- Title Pages
- Preface to third edition
- Preface to second edition
- Preface to first edition
- 1 Introduction
- 2 ANIMALIA (METAZOA)
- 3 Prelude: Phylum Choanoflagellata
- 4 Early animal radiation
- 5 Phylum Silicea
- 6 EURADICULATA
- 7 <i>Phylum</i> Calcarea
- 8 PROEPITHELIOZOA
- 9 <i>Phylum</i> Homoscleromorpha
- 10 EUMETAZOA (GASTRAEOZOA)
- 11 <i>Phylum</i> Placozoa
- 12 NEURALIA
- 13 <i>Phylum</i> Cnidaria
- 14 TRIPLOBLASTICA
- 15 Phylum Ctenophora
- 16 BILATERIA
- 17 ACOELOMORPHA
- 18 Phylum Acoela
- 19 Phylum Nemertodermatid
- 20 <i>Phylum</i> Xenoturbellida
- 21 EUBILATERIA
- 22 PROTOSTOMIA
- 23 SPIRALIA (LOPHOTROCHOZOA)
- 24 SCHIZOCOELIA
- 25 Phylum Annelida
- 26 <i>Phylum</i> Sipuncul
- 27 Phylum Mollusca
- 28 <i>Phylum</i> Nemertin
- 29 <i>Phylum</i> Platyhelminthes
- 30 Phylum Gastrotricha
- 31 GNATHIFERA
- 32 <i>Phylum</i> Gnathostomulida
- 33 <i>Phylum</i> Micrognathozoa
- 34 <i>Phylum</i> Rotifera
- 35 POLYZOA (BRYOZOA s.l.)
- 36 Phylum Entoprocta
- 37 <i>Phylum</i> Cycliophora
- 38 Phylum Bryozo (Ectoprocta)
- 39 BRACHIOZOA
- 40 <i>Phylum</i> Phoronida
- 41 <i>Phylum</i> Brachiopoda
- 42 ECDYSOZOA
- 43 PANARTHROPODA
- 44 <i>Phylum</i> Arthropoda
- 45 <i>Phylum</i> Onychophora
- 46 <i>Phylum</i> Tardigrada
- 47 CYCLONEURALIA
- 48 NEMATOIDEA
- 49 <i>Phylum</i> Nematoda
- 50 <i>Phylum</i> Nematomorpha
- 51 SCALIDOPHORA
- 52 <i>Phylum</i> Priapul
- 53 <i>Phylum</i> Kinorhynch
- 54 <i>Phylum</i> Loricifera
- 55 <i>Phylum</i> Chaetognatha
- 56 DEUTEROSTOMIA
- 57 AMBULACRARIA
- 58 <i>Phylum</i> Echinodermata
- 59 HEMICHORDATA
- 60 <i>Phylum</i> Enteropneusta
- 61 <i>Phylum</i> Pterobranchia
- 62 CHORDATA
- 63 <i>Phylum</i> Cephalochordata
- 64 <i>Phylum</i> Urochordata
- 65 <i>Phylum</i> Vertebrata (Craniata)
- 66 Problematica
- Systematic index
- Subject index
- Plates
Early animal radiation
Early animal radiation
- Chapter:
- (p.15) 4 Early animal radiation
- Source:
- Animal Evolution
- Author(s):
Claus Nielsen
- Publisher:
- Oxford University Press
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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- Title Pages
- Preface to third edition
- Preface to second edition
- Preface to first edition
- 1 Introduction
- 2 ANIMALIA (METAZOA)
- 3 Prelude: Phylum Choanoflagellata
- 4 Early animal radiation
- 5 Phylum Silicea
- 6 EURADICULATA
- 7 <i>Phylum</i> Calcarea
- 8 PROEPITHELIOZOA
- 9 <i>Phylum</i> Homoscleromorpha
- 10 EUMETAZOA (GASTRAEOZOA)
- 11 <i>Phylum</i> Placozoa
- 12 NEURALIA
- 13 <i>Phylum</i> Cnidaria
- 14 TRIPLOBLASTICA
- 15 Phylum Ctenophora
- 16 BILATERIA
- 17 ACOELOMORPHA
- 18 Phylum Acoela
- 19 Phylum Nemertodermatid
- 20 <i>Phylum</i> Xenoturbellida
- 21 EUBILATERIA
- 22 PROTOSTOMIA
- 23 SPIRALIA (LOPHOTROCHOZOA)
- 24 SCHIZOCOELIA
- 25 Phylum Annelida
- 26 <i>Phylum</i> Sipuncul
- 27 Phylum Mollusca
- 28 <i>Phylum</i> Nemertin
- 29 <i>Phylum</i> Platyhelminthes
- 30 Phylum Gastrotricha
- 31 GNATHIFERA
- 32 <i>Phylum</i> Gnathostomulida
- 33 <i>Phylum</i> Micrognathozoa
- 34 <i>Phylum</i> Rotifera
- 35 POLYZOA (BRYOZOA s.l.)
- 36 Phylum Entoprocta
- 37 <i>Phylum</i> Cycliophora
- 38 Phylum Bryozo (Ectoprocta)
- 39 BRACHIOZOA
- 40 <i>Phylum</i> Phoronida
- 41 <i>Phylum</i> Brachiopoda
- 42 ECDYSOZOA
- 43 PANARTHROPODA
- 44 <i>Phylum</i> Arthropoda
- 45 <i>Phylum</i> Onychophora
- 46 <i>Phylum</i> Tardigrada
- 47 CYCLONEURALIA
- 48 NEMATOIDEA
- 49 <i>Phylum</i> Nematoda
- 50 <i>Phylum</i> Nematomorpha
- 51 SCALIDOPHORA
- 52 <i>Phylum</i> Priapul
- 53 <i>Phylum</i> Kinorhynch
- 54 <i>Phylum</i> Loricifera
- 55 <i>Phylum</i> Chaetognatha
- 56 DEUTEROSTOMIA
- 57 AMBULACRARIA
- 58 <i>Phylum</i> Echinodermata
- 59 HEMICHORDATA
- 60 <i>Phylum</i> Enteropneusta
- 61 <i>Phylum</i> Pterobranchia
- 62 CHORDATA
- 63 <i>Phylum</i> Cephalochordata
- 64 <i>Phylum</i> Urochordata
- 65 <i>Phylum</i> Vertebrata (Craniata)
- 66 Problematica
- Systematic index
- Subject index
- Plates