Quote:
Originally Posted by Biochemist
All true, and a good point. I still am surprised (and it seems somewhat counterintuitive) that after "carving into stone" so much of the standardized cell infrastructure, that we could generate such a remarkable diversity in body plans without significant alteration to the standardized machinery...
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Hmmm, he said while scratching his chin
But why would a different body plan (3 body segments vs. 2; 8 legs vs. 6; etc.) need different machinery at the cellular level? The cellular machinery (CM) exists only for the benefit, survival and reproduction of the cell. Well, 99%, anyway. Nothing in the CM "knows" about the "body plan", whether there are 2 heads or 1, or any at all, 2 eyes or 3, antenna or none, etc. The only "external" factors the CM knows about are a select few chemical reactions. Excess potassium or sodium outside the cell will trigger protein gates buried in the cell walls. There exist a few "communication" processes whereby one cell can initiate a process in another. Apoptosis comes to mind. Among some bacteria, if their "wastes" become concentrated enough, all the cells imbedded in that waste change mode or form. Like, growing spores, shutting down metabolism, extruding a protective glue, growing structural fibers between cells, etc.
On the other hand, I could argue that multicelled organisms, like duckweed and giraffes, DO have macro large scale functions that demand a "different" kind of cell. Well duh!!!
They evolved a host of NEW CELL TYPES to support those new functions. The nerve cell has a significantly different CM than a blood cell, or a muscle cell or a bone-growing cell.
So, you were onto something, but just not what you anticipated.
CM did not have to change to support new body
plans.
NEW types of cells with a modified CM evolved to support new body
structures.
Now, given that any animal needs a nervous system and a digestive system, which are infinitely flexible in terms of size and configuration, animals can evolve an arbitrary number of body plans without any further changes to CM. Kind of a "one size fits all" solution.
The paucity of body plans, IMO, is due strictly to the non-viability of most body plans under evolutionary stresses. Eight-legged gazzelles never evolved because eight-legged lizards never evolved because eight-leg-segmented vermicular pseudopedes never evolved because their immediate predecessors took the path of developing a "spine" and becoming chordates at the point where their body plan was 4-symmetric, and mutations toward 8 legs
after that point really fucked up the embryonic nervous system, and left the progeny too clumsy to fend off fangy predators.
Once the complexity of the body plan increased past a certain point, older elemental parameters of the body plan (like, number of legs) get frozen in. More recent "emergent" parameters of the body plan (like, scales vs. hair) can still mutate for a few score MYrs, until those too are frozen in.
