An article published in Frontiers of Microbiology highlights the “the chasm in design between prokaryotic and eukaryotic cells.”
The Dominant Biological Paradigms of Life and Evolution
Three paradigms established in the 19th century, combined with advances in quantitative genetics in the 20th century, led to the dominant ‘textbook’ paradigm of biology where all life is cellular and descends from a common ancestor via the neo-Darwinian process of natural selection (e.g., Keeton and Gould, 1986).
Although the Modern Evolutionary Synthesis and the universal Tree of Life are very powerful paradigms, they are under challenge as several major tenets of the synthesis are being questioned (e.g., Doolittle, 1999; Dagan and Martin, 2006; Koonin, 2009; Koonin and Wolf, 2012). One challenge is their incompatibility with endosymbiotic processes operating at the origin of the eukaryotic domain (Koonin, 2009). Since the mitochondrion initially evolved separately from the ancestor of the eukaryotic cytoplasm, it arose via a symbiotic event (i.e., saltation) rather than an autogenous incremental process expected under the Modern Evolutionary Synthesis paradigm.
The “Grand Chasm” Between Eukaryotes and Prokaryotes
The eukaryotic cell is extraordinarily distinct from the much simpler bacterial and archaeal cells of the prokaryotic domains. It possesses not only a nucleus and a mitochondrion, but also a sophisticated endomembrane system, a complex cytoskeleton and a unique sexual cycle, leaving the gap between cells of prokaryotic and eukaryotic design as the greatest chasm in biology.
Finally, a notorious ‘queen of evolutionary problems’ related to the origin of the nucleus is the unresolved paradox of the origin of the eukaryotic sexual cycle (Bell, 1982). For the sexual cycle to function, two highly complex integrated but temporally and mechanistically unrelated processes must occur. Firstly, meiosis must occur to convert a diploid cell into four haploid daughter cells. This complex process is achieved by a single cycle of chromosomal replication generating a nucleus with 4 N ploidy and is followed by two mitosis-like cell divisions reducing the ploidy of the four daughter cells to 1 N.
The origin of this process is a paradox that has defied any generally accepted explanation for over 50 years and in part revolves around the challenge of determining which came first, meiosis that allows haploid gametes to be formed from a diploid or syngamy that allows 1 N haploid gametes to mate and create a 2 N diploid in the first place.
According to the modern evolutionary synthesis, incremental changes leading to the complex, unique and interrelated eukaryotic systems associated with the nucleus must have each provided an immediate selective advantage to an archaeal cell. Arguing these innovations were beneficial because they allowed the future evolution of complexity in the eukaryotic domain is clearly a teleological argument. It is particularly thought-provoking to explain these discontinuities in terms of incremental benefit when it appears that the eukaryotic system evolved only once in over 3.7 billion years and left no currently recognised intermediates, while the prokaryotic system remained highly efficient and conserved by the bacterial and archaeal domains for over 3.7 billion years.
In his lengthy article, Bell draws attention to evolutionary hurdles to the origin of eukaryotic cells. The “chasm” between eukaryotic and prokaryotic cells is causing a re-think of the universal common ancestor notion. Separate origins, however, of these types of cells would introduce further strain to the unguided evolution theory. Can we surmise that the evidence is more suitably consistent with intelligent design of these types of organisms?