Endosymbiotic Theory They were the first living creatures on Earth: bacteria. Between 1 and 3.5 billion years ago, something happened to spark their development into higher life forms. How these simple organisms could evolve into complex plant ... ... and animal cells with nuclei and organelles is explained by the theory of endosymbiosis. It all began with two single-celled organisms. The larger one may have been an archaebacterium ... – a primordial microbe with ring-shaped DNA. The smaller one was an alphaproteobacterium. Many present-day pathogens that exist inside their host cells belong to this class of bacteria. It is possible that the larger cell used hydrogen as a source of energy, and the smaller organism released hydrogen. The larger one swallowed the smaller one, securing an in-house supply of hydrogen. In the process, the smaller one - the lodger - got a second membrane. The inside membrane was the lodger's own. The outer membrane came from the host cell. Over the course of the next million years, the absorbed cell gradually lost its self-sufficiency. Many of its genes were no longer required for its life as a lodger and so were lost in the course of evolution. Some of the genes were even taken into the genetic material of the host cell. The lodger developed into a cell organelle - a mitochondrion. In the course of evolution, mitochondria have perfected the art of energy production. They now supply the host cell not only with hydrogen, but also the valuable energy source ATP. These molecules are the power stations of the cell. Finally, the cell formed a cell nucleus and packed its DNA into it. Next, it absorbed a cyanobacterium. Cyanobacteria can photosynthesize, that is, convert sunlight and water into oxygen and energy in the form of sugar. A huge advantage for the host cell. Cyanobacteria evolved to form the chloroplasts in plant cells. Again, the absorbed microorganisms lost large proportions of their genomes as time went by. In fact, the cyanobacterium surrendered more than 95% of its genes to the cell nucleus, ... as well as some to the mitochondria. Only a small number of genes came in the opposite direction, from the cell nucleus to the mitochondria. The chloroplasts, however, apparently resist the importation of genes. So far, they have absorbed none. There is ample evidence to support the endosymbiotic theory. Firstly, there is the double membrane surrounding the mitochondria and the chloroplasts. Then the fact that the nuclei of all present-day eukaryotic cells contain genes that originally came from Proteobacteria. Furthermore, both chloroplasts and mitochondria have their own genetic material in a ring of DNA, ... ... and replicate by division – just like bacteria. And endosymbiosis is by no means uncommon. It has occurred repeatedly in the course of evolution. Single-celled algae often merge. But even higher organisms take advantage of protozoa. The sea slug Elysia chlorotica, for instance, has absorbed single-celled organisms, ... ... namely chloroplasts from the alga Vaucheria litorea, which it uses to carry out photosynthesis. Endosymbiosis: a sophisticated strategy of evolution.