We have looked for ways to to identify and group organisms by using morphology and development. This proves somewhat satisfactory for identification even today, however, it has always been problematical for relationship studies. Although less relied upon, it still dictates the classification schemes encountered in textbooks and to some extent, that found in systematic studies.
This is because until recently we have had so little other evidence to use. In 1909, our earliest (then) glimpse into what the ancestors of most invertebrate body patterns found today begins with the discovery of the Burgess shale. It took years to "mine" these fossils and they became the focus of serious study in 1950s. Recently older biota have been found in China (Chengjiang) and Greenland (Sirius Passet). About the same time, we began to appreciate the function of DNA as the information molecule and now molecular evidence is accepted as the most important of all that available to classify invertebrates.
Fossils
Limitations of Fossil Record
No record of many soft-bodied forms.
No evidence of Metazoan ancestors.
No clues to relationships of flatworms (acoelomate) and coelomates.
Why?
History of life
The earliest morphological evidence for life is 3.5 billion years old, fossils of stromatolites (colonies of cyanobacteria) and single, undifferentiated cells, or Prokaryotes.
For 1.6 billion years these simple cells were the only kind of living organism, until the arrival of Eukaryotes, or single cells with differentiated nuclei and cell organelles. Although representing a large leap in complexity, the Eukaryotes were still only single cells or cell aggregates.
It was another 1.4 billion years before complex, multicellular life made an appearance in the form of the Ediacaran faunas followed by all the variety of the true Cambrian animals about 550 million years ago.
Ediacaran or Vendian fauna
Animals first appear in the fossil record around 580 million years ago as frond-likeforms, jellyfish-like imprints, and trace fossils. These fossils appear simultaneously on all continents, except Antarctica, and each assemblage contains roughly the same kinds.
Some of these fossils are simple blobs that are hard to interpret and could represent almost anything. Most have been classified as cnidarians, some are more worm-like with segments or appear to some systematists to be soft-bodied relatives of the arthropods. Others are less easy to interpret and may belong to extinct phyla. Vendian rocks also contain trace fossils, probably made by wormlike animals slithering over mud.
At the base of the Cambrian period about 545 million years ago, all modern phyla of animals and many algae and protists appear or radiated. This event is sometimes called the "Cambrian Explosion", because of the relatively short time over which this diversity of forms appears. We know have several sites where organisms were preserved because they were buried rapidly in a mud slide. Burgess Shale type assemblages show that this is a radiation of soft-bodied as well as skeletonized organisms or Bauplans in general. Between 60% and 80% of the fossil remains are those of soft organisms. These
You will need to link to the Chengjiang, China and Sirius Passet Fromation, Greenland to see Cambrian fossils.
Why the Cambrian explosion?
Most explanations for this radiation involve properties of animals themselves (intrinsic causes), but the coincident events in algae and protozoans suggest perhaps a more ubiquitous, ecologic trigger (extrinsic causes). One possibility is oceanographic changes that increased nutrient supplies to the shallow waters, hence a radiation of trophic links in the food chains of that time. Oxygen levels were increasing for the same reason. After the glacial period, temperatures rose until by the time of the Cambrian itself the climate was warmer than it is today. We also do not know why the innovation stopped. There have been extinctions since the Cambrian but an increase in very different bauplans has not been part of any following radiation of forms. Now we have evidence from two other localities that are older. Chengiang adds many new species to a Burgress shale type setting, so sets the time for the Cambrian explosion back about 10-15 million years. Interestingly enough there are many more chordates and ancestors in this biota.
The Sirius Passet biota are also about 15 million years older but hint at simpler forms (perhaps living deeper?), but also give more evidence for more worm like and mollusk type of fossils.
The Cambrian explosion is still about 700-300 million years later than the molecular data suggest that animals originated, leaving an enormous period of time without a fossil record. Perhaps, the first animals were small, unskeletonized, or destroyed by geologic processes.
Examining rRNA sequences supports basic phylogeny as indicated so far. DNA and some RNA sequence evidence indicate that diversification started earlier but support that diversification occurred rapidly on a geological scale. In fact, most of the classification you will learn for invertebrates in this courses rest on comparative rRNA sequence analysis.
Why not compare structural gene sequences, or those genes that code for eyes, livers, the structures that make up a body?
Genes and the Cambrian explosion.
We need to examine regulatory genes since it is small changes in the developmental pathway that we believe are responsible for the differences we see in phyla or crown clades.
Most of the work to date is on homeobox genes, (or a gene coding for a protein with a particular DNA binding motif known as the homeodomain) that controls axis and overall positioning and are highly conserved in evolutionary time. They encode transcription factors that act as molecular markers for the position of cells along the major body axis.
Example of similarity in some developmental genes:
If you compare a single hox gene (MSX gene) from sea urchins and mice: both of them consist of about 180 nucleotides, and both code for a protein of about 60 amino acids. The sequence of amino acids in the sea urchin and the mouse differ only in a single amino acid. The rest have remained the same over 600 million years of evolution.
Hox genes are so amazingly conservative that geneticists do all sorts of weird things like remove a hox eye gene from a fly and replace it with the equivalent human hox gene and produce an absolutely normal fly with a compound eye.
Example of a difference in body plan that a small change in the expression of a regulatory gene can make.
This conservatism has a major implication, it means that hox gene clusters probably evolved only once and so all animals evolved from a single ancestor. It also substantiates a radiation over a relatively short geological timespan, although it might have taken place before the Cambrian.
However, it does not indicate anything about the bauplan of that ancestor because hox gene clusters can be lost as well as added.
We also need more information on the next level of regulatory genes, the ones beyond those that control the overall pattern. The ones that control what specific organ, etc. develops at a specific time and place.
Take home messages.
By Cambrian have major body or bauplans or so all basic genes. Evolution since then has mostly tinkered with variation within a bauplan. So animals have gotten more diverse, but not usually by the appearance of radically new bauplans. The new phylogeny will be based on what we find out how development is controlled and changed not only to get different bauplans, but variations of a bauplan through time.
