Where are the prokaryotic mice?

By following the lead of Richard Dawkins, we realize that random variations coupled with natural selection can function as a designer-mimic – the blind watchmaker. But as I have noted earlier, all designers are constrained (and thus, in a way, guided) by their design material.

When we survey the living world today, it would be unjustified to assume that the blind watchmaker could craft a world of similarly complex and integrated creatures without proteins for the simple reason that the living world today is a protein-dependent reality. Without the use of proteins as design material, it is not clear what the designer-mimic could actually design.

But what if we moved up the ladder of complexity and considered the two basic cell designs – prokaryotic and eukaryotic.

The prokaryotic cell design is much simpler than the eurkaryotic cell design:

At one time it was thought that bacteria and other procaryotes were essentially “bags of enzymes” with no inherent cellular architecture. The development of the electron microscope in the 1950s revealed the distinct anatomical features of bacteria and confirmed the suspicion that they lacked a nuclear membrane. Procaryotes are cells of relatively simple construction, especially if compared to eucaryotes. Whereas eucaryotic cells have a preponderance of organelles with separate cellular functions, procaryotes carry out all cellular functions as individual units.

And this difference in complexity exists at all levels of the cell design.

Yet might the two different cell designs help us further appreciate the manner in which the designer-mimic is constrained by its design material?

Consider this simple observation – there are no prokaryotic mice. Where is the convergent evolutionary equivalent of a prokaryotic mouse? Why haven’t bacteria been able to match the level of structural complexity that is on dazzling display among the metazoans?
In their paper, “A molecular timescale of eukaryote evolution and the rise of complex multicellular life,” Hedges et al. (BMC Evolutionary Biology 2004, 4:2) estimate the the maximum number of cell types of common ancestors throughout evolutionary history. I’ve put their data into table format below:

Estimation of ancestral numbers of cell types
Organism Cell Type Number
Mammalia 120
Reptilia 120
Amphibia 120
Actinopterygii 120
Arthropoda 69
Agnatha 67
vascular plants 44
mosses 26
Cnidaria 22
Porifera 16
Hymenomycetes 9
Plectomycetes 9
chlorophytes 5
Saccharomyces 3
Mucorales/Blastocladiales 3
amoebozoans 3
Candida 2
Choanoflagellata 2
Euglenozoans 2
diplomonads 2
eubacteria 2
archaebacteria 2
Archiascomycetes 1

Note that while the vertebrate expression of the eukaryotic cell has been able to spawn 120 different cell types, both eubacteria and archaebacteria have not moved beyond a meager 2, even though these prokaryotes are both more numerous and older than eukaryotes. This suggests that had the eukaryotic cell design failed to emerge, the Earth would contain nothing more complex than any extant bacteria in existence today. And this suggests that the blind watchmaker, working with such an extremely adaptable cell as the prokaryotic cell, could not ever design something like a mouse. In order for the blind watchmaker to craft a mouse, it is reasonable to propose that it needed the basic architecture of the eukaryotic cell plan.

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2 responses to “Where are the prokaryotic mice?

  1. Pingback: Intron Distribution «

  2. Pingback: Origin of Eukarya: Follow the Clues «

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