Evolution of Metazoa

Origins and Evolutionary Relationships Among Metazoan Phyla

What Are the Lines of Evidence?

A. Fossil Record

B. Development

C. Ultrastructure

D. Molecular

A. Evidence from the Fossil Record

1. Pre-Cambrian - 600 mya

• Ediacarian deposits in Australia (680 mya)
• First known metazoan fossils
• Resemble cnidarians,annelids, arthropods, echinoderms
• None clearly linked to any existing phyla

2. Cambrian Radiation - 550-480 mya

• Sudden (in geologic time) appearance of modern phyla; gap between Edicarian and Cambrian forms <40 my
• All major body plans present (sponges, cnidarians, protostome and deuterostome bilaterians)
• Burgess shale, British Columbia (530 mya)
  • 100 genera, 16 not in any known phlyum
  • 35-40% are primitive arthropods
  • Sponges, crinoids, brachiopods, molluscs, polychaetes
  • Graptolites (hemichordates), ammonites (cephalopods), conodonts (sister group of jawless vertebrates)
  • Cnidarians occur in later strata

3. Ordovician - 480-400 mya

• Radiation of modern phyla
• Most subphlya and classes present, including vertebrates (age of fishes)
• Dominant groups were mostly those now extinct or less diverse: trilobites, brachiopods, bryozoans

4. Major Subsequent Events

• Land forms appear (e.g. millipedes) in Silurian (380 mya)
• Major radiation of terrestrial forms in Devonian (350 mya), which include arachnids, insects, and amphibians
• Marine forms of Ordovician continue to dominate - with the addition of extensive coral reefs - through Carboniferous
• Major extinctions in the Permian. Modern groups replace ancient (brachiopods-->molluscs) in the Mesozoic Era
• Gradual increase of modern groups (Orders) throughout Mesozoic. Major extinction at end of Cretaceous

5. Limitations of Fossil Record

• No record of many soft-bodied forms
• No clues to relationships of flatworms (acoelomate) and coelomates
• Does not resolve relationships among protostomes, deuterostomes, or misfits (aschelminths, lophophorates)
• No evidence of Metazoan ancestors
• Ediacarian fossils (most do not appear to be modern phyla) raise more questions on the origin of Metazoa

B. Evidence from Development

Basis for Traditional View of Metazoan Relationships

Major Groupings are as follows:

• Diploblastic

  1. Porifera - Asymmetric, Amphiblastula larva

  2. Cnidaria, Ctenophora - Radial, Planula

• Triploblastic - Bilateral Animals

  1. Acoelomate - Platyhelminthes

  2. Pseudocoelomates (=aschelminths)

  - Rotifera, Acanthocephala, Nematode, Tardigrada

  - "pseudocoel" is from persistent blastocoel; not derived from mesoderm as in coelomate animals

  3. Coelomate - Protostome and deuterostome lineages

  a. Protostomes - Mollusca, Annelida, Arthropoda

  - ventral nerve cord, dorsal contractile vessels

  b. Deuterostomes - Echinodermata, Chordata

  - dorsal nerve cord, ventral contractile vessels

  c. Lophophorates (brachipods, bryozoans) - mixture of protostome and deuterostome traits

• Unresolved Issues and Limitations
  • Two sets of "misfit" phyla based on development: pseudocoelomates and the lophophorates
  • Does not provide a common ancestor for metazoans; suggests two or more major lineages

C. Evidence from Ultrastructure

• Widespread Occurrence of Monoflagellate Cells
  • Same 9 + 2 microtubular arrangement
  • An extra basal body in monoflagellate cell suggests it was derived from biflagellate or multiciliated cells
• Suggests a monophyletic origin of Metazoa
• Collar cells of sponges and choanoflagellates
• Similarity in Ultrastructure
  • Connective tissue, blood vessels and nephridia - coelomates derived from flatworm-like acoelomate
• Neural circuitry of arthropod brain - identical configuration of neurons in optic lobes of crustaceans and insects

D. Evidence from Molecular Approaches

• DNA sequences
  • rRNA genes - large, conserved in evolutionary time
  • homeobox genes - controls segmentation & specialization
• Amino acid sequences or protein tertiary structure
  • Hemoglobin and chitin (most phyla)
  • Collagen (all phyla)
• Collagen occurs in every phylum of Metazoa
  • Data incomplete for relationships among phyla
  • Changes in sequence suggest it evolved ~800 mya
• Analysis of rRNA gene sequences:
  • Monophyletic origin of Metazoa
  • Flatworms likely ancestral to other coelomates
  • Diploblastic phlya distinct from triploblasts
• Protostomes and deuterostomes (rRNA)
  • Appear to be distinct monophyletic groups (rRNA)
  • Vertebrates are similar to other deuterostomes
  • All lophophorate phyla cluster with protostomes
  • Pseudocoelomates cluster with protostomes but most occur in different clades (hence aschelminths are polyphyletic)
  • Tardigrades show a close relationship to arthropods
• Protostomes and deuterostomes (homeobox genes)
  • Flies & mice share 6 homeobox genes (common ancestor)
  • Mice have more genes -> body plans & regulatory genes
• Arthropod subphyla still unresolved
  • rRNA and hemocyanin suggest monophyly
  • hemoglobin occurs sporadically in crustaceans and insects and differ considerably in tertiary structure
  • Chitin hardening differs among suphyla: chelicerates (disulfides), uniramians (quinones), crustaceans (CaCO3)

Evidence From Molecular Approaches

Unresolved Issues and Limitations

• Which characters? Conserved genes? (e.g. rRNA)
• Cannot be used for fossils (body plans & homeobox genes?)

Evolutionary Classification (see pages 23 and 24 of text)

Numerical Taxonomy

Evolutionary Systematics

Cladistics - Phylogenetic Systematics - know these terms

- Monophyletic Taxa (Clade)

- Polyphyletic Taxa

- Paraphyletic Taxa