EVOLUTION: QUIET PRE-CAMBRIAN GENES

THE PRE-CAMBRIAN ENVIRONMENT AND ANIMALS

Genes of the Pre-Cambrian can be inferred from common genes and biochemical pathways shared by extant species.  But there is reason for saying that genes that have not left echoes of their voice had an important role in evolution as well.  Those quiet genes, sometimes important for their silence, can also be inferred from analysis of how some characteristics developed, especially the annelid theory of chordate origin that, I maintain, is the best explanation of origin of chordates via pogonophorans and hemichordates.

Introns and exons

Natural selection is effective in eliminating useless features because there will be no selective force maintaining them, other than the chromosomal duplication process.  The portion of the genome active in protein coding is thought to be about 1.5 percent.  The 98.5 percent of the DNA not active in production of RNA needed to produce structural and functional proteins and control substances may have some value in other ways that are less dependent upon their exact nucleotide sequence.  In comparison to human DNA Collins (2006) shows the sequence for protein codes is 100% the same as in a chimpanzee, 75% the same in a chicken, and 35 % in a roundworm; whereas random segments of DNA between the genes only corresponded at rates that were 98% for a chimpanzee, 4 % for a chicken, and -0 % for a roundworm.  Human to human DNA comparisons show about 99.9% identical DNA.

What happened to the fossils?

Very little fossil record exists for times before the Cambrian.  Because fossils of numerous phyla show up in Cambrian rocks, it is apparent that many important evolutionary events occurred earlier.  I will omit discussion of sponge spicules, and tubes that may well have been made by pogonophorans, to go directly to presumed jellyfish fossils.  Pre-Cambrian jellyfish fossils were in the form of doughnut-shaped fossils thought to be casts of sand filled cavities of the bells of jellyfish.

It is not surprising that more ancient fossils are not found because extinction-causing events were more frequent as our orbit was more frequently hit by intrusion into our orbit of asteroids.  Localized destruction was probably much more frequent than the era-ending type giving worldwide unconformity of rock layers.  Even dating of intermediate layers produced by those major events may be difficult due to destruction of potential fossils.  Large areas may have been wiped clear by the event, so dating of sediments above and below an unconformity my show very different times of deposition.  A blended layer might be deposited in such locations and provide an intermediate date of origin.

Surviving extinction

Jellyfish deprived of food have the ability to survive and use their own substance as they "grow" smaller.  In the process they are found to first absorb reproductive organs completely as they shrink in size.  When food becomes reavailable the organs are eventually regenerated.  The finding that corals preceded jellyfish (described in an earlier post) provides support for the polyp-medusa-planarian sequence suggested.

The illustration above indicates how the polyp released from a starving coral could drift free to survive burial by sediments.  Adaptations aiding survival eventually result in the medusa shown in side view above and top view below where a later series of extinction events, depleting planktonic food, selected forms descending to sediments with nutrition enabling them to eventually take on the planarian shape that enabled better bottom feeding.

The jellyfish would naturally invert from the polyp orientation because the ring of tentacles would have a higher density due to concentration of protoplasmic structures and the potential buoyancy of the bell having trapped air bubbles.

The above enabled survival of jellyfish because their sexual generation could provide widely dispersed larvae capable of colonizing new post-extinction locations in shallow waters they required.  On the other hand pogonophorans survived because the abyssal region they inhabit is so extensive that remnants of habitat survived and the sediments themselves were the source of nutrition capable of bridging a long period of no nutrient input.  Also the extinction event may have given a temporary boost to dying and sedimenting food organisms replenishing abyssal food deposits.

Remember the quiet genes

Pogonophorans apparently adapted to the very low food supply by reduction of unneeded structure.  They have been shown to be able to take up amino acids from sediment water at naturally occurring concentrations.  Even though selection is not maintaining the code needed for gut formation, its slow rate of deterioration shown by the percentage comparisons in the second paragraph, enough remained to provide a basis for selection of a new gut much more efficiently than totally new selection would have required.  Most of the segment formation of annelids was lost by pogonophorans as well.  But enough of the process code survived to make the repeated structures - ribs, vertebrae, muscles, blood vessels, nerves - of chordate systems exist.

If every gene were always expressed, it would be difficult to have the diversity of cells making up our bodies.  There must be a very sensitive mechanism capable of turning gene activity on and off.  The process must also be varied in slight ways and sensitive to internal and/or external environmental cues to make well-adapted organisms.

Reference:  Collins, Francis S.  2006.  The Language of God.  Free Press (Simon & Schuster), New York, NY.  295 pp.

Joseph G. Engemann, Emeritus Professor of Biological Science, Western Michigan University, Kalamazoo, Michigan        March 6, 2015