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Animal EvolutionInterrelationships of the Living Phyla$
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Animal Evolution: Interrelationships of the Living Phyla

Claus Nielsen

Print publication date: 2011

Print ISBN-13: 9780199606023

Published to Oxford Scholarship Online: December 2013

DOI: 10.1093/acprof:oso/9780199606023.001.0001

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Early animal radiation

Early animal radiation

Chapter:
(p.15) 4 Early animal radiation
Source:
Animal Evolution
Author(s):

Claus Nielsen

Publisher:
Oxford University Press
DOI:10.1093/acprof:oso/9780199606023.003.0004

Two competing theories have been proposed to explain the origin of metazoans: the colonial theory and the cellularisation theory. According to the colonial theory, the first multicellular organism evolved from a colony of cells, derived from a zygote, which developed cell contacts that enabled exchange of nutrients between the cells and, subsequently, specialisation of cells. The detailed similarity between choanoflagellates and the choanocytes of the ‘sponges’, as well as molecular phylogeny, suggests a sister-group relationship between choanoflagellates. Alternatively, ‘cellularisation’ theories posit the evolution of a turbellariform-metazoan ancestor through compartmentalisation of a ciliate, or a ciliate-like organism. The ancestral metazoan could have been a choanoblastaea having a life cycle with sexual reproduction and haploid eggs and sperm, characteristic of extant metazoans.

Keywords:   metazoans, colonial theory, cellularisation theory, cells, choanoflagellates, choanocytes, sponges, molecular phylogeny, evolution, choanoblastaea

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