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Max Telford is the Jodrell professor of zoology at University College London, UK, and knew Claus Nielsen for three decades, notably through courses at the Kristineberg Marine Research Station.
Andreas Hejnol is director of the Institute for Zoology and Evolutionary Research, University of Jena, Germany, and knew Claus Nielsen for three decades, notably through courses at the Kristineberg Marine Research Station.
Detlev Arendt is a senior scientist at EMBL, Heidelberg, Germany, and a professor of animal evolution at the University of Heidelberg, and knew Claus Nielsen for three decades, notably through courses at the Kristineberg Marine Research Station.
Unravelling the winding paths that evolution has taken is a daunting task. Claus Nielsen was one of the few who dared put forward a comprehensive view of how today’s animal species originated and how they are related. His ‘trochaea’ theory of animal evolution was an important synthesis, integrating morphology, embryology and systematics. This comprehensive view was expanded on in his 2001 book Animal Evolution, which is now in its third edition.
Central to Nielsen’s proposal was the idea that the ancestors of most living animals stem from tiny larva-like creatures that swam using a band of tiny hairs, or cilia. His greatest expertise was in a class of microscopic invertebrates called the Entoprocta — an aquatic group with swimming larvae and sessile (attached to a substrate) adults. He provided a refined understanding of their life cycle, anatomy and position on the tree of life. Nielsen’s knowledge broadened to all invertebrate animal groups and he described dozens of new species of entoprocts, bryozoans, molluscs and annelids.
Nielsen was born in Copenhagen in 1938. His interest in zoology began in his school days, through collecting and identifying shells. In 1956, he began to study marine biology at the University of Copenhagen, where he was inspired by the lectures of Ragnar Spärck, a renowned shellfish biologist. Nielsen learnt that animal classification is a work in progress, and this challenge motivated his later work.
His doctoral thesis focused on entoprocts and, after winning the university’s gold medal, he received his PhD in 1964, and a doctor of science degree in 1972. In 1983, Nielsen moved from the Danish Marine Biological Laboratory in Helsingør (where he had been director from 1970 to 1975) to Copenhagen’s Zoological Museum (now the Natural History Museum of Denmark), which he directed from 1992 to 1996. He remained there for the rest of his career, becoming emeritus professor in 2008.
Nielsen’s trochaea theory built on his studies of larval entoprocts and ectoprocts, which swim with a ring of cilia (trochus in Greek means wheel). He was influenced by zoologist Ernst Haeckel’s biogenetic law (a theory of development and evolution proposed by Haeckel in the 1860s) and zoologist Berthold Hatschek’s 1878 idea of the ‘trochozoon’ as the common ancestor of bilaterally symmetric animals. Many marine groups, such as annelid worms, molluscs, starfish and sea urchins, also have larvae that use cilia to help these mostly sedentary animals to disperse.
Nielsen proposed that the last common ancestor of flies and humans (and all other bilaterally symmetrical animals) resembled such a swimming larva and that it later gave rise to bottom-dwelling adults in the animal groups of protostomes and deuterostomes independently. This was a bold idea with far-reaching implications for our understanding of animal evolution, and it prompted Nielsen to write his influential book.
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