Friday, November 21, 2008

The eight major evolutionary transitions

Regular readers of this blog can vouch for my fascination with the eight stage  theories and would no doubt be sympathetic when I report my exhilaration of finding that none other than Maynard Smith himself has proposed that there are eight major evolutionary transitions till date in the evolution of life. Maynard Smith and  Szathmary in their book The Major Transitions in Evolution had proposed for the following eight transitions :

More details are available at this wikipedia page . now I , independent of any knowledge that this has already been proposed by Maynard Smith as back as in 1995, a few days ago had come up with similar eight transitions in evolution of life forms . Of course there are some differences, but the important thing to note is the similarities(great minds think alike) and of course my model is far more accurate and realistic than Maynard smiths who I believe leap from multi-cellular organisms to humans quite arbitrarily leaving all the phylum in between un addressed.

I'll now briefly note the similarities and also highlight the dissimilarities in our approaches:

The first three stages are identical (first description of Maynard Smith stage and that is followed by my description in a few days ago post) :

1. Transition from Replicating molecules to "Populations" of molecules in compartments
1. Co-Evolution of genes and proteins/ amino-acids

2. Transition from Independent replicators (probably RNA) to Chromosomes
2. Evolution of the chromosome or two strands of DNA

3. Transitions from RNA as both genes and enzymes to DNA as genes; proteins as enzymes (Prokaryotes)
3. Evolution of a simple unicellular prokaryotic-bacteria-like cells

In the fourth stage I differ a bit from Maynard Smith, in that I propose for an intermediate archea type life-feom evolution while they jump straight to prokaroyotes)
4. Trasition from Prokaryotes to Eaukaryotes
4. Evolution of simple unicellular Archea-like cells

In the fifth stage they stress the importance of sex. I stress the importance of organalles, mitochondria and nucleus (specialized cell structures) instead.
5.Transition from Asexual clones to Sexual populations
5.Evolution of simple uni-cellular Eukaryotic like cells

In the sixth stage they move directly to multi-cellular organisms while I introduce intermediate colonies. I believe their fifth stage sexual populations are a substitute for my colonies (both map to protists)
6. Transition from Protists to Multicellular organisms — animals, plants, fungi
6.Evolution of simple colonies of cells (first animal phylum: the porifera or sponges)

In the seventh stage they make a leap and go directly to full-fledged solitary individuals (animals, plants fungi) while I take a more conservative approach and introduce multi-cellular organisms now from colonies.
7. Transition from Solitary individuals to Colonies with non-reproductive castes
7. Evolution of multi-cellular organisms with digestive tracts (second animal phyla coelenterate)

In the eighth and final stage they leap from primates to humans while I stay with multi-cellular organisms but introduce a CNS for the first time.
8.Transition from Primate societies to Human societies with language, enabling memes
8. Evolution of multi-cellular organisms moving towards a CNS( bilaterality) (third animal phyla :Ctenophora (Comb Jellies)):

I believe that after multi-cellular organisms they have made big leaps (which may be justified in some contexts), but I have worked more on micro level and believe that we can gain much more by studying the intermediate phyla too. The important thing to note is the common evolutionary and taxonomic approach and the guiding principles as outlined below for each transition:

Maynard Smith and Szathmary identified several properties common to the transitions:

  1. Smaller entities have often come about together to form larger entities. e.g. Chromosomes, eukaryotes, sex multicellular colonies.
  2. Smaller entities often become differentiated as part of a larger entity. e.g. DNA & protein, organelles, anisogamy, tissues, castes
  3. The smaller entities are often unable to replicate in the absence of the larger entity. e.g. Organelles, tissues, castes
  4. The smaller entities can sometimes disrupt the development of the larger entity e.g. Meiotic drive (selfish non-Mendelian genes), parthenogenesis, cancers, coup d’état
  5. New ways of transmitting information have arisen.e.g. DNA-protein, cell heredity, epigenesis, universal grammar.

Hat Tip: Shared Symbolic Storage blog

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