Endosymbiosis - The Appearance of the Eukaryotes

Also see:
Domains Archaea and Bacteria
Evolutionary origin of eukaryotic cells

Endosymbiosis Introduction

The hypothesized process by which prokaryotes gave rise to the first eukaryotic cells is known as endosymbiosis, and certainly ranks among the most important evolutionary theories. Chloroplasts in plants and mitochondria in other eukaryotes are believed to have evolved through a form of endosymbiosis. There are many variants to the theory, regarding what organism(s) engulfed what other organism(s), as well as how many times and when it occurred across geological time.

Endosymbiotic theory that attempts to explain the origins of organelles such as mitochondria (in animals and fungi) and chloroplasts (in plants) eukaryotic cells was greatly advanced by the seminal work of biologist Lynn Margulis in the 1960s. Endosymbiosis has gained ever more acceptance in the last half century, especially with the relatively recent advent of high throughput sequencing technologies. The biology is messy, and their are many competing theories, so here we tend to converge them in a unified conceptualization [for more detained treatment, visit the "Origins of the Eukara" pages at Palaeos].

Symbiosis and Co-evolution

Symbiosis is ubiquitous among organisms throughout the tree of life, from the species level to the kingdom level, and even to domain level. It is integral to evolution as cooperating organisms gain survival advantage, a quid pro quo. For example, you (and for that matter all herbivores omnivores) could not digest your food without the exquisite symbiosis between your gut and the bacteria therein. Symbiosis played a major role in the co-evolution of flowering plants and the animals that pollinate them. The fossil record indicated that the first flowering plants in the had primitive flowers. Through natural selection, adaptive speciation quickly gave rise to many diverse groups of plants, and, at the same time, corresponding speciation occurred in certain insect groups. Many plants are pollinated by insects and vertebrates (e.g., bats and or birds) that have evolved highly specialized flowers facilitating pollination by a specific group or species that are themselves concomitantly adapted through co-evolution. Such mutualistic associations, where both host and symbiont evolve to accommodate one another abound in the history of life. But, we digress, so let's return to endosymbiosis.

Endosymbiosis Theory and Eukaryotic Origins

The theory holds that the eukaryote mitochodrion evolved from small, aerobic, heterotrophic prokaryotes (that got their energy from organic compound metabolism) that were engulfed by a larger anaerobic eukaryotic cell. The heterotrophic prokaryote used cellular respiration to intake oxygen and convert organic molecules to energy. The prokaryotic cells that were too small to be digested continued to live inside the host eukaryotic, eventually becoming dependent on the host cell for organic molecules and inorganic compounds. Conversely, the host cell would have acquired, by the addition of the aerobic function, an increased output of ATP for cellular activities, leading an improved selective advantage. Was the "engulfer" eubacteria or archaea - yes - it depends on which of competing theories you choose? from Other theories hold that the prokaryotes that gave rise to all eukaryotes were probably from the Domain Archaea, both because several key characteristics and because DNA sequence comparison suggest that Archaeans are more closely related to the eukaryotes than are eubacteria. This is the so-called serial endosymbiosis theory of a monophyletic origin of the mitochondrion from a eubacterial ancestor. That fact that mitochondria have their own DNA, RNA, and ribosomes, supports the endosymbiosis theory, as does the existence of the amoeba, a eukaryotic organism that lacks mitochondria and therefore requires a symbiotic relationship with an aerobic bacterium.

Digging deeper, the symbiosis is analogous to that between plants and their birds and bees symbionts. The aerobic bacterium thrived within the cell cytoplasm that provided abundant molecular food for its heterotrophic existence. The bacterium digested these molecules with oxygen and manufactured enormous energy, so much that extra energy in the form of Adenosine triphosphate was provided to the host cell's cytoplasm. This enormously benefited the anaerobic cell that gained the ability to aerobically digest food. Eventually, the aerobic bacterium could no longer live independently from the cell, evolving into the mitochondrion organelle. Theory posits a later parallel origin of the chloroplasts; a cell ate a photosynthetic cyanobacterium and failed to digest it. The cyanobacterium thrived in the cell and eventually evolved into the first chloroplast. Other eukaryotic organelles may have also evolved through endosymbiosis; it has been proposed that cilia, flagella, centrioles, and microtubules may have originated from a symbiosis between a Spirochaete bacterium and an early eukaryotic cell, but this is not yet broadly accepted among biologists.

Mitochondria and Chloroplasts Cell Powerhouses

We could fairly posit that the evolutionary origin of the eukaryotic cell was"the first time that what went around came around", a quid pro quo with among primitive organisms in deep time. This would make the all eukaryotes chimaeras at a cellular level. The Eukaryotic cell could also be likened to the V8 engine in producing power, as compared to a donkey powering prokaryotic cells. This would have enormous implication for subsequent evolution as earth's oceans atmosphere were oxygenated by photosynthetic bacteria creating extensive stromatolitic reefs. Organisms became multicultural, setting the stage for the Cambrian Explosion when the ancestors of modern eukaryotes' appeared. Mitochondria, the result of endosymbiosis in eukaryotic evolution are the energy-generating V8 engines of eukaryotic cells, where oxidative phosphorylation and electron transport metabolism takes place. Plastids, including chloroplasts, are the corresponding photosynthetic organelles of plant and algae cells.