Metamorphosis: A Natural and Human History Oren Harman Basic Books (2025)
“Metamorphosis is wild,” marvels historian Oren Harman. A caterpillar completely dissolves inside its chrysalis, before reconstructing itself in the shape of a butterfly. The ‘immortal jellyfish’ (Turritopsis dohrnii) reverts from its mature, floating form into a polyp that dwells on the ocean bed when threatened, only to eventually turn back into a mature jellyfish again — never dying of old age.
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How such weird natural phenomena strike at the core of our understanding of biology is the focus of Harman’s beautiful book, Metamorphosis. The author uses the life stories of four scientists to argue that studies of metamorphosis — drastic changes in the body plan of an organism after birth — have been key to our understanding of the mechanisms of genetic inheritance, organismal development and evolution. It is a sweeping biography that is also part biology textbook, part cultural human history and part personal odyssey.
Harman begins in the 1600s, when philosopher Aristotle’s influence on popular theories was still prevalent and science and magic co-existed in the minds of scholars. “In reports of the highest caliber, sightings of new planets lay side by side with sightings of unicorns and dogs who could bark in French,” he notes. Many people still adhered to Aristotle’s theory that spontaneous generation — the miraculous formation of a living organism from dead material — accounted for metamorphoses.
It was against this backdrop that naturalist Maria Sibylla Merian travelled to South America in 1699 to document the life cycles of insects. According to Aristotle, most insects are ‘imperfect’ animals — born in a state that is completely different from their adult form. Driven by her obsession with metamorphosis, Merian published an illustrated book on the insects of Suriname, which remains a classic for its demonstration of caterpillar–butterfly life cycles.
As Harman shows, the work of naturalists in the seventeenth century led to an understanding that, rather than appearing through spontaneous generation, animals arise from fertilized eggs. Later, studies of butterflies developing in their chrysalises contributed crucial insights to the debate on whether an organism builds itself from scratch, as we now know to be the case, or whether the adult form exists inside the egg and simply grows larger.
Biology as history
Next, Harman turns to zoologist Ernst Haeckel, a nineteenth-century adopter of Darwin’s theory of natural selection. Haeckel viewed it as ridiculous to expect that living organisms could be reduced to simply ‘cold’ physical forces, as his university professors thought. Nor did he take to the romanticism of the natural philosophers, who thought that vital forces animated living creatures.
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