This Week in Science History - The Descent of Man

23 February 2009

Interview with

Sarah Castor-Perry

This Week in Science history saw, in 1871, the publication of The Descent of Man by Charles Darwin - perhaps not as famous as the Origin of Species, published 12 years earlier, it was still an influential and controversial book.

Cover page of Charles DarwinIn 1871, Queen Victoria was on the throne in Great Britain, photographs were in black and white, travel was by horse drawn carriage or steam train, Lewis Carroll published Through the Looking Glass, the Impressionist art movement was in full swing led by artists like Monet and Cezanne and slavery was abolished in the US during the Reconstruction after the Civil War.

After the publication of the Origin of Species in 1859, Darwin began working on another book that would allow him to discuss further how humans fitted in to his theory of evolution and also try to explain the process of evolution of traits that were purely for increasing success in mating, or 'sexual selection'.

The first part of the Descent of Man, in which he discusses man's place in evolution and his 'descent from some lower form', was the most controversial. Darwin had not intended to publish his writings on this subject, but as more 'young naturalists' showed enthusiasm for his theory, he felt he should see how far his 'general conclusions' on natural selection applied to man.

It is now accepted by most people that we are closely related to apes less closely related to other mammals, and even less to other vertebrates, other multi-celled organisms and so on, but this understanding has developed with over 130 years of research since the Descent of Man was published.  We have modern DNA evidence that Darwin did not, but a lot of the anatomical evidence that he presents is still relevant to comparisons today. Woodcut depicting the similar appearance of a human embryo (top) and a dog embryo (bottom), from Charles Darwin's Descent of Man (1871).Darwin suggests that the similarity of human embryos to those of other mammals, and the anatomical similarity of humans and apes, down to the fact that we can suffer from the same infectious diseases with similar symptoms, is strong evidence in favour of man being descended from other animals, not specially created.

In the book, Darwin also makes the comparison of mental characteristics such as tool use, altruism and kindness in humans, apes, monkeys and dogs, concluding that ' the difference in mind, great as it is, certainly is one of degree and not kind'. The idea that our mental abilities and moral ideas were merely one level on a scale that also included animals was abhorrent to many, and the debate over intelligence in animals is still going on today.

The question of whether natural selection is still acting on humans is one that Darwin and many of his contemporaries discussed, and in fact is still asked today - David Attenborough, for example, believes that humans have probably stopped evolving.

The provision of medicine, aid and asylums for the poor or sick seemed to Darwin to 'check the process of elimination' by natural selection, allowing these weaker members of society to breed. Although Darwin suggested that this might be bad for the evolution of the human race as a whole, neither he nor his cousin Galton, who argued in favour of eugenics and social Darwinism, would ever have supported the ends to which these ideas were put to by the Nazis and others in the 20th century.

Sir Francis GaltonThe second half of the book is a detailed collection of evidence for sexual selection - the idea that a characteristic might evolve because it gives greater success in mating rather than greater survival.  Darwin suggested that sexual selection could occur by two routes - through competition between members of the same sex for mates (leading to weapons such as horns and antlers), and through mate choice (leading to elaborate plumage or colouration in males to attract females).

We now know that there is a third way sexual selection can lead to the evolution of characteristics- male-female conflict, which can lead to deceptive behaviour to make your mate stick around to help rear young.

In 2009, we're celebrating 200 years since Darwin's birth and 150 years since the publication of the Origin of Species, but this second major work by the great naturalist also deserves recognition. It was a collation of a huge amount of information on sexual selection from mammals and birds to fish and insects, and also an in-depth view on the controversial debate over man's place in evolution from one of the greatest scientific minds of the age.

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