Why eukaryotes developed

The first eukaryotic cells - cells with a nucleus an internal membrane-bound organelles - probably evolved about 2 billion years ago. This is explained by the endosymbiotic theory. As shown in the Figure below , endosymbiosis came about when large cells engulfed small cells. The small cells were not digested by the large cells. Instead, they lived within the large cells and evolved into organelles. From Independent Cell to Organelle. The endosymbiotic theory explains how eukaryotic cells evolved.

The large and small cells formed a symbiotic relationship in which both cells benefited. They supplied energy not only to themselves but also to the large cell. They became the mitochondria of eukaryotic cells. Other small cells were able to use sunlight to make food.

They shared the food with the large cell. They became the chloroplasts of eukaryotic cells. What is the evidence for this evolutionary pathway? Biochemistry and electron microscopy provide convincing support.

The mitochondria and chloroplasts within our eukaryotic cells share the following features with prokaryotic cells:. The "host" cell membrane and biochemistry are more similar to those of Archaebacteria, so scientists believe eukaryotes descended more directly from that major group Figure below. The timing of this dramatic evolutionary event more likely a series of events is not clear. The oldest fossil clearly related to modern eukaryotes is a red alga dating back to 1. However, many scientists place the appearance of eukaryotic cells at about 2 billion years.

Some time within Proterozoic Eon, then, all three major groups of life — Bacteria, Archaea , and Eukaryotes — became well established. The sophisticated cellular compartmentalization and the symbiotic association with mitochondria are thought to have enabled eukaryotes to adopt new ecological roles and provided a precursor to numerous successful origins of multicellularity.

Nevertheless, despite being recognized as the single most profound evolutionary transition in cellular organization, the origins of the eukaryotic cell remain poorly understood. The key events in the evolution of eukaryotes were the acquisition of the nucleus, the endomembrane system, and mitochondria. Existing models for the origin of eukaryotes generally agree that proto-mitochondria entered the cell via phagocytosis.

Likewise, the most widely favored models for the origins of the nucleus assume that it was formed within a prokaryotic cell as the result of invaginations of the plasma membrane - whether by phagocytosis of an endosymbiont that corresponds to the nuclear compartment or by the internalization of membranes that became organized around the chromatin reviewed in [ 7 ] and discussed further below.

Thus, existing theories for the origin of eukaryotes share the assumption that the nucleus is a novel structure formed within the boundaries of an existing, and largely unaltered, plasma membrane [ 8 ] - they are outside-in models.

Here, we set out to challenge the outside-in perspective. Archaea often generate extracellular protrusions [ 9 ]-[ 14 ], but are not known to undergo processes akin to endocytosis or phagocytosis.

Therefore, we suggest that eukaryotic cell architecture arose as the result of membrane extrusion. In brief, we propose that eukaryotes evolved from a prokaryotic cell with a single bounding membrane that extended extracellular protrusions that fused to give rise to the cytoplasm and endomembrane system.

Under this inside-out model, the nuclear compartment, equivalent to the ancestral prokaryotic cell body, is the oldest part of the cell and remained structurally intact during the transition from prokaryotic to eukaryotic cell organization. The inside-out model provides a simple stepwise path for the evolution of eukaryotes, which, we argue, fits the existing data at least as well as any current theory. Further, it sheds new light on previously enigmatic features of eukaryotic cell biology, including those that led others to suggest the need to revise current cell theory [ 15 ].

Given the large number of testable predictions made by our model, and its potential to stimulate new empirical research, we argue that the inside-out model deserves consideration as a new theory for the origin of eukaryotes. Endosymbiotic, outside-in models explain the origin of the nucleus and mitochondria as being the result of sequential rounds of phagocytosis and endosymbiosis. These models invoke three partners - host, nucleus, and mitochondria - and envisage the nuclear compartment being derived from an endosymbiont that was engulfed by a host cell.

Authors have suggested that the host that is, cytoplasm could be an archaeon [ 16 ]-[ 18 ], a proteobacterium [ 19 ]-[ 21 ], or a bacterium of the Planctomycetes, Verrucomicrobia, Chlamydiae PVC superphylum [ 22 ]. The endosymbiont that is, the nucleus has been proposed to have been an archaeon [ 19 ]-[ 22 ], a spirochete [ 16 ], or a membrane-bound virus [ 17 ],[ 18 ]. In general, endosymbiotic models are agnostic as to whether mitochondria were acquired before or after the nucleus.

An exception to this is the syntrophic consortium model, which envisages the simultaneous fusion of a symbiotic community composed of all three partners: cytoplasm, nucleus, and mitochondria [ 23 ],[ 24 ]. This holds that the nucleus evolved when a cell enclosed its sister after cell division, similar to the way in which endospores are formed in certain Gram-positive bacteria.

However, there is no evidence of endospore formation or other engulfment processes in Archaea, making this hypothesis improbable. Recent phylogenomic analyses have revealed that the eukaryotic genome likely represents a combination of two genomes, one archaeal [ 26 ],[ 27 ] and one proteobacterial [ 28 ],[ 29 ].

There is no evidence to support any additional, major genome donor as expected under nuclear endosymbiotic models [ 30 ]. Furthermore, endosymbiotic models including the endospore model require supplemental theories to explain the origin of the endomembrane system, the physical continuity of inner and outer nuclear membranes, and the formation of nuclear pores.

In light of these facts, we do not think that endosymbiosis provides a convincing explanation for the origin of the nuclear compartment [ 2 ],[ 7 ],[ 31 ]-[ 33 ].

Given the problems with endosymbiotic models, we believe that the most compelling current models for the origin of eukaryotes are those that invoke an autogenous origin of the nucleus. These usually suggest that a prokaryotic ancestor evolved the ability to invaginate membranes to generate internal membrane-bound compartments, which became organized around chromatin to generate a nucleus [ 32 ],[ 34 ]-[ 36 ].

In some models, infoldings of the plasma membrane were pinched off to form endoplasmic reticulum ER -like internal compartments that later became organized around the chromatin to form the inner and outer nuclear envelope [ 35 ],[ 37 ]-[ 39 ]. Alternatively, the nuclear membranes could be seen as arising from invaginations of the plasma membrane, so that the early eukaryote cell had an ER and nuclear envelope that were continuous with the outer cell membrane [ 40 ].

In either case, under these models the nuclear membrane is ultimately derived from internalized plasma membrane. Older autogenous outside-in models generally proposed that mitochondria were acquired by a cell that already had a nucleus [ 32 ],[ 34 ],[ 35 ] - in line with the results of early phylogenetic studies [ 41 ].

More recent phylogenetic data have suggested that mitochondria were present in the last eukaryotic common ancestor [ 42 ],[ 43 ].

This has led to the formulation of new autogenous models in which the acquisition of mitochondria predates the formation of the nuclear compartment [ 1 ],[ 23 ],[ 44 ]-[ 46 ]. Under the inside-out hypothesis, the outer nuclear membrane, plasma membrane, and cytoplasm were derived from extracellular protrusions blebs , whereas the ER represents the spaces between blebs Table 1.

Mitochondria were initially trapped in the ER, but later penetrated the ER membrane to enter the cytoplasm proper. Under the inside-out model, the final step in eukaryogenesis was the formation of a continuous plasma membrane, which closed off the ER from the exterior. Inside-out model for the evolution of eukaryotic cell organization.

B We envision the eocyte cell forming protrusions, aided by protein-membrane interactions at the protrusion neck. These protrusions facilitated material exchange with proto-mitochondria. C Selection for a greater area of contact between the symbionts would have led to bleb enlargement and the eventual loss of the S-layer from the protrusions.

D Blebs would have then been further stabilized by the development of a symmetric nuclear pore outer ring complex Figure 2 and through the establishment of LINC complexes that, following the gradual loss of the S-layer, physically connected the original cell body the nascent nuclear compartment to the inner bleb membranes. E With the expansion of blebs to enclose the proto-mitochondria, a process that would have facilitated the acquisition of bacterial lipid biosynthesis machinery by the host, the site of cell growth would have progressively shifted to the cytoplasm, facilitated by the development of regulated traffic through the nuclear pore.

At the same time, the spaces between blebs would have enabled the gradual maturation of proteins secreted into the environment via the perinuclear space through glycosylation and proteolytic cleavage. F Finally, bleb fusion would have connected cytoplasmic compartments and driven the formation of an intact plasma membrane, perhaps through a process akin to phagocytosis whereby one bleb enveloped the whole.

This simple topological transition would have isolated the endoplasmic reticulum from the outside world, driven the full development of a system of vesicular trafficking, and established strict vertical transmission of mitochondria, leading to a cell with modern eukaryotic cell organization.

Only one other paper that we are aware of has proposed that the nuclear compartment corresponds to boundaries of an ancestral cell. The exomembrane hypothesis of de Roos [ 47 ] is, however, quite distinct from the model put forward here. De Roos postulated that the starting point was a proto-eukaryote with a double membrane that secreted membranous extracellular vesicles that fused to form an enclosing plasma membrane.

Moreover, his model relies on an unconventional view of evolutionary history, including an independent origin of eukaryotic and prokaryotic cells. Thus, we will not discuss the exomembrane hypothesis further. In the following sections, we describe the inside-out model in detail. We discuss the cellular processes involved in the generation of the cytoplasmic compartment, the vesicle trafficking system and plasma membrane, and cilia and flagella.

In each section we point to relevant selective drivers and supporting evidence. Finally, we look at some of the implications and testable predictions of the model and conclude by reflecting on the prospects for determining which of the models, inside-out or outside-in, is more likely to be correct.

We take as our starting point a prokaryotic cell similar to an 'eocyte' [ 48 ], an informal name that has come to refer to a member of the archaeal phyla Crenarchaeota, Thaumarchaeota, and Korarchaeota [ 49 ].

Eocytes usually have a single lipid bilayer membrane and a simple cell wall S-layer rich in N-glycosylated proteins [ 50 ]. They also have a relatively well-developed cytoskeleton that includes homologs of actin and tubulin [ 51 ]-[ 53 ] and the membrane-manipulating protein ESCRTIII [ 54 ]-[ 58 ]. While the inside-out hypothesis is not formally dependent on the veracity of the eocyte hypothesis, as we show below, the eocyte hypothesis poses a significant challenge to any outside-in hypothesis proposed to date.

Under the inside-out model, the pre-eukaryote developed outward protrusions Figure 1 A,B. Many Archaea, including some eocytes [ 11 ],[ 13 ],[ 62 ], exhibit such structures [ 9 ]-[ 14 ],[ 62 ], but they are rarely seen in bacteria [ 54 ],[ 63 ]. In almost all cases where the images are clear, protrusions are bounded by an S-layer.

However, if scission were suppressed, long-lived protrusions could be formed. The stable protrusions formed by suppression of scission would have increased the surface-to-volume ratio of the host cell.

The idea that an eocyte might produce extracellular protrusions as a means to increase its surface area is justified by the observation that protrusion formation is stimulated in the crenarchaeote Stettaria hydrogenophila in response to reductions in the concentration of extracellular sulfur [ 9 ].

Moreover, Archaea with protrusions associated with cell-cell contacts have been seen in mixed microbial communities in biofilms [ 12 ]. The potential selective value of extracellular protrusions is also illustrated by a number of living eukaryotic groups, such as foraminiferans and radiolarians, which have a central cell body enclosed within a rigid test that has pores through which protrusions project.

This arrangement allows cells to interact directly and dynamically with the external environment while retaining their genetic material in a protective keep. These phyla are ecologically successful, with many thousands of living and extinct species [ 64 ]. The rapid radiation of foraminiferans in the Cambrian, not long after the evolution of rigid tests [ 65 ], makes clear the potential advantages of a cell increasing its surface area while retaining its chromatin in a protective inner compartment.

Further, it is noteworthy that in some rhizarian subgroups, pseudopodia fuse with one another to generate an extra-testal compartment that is loosely analogous to a continuous cytoplasm forming via the fusion of extracellular blebs. Little is currently known about the cell biology of archaeal protrusions.

Specifically, it is unclear how protrusions are formed and stabilized. How cells generate stable protrusions is important for the model, since this corresponds to the first step in the evolution of the cytoplasm Figure 1. COPII-like proteins do not associate with membranes directly, but interact with membranes via diverse membrane-binding proteins [ 67 ],[ 68 ]. Nonetheless, they play a conserved role in stabilizing positive membrane curvature [ 69 ], making them a natural candidate for having an ancestral role in stabilizing the bases of extracellular protrusions - a cellular location that corresponds to the nuclear pore of modern eukaryotes.

Example of epibiotic bacteria associated with archaeal cells. Model for the evolution of nuclear pores and cytoplasmic blebs. A Membrane protrusions are formed that extend through holes in the cell wall S-layer, shown in gray of the eukaryote ancestor. Protrusions could initially have been coated with an S-layer that was later lost. Additionally, blebs may have been stabilized by an internal cytoskeleton red , like that provided by microtubules in modern day flagella, and by components of LINC complexes that connect the cell membrane and underlying structures to the S-layer gray.

B Lateral spreading of the bleb is aided by the movement of LINC proteins to the inner bleb membrane and by the recruitment of a second, outer ring of nuclear pore proteins to stabilize positive curvature outside of the cell wall. Under the inside-out model, the structural components of the nuclear pore constituted the very first eukaryotic innovation, playing an essential role in ensuring the stable attachment of extracellular protrusions to the cell body.

This hypothesis leads one to expect the outer ring of the NPC to be the most highly conserved portion of the complex - as is the case [ 72 ]. Moreover, in line with the idea that the complex evolved to stabilize long-lived protrusions, NPCs are among the most stable proteins in eukaryotic cells [ 75 ],[ 76 ].

Within eukaryotes, there is now abundant evidence that structural components of the nuclear pore for example, Nup are homologous to COPII proteins that drive the budding of endomembrane vesicles [ 40 ],[ 68 ],[ 69 ],[ 72 ],[ 74 ],[ 77 ],[ 78 ].

This led Devos and collaborators to propose the protocoatomer hypothesis [ 40 ],[ 80 ], which assumes an outside-in origin of the nucleus. They proposed that an ancestral protein involved in maintaining positive curvature around vesicles and at the edges of ER sheets underwent gene duplication, and some copies became specialized to function at nuclear pores - which are seen as being topologically equivalent to the edges of ER sheets.

Under the inside-out model, this same homology is interpreted differently: proteins whose original function was to stabilize positive membrane curvature in the nuclear pore were later co-opted for a new function in vesicle formation. To distinguish between these theories it will be important in future work to conduct a phylogenetic analysis of COPII and NPC proteins, rooted with appropriate prokaryotic sequences, to determine if the trees better support the inside-out or protocoatomer interpretation.

We suggest that external protrusions evolved in the original proto-eukaryote to facilitate resource exchange with ectosymbiotic bacteria that ultimately gave rise to modern day mitochondria. A number of modern bacteria form ectosymbiotic associations with specific hosts for examples, see [ 81 ]-[ 83 ] , including archaeal species. This illustrates the ecological plausibility of progenitors of mitochondria being ectosymbiotic bacteria that entered into a metabolic mutualism with the progenitor of the host cell.

This type of association would be augmented by a progressive increase in host cell surface area. Something similar is seen in the foraminiferan Bolivina pacifica , which increases its membrane surface area in parts of the cell that underlie prokaryotic ectosymbionts [ 85 ]. Thus, selection for an increase in the surface area available for metabolic exchange with ectosymbiotic bacteria could have driven production and proliferation of extracellular protrusions.

The nature of the material exchange between the eukaryotic host and proto-mitochondria has been a matter of debate [ 23 ],[ 24 ],[ 44 ],[ 45 ]. Possibilities include hydrogen, sulfur, hydrogen sulfide, organic acids, and ATP.

Nonetheless, the idea that efficient transfer between proto-mitochondria and a symbiotic archaeon selected for an increasing surface area of contact is shared by both the hydrogen and inside-out hypotheses. Typically, analyses have identified mitochondria as very close relatives of Rickettsiales [ 87 ],[ 88 ], a group of intracellular parasites of eukaryotes that co-opt the host cell's phagocytic machinery to enter cells in food vacuoles, and then enter the cytoplasm proper by lysing the food vacuole membrane [ 87 ].

However, even if mitochondria are eventually confirmed as close relatives of Rickettsiales, for reasons discussed below we do not consider it likely that the ancestor of mitochondria entered its proto-eukaryotic host by phagocytosis. Instead, we propose that mitochondria are derived from ectosymbionts, and that the endoparasitic capabilities of Rickettsiales evolved later.

Material exchange with a mutualistic epibiotic bacterial community would have favored both loss of the S-layer overlying protrusions and lateral expansion of protrusions into larger blebs, increasing both cell volume and surface area Figure 1 B-D. Such an expansion would have trapped populations of bacteria between the folds of adjacent blebs and the underlying cell wall Figure 1 C,D. This would have ensured sustained close contacts between host cytoplasm and proto-mitochondria, increasing the probability of vertical proto-mitochondrial inheritance, and helping to exclude parasitic microbes.

At some point, either before or after further elaboration of the cytoplasmic compartment Figure 1 E,F , mitochondria moved into the cytoplasm by penetrating the ER membrane.

This seems plausible since rickettsialean bacteria, which are often found within the ER and Golgi of modern eukaryotes [ 89 ], gain entry to the cytoplasm proper by lysis of the confining host-cell membrane [ 87 ]. It is striking in this light that mitochondria in modern eukaryotes retain close metabolic, physical, and regulatory linkages with ER [ 90 ]. The ER has even been found to play a critical role in mitochondrial fission [ 91 ],[ 92 ]. The extent to which membrane protrusions swelled beyond the S-layer would have depended on the relative osmotic pressure of the cell and its environment, and the sophistication of osmoregulation.

While data on osmoregulation in Archaea remain sparse [ 93 ],[ 94 ], it is noteworthy that many archaeal cells live in conditions of high external osmolytes where the thinning or loss of the S-layer would not cause cells to burst. Thermoplasma , for example, appears to lack a cell wall entirely [ 95 ]. We propose that with the progressive growth of the external cytoplasmic compartment, adjacent blebs pressed against one another to generate a continuous network of inter-bleb crypts, homologous to the lumen of the nuclear envelope and the ER of modern eukaryotes Figure 1 D.

This would provide a simple explanation for the continuity of ER and the nuclear envelope, a common feature of all eukaryotes [ 96 ] even within the context of syncytia generated via incomplete cell division [ 97 ],[ 98 ].

Furthermore, since the location of the original glycoprotein-rich archaeal cell wall is topologically equivalent to the perinuclear space in modern eukaryotes, the model parsimoniously explains why the N-linked glycosylation pathway, which operates in the lumen of the nuclear envelope and ER to modify proteins destined for secretion, is homologous to that used to modify S-layer proteins in Archaea [ 99 ],[ ].

The stabilization of blebs would have been facilitated by the evolution of an outer ring of nucleoporins supporting a second area of positive curvature on the outside of the cell wall, giving rise to the partial inside-out symmetry of the NPC Figure 3. Additionally, the nucleus would have been stabilized by the co-option of proteins used to anchor the cell membrane to the inner surface of the S-layer.

Under the model, these would have given rise to LINC complexes [ ],[ ]. In vertebrates, where nuclear envelope structure is best understood, the key components of LINC complexes are SUN-domain proteins on the nucleoplasmic side and KASH-domain proteins on the cytoplasmic side [ ]-[ ]. Torsin, which sits within the perinuclear space, interacts with SUN-KASH domain proteins [ ],[ ], as well as other linkers [ ],[ ]. These proteins function together to ensure the structural integrity of the nuclear envelope.

Moreover, Torsin has been shown to play a role in nuclear bleb formation during ribonuclear protein granule export [ ],[ ] and in the control of ER morphology [ ]. Some of these functions are clearly ancient, given that SUN-domain proteins play a similar role in plant nuclei [ ],[ ].

Under the inside-out model, it seems likely that LINC complexes would be descended from archaeal S-layer glycoproteins.

We speculate further that LINCs originally functioned to connect the archaeal plasma membrane and perhaps cytoskeleton to the S-layer. Later, following the growth of cytoplasmic blebs, it is easy to imagine how gene duplication and the recruitment of new proteins could have connected the inner membranes of each bleb to remnants of the S-layer to create a perinuclear lumen and a double nuclear envelope.

Although this scenario is attractive, most of the what we know about the structure of the nuclear envelope comes from animal systems, and the identity of potential homologs in archaea remains unknown. By contrast, SUN-domain protein are found in all eukaryotic groups and have structural homology to carbohydrate-binding motifs [ 72 ], which are also present in some archaeal proteins. Thus, it will be important to characterize the closest archaeal homologs of these nuclear envelope scaffolding proteins to determine whether they play a role in anchoring the plasma membrane to the S-layer, as we predict.

The majority of the structural lipids within eukaryotic cell membranes are quite distinct from archaeal lipids [ ],[ ]. In fact, they bear many similarities to those found in bacteria [ ]. Bacterial and eukaryotic membranes are primarily composed of ester-linked, straight-chain fatty acids and utilize glycerolphosphate lipids, whereas archaea have ether-linked fatty acids derived from highly methyl-branched isoprenoids and utilize a glycerolphosphate backbone [ ].

Additionally, both eukaryotes and some bacteria, but not archaea [ ], produce triterpenoids for example, hopanoids and sterols that help modulate membrane fluidity. This strongly suggests that eukaryotes acquired their bacterium-like lipids from mitochondria. This conclusion is reinforced under the eocyte hypothesis, which embeds the eukaryotes within the Archaea, implying a late and dramatic switch from archaeal to bacterial lipid biochemistry.

It seems likely that the transfer of genes for lipid biosynthesis from proto-mitochondria to proto-eukaryotes occurred prior to the development of an elaborate vesicle trafficking system and phagocytosis. If this were not the case, one would have to postulate that numerous proteins that had evolved to manipulate archaeal membranes tolerated the shift towards bacterial membranes, which have distinct chemical and biophysical properties [ ],[ ]. While one can envisage a few membrane-interacting proteins, especially those with simple modes of interaction as seems to be the case for ESCRTIII [ ] , being able to retain functionality during a transition from archaeal to bacterial membranes, we think it likely that most membrane-manipulating machinery of eukaryotes arose after membranes were bacterium-like.

Furthermore, it is hard to see how processes like phagocytosis, which rely both on a large cell size and dramatic, energy-intensive membrane remodeling events could have occurred in an archaeal proto-eukaryote lacking mitochondria [ 1 ].

The contention that phagocytosis evolved after the acquisition of mitochondria as previously suggested [ 8 ] can be further justified by consideration of the physical properties of archaeal lipids. Archaeal membranes typically retain their physical properties across a wide range of temperatures, whereas bacterial and eukaryotic membranes are tuned to keep them close to the phase transition boundary at physiological temperatures [ ].

The latter property is thought to allow the formation and dissolution of distinct lipid domains, which permits the dynamic and reversible membrane deformations that are characteristic of eukaryotic cells [ ]. These considerations support the idea that the physico-chemical properties of bacterial membranes were an essential precursor to the evolution of dynamic mechanisms such as endocytosis and phagocytosis.

These facts are hard to reconcile with outside-in models, which typically view phagocytosis as the means by which proto-eukaryotes established a close, symbiotic relationship with proto-mitochondria.

By contrast, the inside-out model implies that symbiosis arose by the passive trapping of proto-mitochondria in inter-bleb spaces, and did not require complex membrane manipulating machinery besides the ability to generate protrusions - a feature common in many modern-day archaea. Under the inside-out model, the structural lipids present in modern eukaryotes would have been first acquired from mitochondria via traffic across ER-mitochondrial contact sites, which are conserved across eukaryotes and apparently ancient [ ].

Given this, there are a number of striking observations. First, mitochondria retain a critical role in eukaryotic fatty acid metabolism and in lipid synthesis, generating many of their own lipids, such as cardiolipin [ ],[ ]. Second, the ER is the major site of lipid and membrane synthesis in modern eukaryotes, with many of the enzymes involved found concentrated at ER-mitochondrial contact sites [ ].

And third, connections between ER and mitochondria remain important sites of lipid traffic in modern eukaryotes [ ]-[ ]. Thus, the spatial organization of lipids and lipid synthesis in modern cells is easy to understand under the inside-out model as a by-product of the gradual evolution of a symbiotic relationship between the host and mitochondria the original site of endomembrane lipid synthesis situated in the spaces between cytoplasmic blebs.

For a time it is likely that membranes were formed that contained a mixture of archaeal and bacterial lipids [ ] prior to gradual reductions in the archaeal contribution. The primary use of only one type of structural lipid may have been driven in part by the difficulties of reconciling metabolic pathways that use different chiral forms of the lipid glycerol backbone, with the mesophilic environment removing any intrinsic benefit of ether-linked lipids.

Interestingly, though, modern eukaryotic cells do produce some lipids with ether-linkages [ ],[ ], some of which have been implicated in the generation of mechanically rigid membranes during cell division [ ].

These facts raise the possibility that use of archaeon-like lipids in cell division helped ESCRTIII to survive the transition from archaeal to eukaryotic cell biology. In contrast to the structural lipids of eukaryotes, inositol lipids, which are ubiquitous in eukaryotes but represent a tiny fraction of total lipids in membranes [ ], are common to eukaryotes and archaea, but not bacteria [ ].

This implies that inositol metabolism was originally associated with the proto-nuclear compartment, thus explaining why inositol lipids are actively imported into mitochondria rather than being synthesized there [ ],[ ]. This may also account for the fact that inositol lipids, and the enzymes that generate them, are found in the nuclei of modern eukaryotes - something that has long perplexed researchers in the field [ ],[ ].

Instead of a structural role, inositol lipids are important regulatory molecules, modulating cell growth [ ],[ ] and marking cytoplasmic compartment identity [ ]. This is reasonable under the inside-out model: inositol derivatives were present throughout eukaryotic evolution, allowing their phosphorylation states to be deployed as signals [ ],[ ] for facilitating nuclear control over an increasingly large and elaborate cytoplasmic compartment.

Despite the presence of blebs and proto-mitochondria at early stages in its evolution Figure 1 A-D , the proto-eukaryote would have had the same topology as the ancestral eocyte. It retained a single, continuous bounding membrane, albeit one that was much more extensive and contorted than the ancestors'.

Thus, at this stage there would have been no distinction between nuclear division and cell division. Moreover, cell cycle progression and cell division would have likely been regulated in a manner similar to that seen in modern day Archaea, and using homologous proteins [ 58 ]. Likewise, proteins controlling chromosomal architecture histones and DNA replication are of archaeal origin [ ]. Strikingly, in many archaea, the scission event completing cell division is driven by the action of the ESCRTIII complex [ 56 ]-[ 58 ], just as appears to be the case in eukaryotes [ ].

Under the inside-out model, it is relatively easy to see how cell division could have been achieved in an early proto-eukaryotic cell, even one that had links between blebs, using pre-existing ESCRTIII machinery Figure 4. After division, each daughter cell would have acquired a subset of the nuclear pore-associated blebs, with naked cell surface being covered by the movement of pores and through the action of LINC complexes [ ], which would attach flanking bleb membranes to the exposed portion of the proto-nucleus Figure 4.

However, in a proto-eukaryote with a well-developed cytoplasmic compartment, the simple division of the nuclear compartment would not have guaranteed a fair segregation of cell mass between the two daughter cells.

After loss of the original cell wall, this problem could have been solved through the evolution of partially open mitosis Figure 4. Because the inner nuclear membrane is topologically continuous with the outer bleb membrane, this would have required little additional innovation, only the partial disassembly of nuclear pores and LINC complexes as seen in some eukaryotic cell divisions [ ]. Following division, the nuclear-cytoplasmic boundary would have been re-established through the rebinding of nuclear membranes by chromosome-associated NPC and LINC components.

Model for the evolution of cell division. Cell division is depicted for the ancestral eocyte A , and at two intermediate stages in the evolution of eukaryotes, before B or after C bleb fusion. Following the acquisition of blebs, ESCRTIII is used to drive the scission of cytoplasmic bridges connecting cells likely aided by the archaeal-derived actin cytoskeleton [ 51 ] , while LINC complexes and the formation of new nuclear pores restore cell and nuclear organization following division.

Mitochondrial segregation is likely aided by host induced Dynamin-mediated scission within the endoplasmic reticulum not depicted , as observed in modern eukaryotes [ 91 ]. Instead, there is a loss of compartment identity as nuclear and cytoplasmic compartments mix and nuclear membranes become indistinguishable from cytoplasmic ER [ ]-[ ].

Under the inside-out model it is easy to see that open and closed mitosis are not as different as often assumed, and to imagine cells switching between open and closed modes of mitosis by modifying the extent to which LINC and NPCs remain associated with the nuclear membranes during cell division. This offers an explanation for the frequent occurrence of evolutionary transitions between these two modes of mitosis [ 2 ],[ ].

Under the inside-out model, the recruitment of additional proteins to the NPC enabled the controlled movement of membrane lipids and the flow of aqueous material between the nuclear and bleb cytoplasmic compartments.

This includes the regulated transport of mRNA and ribosomes [ ],[ ] to generate distinct domains of protein translation: nuclear and cytoplasmic. In such a situation, it is easy to imagine that it might be beneficial for certain transcripts to be translated in the cytoplasmic domain and that this might have resulted in the evolution of mechanisms for targeting some transcripts for transport to the cytoplasm and for preventing their premature translation in the nucleus.

We speculate that mRNA cap formation and polyadenylation evolved originally for this purpose: tagging certain transcripts for translocation through the nuclear pore and limiting intranuclear translation. It is noteworthy that, in some systems, mRNA processing [ ],[ ] and mRNA export [ ] are regulated by phosphoinositol lipids which, as suggested above, might have had an ancestral role in coordinating growth of the nuclear and cytoplasmic compartments.

Through the regulated transport of mRNA and proteins between nuclear and cytoplasmic compartments it would have become possible to separate core metabolic processes in the cytoplasm from DNA replication, transcription, and ribosome assembly in the nucleoplasm.

At some point before about 3. Cyanobacteria used water as a hydrogen source and released O 2 as a waste product. Originally, oxygen-rich environments were probably localized around places where cyanobacteria were active, but by about 2 billion years ago, geological evidence shows that oxygen was building up to higher concentrations in the atmosphere. Recall that the first fossils that we believe to be eukaryotes date to about 2 billion years old, so they appeared as oxygen levels were increasing.

Eukaryotes, having probably evolved from prokaryotes, have more complex traits in both cell and DNA organization. Prokaryotic cells are known to be much less complex than eukaryotic cells since eukaryotic cells are considered to be present at a later point of evolution. It is probable that eukaryotic cells evolved from prokaryotic cells.

Differences in complexity can be seen at the cellular level. The single characteristic that is both necessary and sufficient to define an organism as a eukaryote is a nucleus surrounded by a nuclear envelope with nuclear pores. In contrast, prokaryotic DNA is not contained within a nucleus, but rather is attached to the plasma membrane and contained in the form of a nucleoid, an irregularly-shaped region that is not surrounded by a nuclear membrane.

Eukaryotic DNA is packed into bundles of chromosomes, each consisting of a linear DNA molecule coiled around basic alkaline proteins called histones, which wind the DNA into a more compact form.

Prokaryotic DNA is found in circular, non-chromosomal form. In addition, prokaryotes have plasmids, which are smaller pieces of circular DNA that can replicate separately from prokaryotic genomic DNA. Because of the linear nature of eukaryotic DNA, repeating non-coding DNA sequences called telomeres are present on either end of the chromosomes as protection from deterioration. Mitosis, a process of nuclear division wherein replicated chromosomes are divided and separated using elements of the cytoskeleton, is universally present in eukaryotes.

The cytoskeleton contains structural and motility components called actin microfilaments and microtubules. All extant eukaryotes have these cytoskeletal elements. Prokaryotes on the other hand undergo binary fission in a process where the DNA is replicated, then separates to two poles of the cell, and, finally, the cell fully divides. Because eukaryotes have mitochondria and prokaryotes do not, eukaryotic cells contain mitochondrial DNA in addition to DNA contained in the nucleus and ribosomes.

The mtDNA is composed of significantly fewer base pairs than nuclear DNA and encodes only a few dozen genes, depending on the organism. Eukaryotes may have been a product of one cell engulfing another and evolving over time until the separate cells became a single organism. This major theme in the origin of eukaryotes is known as endosymbiosis, where one cell engulfs another such that the engulfed cell survives and both cells benefit.

Over many generations, a symbiotic relationship can result in two organisms that depend on each other so completely that neither could survive on its own. Endosymbiosis : Modern eukaryotic cells evolved from more primitive cells that engulfed bacteria with useful properties, such as energy production. Combined, the once-independent organisms flourished and evolved into a single organism. The endosymbiotic theory was first articulated by the Russian botanist Konstantin Mereschkowski in Mereschkowski was familiar with work by botanist Andreas Schimper, who had observed in that the division of chloroplasts in green plants closely resembled that of free-living cyanobacteria.

Schimper had tentatively proposed that green plants arose from a symbiotic union of two organisms. Ivan Wallin extended the idea of an endosymbiotic origin to mitochondria in the s. These theories were initially dismissed or ignored. More detailed electron microscopic comparisons between cyanobacteria and chloroplasts combined with the discovery that plastids organelles associated with photosynthesis and mitochondria contain their own DNA led to a resurrection of the idea in the s.

The endosymbiotic theory was advanced and substantiated with microbiological evidence by Lynn Margulis in Chloroplasts in plants : A eukaryote with mitochondria engulfed a cyanobacterium in an event of serial primary endosymbiosis, creating a lineage of cells with both organelles.

These cyanobacteria have become chloroplasts in modern plant cells. The cyanobacterial endosymbiont already had a double membrane. In she argued that eukaryotic cells originated as communities of interacting entities, including endosymbiotic spirochetes that developed into eukaryotic flagella and cilia.

This last idea has not received much acceptance because flagella lack DNA and do not show ultrastructural similarities to bacteria or archaea. The possibility that the peroxisome organelles may have an endosymbiotic origin has also been considered, although they lack DNA.

However, it now appears that they may be formed de novo , contradicting the idea that they have a symbiotic origin. This hypothesis is thought to be possible because it is known today from scientific observation that transfer of DNA occurs between bacteria species, even if they are not closely related. Bacteria can take up DNA from their surroundings and have a limited ability to incorporate it into their own genome. Mitochondria are energy-producing organelles that are thought to have once been a type of free-living alpha-proteobacterium.

One of the major features distinguishing prokaryotes from eukaryotes is the presence of mitochondria. Mitochondria arise from the division of existing mitochondria.

They may fuse together. They move around inside the cell by interactions with the cytoskeleton. However, mitochondria cannot survive outside the cell. As the amount of oxygen increased in the atmosphere billions of years ago and as successful aerobic prokaryotes evolved, evidence suggests that an ancestral cell with some membrane compartmentalization engulfed a free-living aerobic prokaryote, specifically an alpha-proteobacterium, thereby giving the host cell the ability to use oxygen to release energy stored in nutrients.

Alpha-proteobacteria are a large group of bacteria that includes species symbiotic with plants, disease organisms that can infect humans via ticks, and many free-living species that use light for energy. Several lines of evidence support the derivation of mitochondria from this endosymbiotic event. Most mitochondria are shaped like alpha-proteobacteria and are surrounded by two membranes, which would result when one membrane-bound organism engulfs another into a vacuole.

The mitochondrial inner membrane involves substantial infoldings called cristae that resemble the textured, outer surface of alpha-proteobacteria. The matrix and inner membrane are rich with enzymes necessary for aerobic respiration.