The unseen world: reflections on Leeuwenhoek (1677) ‘Concerning little animals’

Leeuwenhoek's 1677 paper, the famous ‘letter on the protozoa’, gives the first detailed description of protists and bacteria living in a range of environments. The colloquial, diaristic style conceals the workings of a startlingly original experimental mind. Later scientists could not match the resolution and clarity of Leeuwenhoek's microscopes, so his discoveries were doubted or even dismissed over the following centuries, limiting their direct influence on the history of biology; but work in the twentieth century confirmed Leeuwenhoek's discovery of bacterial cells, with a resolution of less than 1 µm. Leeuwenhoek delighted most in the forms, interactions and behaviour of his little ‘animalcules', which inhabited a previously unimagined microcosmos. In these reflections on the scientific reach of Leeuwenhoek's ideas and observations, I equate his questions with the preoccupations of our genomic era: what is the nature of Leeuwenhoek's animalcules, where do they come from, how do they relate to each other? Even with the powerful tools of modern biology, the answers are far from resolved—these questions still challenge our understanding of microbial evolution. This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society.