Backwards before we go forwards

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).

Tree of life. Image from the BBC

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.

The image below is one of the oldest known animal fossils, Charnia, which is similar in its body shape to the sea pens I covered in an earlier post, although it is not thought to be related to it.

Charnia fossil, image from Wikipedia

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)

Modern day sea pen. Similar in shape to a Charnia, but not thought to be related. Image from UCMP

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.

Dickinsonia fossil. Thought to be similar to some species of Cnidaria today, although discussion is still ongoing.  Image from wikipedia

The final image shows one of the Cnidaria thought to be similar to Dickinsonia.

Fungia coral, image from wikipedia

And one more of the Fungia coral

Fungia coral, image from UCMP Berkeley

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)

Microbial mat on the sea floor (white and orange sections on the photo). Image from NOAA Ocean Explorer

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.

Ediacaran trace fossil showing horizontal burrowing (much like traces you see of worms on the beach) Image from wikipedia

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.

Helminthoidichnites fossils (Remember how Platyhelminthes we covered earlier means flat worm? the Helminthes part means worms, so this is worm-like fossils) Image from Amherst College

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.

References:

Charnia growth patterns: http://oxford.academia.edu/MartinBrasier/Papers/416927/Charnia_and_Sea_Pens_Are_Poles_Apart

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3 thoughts on “Backwards before we go forwards

  1. Here are some questions from an ignoramus with slightly flippant tendencies.
    What’s the Cambrian period and why’s it called that?
    How can you write about fossils and say that they’re not set in stone?
    Is there a point at which the Tree of Life isn’t linear with branches, but more like a network? Perhaps you’ve reached that point already in your story.

    • Fossils are indeed set in stone (luckily), however, the relationship between organisms which are only found in stones, and those alive today is not so stone-like 🙂 I will not be spending too much time with fossils, not because they are not interesting, but because I feel I can show the progression much better through living organisms (and videos are cooler than fossil pictures!)

      The Cambrian period is a geologic period that runs from approx 540-490 million years ago, and it is from early in this period that we first get large numbers of fossils. This is for a number of reasons, but mainly because organisms started having shells, and hard bodies, which are easier to fossilize than soft bodies.

      The Cambrian period is called that because it is the Latin name for Wales, and we have the best examples of our fossils from that period in Wales. This site gives a nice overview of the geology of the UK, and you can see if you click the “Make a map” button, the geologic periods, and if you mouse over them you can see their age. http://www.bgs.ac.uk/discoveringGeology/geologyOfBritain/makeamap/home.html
      Generally, in the UK, the very oldest rocks are in Scotland, especially the islands, and in England, as you move West to East, the rocks are younger.

      The Tree of Life is never really linear, it is more of a radial pattern, with a central starting point. It is not A goes to B goes to C, but rather A goes to B goes to C, M and Z. If you look at the link here, it shows the phylogenetic tree of just bilateral organisms, and from the left side of it, it splits into 3, which then splits further, so for example, the Platyhelminthes (flatworms), do not directly lead onto any other group, but are themselves an important step along the way, and are an earlier form of life than say, Arthropods, which share a common ancestor with flatworms
      http://tolweb.org/Bilateria/2459

      Finally, there is not one full tree of life (as far as I know), partially because it would eat insane amounts of resources to download and open it, but also because relationships between organisms are still being analysed. In the US, there is a large scale project under way to reconstruct relationships between organisms, and there is a link to it here: http://www.phylo.org/atol/

      For examples of how phylogenetic trees can look, and how complex they can be, here is a generalised one, showing around 3000 species.
      http://www.zo.utexas.edu/faculty/antisense/tree.pdf

      Usually they are shown as linear relationships for ease of reading, as with this one from Nature, for dogs http://www.nature.com/nature/journal/v438/n7069/fig_tab/nature04338_F10.html

      This one here, from bioblogs, shows the same thing, but in a radial diagram, and is much less easy to read. http://www.biology-blog.com/blogs/permalinks/3-2010/dogs-likely-originated-in-the-middle-east.html

      I think I covered everything! I hope I got it all right, as I am learning this stuff as I write about it!

  2. Pingback: Recap « skepticalsquirrel

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