Here is How the Cytoskeleton Evolved

Not Easy to Explain

Though illustrations of the cell often depict it as a bag full of various organelles and folded membranes, this fundamental unit of life is actually organized upon a highly-structured three-dimensional truss structure known as the cytoskeleton. Until the early 1990s the cytoskeleton had been observed only in the more complex eukaryotic cells. But a series of detailed studies emerged indicating that the other two domains of life (bacteria and archaea) also have cytoskeletons. The wikipedia entry gives a good introduction to this subject:

The cytoskeleton is a network of fibers composed of proteins contained within a cell's cytoplasm. Although the name implies the cytoskeleton to be stable, it is a dynamic structure, parts of which are constantly destroyed, renewed or newly constructed.

In most cells of all domains of life (archaea, bacteria, eukaryotes) a cytoskeleton is found (notably in all eukaryotic cells which includes human, animal and plant cells). The cytoskeletal systems of different organisms are composed by similar proteins. However, structure, function and dynamic behaviour of the cytoskeleton can be very different, depending on organism and cell type. Similarly, within the same cell type the structure, dynamic behaviour, and function of the cytoskeleton can change through association with other proteins and the previous history of the network.

The cytoskeleton of eukaryotes (including human and all animals cells) has three major components: microfilaments composed of the protein actin and microtubules composed of the protein tubulin are present in all eukaryotic cells. By contrast intermediate filaments, which have more that 60 different building block proteins have so far only been found in animal cells (apart from one non-eukaryotic bacterial intermediate filament crescentin). The complexity of the eukaryotic cytoskeleton emerges from the interaction with hundreds of associated proteins like molecular motors, crosslinkers, capping proteins and nucleation promoting factors.

There is a multitude of functions the cytoskeleton can perform: It gives the cell shape and mechanical resistance to deformation; through association with extracellular connective tissue and other cells it stabilizes entire tissues; it can actively contract, thereby deforming the cell and the cell's environment and allowing cells to migrate; it is involved in many cell signaling pathways; it is involved in the uptake of extracellular material (endocytosis); it segregates chromosomes during cellular division; it is involved in cytokinesis - the division of a mother cell into two daughter cells; it provides a scaffold to organize the contents of the cell in space and for intracellular transport (for example, the movement of vesicles and organelles within the cell); it can be a template for the construction of a cell wall. Furthermore, it forms specialized structures such as flagella, cilia, lamellipodia and podosomes.

A large scale example of an action performed by the cytoskeleton is muscle contraction. During contraction of a muscle, within each muscle cell, myosin molecular motors collectively excert forces on parallel actin filaments. This action contracts the muscle cell, and through the synchronous process in many muscle cells, the entire muscle.

Evolutionary theory predicts there to be an evolutionary progression of cytoskeleton designs, as this key aspect of the cell design evolved. But this is not what the science reveals.

For example, in eukaryotes, the proteins actin and tubulin are the building blocks for the microfilament and microtubule structures, respectively. In bacteria and archaea these roles are performed by proteins such as MreB and FtsZ, respectively. But these cousin proteins do not reveals signs of an evolutionary progression. The actin and tubulin proteins show very few changes between different species. In fact they are among the most highly conserved proteins in the eukaryotes.

Even between species as different as yeast and rabbits there is only about a 12% difference in the respective actin proteins. Therefore there is no sign of how a gradual progression of protein evolution could have arrived at the actin and tubulin building block proteins. Importantly, this includes the MreB and FtsZ proteins. The sequence relationships between actin and MreB, and between tubulin and FtsZ, are essentially what we find between any two randomly selected proteins. With evolution we must believe that molecular evolution traversed an enormous gap without leaving a trace of sequence evidence.

This finding is not restricted to the molecular sequence data. The function and distribution of the bacterial components vary dramatically from what we find in the eukaryotes. As one review paper admitted,

it has become clear that there is no simple relationship between the cytoskeletons of prokaryotes and eukaryotes. Moreover, there is considerable diversity in both composition and function between cytoskeletons in different lines of prokaryotes and eukaryotes.

In fact the bacterial designs are highly divergent amongst themselves. Molecular sequences, proteins used, lateral interactions within the filament, polarity (left-handed versus right-handed filaments), and so forth, are all inconsistent across the bacteria. It is not a story of an evolutionary progression.

Another surprise for evolutionists is much of the eukaryotic cytoskeletal functionality must trace back to the first eukaryotic cell—the so-called LECA or Last Eukaryotic Common Ancestor. It is yet another case of complexity pushed farther and farther back in history, to the era of early evolution where the supposed evolution of such complexity is hidden in evolutionary gaps. Here is a particularly candid admission from our review paper:

One of the most surprising results of our increasing ability to probe the characteristics of the LECA has been how much of the biological complexity in extant cells can be traced back to this ancestral cell. The LECA possessed much of the complexity now seen in the replisome, the spliceosome, and the endocytic system, as well as the machineries necessary for meiosis and phagotrophy. Moreover, comparative analysis of the genome of the free-living excavate Naegleria gruberi identified ∼4,000 protein groups that probably were present in the LECA.

This “complexity early” model of eukaryotic evolution is mirrored in the cytoskeleton (Fig. 2 D). Somewhere in the evolutionary space between prokaryotes and the LECA, single proto-tubulin and proto-actin molecules diversified into multiple specialized forms. Three classes of motors arose independently, and evolved to include at least nine classes of dynein, eleven classes of kinesin, and three classes of myosin. As well as these, the axoneme formed, with 100–200 associated proteins, many of which have no prokaryotic orthologues. Between the prokaryotes and the LECA, a revolution occurred in cytoskeletal biology.

Such complexity cannot have appeared fully formed, but arose by stepwise elaborations of cell structure (and genetic repertoire). However, the large number of simpler intermediate forms that must have existed appear to have left no descendants. This is perhaps because a great many of these changes occurred in a relatively short time, with one innovation creating a favorable landscape for the evolution of the next. Alternatively, all descendants of these intermediate forms have been simply out-competed by the arrival of the LECA, with its mitochondrial endosymbiont, endomembrane system, and sophisticated cytoskeleton. What is clear is that since this complex LECA, the diversification into many eukaryotic lineages has often been accompanied not by the addition of further classes, but by loss of ancestral ones. Some of these losses are associated with loss of specific structures or functions (such as axonemal motility), but there appears to be a remarkable flexibility in the precise repertoire of many of these ancient families that is required for eukaryotic cell function.

From a scientific perspective, it would be difficult to imagine a more absurd narrative. Evolutionary explanations, such as this one, are the height of creative story-telling, contorting the theory to try and fit awkward facts.

h/t: La Victoria