Charles Darwin was a 19th-century scientist who theorized that we are all descended from a common ancestor. He explained this in his seminal work, "On The Origin of Species," in which he theorized that species evolve by natural selection.

On The Origin of Species

Darwin published his book "On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life," in 1859. It outlined his famous theory of natural selection, or "survival of the fittest."

Natural Selection

Natural selection is the theory that creatures best suited to their environment will survive and reproduce, while weaker creatures of the same species will gradually die off.

Example

Dark-colored moths camouflage better against soot-covered trees than light-colored moths. As a result, light moths, easily spotted by predators, die out, while dark moths thrive and reproduce until they outnumber light moths.

Pigeons

Darwin spent time with pigeon breeders, who chose pigeons with desirable traits, such as ruffled necks, for breeding. Darwin saw this as similar to the way creatures with desirable traits in nature produce offspring who are similarly adapted to survive.

Common Ancestor

Darwin believed that over time, natural selection can result in new species, such as fish that become frogs. This explains how all creatures on Earth, including humans, can be descended from a common ancestor.

Source:

Biology-online.org: Theory of Natural Selection; Jan. 1, 2000

Darwin and Natural Selection; Dennis O'Neil; Jan. 8, 2010

Allaboutscience.org: How does Darwin's theory of natural selection explain the origin of species?