XII.5 Multicellular organisms can overcome certain evolutionary constraints imposed on unicellular organisms; they can, for example, grow substantially bigger and can also have an incomparably more complex constitution
The size of a single-cell organism is limited by the size of a functional living cell.The size of modern cells is simultaneously apparently limited by the effectiveness of those biochemical and physiological processes involving diffusion of the reactants.In a prokaryotic cell of normal size, processes connected with diffusion or transport of molecules across a membrane tend to be important rather than the actual diffusion of molecules in the cytoplasm.In a eukaryotic cell which, in the typical case, has a volume 2,000x to 10,000x greater than a typical prokaryotic cell, the most important process would be the rate of diffusion of reactant molecules inside the cytoplasm.The time required for the transfer of particles with size similar to that of the molecule of a normal enzyme from one end of a cell to the other through simple diffusion has been estimated at more than 26 minutes (Wheatley 1985), and this is assuming that its free movement is not prevented by membrane structures or cytoskeletons.If we consider that there are only a very few molecules of some vital enzymes in the cell (Snol 1979), it becomes apparent that cells with large volumes could encounter substantial difficulties in overcoming the limited efficiency of internal processes through the rate of diffusion alone.It is highly probable that especially eukaryotic cells must have formed mechanisms that allowed them to at least partly avoid this limitation.These mechanisms include mixing the reactants by flow of the cytoplasm (Wheatley 1985), various targeted molecular “motors” mediating transport of bonded species to the sites where they are needed along the cytoskeletal components (Hayden & Allen 1984)or transport of free particles by electrophoresis (Cooper, Miller, & Froser 1999; DeLoof 1985; DeLoof 1986) or even isoelectric focusing in the cytosol (see Fig. VIII.6)Flegr, J. 1990 Does a cell perform isoelectric focusing? BioSystens 24, 127-133.an title="Flegr, J. 1990 Does a cell perform isoelectric focusing? BioSystens 24, 127-133."> (Flegr 1990).It is quite possible that it was just the evolutionary formation of such mechanisms, forming the basis for the functioning of large cells that facilitated the subsequent development of phagocytosis and thus, as a consequence, also the formation of modern eukaryotic cells containing organelles of the mitochondrial and plastid type formed by endosymbiosis.
It is not very apparent whether and how the formation of multicellular organisms is connected with the formation of eukaryotic cells.True multicellularity does not occur in prokaryotes.To a certain degree, cooperation of genetically and frequently even phylogenetically unrelated cells forming a multi-species consortium, for example biofilms, plays a similar role here; the individual cells can be very closely interconnected both functionally and morphologically within these consortia (Shapiro 1988). The reason why true multicellular organisms are encountered only in eukaryotes could be purely a result of historical chance.The highly improbable formation of sexual reproduction, which subsequently led to the transition of Darwinian evolution into Dawkinsian evolution (which alone creates preconditions for close cooperation of mutually genetically related but not identical cells, see XII.4.3) probably occurred only once, in the predecessors of modern eukaryotic organisms.
In multicellular organisms, difficulties associated with the limitation of the effectiveness of biochemical and physiological processes by the rate of diffusion are far less important.Even very small cells can form the large bodies of organisms in which the transport of reactants is performed through quite different means than diffusion, i.e. particularly through the creation of specialized tissues and organs permitting active ventilation, blood circulation, etc.Through the formation of multicellularity, organisms apparently overcame one of the most important evolutionary constraints of a physical nature, where the direct consequence of this evolutionary step was completely unparalleled development of the forms of life occurring on this planet.Multicellular organisms appeared in the fossil record approximately 600 million years ago (Morris 1993)and the vast majority of present-day strains appeared during the Cambrian, in a time interval of the order of tens of millions of years (Bowring et al. 1993).Simultaneously, it is necessary to bear in mind that life spread massively on the Earth approximately 3500 million years ago and most probably 300 million years sooner (Mojzsis et al. 1996).Contemporary results of molecular phylogenetics indicate that the individual strains of multicellular organisms branched off from one another long before the beginning of the Cambrian during a much longer period of time, probably of the order of hundreds of millions of years (Fortey, Briggs, & Wills 1997; Wray, Levinton, & Shapiro 1996; Wang, Kumar, & Hedges 1999, Douzery et al. 2004). Simultaneously, these results indicate that multicellular organisms, i.e. metazoa, fungi, metaphyta and rhodophyta, probably had a multicellular predecessor (Kumar & Rzhetsky 1996).However, on the basis of molecular data it is, in principle, not possible to determine anything about the anagenesis of the studied organisms and thus not even the period of time over which the members of this developmental branch evolved as single-cell organisms, when and how many times their transition to multicellular organisms occurred and how long the development of modern strains of multicellular organisms lasted.Thus, it will be very difficult to determine whether the formation of multicellularity is actually connected with the emergence of sexuality (or with any other evolutionary or environmental event) and to what degree they agree in time.Because of the existence of enormous diversity and disparity in eukaryotic unicellular organisms, it is, however, clear that the emergence of sexuality and the related transition to Dawkinsian evolution could be a necessary but not a sufficient precondition for the formation of multicellularity.
The considered scenario of the formation of multicellularity, i.e. the formation of eukaryotic cells – the chance formation of sexuality just in eukaryotic organisms – transition from Darwinian to Dawkinsian evolution – the formation of multicellular organisms with germinal and somatic tissues – is, of course, not the only possible one.A different scenario for the formation of multicellular organisms will be described in Chapter XIII.18.104.22.168.It is based on the following chain of causes and results:the formation of sexual reproduction – formation of diploidy – preferential development of trans-mechanisms of regulation of gene expression – formation of cell differentiation (i.e. formation of phenotype and functionally different cells with identical genomes) – formation of specialized multicellular tissues, organs and thus multicellular organisms.