Endosymbiosis


Endosymbiosis

Eric R. Pianka


Bacteria arose several billion years ago in Earth's primeval seas. Because they have no cell nuclei, they are known as prokaryotes
(pro = before, karyote = nucleus). Bacteria invented and perfected three fundamental metabolic processes: (1) photosynthesis, (2) fermentation, and (3) respiration. Photosynthesis is the process by which plants use light energy to convert inorganic carbon into energy rich organic molecules. Oxygen is a byproduct of photosynthesis. Indeed, photosynthetic bacteria terraformed planet Earth, creating its oxygen atmosphere. Fermentation produces energy under anaerobic conditions, whereas respiration releases energy under aerobic conditions. All three processes are vital to life.

In contrast, because "higher" organisms are more complex and have cell nuclei, they are known as eukaryotes (eu = true, karyote = nucleus). In addition to having a nucleus, eukaryotic cells house cell organelles known as mitochondria and chloroplasts, which are thought to be remnants of prokaryotes for many reasons.














Mitochondria and chloroplasts (the latter are present only in plants) house a circular DNA structure homologous to that found in bacteria. These DNAs code for synthesis of enzymes essential to metabolism in all living things. Chloroplasts produce rubisco, a vital enzyme in the metabolic machinery responsible for photosynthesis, the process by which plants capture energy from light and convert it into chemical energy.

Mitochondria are symbiotic cellular powerhouses housing ancient mitochondrial genes that first evolved billions of years ago and have presumably been maintained by stabilizing natural selection ever since. Mitochondrial genes code for ribosomal RNAs and proteins such as dehydrogenase, oxidase, and ATPase enzymes. These enzymes drive both anaerobic and aerobic metabolic processes. Without these vital enzymes, eukaryotes could not exist. The figure below depicts oversimpified yet fundamental metabolic pathways shared among all living organisms.


In his book "River Out of Eden," Dawkins describes endosymbiosis as follows:

"Each one of us is a community of a hundred million million mutually dependent eukaryotic cells. Each one of those cells is a community of thousands of specially tamed bacteria, entirely enclosed within the cell. A single animal or plant is a vast community of communities, packed in interacting layers, like a rain forest. As for a rain forest itself, it is a community seething with perhaps ten million species of organisms, every individual member of every species being itself a community of communities of domesticated bacteria."

Even our own blood plasma reflects our ancient origin: it is very close to sea water. We are but one small branch on the tree of life. We share most of our genes with other organisms, including bacteria and fungi. Space travel simply won't be possible unless we carry a substantial biodiversity out with us.