Answer:
The theory of evolution is that species aren't fixed; new species can evolve from existing species. The earliest recorded theory of evolution dates back to the ancient Greeks - Anaximander in 650BC proposed that life originated in the sea and later moved to the land.
The theory was postulated again in the 18th century; fossils had been discovered of extinct animals - including of "primitive" forms of modern animals. At the time, multiple floods were proposed to explain fossils of extinct animals - although it was never explained where all the water went in between floods! The discovery of fossils of extinct animals with fossils of existing animals was very hard to explain with flood theory - how could some animals survive a flood and others not?
To explain evolution several theories have been proposed; Larmarck suggested that offspring inherited characteristics from parents that they had acquired during life - for example, giraffes stretching to reach high leaves on trees grew longer necks - the longer necks were passed on to their offspring. The problem with Larmarckian evolution is that there wasn't a mechanism to explain how acquired characterstics could be passed on; if you have a scar on body from an injury, your children aren't born with the same scar.
It was the careful research of Charles Darwin and Alfred Wallace that led to the natural selection theory - which provides a mechanism for evolution. Darwin, in particular, provided a large amount of evidence to support the theory.