Zoology 10th Edition by Stephen Miller,John Harley

Exercise 3

Multicellularity arose approximately 600 million years ago. The evolutionary events leading to multicellularity are largely unknown. The coenocytial hypothesis is the idea that multi-cellularity may have arisen by the formation of cell boundaries in a large coenocytic protist. The colonial hypothesis is the idea that multicellularity may have arisen as dividing cells remained together. Similarities of choanoflagellate protists and sponge choanocytes, and between choanoflagellate and animal genes, suggest close evolutionary ties between these protists and animals.
FIGURE 9.3 Phylum Porifera. Many sponges are brightly colored with hues of red, orange, green, or yellow. ( a ) Verongia sp. ( b ) The elephant ear sponge ( Agelas clathrodes ).
Assuming that the choanoflagellates are the ancestors of the Animalia, would the colonial hypothesis or the coenocytial hypothesis be a more likely explanation of animal origins? Explain.