LIFE

What do we think we know about the origin of life? Of eukaryotic cells? Of sex? These "major transitions" are described by John Maynard Smith and Eörs Szathmáry (1999). Groups of four students will investigate sets of alternative scientific hypotheses related to origin of life, origin of eukaryotes, and origin & maintenance of sex to see how evidence is used to determine which hypothesis is best, resulting in scientific progress in understanding natural phenomena. Each group will present to the class both a written (1000-word) and an oral (20-minute) report.

A Mesophilic Origin of Life

Melissa Tauscher, David Simpson and Kevin Long

Nearly all scientists agree that life on Earth began billions of years ago. Most will also agree that RNA appeared first and found a way to replicate itself, an essential step in the early stages of life. Without this ability of RNA, only short pieces of DNA could be copied and no enzymes could be created to copy longer strands which would be necessary for the formation of enzymes[1]. Beyond that, there is very little agreement about the origin of life, including what the original ancestor of all life was like. This has led to a significant amount of debating with very few answers. Scientists are still debating how life originated: was it a series of successive steps or a spontaneous gathering of the necessary materials? The problem with the second theory is that the odds of this happening are about the same as a tornado assembling a 747 aircraft in a junkyard.[2]

As of now, we still have to ponder whether the ancestor of all life, or LUCA, was a single being or a community of organisms sharing genes. Another question entails whether the last universal common ancestor (LUCA) dwelled in a hot-water environment or a cooler place, possibly near the surface of the ocean. The experts are split almost evenly on this question and both sides present convincing evidence for their side of the argument, but we will focus primarily on the side that believes that LUCA was not thermophilic, and, in fact, lived in a cooler surrounding. This cooler surrounding would have likely been near the surface of the ocean rather than in the vicinity of a thermal vent near the ocean’s floor.

The other side argues that Earth was warmer 3.5 billion years ago, when life is believed to have been spawned. This, coupled with the discovery of archaea nearly 30 years ago, is sufficient evidence for some to believe that LUCA had to be a thermophilic, or heat-loving being. The archaea share characteristics with bacteria and eukaryotes, and the modern groups most nearly related to the ancestor of archaea and bacteria are hyperthermophiles.

The argument that attempts to refute this states that over such a long period of time, mutations and inherited traits can overwrite one another and significantly alter the genetic composition of organisms in different generations. It is true that an incredible amount of evolution has taken place over 3.5 billion years, so it is possible that the “closely-related” single-celled organisms could have evolved back and forth between thermophilic organisms and those of cool environments several times in that time span.

Attempts to use the ribosomal RNA (ribonucleic acid) of LUCA to discover the temperature which it preferred have also ended in split results, but one reconstruction of a putative ribosomal RNA sequence for LUCA has shown it to have been in favor of cool water.[3] It has previously been shown that purines, necessary for life, can be produced in the presence of amines at temperatures near -20°C.[4] These purines consist of a six-membered and a five-membered nitrogen containing ring.

Recent studies published in Science magazine show that ancestral RNA had a moderate guanine and cytosine composition, well below that of known hyperthermophiles and much more similar to organisms that live at moderate temperatures. In fact, the studies show that these genes could not have withstood temperatures above 70°C.[5] The guanine and cytosine are more stable than adenine and uracil pairs at high temperatures because of an additional hydrogen bond.

Similar reports have been published in Nature in the past few years. A reanalysis of bacterial phylogeny based on ribosomal RNA found that hyperthermophilic bacteria did not emerge first. This implies that bacteria had a mesophilic ancestor, which is one that functions well in moderate conditions. The report speculates that Planctomycetales could have been the first group of bacteria to emerge and that the emergence of hyperthermophilic bacteria came as a secondary adaptation to life at high temperatures. A hyperthermophile-specific enzyme, reverse gyrase, was acquired from archaea, and is the best evidence that the original bacteria ancestor was not a thermophile, as reverse gyrase is not an original bacterial enzyme, but is necessary to sustain life at high temperatures. The findings in Nature regarding cytosine and guanine are consistent with this theory of a “cold” LUCA, as well as their phylogeny, which shows hyperthermophilic bacteria to be a monophyletic group that has appeared lately, relatively speaking.[6] A monophyletic group has a single common ancestor to which all members can be traced.

However, this does not prove that the original ancestor of life was a “cold” organism, but it certainly brings new evidence forward that suggests such a possibility. Throughout human history there has been a reoccurrence of false assumptions, and then these assumptions have been proven false by new scientific discoveries. This may very well be the case when we consider our original ancestor. We still have yet to find definitive scientific evidence for either side, but the most recently discovered evidence (and our best assumption) proposes that LUCA was, in fact, not a thermophile. While this sheds more light on the origins of life, we must still continue to search for more evidence, especially in RNA sequences, until an answer can be established.

Brochier, C., and H. Philippe. 2002. A non-hyperthermophilic ancestor for Bacteria. Nature 417:244.

DeDuve, Christian. 1991. Blueprint for a Cell: The Nature and Origin of Life 100-105.

Levy, M., and S.L. Miller. 1999. The prebiotic synthesis of modified purines and their potential role the RNA world. Journal of Molecular Evolution 48:631-637.

Smith, John Maynard., and Szathmary, Eros. 1999. The Origins of Life: From Birth to the Origins of Language 1-14.

Artificial Protocells

Summer Carr, Lindy Russell, Chenoa Allen

As science and technology advance, an understanding of the origin of life becomes a feasible possibility. Artificial life research seeks to mimic life and to gain knowledge of the origin of life through reenacting it. This research strives to create the simplest possible “organism” that fulfills all of the requirements of life.

These researchers define life as that which can evolve, self-reproduce, metabolize, adapt to environmental changes, and, ultimately, die. A fundamental concept behind research attempts is emergent properties, those that arise in more complex levels and are not predictable from the properties of lower hierarchical levels. Researchers use two approaches to create protolife: the “bottom up” approach strives to create life from nonliving components, while the “top down” approach attempts to simplify living cells to create the simplest possible cell and thus give a glimpse of a possible form of the first living cell. The “bottom up” approach seems to be the more widely attempted method because it is believed that if nature had to start with simple building blocks, we should do the same.

There are two theories about the order in which life emerged; “one […] assumes the primacy of metabolism and cellular organization [while] the other […] assumes the primacy of reproduction and genetic information” (Lifson). Thus, much relevant research focuses on determining whether the origin of life began with self-replicating, information storing molecules (such as RNA or PNA) or with other molecules such as lipids or proteins (which form, for instance, encapsulating membranes). Whatever the order in which these characteristics evolved, there are three components a protocell must have: a container, a metabolism, and an information-storing molecule(s) (Rasmussen, et al.). Much research has been and continues to be done in this discipline.

The first important experiment concerning the production of artificial life took place in 1953 utilizing the bottom-up approach. Grad student Stanley Miller approached scientist Harold Urey about the possibility of a pre-biotic synthesis experiment using a reducing gas mixture. They used water vapor produced by heating to simulate evaporation from oceans and mixed it with methane, ammonia and hydrogen to mimic a water vapor-saturated primitive atmosphere. They subjected their artificial world to electric discharge, and after only two days of sparking Miller was able to detect glycine. This gave scientists hope that the building blocks of life might one day be reproduced in a manner similar to that experiment (Bada and Lazeano).

Obviously, many advances have been made since Urey and Miller, and more recently one prominent experiment has been conducted by Jack Szostak (Harvard Medical School) and a group of grad students working under him. Szostak created an evolutionary cycle by stringing together random nucleotides to create RNA molecules and gave them the task of latching on to another molecule. They extracted those who completed their task to make trillions of new copies with some random mutations. The evolved RNA strands are called aptamers, and by 2002, ribozymes had been produced that could grab RNA molecules and use them to add nucleotides onto an RNA fragment (up to 14 nucleotides with 97% accuracy). Scientists are still a few steps away from creating life, but they admit that they are close.

According to the supposed nature of the emergence of life on Earth, it must be possible to create life if the appropriate conditions can be determined and provided. The processes that led to life were chemical and thus are bound by certain rules; therefore, these processes are inevitable given the proper environment. The goal of modern experiments is to first determine and then manufacture the proper environment in which life will inevitably naturally emerge (“The Beginnings of Life on Earth”).

Many experiments are being planned to try to create artificial life. Organizations such as the IBEA plan to conduct such experiments such as developing a synthetic chromosome (“IBEA Receives $3 Million”). Despite the many challenges, such as determining when a molecular aggregate is truly ‘alive’ and the current inability of scientists to create a living cell from scratch, scientists continue to strive to create artificial life by “trying to define the minimum number of genes it need[s] to survive by stripping out all the unnecessary ones” (“Scientists Create a Virus”). Recently, a group of scientists announced “significant progress […] toward creating an artificial organism” (“Scientists Create a Virus”).

The creation of artificial could have many consequences and benefits for the world in which we live. The artificially created cells could be engineered to perform “useful functions” such as the self-replication and self-repair of genes (Rasmussen, 964). Artificially created cells could also have “uses ranging from pollution control to clean energy production” (“Scientists Create a Virus”). These organisms could be a “biology-based solution for some of our most pressing energy and environmental challenges” (“Scientists Create a Virus”). If it were possible to manufacture life here on Earth, it would also give us an opportunity to experiment with the different conditions necessary for creating and sustaining life, which could give us clues to where other life might exist in our universe (Zimmer 40).

“IBEA Receives $3 Million Dept. of Energy Grant for Synthetic Genome Development.”

Lifson, Shneior. “On the Crucial Stages in the Origin of Animate Matter.” Journal of Molecular Evolution (1997) 44:1-8.

Rasmussen, Chen, et al. “Transitions from Nonliving to Living Matter.” Science.

Bada, Jeffrey L. and Antonio Lazeano. “Prebiotic Soup—Revisiting the Miller Experiment.” Science. Vol. 300: 745-56. May 2003.

“Scientists Create a Virus.” CreationTalk.com Forum. Internet document accessed

Hot Stuff Baby!!!

Amanda Banning, Jonathan Hatcher, Kristina Hollowell

Before any speculation toward the origin of biotic forms, what was present at the formation of the earth that could result in inorganic, then organic, and later biotic creatures? Early atmospheric conditions have been theorized to be present due to planetesimal collisions releasing gases present in the Earth, after the initial atmosphere of Hydrogen and Helium escaped Earth’s gravity assisted by heat energy. The earlier atmosphere is believed to have consisted mainly of carbon, hydrogen, nitrogen, and oxygen (bonded to other elements) in such forms as CO₂/CO, N₂, and H₂0. Stanley Miller, through experimentation, shows that given an energy source like heat or electric charge it is possible to form reactions that create complex molecules, and through subsequent experiments nucleic acids like adenine were even formed. This is the premise for the “hot” theories of the origin of life. Given there are many derivative possibilities like process evolution, chemoautotrophic, and photoautotrophic origins, the basis is that given an energy source (heat) basic elements can form and break bonds to become increasingly complex.

Given the theories have technically been progressing since 1922 and A.I. Oparin’s hypothesizing, the major strides have been in recent research. Through studies of volcanic activity, fossils, and archaebacteria, speculation leans heavily toward evidence provided by “hot” theory experiments. Given that it is quite plausible and possible that the early earth had the suggested “hot” environment providing heat and monomers that can combine to become polymers, the main step to come into question is, when did these polymers amount to life? “Life for Dummies” would suggest that life requires a way to replicate (RNA/DNA), which involves metabolic pathways like those involving acetyl-CoA. This is the point at which most theories diverge. How were these processes created?

Gunter Wachtershauser’s experiments forced the scientific community to recognize that life originating in “hot” conditions was a plausible theory. Wachtershauser understood that molecules needed a meeting place to be able to form and he proposed that the surface of iron-sulfur minerals and pyrite proved to be very favorable. He was able to prove that the driving force in the creation of amide bonds could be pyrite formation. Using conditions similar to that of volcanic vents Wachtershauser and Claudia Huber joined two carbon atoms to form activated acetic acid and eventually was able to link amino acids into short peptides (Hagmann 2006). Since Wachtershauser's discovery further experiments have been conducted which support his theory.

One experiment supports that under hot conditions elements crucial to metabolism could form. Pyruvic acid is essential for extant intermediary metabolism. Using a high temperature and high pressure, 250 degrees Celsius and 200 MPa, the experiment showed that pyruvic acid forms from formic acid in the presence of nonylmercaptane and iron sulfide (Wachtershauser 1307-1308). Also, it has been suggested that COS could be an intermediate in the hydrothermal formation of dipeptides from amino acids, but only in the presence of nickel and iron sulfides (Leman 283-286). This tells us that this reaction probably occurred in regions close to volcanic sources where these minerals are present. To make the cycle complete, it is necessary that peptides turn back into amino acids. A net conversion of CO to CO2 are the driving force of both the anabolic and catabolic segments of the peptide cycle. Through several experiments it was shown that hydantoin could have formed in the presence of CO-laden volcanic vents. Also, it was shown that hydanoins are intermediates in the breakdown of peptides (Huber 938-940). Therefore, the entire cycle of peptides could occur under the same conditions.

Another experiment shows that mineral-catalyzed reduction of N₂ to ammonia might have provided a significant source of ammonia to the Hadean Ocean. The experiment was performed under conditions of temperatures between 300 to 800 degrees Celsius and pressures of 0.1-0.4 GPa, which are usual conditions of crustal and oceanic hydrothermal systems. This is important because a reducing environment rich in ammonia is more efficient in synthesizing nitrogen bearing organics than an environment dominate in N₂(Brandes 365-367). An additional experiment discusses how the same reaction of N₂ to ammonia would have been present in hydrothermal environments making them ammonia rich environments in the prebiotic world which would have provided favorable conditions for early life. The hydrothermal vents could have made a contribution to the atmosphere which would have allowed the Earth to stay above a freezing temperature. The interaction between the surface and subsurface environments may have been important for the origin of life (Chyba 329-330).

Opponents to the theory that life began in “hot” conditions create obstacles for the hypothesis. One is that DNA bonds, and organic bonds in general, break at hot temperatures. To counter this problem, it has been proposed that the heat vents merely provided the “ingredients” needed for life, and that the life processes actually began away from the site of the extreme heat, in a sort of gradient theory (Szaflarski).

Another theory similar to the gradient theory is that life didn’t actually begin in hot vents or in the hot temperatures, merely that the hot conditions created the molecules that warmed the surface, where life formed. Says (Cody 1339), “From the perspective of an ideal environment suitable for the emergence of life, a promising hypothesis would have the synthesis of carbonylated iron-sulfur catalysts occurring at higher temperatures and pressures (e.g., 200 to 300 degrees C and 100 to 200 Mpa), followed by advection of such compounds to lower temperature regimes (e.g., 90 to 150 degrees C)”. Also, recent studies show that certain bacteria, known as thermophiles, can live under “hot” conditions.

Other opponents of the pre-biotic broth theory say that needed elements were not present in the early world. There is “no evidence that the methane-ammonia atmosphere necessary for prebiotic synthesis ever existed” (Luskin and Hankins). To counter theories that question the source of oxygen, which Earth’s early atmosphere is believed to lack, Leman, Orgel, and Ghadiri propose that the oxygen needed could have been derived from nitrate, nitrite, or ferricyanide ions (285). Scientists also have to consider that there might have been conditions that are not present or inexplorable today, which could impact the formation of pyruvic acid as well as the decomposition of formic acid, suggests Wachtershauser (1308).

It has yet to be discovered the true origin of life, but evidence does strongly suggest the possibility of life originating in hot conditions.

Brack, Andre, ed. The Molecular Origins of Life. Cambridge: Cambridge University

Brandes, Jay A., et al. “Abiotic Nitrogen Reduction on the Early Earth”. Nature

Chyba, Christopher. “Buried Beginnings”. Nature 395 (24 September 1998): 329-330.

Cody, George D., et al. “Primordial Carbonylated Iron-Sulfur Compounds and the

Edwards, Matthew R. “From a Soup or a Seed? Pyritic Metabolic Complexes in the

Hagmann, Micheal. “Between a Rock and a Hard Place”. Science 295 (15 March 2002):

Huber, Claudia, et al. “A Possible Primordial Peptide Cycle”. Science 301 (15 August

Leman, Luke, Leslie Orgel, and M. Reza Ghadiri. “Carbonyl Sulfide – Mediated

Luskin, Casey, and Reid Hankins. “Problems with Purely Natural Explanations for the

Szaflarski, Diane. “Possible Sites for the Origin of Life”. Cruising Chemistry. 22

Wachtershauser, Gunter. “Life as We Don’t Know It”. Science 289 (25 October 2000):

Zubay, Geoffrey. Origins of Life on the Earth and in the Cosmos. San Diego: Academic

Cell Fusion: Prokaryotic Genome Fusion Lead to First Eukaryote

Leslie Epling Drew Guider Travis Russell

Endosymbiotic theory commonly asserts that the eukaryotic endomembranes and cytoskeleton originated and formed a type of proto-eukaryotes that started engulfing and using other prokaryotes, thus explaining the presence of bacterial genes in mitochondria. Many scientists are now contesting this idea based on new data. This new data shows that some of the machinery for transcription and translation of DNA may be from archaea, instead of the theory that most or all genetic information came from an engulfed bacterial cell. “In particular, it is turning out that eukaryotic nuclear genomes carry many genes of bacterial (sometimes α-proteobacterial) origin which have nothing to do with mitochondrial functions.” (2) The tree of life may not necessarily be correct when it shows Eukarya coming off either the bacterial or archaeal branch of the tree. Instead, we are proposing that new theories suggesting genomic fusion may be more accurate.

The relationship may have started out through necessity, which lead to increased surface contact and eventual lateral gene transfer. Scientists not only have evidence that this may have happened instead of endosymbiosis, but also reasons why cell fusion may have occurred. Martin and Muller think that the excretion of hydrogen and carbon dioxide by α-proteobaceria benefited methane-producing archaeans that used hydrogen and carbon dioxide as sources of energy and carbon. “In the absence of an outside source of hydrogen, the Archaean (nominally the host) became dependent on the α-proteobacterium (the symbiont). Selection of host genes produced tighter and tighter physical association and greater surface contact between the two.” (2)

Researchers recently used thirty different organisms that are selected from the categories Eukarya, Archaea, and Bacteria, looking for genes linked to eukaryotic organisms. With all of the organisms obtained having about the same number of genes, they were able to use computer analysis to determine that ancient prokaryotes, most likely one bacteria and one Archaea, laterally transferred genes to survive. (1)

Evolution of eukaryotes seemed to be previously based on rRNA as the genetic material used for lateral gene transfer. “The usefulness of rRNA in exploring biodiversity within the three domains in unparalleled, but the proposal for a natural system of all life based on rRNA alone has come increasingly under fire.” (4) If rRNA alone were used to determine the tree of life, then Eukarya and Archaea would be sisters. New data proposed by Rivera, Lake, and others shows that lateral gene transfer is possible, and reconciles a tree that seems more accurate than ever before.

“Recently a new algorithm, conditional reconstruction, based on the two character states of gene presence and absence has been developed, which can reconstruct genome fusions.” These new rings formed due to the new algorithm are consistent with several different accepted phylogenetic trees—all at once. “From these results, we infer that the eukaryotic nuclear genome was formed from the fusion of the genomes of a relative of a proteobacterium and a relative of an archaeal eocyte.” The data that was analyzed according to the newest computer algorithms suggests that eubacterial genes are the operational genes and archaean genes are the informational genes in the first eukaryotic cell fused from these two prokaryotic ancestors.(3)

“Further, he added, the results provide a new picture of evolutionary pathways. “At least two billion years ago, ancestors of these two diverse prokaryotic groups fused their genomes to form the first eukaryote, and in the processes, two different branches of the tree of life were fused to form the ring of life,” Lake said.”(1) New DNA analytic technology that employs computational tools can successfully compare genomes. In the study performed by Lake and Rivera, thirty organisms were used, all with approximately the same number of genes. The comparative analysis showed that the first eukaryote was probably formed from the fusion of a bacteria like organism and an Archaea, as a mutually advantageous relationship. (1)

The new data that suggests that the tree of life may not be fully accurate does not discount most widely accepted parts. It is important to explain that all the cell fusion theory is saying is that instead of one prokaryote engulfing another to form organelles including a nucleus, that two different prokaryotes evolved separately and fused to form the first eukaryote. How does this change the way we think about the LUCA or last universal common ancestor? It makes us think that instead of it being one prokaryote, it may have been a eukaryote formed from two prokaryotes. In addition, we are not trying to determine in this short paper whether Archaea or Bacteria evolved first; instead, we are simply stating that the endosymbiosis theory of Lynn Marguiles leaves far too many gaps in the foundation of eukaryotic existence for it too be widely accepted without any further examination.

Works Cited:

(1)”Complex Cells Likely Arose from Combination of Bacterial and Extreme Microbe

Genomes.” NSF Press Release. 8 Sept 2004.

http://www.nsf.gov/od/lpa/newsroom/

(2)Doolittle, W. Ford. “A Paradigm Gets Shifty.” Nature. Volume 392. 5 Mar 1998.

(3)Lake, James; Rivera, Maria. “The Ring of Life provide evidence for a genome fusion

origin of eukaryotes.” Nature. Volume 431. 9 Sept 2004.

(4)Embley, T. Martin; Martin, William. “Early Evolution Comes Full Circle.”

Endosymbiosis

Endosymbiosis is the theory that eukaryotic cells were formed when a prokaryotic cell ingested some aerobic bacteria. The first step of the evolution of a eukaryotic cell is the infolding of the cellular membrane. This process takes place when the plasma membrane folds inwards and develops an envelope around a smaller prokaryotic cell. Once the smaller cell is engulfed, it becomes dependent upon its host cell. It relies on the host cell for organic molecules and inorganic compounds. However, the host cell also benefits because it has an increased output of ATP for cellular activities and becomes more productive. This ATP comes from the mitochondrion (the aerobe) that is engulfed.

All eukaryotic cells contain the mitochondrion that is made through this process. However, only some of the eukaryotic cells (plant cells) form chloroplasts through endosymbiosis after the mitochondrion is formed. Some of the prokaryotic cells ingest cyanobacteria. These bacteria contain photosynthetic pigments that are useful in photosynthesis. The cyanobacteria become dependent upon the host cell and can no longer survive on its own. Over time, it becomes the chloroplast, a main organelle of plant cells. The chloroplast is then able to convert energy from the sun to energy-rich sugar molecules which are then converted to chemical energy in the form of ATP. (1)

The evidence for endosymbiosis is most prevalent in the mitochondria and chloroplasts of cells. The ribosomes of mitochondria and chloroplasts resemble that of prokaryotic ribosomes because of their similar size, 70s. Mitochondria, chloroplasts, and prokaryotes all divide by binary fission. The genome of mitochondria and chloroplasts most resemble prokaryotes and they hare similar DNA replication, transcription, and translation as that of bacteria. The genomes have a single circular molecule of DNA and there are no histones associated with the DNA.

The protein-synthesizing machinery in mitochondria and chloroplasts resemble prokaryotes. This is shown through their ribosomal RNA and the structure of the ribosomes. The ribosomes are similar in size and structure to bacterial ribosomes. fMat is always the first amino acid that is in the mitochondria and chloroplasts transcripts. The antibiotics that act by blocking protein synthesis in bacteria also block protein synthesis in mitochondria and chloroplasts. These antibiotics do not interfere with protein synthesis in the cytoplasm of the eukaryotes. The inhibitors that effect the protein synthesis of eukaryotic ribosomes do not change the protein synthesis of the bacteria, mitochondria, or chloroplasts.

Mitochondria and chloroplasts have two membranes that surround them. The inner membrane is probably from the engulfed bacterium and this is supported by that the enzymes and proteins are most like their counterparts in prokaryotes. The outer membrane is formed from the plasma membrane or endoplasmic reticulum of the host cell. The electron transport enzymes and the H+ ATPase are only found in the mitochondria and chloroplasts of the eukaryotic cell. (2)

Currently, there are two major competing theories for the endosymbiotic origin of eukaryotic cells. The first theory claims that the eukaryotic cell is a combination of an archaeon with a bacterium. This hypothesis is known as the AB hypothesis. A major difficulty with this hypothesis is that the prokaryotic host cell must have actually been able to engulf another cell. The engulfing of other cells requires an internal cytoskeleton, which interacts with the plasma membrane. This structure, in the absence of a cell wall, allows phagocytosis to happen. Prokaryotes, whether they are archaea or bacteria, do not contain a complex internal cytoskeleton, and do have a cell wall. For these reasons, they are unable to engulf another cell. Also, a whole set of new cellular structures, other than the cytoskeleton, would have to be constructed from prokaryotes that did not have them.

These ominous difficulties with the AB hypothesis lead to the second theory. This hypothesis assumes that the nucleus is formed from the endosymbiosis of archaea and bacteria in a third cell. This third cell is called the chronocyte, leading the name of this hypothesis to be the ABC hypothesis. The simplest prediction of this theory is that all that was lacking in the AB hypothesis was fulfilled by the proteins that evolved from the chronocyte.

There are some basic parts in eukaryotic cells that support the ABC hypothesis. The first is the cytoskeleton. Found in the cytoskeleton are the eukaryotic signature proteins actin and tubulin. They have structural and weak sequential similarities to proteins found in bacteria and archaea. The assumption is that there was a common ancestral protein and that it existed in the progenote, which is the cellular domain that was an ancestor to both the chronocyte and the prokaryotic cells. Ergo, when on efinds a protein in eukaryotic cells that is similar structurally, but has little or no sequential likeness to those found in prokaryotic cells, the best solution tha tone can offer is that these proteins were shared by a common ancestor.

A second supporting part of the ABC hypothesis is the plasma membrane. The prokaryotes have perfected the use of a proton gradient across a membrane to create ATP. The eukaryote has perfected the interaction of cytoskeletal proteins with its membrane, adjusted by calcium ions. This distinction could be the major driving force for endosymbiosis. The eukaryote could engulf a prokaryotic cell and it would benefit from the ability of the prokaryotic cell to generate ATP from a proton gradient. This was the case in the formation of the mitochondrion from the aerobic bacterium and the formation of the chloroplast from a cyanobacterium. Also, this may have been the case in early stages of the formation of the nucleus as well.

In conclusion, the nucleus is an endosymbiont of bacteria and archaea. The host cell most likely did not come from the bacteria. The host cell, chronocyte, was not a prokaryotic cell but one that had a cytoskeleton composed of actin and tubulin and a complex membrane system. The chronocyte contributed to the end product that is the euaryotic cell. Its contributions were the cytoskeleton, endoplasmic reticulum, Golgi apparatus, and major intracellular control systems. (3)

3) Hartman, Hyman, and Fedorov, Alexei. The Origin of the eukaryotic cell: A genomic investigation. 5 February 2002.

Great Ideas Project: Origin of Sex

Sarah Culp, Matt Lazenka, Matt Schact

Sex, though usually used in terms of reproduction, is actually quite separate: it refers to the splitting and recombining of genetic material through the meiosis (fission) and fertilization (fusion) of genomes in such a way that, when they are reproduced, the new generation of cells contains a different set of genes than that of its parents. Sex is by no means necessary for reproduction. Asexual reproduction, or parthenogenesis, is actually about twice as efficient for population growth; there is thus a “two-fold cost” of sex in reproduction. There are many benefits to sex, however, that outweigh its inefficiency. Among these benefits are the opportunities for the repair of damaged DNA, the rapid recombination of genes to more readily adapt to changing environments, and the elimination of the accumulation of deleterious mutations from a population. It is easy to find benefits to sex; finding a theory for the origin of sex is not nearly as simple. There are many contested theories for what made organisms evolve in a pattern which is less efficient for population growth, but no sure-fire answers.[1]

The predominant theory for the origin of sex has always been the benefits of DNA repair. In an asexual haploid cell, if both sides of a DNA strand were damaged—for instance, if the thymine and adenine erred and became unrecognizable—since the cell would have only one copy of the information, there would be no chance for the cell to repair the damage and the cell would either die or be able to pass on only grossly mutated genes. However, an asexual diploid cell and a sexual cell each contain an extra set of chromosomes providing the template to repair that damaged DNA strand. Using DNA repair as the strongest argument for sex is, therefore, problematic as it gives asexual diploids an equal footing with sexual organisms in that regard. DNA repair in and of itself does not provide a satisfactory explanation for the necessity of sex, only a strong argument for diploidy.

In his book, The Origins of Life, Szathmáry has used the Stochastic Corrector Model to render believable situations of how asexual diploid cells fare equally as well as sexual cells when faced with similar environmental conditions. Szathmáry attributes the asexual diploid’s success to gene redundancy. In fact, Michod’s models (“Origin of Sex for Error Repair”) parallel Szathmáry’s in that they too show that selection favors asexual diploids over sexual cells, but only when the costs of diploidy are small, mortality rates low, and damage rates high.[2] To make a strong case for sexual cells in selection, there must be moderate to high costs for diploidy, high mortality and high damage rates. With these models, one must look for an environmental cause outside DNA repair to sustain a dominant sexual society.[3]

Most theories of the origin of sex include either facultative or obligate sexual cycles. Facultative means that sex is optional and that an organism can reproduce either sexually, through the fusion of haploid gametes produced in meiosis, or asexually, by simple cell fission. An obligate sex cycle on the other hand is bound to the mechanism of sex and that is the only mode of reproduction. Simply stated, organisms that have facultative sexual cycles can reproduce sexually or asexually, whereas organisms that reproduce in an obligate sexual cycle are forced to reproduce sexually or not at all.

The primary content of the article “The First Sexual Lineage and the Relevance of Facultative Sex” is that facultative sex was most likely the sexual cycle which developed first, at the origin of sex. In fact, facultative sex seems to have all the advantages of sex such that the question is raised why obligate sex ever came about. A creature which reproduces through a facultative sexual cycle can reproduce with the speed and efficiency of the asexual mechanism, while also enjoying the ability to recombine DNA which benefits sexual reproduction. This article supposes that the arrival of obligate sex may have something to do with the evolution of increasingly complex multi-cellular organisms. Nevertheless, this article dodges the question of why mammals don’t reproduce through a facultative sexual cycle.[4][5][6]

One recent theory on the origin of sex is that UV radiation stirred evolution in such a way as to make sex advantageous. UV radiation is a mutagen, and exposure thereto causes genes to mutate in ways that may or may not be good for the organism. Sex splits the homologous pairs of chromosomes and allows them to recombine in haploidy before they are passed on, so that daughter cells receive different combinations of beneficial and harmful mutations. Selection favors this process because the cells that receive only harmful mutations will die off, and cells with more beneficial mutations will become stronger and more fecund. It also allows for a generation of daughter cells with no mutations, all of which cut down on the accumulation of deleterious mutations. UV radiation also affects metabolic processes and causes DNA damage, which makes sex advantageous in that it provides for a second copy of all DNA and allows for recombinational DNA repair.[7]

Another theory is that sex originated as a way to protect cells from infection by plasmids and other parasitic bodies. According to this theory, cells that come to contain parasites through phagocytosis or another method of ingestion will co-evolve with the parasites, allowing the primary cell protection against further infection and the secondary body assurance of reproduction. Sex must emerge to keep this relationship stable, preventing the parasite from taking over the host by breaking up the symbiont genomes and asserting host control over replication.[8]

[1] Smith, Maynard and Szathmáry, Eors (1999). The Origins of Life. New York: Oxford University Press

[2] Michod, Richard E. (1998). Origin of Sex for Error Repair. Theoretical Population Biology, 53, 60-74

[3] Mauro Santos, et. al. (2003) Origin of Sex Revisted. Origins of Life and Evolution of the Biosphere, 32, 405-432

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