Symbiogenesis, gradualism, and mitochondrial energy in eukaryote origin

The origin of eukaryotes is one of the big questions in evolution. Many different ideas about eukaryote origin currently coexist in the literature that weight the evolutionary significance of mitochondria differently. Gradualist theories depict the origin of eukaryotic cell complexity and the origin of mitochondria as independent, unrelated processes that occurred in series. Symbiogenic theories depict the origin of eukaryotic cell complexity as emerging from the symbiosis that gave rise to mitochondria. Since it was introduced over 100 years ago, the idea of symbiogenesis has been controversial. It posits that rarely in Earth history, and only in microbes, a mechanism of evolution has operated in nature that forges new taxa at highest ranks via the endosymbiotic combination of two cells into one. The role of symbiogenesis in evolution versus the gradualist paradigm of constant and incremental mutation accumulation is still debated. A perceived problem with symbiogenesis is that it operates discontinuously and rarely during evolution. Moreover when it does operate, major evolutionary transitions, such as the origin of eukaryotes, plants, and algal groups are the outcome. Here I briefly contrast current symbiogenic and gradualist views on eukaryote origin regarding phagocytosis, the host for mitochondria, eukaryote anaerobes, the eukaryote endomembrane system, gene transfers from organelles, lateral gene transfer, and the number of endosymbiotic partners in eukaryote history. Special attention is given to energy conservation in mitochondria, which fostered eukaryotic complexity by lifting energetic constraints on protein synthesis in mitochondrion bearing cells relative to their prokayotic ancestors.