Darwin used this tree figure in The Origin of Species to illustrate his idea of “descent with modification”, with the branches representing the diversity of species all interconnected back through time.
However, some 50 years earlier, in fact in the year of Darwin’s birth 1809, Jean-Baptiste Lamarck used a tree of sorts to depict evolution in his book Philosophie Zoologique. Lamarck is best known for wrongly believing that evolution happened by the inheritance of acquired characteristics, but he should be given credit for being the first scientist to develop a theory of evolution – unfortunately he just had the mechanism wrong.
But ideas about trees were around even before Lamarck’s time. Perhaps the earliest description of a tree of life comes from Russian naturalist Peter Simon Pallas, in 1766:
But the system of organic bodies is best of all represented by an image of a tree which immediately from the root would lead forth out of the most simple plants and animals a double, variously contiguous animal and vegetable trunk; the first of which would proceed from mollusks to fishes, with a large side branch of insects sent out between these, hence to amphibians and at the farthest tip it would sustain the quadrupeds, but below the quadrupeds it would put forth birds as an equally large side branch.
The earliest published tree diagram likely comes from 1801, when French botanist Augustin Augier published his Arbre Botanique, a detailed tree diagram complete with leaves that depicted his view of the relationships between members of the plant kingdom.
Of course the theory of evolution has advanced somewhat since Darwin’s time and we now know that the idea of a tree of life is a little too simplistic. As explained in a recent article in New Scientist (which was published with the outrageously inflammatory cover “Darwin was wrong” that no doubt excited a few creationists), some species relationships, particularly of the earliest organisms, are better described by a network rather than a tree, which acknowledges that hybridisation and horizontal gene transfer play a big role in evolution.
If you want to read more on the history of the tree of life, have a look at this article by David Archibald from San Diego State University.