I’ve long found it fascinating that every living thing on this planet can be cleanly split into two categories – prokaryotes and eukaryotes. The prokaryotes consist of all the bacteria while the eukaryotes include animals, plants, fungi, and various protozoa. The core life processes of the two cells are much the same, being built around the triad of proteins, RNA, and DNA, relying on the ribosome to build the proteins that synthesize everything else, including RNA and DNA, using ATP as the primary energy currency, and using lipid bilayer membranes to compartmentalize. So what makes the two cell plans so different?
Below is a nice figure that helps you answer this question.
As you can see, there are two primary differences: size and level of compartmentalization. Typical eukaryotic cells are much larger than bacteria and show a much more extensive level of compartmentalization given the numerous membrane-bound organelles and membranous folds.
Yet a question to ponder is why there are two cell types and only two cell types? The non-telic perspective would explain this (away?) as simply an artifact of a contingent past. There is no reason to ponder the question “why?” It just happened that way. But the telic perspective allows us to think of these two cell plans at a level that runs deeper.
I’ve long found it fascinating that these two types so nicely fit within a teleological template. The prokaryotes are the terraformers, as they are built to disperse, infiltrate, and transform. Many aspects of their cell biology fit these objectives perfectly, whether it be their high reproductive rates, remarkable ability to adapt to all sorts of extreme environments, or their ability to physiologically and genetically communicate with each other. As Craig Venter says, “We live on a microbial planet.”
But why terraform in the first place? That objective assumes the Earth needed to be transformed to adopt and facilitate later events. Enter the eukaryotic cell. Why does it even exist? The biosphere successfully existed for a very long time without it and when it comes to “life as a cell,” it is needlessly complex in just about everyway. Yet once we realize that nothing akin to an animal would have ever evolved without this cell design, suddenly its existence makes sense in the light of foresight. It exists because it is the cell type that can be used to facilitate the emergence of animal-type and plant-type complexity. Remember, once the eukayotic cell plan emerged, it did not need to be re-tooled any further to make metaozoan life possible. The potential for such later evolution was embedded in the very design of the cell type.
What is most interesting is that over the last year I have highlighted three features of the eukaryotic cell that all converge on the same point. It began with a discussion of protein-coding introns, where I responded to the faulty idea that introns do not fit into a teleological perspective. On the contrary, we uncovered good reason to think introns facilitated the emergence of metazoan-type complexity.
Later, I discovered a putative homolog of beta-catenins in green algae. As we looked more closely, the evidence held and, what’s more, we uncovered the plausible scenario where beta-catenins, needed for form epithelial tissue, were front-loaded to emerge because of their uncanny similarties with the alpha-importins.
Finally, I brought to your attention another team of scientists who have raised the strong argument that mitochondria were needed for metazoan life to emerge.
Yet I now want to bring to your attention that these three independent considerations all converge on the same point – the nucleus. Protein-coding introns exist because the nucleus separates RNA-processing events from translation by ribosomes. Beta-catenins exists because alpha-importins are needed to shuttle proteins in and out of the nucleus. The mitochondria exist because their genes were successfully shuttled to the nucleus. Add it all up and we can begin to appreciate that the nucleus, long recognized as the defining feature of eukaryotic cells, may very well have been the key feature that poised such cells for the eventual emergence of animals and plants in a world prepared for them by bacteria.