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. Review: General Pattern of Development
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.
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.
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:
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.At this time there is not enough evidence to say with certainty what the Edicarians were -- the fossils are widespread but rare.
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).
1. The cellular level of organization -
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.
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 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
The Cnidaria are said to be diploblastic: made up of two tissue layers -
3. two body forms: medusa and polyp:
Flatworms include many important parasites; but don't overlook the free- living forms (some can be quite nice):
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.
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)