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

Abstract

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.

Keywords: Leeuwenhoek; animalcule; bacteria; eukaryote; protozoa; tree of life.

Figure 1.

Portrait of Leeuwenhoek by Jan Verkolje, 1686, at age 54. Copyright © The Royal Society.

Figure 2.

First and last pages of Leeuwenhoek's 1676 letter to Oldenburg, in the hand of a copyist. Copyright © The Royal Society.

Figure 3.

(a) Rotifers, hydra and vorticellids associated with a duckweed root, from a Delft canal. From Leeuwenhoek [16]. (b) Bacteria from Leeuwenhoek's mouth; the dotted line portrays movement. From Leeuwenhoek [17]. Copyright © The Royal Society.

Figure 4.

(a) Replica of a single-lens microscope by Leeuwenhoek (Image by Jeroen Rouwkema. Licensed under CC BY-SA 3.0 via Wikimedia Commons). (b,d) Photomicrographs taken using simple single-lens microscopes including one of Leeuwenhoek's originals in Utrecht, by Brian Ford (Copyright © Brian J. Ford). (b) An air-dried smear of Ford's own blood through the original van Leeuwenhoek microscope at Utrecht, showing red blood cells and a granulocyte with its lobed nucleus (upper right; about 2 µm in diameter). (c) Spiral bacteria (Spirillum volutans) imaged through a replica microscope with a lens ground from spinel; each bacterial cell is about 20 µm in length. (d) The intestinal protist parasite Giardia intestinalis imaged through a replica soda-glass produced by Brian Ford [28,29].

Figure 5.

A tree of life drawn by Bill Martin in 1998, reflecting whole genomes. The tree shows the chimeric origin of eukaryotes, in which an archaeal host cell acquired bacterial endosymbionts that evolved into mitochondria; and the later acquisition of chloroplasts in Plantae. Reproduced with permission from [51]. Copyright 1999 © John Wiley & Sons, Inc.