Darwin’s Origin of Species is the foundation of evolutionary biology, and is unquestionably one of the most important and influential texts in all of science. His thoughts on natural selection and how new species can branch out from a common ancestor can be traced back 20 years earlier to a series of notebooks that emanated from observations during his HMS Beagle voyages and subsequent readings and conversations in London. Of particular note is this remarkable Tree of Life sketch (1837), its branching lines representing his idea of the relatedness of all species – their origin from a single ancestor, their divergence over time, and the need for extinction to explain “immense gaps” between species and formation of genera. Ultimately, in Origin of Species, he writes “As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications.”….and it all started, as it should, with “I think.”