As we move along the evolutionary line, phyla become larger and larger. As an example of this, the phylum Cnidaria contains jellyfish, corals and sea anemones, and a few other species. Human on the other hand, are in the phylum Chordata, which includes ALL organisms with a central nerve cord, with or without vertebrae, so this means that all fish, birds, amphibians, reptiles and mammals, as well as a large number of organisms such as sea squirts and lancelets.
Up til now I have been broadly covering one phylum at a time (I skipped some from the previous phylum because they are parasitic, and are later developments), but from now on I will mostly be looking at how the development of life occurred, as we head along the tree to where we are today. I am skipping over some organisms such as Rotifera, and parasites as whilst these are interesting, they are are not vital to the journey up the tree, as well as terrestrial species of Platyhelminthes etc because at this stage in the story we are still in the marine environment. This means I may not be directly following through one phylum into another, but rather showing examples of organisms which illustrate specific points along the tree, as well as showing some organisms which, for whatever reason, did not make it through to today. I find this fascinating, and I hope I can convey that across to you.
To illustrate what I mean about jumping around, the image below is the tree of life. We have covered so far, the bacteria, the jellyfish (centre top of the diagram), and flatworms (about halfway down the right hand side of the diagram). We are in the Ediacaran period, just about to move into the Cambrian (the 3rd and 4th rings around the diagram).
Alright, so after that slightly boring introduction, where are we going to today?
I was planning on showing you where our journey takes us after flatworms, but in researching this, I decided it was better to lay out a little background first, so today is about fossils, and what life was like in the early seas.
This image is of a living sea pen, so you can see the physical similarity between them. Discussion as to whether they are related is centered around where the growth of new polyps occurs. In living sea pens, new growth happens at the base of the fronds (near the “stem”), and so pushes the branches outwards as it grows, whereas in Charnia, the new growth appears to have occurred at the end of the branches (See references for link to journal paper, Antcliffe & Brasier 2007)
Other common fossils from the Edicarian (or Vendian) period resemble other Cnidarians, but again, discussion continues as to whether they are indeed like these, and related to them, or members of extinct groups of organisms.
The final image shows one of the Cnidaria thought to be similar to Dickinsonia.
And one more of the Fungia coral
There are other fossils from the Precambrian period which have physical similarities to jellyfish or sea anemones today, but their exact nature, and what their body shape was exactly, is a subject of ongoing discussion among paleobiologists (People who study fossil biology). In any event, it appears that few of the Precambrian fossils which we find have living relatives today, and the majority of organisms alive today arose during the Cambrian period, and so it is this period I will be focusing on. If you are interested in Precambrian fossils, there is an amazing location in Newfoundland called Mistaken Point, and several location in Russia which have fossils from this period. Sites I have been browsing around while reading up on this have been UCMP (University of California Museum of Paleontology) and their site on the Vendian period and also Fossil Museum.net
Flatworms are thought to have arisen when there was not a great deal of sedimentation on the sea floor, and predators were not very mobile, especially on the bottom of the sea. Jellyfish mostly move up and down the water column, and corals are not mobile. However, as time progressed, sediment accumulated on the sea floor, and as smaller flatworms may have been prey for larger ones. Flatworms are a good shape for moving along the sea floor, but not so great for burrowing to hide from predators, or find food in the sediment.
There were places to hide in the early seas, there were microbial mats (the mats were likely from the blue-green algae, or cyanobacteria, which I talked about here), and today they are found in many extreme environments as well. The image below shows a mat formed by organisms which use methane as a food source (methanotrophs)
It is thought that organisms could hide under these mats, or feed on the microbes which make them up, and burrowing fossils have been found from the Precambrian period, which seem to indicate that there were organisms which could burrow horizontally, that is, dig a little into the sediment and move across the floor just below the surface.
These burrowing fossils would seem to indicate that the organisms were round rather than flat, and the image below shows more clearly the comparison I made with the traces you see of worms on beaches.
So, to recap the rambling post above, the history of life in the Precambrian is a bit messy, and whilst there is evidence of life forms which are similar to those today, it seems that they are not direct ancestors, so as we move forwards, whilst I will cover some Precambrian stuff, most of the organisms I cover will be from the Cambrian period onwards.
I hope I managed to give a bit of an explanation of why flatworms may have evolved towards rounded shapes, and the next post will attempt to cover the movement from these rounded worms to the next group. The explanations I have written today are by no means set in stone, and are among many different explanations for life in the Precambrian. One of the things I love most about science is that ideas are always open to challenges, and what is considered as a possibility, even written in journals, may be discounted if sufficient evidence arises against it.
For the more sciencey people reading, I have had some difficulties within the Precambrian, as the phyla present there are not directly related to those around today, yet seem to have morphological similarities (annelids and cnidaria etc). I felt that it was important to take a detour into this period, as many people are unaware of the existence of life in the Precambrian due to the amount of times that “the Cambrian explosion” is used for the beginning of complex life. I have tried to give a possible reason for the change in morphology from flatworms onwards, and next post will move into brachiopoda as I think these came along next, then onto the basic mollusca.
I will try my best to make the next post less technical and heavy, however, I think that the readers of this blog do not want the information watered down too much, and you are more than welcome to ask if there is anything you do not understand, and I will do my best to answer it, or try to point you in the direction of information on the subject.
Charnia growth patterns: http://oxford.academia.edu/MartinBrasier/Papers/416927/Charnia_and_Sea_Pens_Are_Poles_Apart