Ivan Wallin advocated the idea of an endosymbiotic origin of mitochondria in the 1920s. The Russian botanist Boris Kozo-Polyansky became the first to explain the theory in terms of Darwinian evolution. In his 1924 book A New Principle of Biology.

When did endosymbiosis first appear?

They first appear in the fossil record around 4 billion years ago.

When did mitochondria first appear?

Mitochondria arose through a fateful endosymbiosis more than 1.45 billion years ago.