BIOL1030 Chap.3 Animal Origins and Early Diversification
Animal Origins and Early Diversification
Animal Origins and Early Diversification covers the Week 3 lectures on how animals began and diversified.
It places the first animals in the Ediacaran Period, introduces choanoflagellates as the closest relatives of animals, and dissects sponges: asconoid, syconoid and leuconoid body plans, the pinacoderm, mesohyl and choanoderm, and the porocytes, pinacocytes, mesohyl cells and choanocytes that move water and capture food.
It then surveys cnidarians, from corals and anemones to box jellyfishes, and the comb jellies with their adhesive colloblasts, using them to weigh two trees for the origin of nerves and muscles. The chapter ends among the deuterostomes, with echinoderm larvae, hemichordates, tunicates, lancelets and the synapsid skull.
What this chapter covers
- 01
The Ediacaran Period and the first animals
- 02
Choanoflagellates and the choanocytes of sponges
- 03
Sponge body plans, layers and cell types
- 04
Cnidarians: Anthozoa, Medusozoa and reef holobionts
- 05
Ctenophores and the origin of nerves and muscles
- 06
Echinoderms, hemichordates and the chordate ancestors
Worked example · free
Count the events on two trees for the origin of nerves
- 1Tree 1: one gain of nerves and muscles on the branch leading to comb jellies, cnidarians and bilaterians explains every group.
- 1Tree 1 implies nerves and muscles evolved once and the earliest animals were probably built like sponges.
- 1Tree 2: either two separate gains, in comb jellies and in the cnidarian and bilaterian branch, or one early gain followed by a loss in sponges.
- 1Tree 2 needs two events either way and leaves the appearance of the first animals open.
Key terms
- Choanocyte
- A sponge cell whose beating flagellum pumps water while its collar of microvilli traps food.
- Cnidocyte
- The stinging cell that defines cnidarians such as corals, anemones and jellyfishes.
- Colloblast
- An adhesive prey-capturing cell of comb jellies, thought not to be homologous to cnidocytes.
- Temporal fenestra
- An opening in the skull of synapsids that anchors large jaw muscles for a more efficient bite.
Animal Origins and Early Diversification FAQ
Why are choanoflagellates important for understanding animal origins?
They are the closest living relatives of animals, single-celled or colonial protists with a flagellum ringed by a collar. Sponge choanocytes have the same collar and flagellum design, so comparing the two helps reconstruct what the earliest animal cells could do before true tissues existed.
How does a sponge feed without a mouth or gut?
Water is the delivery system. Porocytes let water in, the flagella of choanocytes pump it through internal chambers, and their microvillar collars catch bacteria and microalgae. Mesohyl cells then distribute materials, and the water leaves through the osculum at the top.
Why do comb jellies matter for the origin of nerves?
Comb jellies have nerves and muscles but sit close to the base of the animal tree. If they branched off before sponges, nerves and muscles must have evolved twice or been lost in sponges, which would mean we cannot assume the first animals looked like sponges.
Why are lancelets used to picture the first chordates?
Lancelets have retained many ancestral chordate characters, while tunicates are highly derived and their adults look little like their ancestors. That makes the lancelet body plan a better guide to where chordates, and later vertebrates, started. Exam answers should name the retained characters as the reason, not simply the group.
What does a choanocyte do in a sponge?
Choanocytes line the sponge's internal chambers. Each beats a flagellum to drive water through the body, and its collar of microvilli traps bacteria and microalgae as food. Their close resemblance to free-living choanoflagellates is one reason the two groups are compared when biologists ask what the first animals were like.
Exam move
Treat this chapter as three pictures you should be able to draw. First, a cross-section of a sponge with the three layers and four cell types labelled, and arrows for water flow from pores to osculum. Second, the two competing animal trees side by side, with gains and losses of nerves and muscles marked, so you can explain either conclusion.
Third, a short deuterostome ladder from echinoderms and hemichordates to tunicates, lancelets and vertebrates. Then link each cnidarian and ctenophore example from the slides to its group, and use the optional limb evolution reading only after the core pictures are secure.
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