The Evolution of Early Animal Complexity

In animal evolution, structural complexity has increased in the evolution of animals so that there is considerable specialization and division of labor within body tissues.Some of the trends seen in animal evolution are:

I. Grades of Organization

An animal (or a plant for that matter) is composed of many units organized into successive units: Molecules are the units of organelles, Organelles are the units that make up cells, Cells are the units that make up tissues, Tissues are the units that make up organs, and Organs make up organ systems

Each level is more complex than the one before and, as a general rule, a more recent evolutionary product.

A. Protoplasmic grade of organization.
All life functions are confined within the boundaries of a single cell. Within the cell, the protoplasm is differentiated into organelles capable of carrying out specialized functions. (e.g., the protists)

B. Cellular grade of organization.
Cellular organization is an aggregation of cells that are functionally differentiated. A division of labor is evident, so that some cells are concerned with, for example, reproduction, others with nutrition. Such cells do not become organized into true tissues but may form definite patterns or layers. Sponges are at this level of organization.

C. Tissue grade of organization.
Cells all of one type begin to function in a unified way to accomplish a task. Cnidarians are usually considered to be at this level of organization.

D. Organ grade of organization.
The aggregation of different kinds of tissues into organs is a further advancement in the evolution of animals. Organs appear first in the Platyhelminthese (flatworms).

E. Organ system grade of organization.
When organs work together to perform some function (circulation, respiration, reproduction, digestion, etc.) we have the grade of organization seen in all animals that evolved after the flatworms.

II. Increased Complexity of Development
Embryology is the study of the progressive growth and differentiation that occurs during the transformation of a fertilized egg to a new individual. A brief summary of development is necessary for understanding the early evolution of animals.

A. Review: General Pattern of Development

1. Fertilization - In all animals, germ cells produce by meiosis eggs or sperm. The fusion of an egg or sperm to form a zygote is called fertilization. This is the starting point for development.

2. Cleavage - The division of the zygote into smaller and smaller cells.

3. Blastulation - cleavage eventually gives rise to a hollow ball of tiny cells called a blastula.

4. Gastrulation - The sorting out of cells of the blastula into layers (ectoderm, mesoderm, endoderm) that become committed to the formation of future body organs.

5. Differentiation - the formation of body tissues and organs. The basic body plan of the animal is established.

6. Growth - increased size of the animal.

Differentiation is a key feature of multicellular life:

Emergence of a New Field: Evolution and Development (Evo/Devo) - new insights into the origin and evolution of multicellular organisms.

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.

For example, use of the milk sugar lactose by E. coli in your digestive tract. E coli has three genes that produce enzymes to break down lactose and release ATP. These genes are preceeded on the DNA strand by a promoter (a base sequence that signals the start of a gene) and an operator (an intervening sequence with an active binding site):

Lactose isn't always present, so the bacterium does not need to make the enzymes necessary to break it down. The regulatory gene makes a repressor protein that binds with the operator and stops the production of the enzyme:

When lactose is present, it binds with the repressor protein so that the receptor site in the operator is empty, and the gene is transcribed:

You could also have an activator protein being made that turns an operator on rather than off.

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.

B. Constraints

1. Size -
As a body increases in size, the ratio of surface area to volume increases.

Volume increases by body length 3
Surface area increases by body length2

In small animals the surface area is great enough in proportion to the volume to exchange gases and waste by diffusion as well as move nutrients around the body by diffusion.

Larger animals must have organ systems to pump gases and wastes in and out (e.g., such as a circulatory system).

2. Habitat - animals that live in marine water are isotonic with it; animals that live in freshwater must have a system of eliminating the water that leaks into their body by osmosis (they have an internal concentration of salt higher than the surounding environment); terrestrial animals must cope with drying conditions.

Body weight in and out of water

Protect gametes in and out of water

wastes in and out of water

3. Speed of locomotion

Sessile (not moving) or slow moving animals must be aware of potential predators or prey moving in on any side, so they are often radial or bi-radial in symmetry

Rapidly moving animals must be more concerned about the environment that they are moving into, so they often have bilateral symmetry with cephalization (a "head end").

This, in turn will affect how an animal feeds: a motile animal can move towards food; a sessile animal has to bring the fodd to itself (by setting up a water current, for example).

But organisms also carry "phylogenetic baggage" - for example a sessile vertebrate will tend to have more radial symmetry, but it will still have bones!

III. Survey of Early Animal Evolution

At the end of the Precambrian the continents were joined in a single supercontinent called Rodinia (from the Russian word for "homeland", rodina).

As the first part of the Paleozoic began (the Cambrian), Rodinia began to fragment into smaller continents which did not always correspond to the ones we see today:

Most of North America lay in warm southern tropical and temperate latitudesand the east coast bordered Greenland and Britian.
Siberiawas a separate continent due east of North America.
Baltica -- what is now Scandinavia, eastern Europe, and European Russia -- lay to the south.
What is now China and east Asia was fragmented at the time, with the fragments north and west of Australia.
Most of the rest of the continents were joined in a supercontinent; South America, Africa, Antarctica, India, and Australia are all present.

The climate was mild with no glaciation.

B. Ediacaran Organisms

Until the middle of the 20th century, it was generally believed that animals did not evolve until the Cambrian. In 1947, R.C. Sprigg discovered a strange group of fossils named the Edicaran animals. They appear to have led a placid existence on the ocean floor, absorbing nutrients from seawater or manufacturing them with the help of symbiotic bacteria:

There is a debate about the significance of these organimsms:
1. Some scientists argue that these organisms have a unique body construction with their bodies into compartments so plumped with protoplasm that they resembled air mattresses. Thus they represent a type of organism that is now totally extinct and may even be considered to be a separate kingdom from the animals.

2. Other scientists suggest that the fossils are remains of primitive representatives of still living animal phyla (e.g., flatworms, cnidarians, etc) or of fungi (e.g., perhaps lichens).
At this time there is not enough evidence to say with certainty what the Edicarians were -- the fossils are widespread but rare.

C. The Cambrian Explosion
The end of the Precambrian and the beginning of the Paleozoic (the Cambrian Period) marks an important point in the history of life on earth; it is the time when most of the major groups of animals first appear in the fossil record. This event is sometimes called the "Cambrian Explosion," because of the relatively short time over which this diversity of forms appears.

1. The cellular level of organization -

a. Phylum Porifera (Cambrian) -- cell differentiation, but no organs or true tissues

Sponges belong to the phylum Porifera.

When sponges reproduce sexually, the zygote divides (=cleavage). The cells divide to form a flattened blastula with cilia lining the blastocoel. This blastula turns inside out to form a solid, free-swimming planula larvae.

The swimming planula larvae settles down and the cells differentiate into one of 4 types (no gastrula is formed and tissues do not develop):
i. Pinacocytes - thin flattened cells that cover the exterior surface of the sponge.

ii. Porocytes - bead-shaped cells that allow water to flow through from the surrounding ocean into the internal canals and chambers of the sponge.

iii. Choanocytes - collar cells that line the internal canals and chambers of the sponge. The beat of the flagellum on the collar cell pulls water through the sieve-like collar and forces it through the top of the collar. Particles of food become trapped on the collar, are carried down to the cell surface where they are ingested.

iv. Amebocytes - ameboid cells move about in the mesoglea (a non-cellular layer between the pinacocytes and choanocytes. They aid in transport of food and nutrients and secrete the spongy-to-hard skeleton of sponges.

There is no nervous system, no muscular system, and no digestive system.

Given that sponges are almost all sessile, marine organisms, what can you conclude about their symmetry, need for complex organ systems, and reproduction? The few freshwater forms use a contractile vacuole to regulate water content.

B. Are archaeocyathids sponges?

Among the most frequently found fossils in the Early Cambrian are archaeocythids -
curious champagne glass-shaped organisms. Their body construction is basically that of two cups, one inside the other, each with sieve-like walls.

The two cups are held apart by thin radial walls. The base of the organism was obviously adapted for anchoring it to the sea floor.

Archaeocyath species were very important members of early Cambrian communities. They diversified into hundreds of species during this time period and some of these species contributed greatly to the creation of the first reefs.

Reef ecosystems tend to support a wide variety of organisms both in the present and in the past. Despite their great success in terms of numbers, the archaeocyaths were a short-lived group. They were almost completely non-existent by the middle Cambrian, some 10 to 15 million years after their first appearance.

2. The Tissue Level of Organization - Phylum Cnidaria (Cambrian)

The phylum Cnidaria is a very successful group of more than 9000 species.
It includes sea anemones, jellyfish, corals, sea whips, and sea fans.

The phylum takes its name from the cells (cnidocytes) which contain the stinging structures nematocysts.

These characteristic cells are found in no other phylum and may account for the long-term success of this group - the members of the group are sessile or slow moving but still manage to capture fast-moving prey using the nematocysts and the nematocysts make a good defense against predators.

1. Review Development

2. Basic body plan
The gastrovascular cavity with a single opening that serves as both the mouth and anus.

The Cnidaria are said to be diploblastic: made up of two tissue layers -

Result: an ectoderm (epidermis) and endoderm (gastrodermis) separated by a noncellular mesoglea (nerve cells branch through the mesoglea):

The nerve net has no directionality and cellular-muscular system does not provide powerful locomotion (why this is is described in class)

3. two body forms: medusa and polyp:

Most Cnidaria are marine; freshwater forms use a contractile vacuole to regulate water content.
3. The Organ Level of Organization - Phylum Platyhelminthese

Flatworms include many important parasites; but don't overlook the free- living forms (some can be quite nice):

Triploblastic - 3 tissue layers

Motility from cilia and muscular system (better locomotion than cnidaria because muscles can use mesoderm as a stronger lever than mesoglea).

Result cephalization with photosensitive cells (eyespots) and "brain" (large ganglion) at anterior end. Nerve ladder.

Other organs appear -
Reproductive system (hermaphrodites)
Osmoregulatory system (flame cells) - allows them to invade freshwater

But there is still a sac-like digestive track (mouth and anus same opening), and no circulatory or respiratory system (this limits the size of flatworms)