Our current exhibition on the history of the Royal Society has really been drawing the crowds, and we’ve been picking up some fascinating snippets from our visitors. We recently hosted the Royal Society’s new Population Working Group, which includes Professor Joel Cohen, a mathematical biologist at Rockefeller and Columbia universities in New York, and author of books including ‘How many people can the Earth support?’ (1995).
 
On spotting some papers from the Dutch microscopist Antoni van Leeuwenhoek, Professor Cohen mentioned a little-known early estimate of the world’s capacity to support people. In a letter to the Royal Society dated 25 April 1679, Leeuwenhoek gave a figure of roughly 150 billion “little animals in the milt of a cod”, calculated from microscopical observations of the seminal fluid of fish. He said that this number greatly exceeded the highest possible number of people on the Earth, which he worked out to be 13.4 billion – possibly the earliest quantitative estimate of maximum population.

Leeuwenhoek's microscope

Leeuwenhoek wrote: “If we assume that the inhabited part of the earth is as densely populated as Holland [which then had a population of perhaps one million people]… the inhabited earth being 13,385 times larger than Holland yields . . . 13,385,000,000 human beings on the earth” In the 20th century much larger and much smaller estimates of how many people the Earth can support were offered, Cohen remarked – all of them based, like Leeuwenhoek’s, on incomplete data and questionable assumptions.

Cohen was very enthusiastic about the exhibition, saying: “I am thrilled to see a first edition of Newton’s Principia and the manuscript in his own hand. It is great fun to learn about the colourful characters in the Royal Society’s early history.”

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