- 1. Division of Labor
- 2. Larger Body Size (surface area to volume ratio)

- 1. First metazoans are from 700-650 mya in the later precambrian in the fossil deposits of the Edicara Hills of South Australia. (too advanced)

- 2. 800 mya trace fossils of burrowing organism from North Australia.

- 3. Not recognizable as extant taxa.

Therefore our theories come from living taxa.

All agree metazoan came from protozoa because of similarity of cell structure. But beyond this there is little agreement. There are basically two theories for the origin of animals:

1. Syncitial Theory
2. Colonial Theory

Syncitial Theory for the Origin of Animals

This theory makes three points:

1. Animal kingdom is polyphyletic sponges and cnidarians are not related to other animals or each other.

2. The flatworms (like planaria) arose by cellularization of the cytoplasm of a large, multinucleate ciliated protist

3. Sponges arose separately from flagellates.

All animals have vesicular nuclei

Ciliates have a macronucleus and micronucleus (these are not just big and little nuclei - they are structurally and functionally different from vesicular nuclei).

Supposed homologies:

Same size and appearance as small flatworms

Both excrete mucus

cytostome/mouth

conjugation/internal fertilization

Stephanopogon is not related to animals or to ciliates and therefore can not be the living representative of the ancestor to either group.

According to this theory the animals:

1. Are monophyletic (sponges and cndarians are related to the rest of the animals).

All these organisms share:

collar cells

collagen

blastula->gastrula development

Within this theory are two subtheories:

1. Plakula theory
2. Blastae theory

1. Plakula Theory

This theory was originally proposed by Butschlei (in 1884) when he stated that he believed that the most primitive metazoan after the sponges would be a flattened, disk-shaped, benthic organism.

2 layered flagellate colony

division of labor between the digestive cells facing the substrate and the upper cells (form a protective covering) - reproductive cells could lie on the upper surface or lie in patches between the other two layers.

eventually the edges fuse to form a digestive tract.

In recent times this story has been supported by Karl Grell through the discovery of Trichoplax.

Cross-section through a placozoan:

This story is supported by many marine ecologists who believe that nutrient distribution requires primitive metazoans to have been benthic.

Problem:

Phylogenetic analysis of the organisms suggests that Trichoplax is a cnidarian, not a primitive animal.

2. Blastae Theory

Haeckel (1874) - starting with the principle that ontogeny recapitulates phylogeny, there must be stages comparable to:

zygote (protist - in this case a choanoflagellate such as Codonsiga)

blastula (choanoflagellate Sphaerocea)

gastrula (originally thought of it as a cnidarian, but Libby Hyman in the 1960s argued that the earliest animals - ctenophores, cnidarians, and flatworms- had a blastula that became a solid ball of cells (called a planuloid ancestor).

One of the main differences between unicellular and multicellular organisms is cell differentiation. That is, cells become specialized for a specific function and, usually, take on a characteristic morphology. For example, your body has epithelial cells lining most of its surfaces. If you take an epithelial cell and grow them in tissue culture they will multiply but they will remain epithelial cells.

How do cells get locked into one type? In the early 1900's, scientists suggested that the genes that control a particular kind of cell (such as an epithelial cell, for example) are passed on to those cells during development and all other genes (such as those that control muscle cells) are filtered out. As you well know, that isn't the case - each of your cells has all the genes needed to make an exact copy of you. This means that some genes are not used in some cells -- for example, muscle cell genes are not used in epithelial cells.

Cell differentiation then depends on different genes being active in different cells. This occurs through a process called gene regulation. In gene regulation, one gene (called a regulator gene) acts a a switch that turns other genes on or off. A basic version of this process occurs in bacteria and protists as well as multicellular organisms.

Locking a cell into one type

In some protists (ciliates have been particularly well studied), some genes get turned off and stay off even after mitosis. The whole genome doesn't get reactivated until the protist goes through meiosis. This occurs when either a protein or a methyl group is added to the DNA blocking transcription of the gene at that site. When the DNA is replicated in mitosis, an enzyme copies the methyl group onto the new piece of DNA. When the cell goes through meiosis, however, the methyl groups are removed and the entire DNA is reactivated.

Master Control Genes of Differentiation

In the first steps of development in multicellular organisms, cells divide and spread but there is no differentiation. Embryos at this stage do, however, have an anterior to posterior gradient that appears to be inherited from the egg's cytoplasm. This gives the embryo an anterior "head" end and a posterior "tail" end.
Cell differentiation is under the control of a set of master genes known as the Hox genes. The Hox genes are a family of genes arranged in a sequence on a chromosome. The anterior-most gene controls the head end of the body by regulating which genes in the anterior cells are activated and which are methelyated (and so turned off). Developmental biologists have investigated the role of these genes in embryos of fruit flies by turning hox genes on and off - for example, by turning off the first gene and turning on the second gene in the anterior end of the animal they can cause legs instead of antenae to grow out of the head.
All animals have Hox genes, but animals that evolve later have more genes and more complex, segmented bodies. Also the genes that are controlled change over time - it may be that the origin of new phyla occurs because of these switches.