Chapter 31 – Animal Origins & the Evolution of Body Plans
31.1 What Evidence Indicates the Animals Are Monophyletic
Traits that distinguish animals from other groups:
- Unlike bacteria, archae, and most microbial eukaryotes, all animals are multicellular. Animals develop from single-
celled zygote into a multicellular adult.
- All animals are heterotrophs.
- Fungi are also heterotrophs, but unlike fungi, animals use internal processes to break down materials from
environment into organic molecules they need most.
- They can move.
- may have resembled modern choanoflagellates, protists that are closest living relatives of animals
Proof of Animal Monophyly:
- Gene sequence, such as the ribosomal RNA genes.
- Organization and function of Hox genes that are present in all animals.
- Types of junctions between cells that are present in all animals.
- Animals have a common set of extracellular matrix molecules, ex. Collagen & proteoglycans
Embryonic develop wrt phylogeny:
- First few cell division of zygote = cleavage (# of cells doubles)
- Different cleavage patterns exist amongst different animals.
- During early development of animals, distinct cell layers form, which later develop into organs and
- The embryos of diploblastic animals have 2 of the cell layer – outer (called ectoderm) and inner
- The embryos of triploblastic animals have above 2 and a 3rd layer (called mesoderm) which lies
between the other 2.
- During early development, a hollow ball indents to form a cup-shaped structure, process =
- The opening of the cavity formed by this indentation is called the blastopore. The pattern of formation after the
appearance of the bastopore divides the triploblastic animals into 2 main groups: protostomes and deuterostomes.