually disappear.

Although these ideas were incorrect, people during Lamarck's time did not have an understanding of genetics. However, he was one of the first to suggest that adaptations to the environment could be inherited. Even later on, Darwin, would credit much of Lamarck's ideas.

On his expedition to South America and the Galapagos Islands, Charles Darwin observed many patterns which he explained with his theory of evolution by natrual selection.

Darwin noticed that all the organisms in the different regions were dissimilar to those in Europe, but were similar to one another. This, Darwin argued, meant that it was unlikely that all organisms were created all at once. He argued that there were clusters of similar organisms throughout the regions of the world, rather than randomly distributed, like a single event explanation would provide.

As well, Darwin observed that the animals on the Galapagos Islands off the coast of South America closely resembled those on the coast itself. Due to the similar conditions in those environments, Darwin concluded that evolution had something to do with the environment.

Upon closer inspection, Darwin noticed that the species on each individual island actually varied slightly. These were mostly adaptations for different conditions on the different islands. He reasoned that these animals must have had a common ancestor, and each species adapted best for life on their island.

Darwin knew that traits could be passed from generation to generation through sexual reproduction. He thought that organisms with favourable traits would be able to survive and mate, passing on the favourable traits. As time went one eventually the entire population would have these traits. This is called survival of the fittest, or natural selection.

Instead of using the term evolution, Darwin carefully chose descent with modification to describe this process. Darwin though evolution meant progress, but natural selection doesn't always take species forwards, but only ensures survival.

While Darwin was trying to conclude a theory based on his observations, he was stuck on how populations changed.

The answer to this was published by economist Thomas Malthus in his Essay on the Principles of Population. He proposed that populations created much more offspring than the environment could support. The ones that wouldn't be able to fight for their food and survival would eventually die of starvation or disease.

This idea was key to Darwin's theory and the idea of survival of the fittest, as it said that individuals without traits to help them survive would slowly die off, while the ones who possess such traits would thrive.