17th Century

Most people believed in Creationism, which considered that all life was created just as it is now. This was not based on any evidence, but was instead a belief.

18th Century

Naturalists began systematic classification systems (especially Linnaeus 1707-1778) and noticed that groups of living things had similar characteristics and appeared to be related. So their classifications looked a bit like a family tree.

European naturalists travelled more widely and discovered more fossils, which clearly showed that living things had changed over time, so were not always the same. Extinctions were also observed (e.g. dodo), so species were not fixed.

19th Century

Lamark (1809) proposed a theory that living things changed by inheriting acquired characteristics. e.g. giraffes stretched their necks to reach food, and their offspring inherited stretched necks. This is now known to be wrong, since many experiments (and experience) have shown that acquired characteristics are not inherited, but nevertheless Lamark's theory was the first to admit that species changed, and to try to explain it.

Charles Darwin (1859) published "On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life", which has been recognised as one of the most important books ever written. A very similar theory was also proposed by Alfred Wallace, and Darwin and Wallace agreed to publish at the same time.