Monday, September 15, 2008

Evolution of Life: the eight stage process repeating again and again?

This post is regarding the evolution of Life-forms on earth. I'll start from the primordial soup/ sandwich and try to show how life developed in stages and how development of a particular life-form was an adaptation to a particular adaptive problem. My thesis is that life should evolve in eight stages each , with each evolutionary stage solving one adaptive problem.

For reference, I have heavily used this post titled 'The Making of Cat' by Roger Berton and Nancy Creek. I would however present the finding in my own idiosyncratic way , using as my reference the eight-fold evolutionary/ developmental stages. I have also used the 21 major animal phyla classification as present on Wayne's Word site.

  1. Co-Evolution of genes and proteins/ amino-acids: Life first originated in the primordial soup/sandwich of molecular compounds. Proteins may be thought of as chemicals (enzymes) that helped speed up the chemical process in desired direction and provided stability to the gene-protein complex, while at the same time destabilizing other combination of compounds; while genes as replicators that ensured that the gene-protein complex could not only survive but reproduce or help make copies of oneself. Here the first problem was that of how to avoid being broken-up by other proteins/ enzyme that worked to break other chemical compounds in the soup. Thus the evolution of genes and proteins was primarily driven by how they could become stable and get into such stable configurations that the corrosive influence of the primordial soup could be withstood and an identity asserted!
  2. Evolution of the chromosome or two strands of DNA: Once stable gene-protein couplings could come together the next problem was how to extract the maximum from the primordial soup for self-maintenance and self-enhancement. The problem was solved by genes and non-genetic code coming together to form a DNA strand and then two DNA strands and a layer of water coming together to form a chromosome. A similar approach was taken by viruses, but it contained RNA instead of DNA and hence juts a single strand, which proved ineffective against the double helix. Eventually, though viruses continue to evolve, life evolved in the direction of DNA.
  3. Evolution of a simple unicellular prokaryotic-bacteria-like cells: Once chromosomes outwitted viruses, the next problem facing them was how to maximally defend against predators (other destabilizing compounds) and also eat or grow maximally (use the soup maximally). Here they thought that forming alliance was a good step. So a few chromosomes came together and the chromosomes and the proteins they made, especially the outer cellular wall, gave rise to simple prokaryotic cells. These cells were simple- no nucleus, no specialized organelles. The key was that 2 or 24 chromosomes were better than single chromosomes.
  4. Evolution of simple unicellular Archea-like cells: It is assumed that Archea is just a type of bacteria or Prokaryotes, but it has been proposed that these are more similar to Euaryotes than prokaryotes and may be the missing link in evolution and may have been the common ancestor of eukaryotes. Anyway, the problem facing the primordial animal after the first three problems had been faced was how to share resource optimally between one and one's offspring. The reproduction was still asexual but different asexual techniques like binary fission, multiple fission, fragmentation, budding etc were tried. Techniques like horizontal gene transfer came into picture. The whole idea being what is the best parental investment while reproducing asexually. Here also for the first time, DNA contained introns or non-coding DNA (whose significance, we still do not know!!).
  5. Evolution of simple uni-cellular Eukaryotic like cells: It is generally agreed that eukaryotes evolved from simple prokaryote-like cells, or better still Archaea like cells.
    These cells are more specialized and have a nucleus as well as other specialized structures enclosed in membranes. It is my thesis that this centralization of DNA in nucleus and also concurrent appearing of different specialized organelles like mitochondria was key step in evolution, that for the first time made permissible a central command system (nucleus). The adaptive problem to be solved was how to help those specialized structures that were related or kin-like from conflicting demands on the cytoplasm (the common pool) and a central command center (nucleus ) evolved!
  6. Evolution of simple colonies of cells (first animal phylum: the porifera or sponges) : Once a central command (nucleus) originated that could control the organelles within, it's command was turned outwards to manage conflicts with other similar cells and form a co-operating colony of identical cells. This was the biggest leap-to-date and gave rise to multi-cellular organisms.These were simple in the sense that all cells were the same : there was no specialization: no digestive tract. There was also radial symmetry. The problem to be solved was how to know which cells would co-operate and which not (akin to reading the cells mind or having a theory-of-cell-mind module) . Somehow, I believe that having radial symmetry sort of solved this trust problem.
  7. Evolution of multi-cellular organisms with digestive tracts (second animal phyla coelenterate): These are the modern day jelly fishes and corals. They solved the internal communication problem that was facing them. How to tell each cell what to do. Some cells specialized as digestive tract based on signaling during development. There are three classes : Hydrozoa (Hydra),Scyzophoa (jelly fish), Anthozoa (anemones and corals ) of these. Reef corals may form (1) fringing reefs extending out to 0.4 kilometers from shore; (2) barrier reefs separated by a lagoon of considerable width and depth from a shore; and (3) atolls or circular reefs that encircle a lagoon of water and not enclosing an island. this is just to highlight the importance of number three at stage seven of evolution! I also believe that for the first time reproduction sexually became paramount and gave rise to germ-line gametes of sperms and eggs and also soma cells that reproduced by mitosis and not meiosis. Specialization of cells into structures like Gonads became possible; just like the digestive tract, once the problem of internal communication and command was solved. Please also note that for the first time we have a polyp type or medusa like stage.
  8. Evolution of multi-cellular organisms moving towards a CNS( bilaterality) (third animal phyla :Ctenophora (Comb Jellies)): These have biradially symmetric bodies. It is my contention that a move from radial to biradial may have arisen just by chance and due to sexual selection and may have ultimately kled to bilaterally symmetric bodies, which somehow necessitated or gave rise to the CNS. Externally there are eight plates of fused cilia that resemble long combs; the rows of ciliated comb plates are used for locomotion. These are also bio-luminescent , perhaps another property to make them attractive to mates and arose out of sexual selection. The problem to be solved : attracting 'right' mates; the solution bio-luminescence and move towards bilateral symmetry. These are also solitary creatures and have no polyp stage.
This brings us finally to the completion of first round of evolution, with the move from genes to fully-functional multi-cellular animals; but still simple and not having a CNS. After this CNS somehow developed along with bilaterality and a new chain of evolution started. I've thus reset the count of evolutionary stage to 1.
  1. Phylum Platyhelminthes (Flatworms): bilateral symmetry with CNS,No body cavity.
  2. Phylum Nemertea (Ribbon Worms)
  3. Phylum Rotifera (Rotifers): Coelem incomplete.
  4. Phylum Gastrotricha (Gastrotrichs).
  5. Phylum Nematomorpha (Horsehair Worms).
  6. Phylum Nematoda (Nematodes): a special level of evolutionary jump and that is why we scientists study this a lot.
  7. Phylum Acanthocephala (Spiny-Headed Worms).
  8. Phylum Bryozoa (Bryozoans): body with, for the first time, a true coelom.
And of course this paves way for the next wave of evolution of protosomians: Blastopore forms mouth, schizocoelom present. Their list goes as follows: again evolutionary stage reset to 1.

  1. Phylum Tardigrada (Tardigrades).
  2. Phylum Brachiopoda (Brachiopods).
  3. Phylum Mollusca (Mollusks).
  4. Phylum Annelida (Segmented Worms).
  5. Phylum Sipunculoidea (Peanut Worms).
  6. Phylum Arthropoda (Arthropods): Evolutionary jump. Body consisting of three parts: head, thorax and abdomen.
  7. Phylum Chaetognatha (Arrow Worms). Phylum Echinodermata (Echinoderms).I'll like to club these two together.
  8. Phylum Hemichordata (Acorn Worms):

And then we come to another major evolutionary jump or invention: the spinal chord: the phylum chordata or vertebrates, having a spinal chord. The classes within vertebrates (chordata):
  1. Class Osteichthyes (bony fishes) : driven by avoiding predation
  2. Class Amphibia (Amphibians): driven by exploring surrounding
  3. Class Reptilia (Reptiles): driven by forming alliances between small groups
  4. Class Aves (Birds): driven by best reproductive/parental strategy
  5. Class Mammalia (Mammals): driven by kin-related concerns?/ specialization/ division of labor??
From the above it seems that much more good things (than mere humans/mammals) are in the offing!! I have bought (and actually generated the argument) the argument hook , line and sinker, what about you!

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3 comments:

Mike Haubrich, FCD said...

Are you saying that evolution still needs three more classes of chordata? That must be what the "more good things to come" phrase at the end of your post refers to.

Sandy G said...

Mike,

Evolution to me is a never ending process. I believe that there is room for more species and more classes to evolve. at least three more classes should evolve in chordata before we see an evolutionary leap. that is why the statement that much more good things to come!!

Pranav said...

Excellent summary & excellent coverage!
Started out with reading your consciousness related post(first in the series), but after sometime found myself here instead :)

A small helpful(IMVHO)addition would be to add the time taken by each stage (in % till date).

Also, double stranded and DNA virus ancestors (whose evolution pre-dates eukaryotes) might deserve a mention for cracking the double helix and continuing to cause much botheration to fellow beings, even till today :)

Thanks for the post!