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 events. Endosymbiotic theory, that attempts to explain the origins of eukaryotic cell organelles such as mitochondria in animals and fungi and chloroplasts in plants was greatly advanced by the seminal work of biologist Lynn Margulis in the 1960s. Mitochondria are one of the many different types of organelles in the cells of all eukaryotes. In general, they are considered to have originated from proteobacteria (likely Rickettsiales) through endosymbiosis. Chloroplasts are one of the many different types of organelles in the plant cell. In general, they are considered to have originated from cyanobacteria through endosymbiosis. Endosymbiosis has gained ever more acceptance in the last half century, especially with the relatively recent advent of sequencing technologies. 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. The biology is messy, and there are many competing theories, so here we tend to converge them in a unified conceptualization.
Endosymbiosis Leads to Mitochondria
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 that manufactured enormous energy in the form of adenosine triphosphate (ATP), and so much so that extra ATP was available to the host cell's cytoplasm. This enormously benefited the anaerobic cell that then gained the ability to aerobically digest food. Eventually, the aerobic bacterium could no longer live independently from the cell, evolving into the mitochondrion organelle. Such aerobically obtained energy vastly exceeded that of anaerobic respiration, setting the stage for vastly accelerated evolution of eukaryotes.
Endosymbiosis Leads to Chloroplasts
Endosymbiotic 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.
Evidence for Endosymbiotic Theory
Mitochondria have very similar characteristics to purple-aerobic bacteria. They both use oxygen in the production of ATP, and they both do this by using the Kreb’s Cycle and oxidative phosphorylation. Similarly, chloroplasts are very similar to photosynthetic bacteria in that they both have similar chlorophyll that harnesses light energy that is converted into chemical energy. Although there are many similarities between mitochondria and purple aerobic bacteria and chloroplasts and photosynthetic bacteria, they appear to be slight and explainable by subsequent evolution.
Mitochondria and chloroplasts are similar in size to bacteria, 1 to 10 microns. Mitochondria and chloroplasts DNA, RNA, ribosomes, chlorophyll (for chloroplasts), and protein synthesis is similar to that for bacteria. This provided the first substantive evidence for the endosymbiotic hypothesis. It was also determined that mitochondria and chloroplasts divide independently of the cell they live in. Mitochondria having their own DNA and dividing independently of the cell is what ultimately results in only mitochondrial DNA being inherited by one’s mother since only an egg cell has DNA while a sperm cell does not.
Both mitochondria and chloroplasts have double phospholipid bilayers. This appears to have arisen by mitochondria and chloroplasts entering eukaryotic cells via endocytosis. Both purple, aerobic bacteria (similar to mitochondria) and photosynthetic bacteria (similar to chloroplasts) only have one phospholipid bilayer, but when they enter another cell via endocytosis, they are bound by a vesicle which forms the second layer of their double phospholipid bilayer.
What Does this All Mean
Eukaryotic cells, made possible by endosymbiosis, were powerful and efficient. That power and efficiency gave them the potential to evolve new characteristics: multicellularity, cell specialization, and large size. They were the key to the spectacular diversity of animals, plants, and fungi that populate our world today. Nevertheless, as we close the history of early life, reflect once more on the remarkable but often unsung patterns and processes of early evolution. Often, as humans, we focus our attention on plants and animals, and ignore bacteria. Our human senses cannot directly perceive the unimaginable variety of single cells, the architecture of organic molecules, or the intricacy of biochemical pathways. Let your study of early evolution give you a new perspective – a window into the beauty and diversity of unseen worlds, now and throughout Earth’s history. In addition to the mitochondria that call your 100 trillion cells home, your body contains more bacterial cells than human cells. You, mitochondria, and your resident bacteria share common ancestry – a continuous history of the gift of life.