Why eukaryotes evolved from prokaryotes

Fossil records indicate that eukaryotes evolved from prokaryotes somewhere between 1. Two proposed pathways describe the invasion of prokaryote cells by two smaller prokaryote cells. They subsequently became successfully included as part of a now much larger cell with additional structures and capable of additional functions. Research conducted by Lynn Margulis at the University of Massachusetts supports the hypothesis that two separate mutually beneficial invasions of a prokaryote cell produced the modern-day mitochondria and chloroplast as eukaryotic organelles.

In this model, ancestral mitochondria were small heterotrophs capable of using oxygen to perform cellular respiration and thereby create useful energy.

They became part of a large cell either by direct invasion as an internal parasite or as an indigestible food source. Later, a second invasion brought ancestral chloroplasts, which are thought to be small, photosynthetic cyanobacteria. Modern-day supporting evidence for endosymbiosis shows that both the mitochondria and chloroplasts have their own genes, circular DNA and RNA, and reproduce by binary fission independent of the host's cell cycle. Curr Opin Genet Dev. Cell Cycle. Collins L, Penny D: Complex spliceosomal organization ancestral to extant eukaryotes.

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Int J Parasitol. Download references. I thank Yuri Wolf for providing the data used in Figure 2 , Bill Martin for helpful discussions and Tania Senkevich for critical reading of the manuscript. You can also search for this author in PubMed Google Scholar. Correspondence to Eugene V Koonin. Reprints and Permissions. Koonin, E. The origin and early evolution of eukaryotes in the light of phylogenomics.

Genome Biol 11, Download citation. Published : 05 May Anyone you share the following link with will be able to read this content:. Sorry, a shareable link is not currently available for this article. Provided by the Springer Nature SharedIt content-sharing initiative. Skip to main content. Search all BMC articles Search. Download PDF. Abstract Phylogenomics of eukaryote supergroups suggest a highly complex last common ancestor of eukaryotes and a key role of mitochondrial endosymbiosis in the origin of eukaryotes.

Eukaryotes The origin of eukaryotes is a huge enigma and a major challenge for evolutionary biology [ 1 — 3 ]. The supergroups of eukaryotes and the root of the eukaryotic evolutionary tree Although several eukaryotic kingdoms, such as animals, fungi, plants and ciliates, are well defined and seem to be monophyletic beyond reasonable doubt, deciphering the evolutionary relationships between these kingdoms and numerous other groups of unicellular eukaryotes also called protists turned out to be daunting.

Figure 1. Full size image. The last common ancestor of eukaryotes Comparative analysis of representative genomes from different eukaryotic supergroups enables the reconstruction of the gene complement of LECA using maximum parsimony MP or more sophisticated maximum likelihood ML methods [ 62 — 64 ].

The archaeal and bacterial roots of eukaryotes Eukaryotes are hybrid organisms in terms of both their cellular organization and their gene complement.

Figure 2. Figure 3. Mixed origins of the key functional systems of eukaryotes Some of the most compelling indications on the course of evolution and the nature of ancestral forms come from signature genes that are uniquely shared by two or more major lineages and from detailed evolutionary analysis of well characterized functional systems, in particular the signature systems of the eukaryotic cell.

Table 1 Apparent origins of some key functional systems and molecular machines of eukaryotes Full size table. Figure 4.

Eukaryogenesis: where did the eukaryotes come from? Figure 5. Box 1. General concepts in the evolution of the eukaryotes. References 1. Chloroplasts are one type of plastid , a group of related organelles in plant cells that are involved in the storage of starches, fats, proteins, and pigments.

Chloroplasts contain the green pigment chlorophyll and play a role in photosynthesis. Genetic and morphological studies suggest that plastids evolved from the endosymbiosis of an ancestral cell that engulfed a photosynthetic cyanobacterium. Plastids are similar in size and shape to cyanobacteria and are enveloped by two or more membranes, corresponding to the inner and outer membranes of cyanobacteria. Like mitochondria, plastids also contain circular genomes and divide by a process reminiscent of prokaryotic cell division.

The chloroplasts of red and green algae exhibit DNA sequences that are closely related to photosynthetic cyanobacteria, suggesting that red and green algae are direct descendants of this endosymbiotic event. Mitochondria likely evolved before plastids because all eukaryotes have either functional mitochondria or mitochondria-like organelles. In contrast, plastids are only found in a subset of eukaryotes, such as terrestrial plants and algae.

One hypothesis of the evolutionary steps leading to the first eukaryote is summarized in [Figure 2]. The exact steps leading to the first eukaryotic cell can only be hypothesized, and some controversy exists regarding which events actually took place and in what order. Spirochete bacteria have been hypothesized to have given rise to microtubules, and a flagellated prokaryote may have contributed the raw materials for eukaryotic flagella and cilia. Other scientists suggest that membrane proliferation and compartmentalization, not endosymbiotic events, led to the development of mitochondria and plastids.

However, the vast majority of studies support the endosymbiotic hypothesis of eukaryotic evolution. The early eukaryotes were unicellular like most protists are today, but as eukaryotes became more complex, the evolution of multicellularity allowed cells to remain small while still exhibiting specialized functions. The first eukaryotes evolved from ancestral prokaryotes by a process that involved membrane proliferation, the loss of a cell wall, the evolution of a cytoskeleton, and the acquisition and evolution of organelles.

Nuclear eukaryotic genes appear to have had an origin in the Archaea, whereas the energy machinery of eukaryotic cells appears to be bacterial in origin. The mitochondria and plastids originated from endosymbiotic events when ancestral cells engulfed an aerobic bacterium in the case of mitochondria and a photosynthetic bacterium in the case of chloroplasts.

The evolution of mitochondria likely preceded the evolution of chloroplasts. There is evidence of secondary endosymbiotic events in which plastids appear to be the result of endosymbiosis after a previous endosymbiotic event.