What is life?

Abstract

It was once believed that the smallest living organisms were insects, but when the microscope was invented early in the seventeenth century it revealed a whole new realm of nature. On noticing in 1665 that cork has a distinctive structure, Robert Hooke, the curator of the Royal Society of London, introduced the word ‘cell’. In Holland in 1675, while analysing a drop of water, Anton van Leeuwenhoek found organisms that were too small to be seen by the naked eye and realised that insects were not the smallest creatures after all. Further investigation revealed that these organisms were a type of animal that was able to move by flexing small flagellae (whips) or cilia (hairs). In France in 1824, R.J.H. Dutrochet proposed that living matter is composed of cells. This was confirmed for plants by M.J. Schleiden in Germany in 1838, and for animals by Theodor Schwann in Germany in 1839. In each case the cells were typically between 5 and 40 micrometres in diameter. In 1839 the Czech physiologist J.E. Purkinje named the colloidal fluid in cells protoplasm (first form) and the German anatomist M.J.S. Schultze later demonstrated the similarity of protoplasm in plant and animal cells, and said it was the “physical basis of life”. The spermatozoa (animal seed) had been seen in 1677 by Johann Ham, one of van Leeuwenhoek’s assistants, and in 1827 the German physiologist K.E. von Baer identified the mammalian ovum (egg), but it was not until the 1870s that microscopes became sufficiently powerful to reveal that the fertilisation of an egg by a sperm led to the division process that formed a multi-cellular organism. In 1845 K.T.E. Siebold discovered that the organisms that van Leeuwenhoek had observed in water droplets were “animals whose organisation is reducible to one cell”, and in 1848 he defined them collectively as protozoa (first animals) because they appeared to be the basis of the ‘animal kingdom’.