Which domain evolved first

Others have argued that the three domains of life arose simultaneously, from a set of varied cells that formed a single gene pool. Two of the three domains, Bacteria and Archaea, are prokaryotic. Based on fossil evidence, prokaryotes were the first inhabitants on Earth, appearing 3.

These organisms are abundant and ubiquitous; that is, they are present everywhere. In addition to inhabiting moderate environments, they are found in extreme conditions: from boiling springs to permanently frozen environments in Antarctica; from salty environments like the Dead Sea to environments under tremendous pressure, such as the depths of the ocean; and from areas without oxygen, such as a waste management plant, to radioactively-contaminated regions, such as Chernobyl.

Prokaryotes reside in the human digestive system and on the skin, are responsible for certain illnesses, and serve an important role in the preparation of many foods. Prokaryotes in extreme environments : Certain prokaryotes can live in extreme environments such as the Morning Glory pool, a hot spring in Yellowstone National Park. Archaea are believed to have evolved from gram-positive bacteria and can occupy more extreme environments.

When and where did life begin? What were the conditions on earth when life began? Prokaryotes were the first forms of life on earth, existing for billions of years before plants and animals appeared. The earth and its moon are thought to be about 4. This estimate is based on evidence from radiometric dating of meteorite material together with other substrate material from earth and the moon. Early earth had a very different atmosphere contained less molecular oxygen than it does today and was subjected to strong radiation; thus, the first organisms would have flourished where they were more protected, such as in ocean depths or beneath the surface of the earth.

Also at this time, strong volcanic activity was common on Earth. It is probable that these first organisms, the first prokaryotes, were adapted to very high temperatures. Early earth was prone to geological upheaval and volcanic eruption, and was subject to bombardment by mutagenic radiation from the sun. The first organisms were prokaryotes that could withstand these harsh conditions.

Although probable prokaryotic cell fossils date to almost 3. Instead, chemical fossils of unique lipids are more informative because such compounds do not occur in other organisms. Some publications suggest that archaean or eukaryotic lipid remains are present in shales dating from 2.

Such lipids have also been detected in Precambrian formations. The archaeal lineage may be the most ancient that exists on earth. Within prokaryotes, archaeal cell structure is most similar to that of gram-positive bacteria, largely because both have a single lipid bilayer and usually contain a thick sacculus of varying chemical composition.

Archaea and gram-positive bacteria also share conserved indels in a number of important proteins, such as Hsp70 and glutamine synthetase.

It has been proposed that the archaea evolved from gram-positive bacteria in response to antibiotic selection pressure. This is suggested by the observation that archaea are resistant to a wide variety of antibiotics that are primarily produced by gram-positive bacteria and that these antibiotics primarily act on the genes that distinguish archaea from bacteria. The evolution of Archaea in response to antibiotic selection, or any other competitive selective pressure, could also explain their adaptation to extreme environments such as high temperature or acidity as the result of a search for unoccupied niches to escape from antibiotic-producing organisms.

Microbial mats or large biofilms may represent the earliest forms of life on earth; there is fossil evidence of their presence starting about 3. A microbial mat is a multi-layered sheet of prokaryotes that includes mostly bacteria, but also archaea.

Microbial mats are a few centimeters thick, typically growing where different types of materials interface, mostly on moist surfaces. The various types of prokaryotes that comprise the mats use different metabolic pathways, which is the reason for their various colors. Prokaryotes in a microbial mat are held together by a glue-like sticky substance that they secrete called extracellular matrix.

Chimneys, such as the one indicated by the arrow, allow gases to escape. The first microbial mats likely obtained their energy from chemicals found near hydrothermal vents. With the evolution of photosynthesis about 3 billion years ago, some prokaryotes in microbial mats came to use a more widely-available energy source, sunlight, whereas others were still dependent on chemicals from hydrothermal vents for energy and food.

Fossilized microbial mats represent the earliest record of life on earth. A stromatolite is a sedimentary structure formed when minerals are precipitated out of water by prokaryotes in a microbial mat. Furthermore, Archaea possess ether-linked lipid membranes which differ from the ester-linked lipid membranes both found in Bacteria and Eukarya.

This difference in the composition of the lipid membranes is in contrast to the fusion hypothesis. According to the authors, co-evolution between early Archaea and Bacteria as well as Eukarya occurred in phases III to V and was included significant horizontal exchange of genetic information between these three emerging domains of life.

They end by providing some interesting questions that arise from this model and propose some avenues for future research in that area. For a more detailed description of the Archaea-first hypothesis, please also take a look at the video abstract for this article provided by the authors. Finally, the three-domain structure of Woese's tree Figure 1a shows that evolutionary history is decoupled from biological organization.

Indeed, archaea and bacteria appear very similar biologically members of both groups consist of tiny cells without much internal structure and different from eukaryotes. However, until scientists determined the position of the LUCA what evolutionary biologists call the root position in the tree, all three domains appeared equal. With the progress of gene sequencing in the s, many scientists performed phylogenetic studies to compare universally conserved proteins, such as protein subunits of the ribosome or of RNA polymerase.

Their results supported the three-domain classification. Moreover, evolutionary biologists developed approaches to deduce the root position of the tree.

Strikingly, they placed the LUCA between bacteria on one side and archaea together with eukaryotes on the other side, implying that archaea and eukaryotes share a common ancestor to the exclusion of bacteria Figure 1b; Gogarten et al.

This finding emphasizes that similarity of cellular organization and common ancestry are two very different things. The discovery of Archaea as a distinct, new domain of cellular life stimulated extensive studies into the molecular biology of these microbes, many of which thrive in unusual, extremely hot or salty environments. From these studies, researchers learned that the three domains are indeed fundamentally different at several cell biological levels, and not just in universal genes like the 16S rRNA.

How do the domains of life differ? Scientists identified two key distinctions related to the DNA replication system and the membrane. The replication system of archaea is largely unrelated to that of bacteria, but it is homologous to the replication machinery of eukaryotes.

Conversely, the archaeal membrane and the proteins involved in its formation are unique, whereas bacteria and eukaryotes share homologous membranes. Thus, archaea and bacteria differ with respect to the origin of some of their central cellular systems, whereas eukaryotes seem to combine important features of both archaea and bacteria.

Evolutionary biologists used the sequences of multiple genomes of diverse life-forms to construct and compare thousands of phylogenetic trees for individual genes. Unexpectedly, when comparing these trees they learned that genes generally have distinct evolutionary histories, and the trees built for different genes show different branching orders topologies.

The diversity of gene tree topologies is particularly pronounced among prokaryotes. For example, when scientists build trees for the numerous genes encoding metabolic enzymes or membrane transport proteins, the separation of archaea and bacteria is almost never precisely reproduced; instead, the archaeal and bacterial branches are mixed. What do these horizontal connections represent? They represent horizontal gene transfer HGT , the exchange of genes between different species.

Indeed, scientists have described mechanisms of HGT, even between archaea and bacteria. Numerous theoretical and experimental studies indicate that HGT is the principal mechanism of evolutionary innovation in prokaryotes Pal et al.

One well-known, medically important example is the spread of antibiotic resistance among pathogenic bacteria. The importance and ubiquity of HGT notwithstanding, comprehensive comparative analyses of phylogenetic trees have shown that the treelike structure roughly corresponding to the rRNA phylogeny represents a central trend in the evolution of prokaryotes.

These trees apparently reflect the concerted evolution of a core set of highly conserved, essential genes, most of which encode proteins involved in information transmission Puigbo et al. In eukaryotes, HGT appears to be much less common than in prokaryotes. Nevertheless, eukaryotic genes seem to differ in their origins.

The majority are most closely related to bacterial homologs, whereas a minority appear to be of archaeal origin Esser et al. What purposes do these genes serve in eukaryotes? The "archaeal" genes in eukaryotes primarily, albeit not exclusively, encode proteins involved in information processing translation, transcription , and replication.

The "bacterial" genes encode mostly operational proteins, such as metabolic enzymes and membrane transporters. Figure 2 Figure Detail Thus, eukaryotes are archaebacterial genetic chimeras; that is, they have combinations of genes from two very different organisms. How could eukaryotes have genes from two different organisms?

The remarkable, unique process that explains this phenomenon is endosymbiosis, the invasion of one host cell by another, followed by degradation of the invader endosymbiont , which becomes an organelle, like the mitochondrion. All known eukaryotic cells contain mitochondria, or related organelles, which play central roles in energy conversion.

These mitochondria retain common features with bacterial cells, including a small genome and a mitochondrial translation system, which reveal beyond a doubt that they originated from a specific bacterial group, the a-proteobacteria. Many bacterial genes were transferred from the genome of the endosymbiont to the eukaryotic nuclear genome during evolution of the mitochondria.

How do scientists believe this transfer could have occurred? One hypothesis holds that the host of the mitochondrial endosymbiont was a primitive eukaryotic cell sometimes called an archaezoan that possessed the signature structures of eukaryotes, including the nucleus, and was capable of phagocytosis Figure 2a. The alternative hypothesis is that the host of the endosymbiont was an archaeon, and the endosymbiosis triggered the evolution of eukaryotic innovations Figure 2b.

Making a rigorous choice between the two hypotheses is extremely difficult. Unlike the archaezoan hypothesis, however, the endosymbiotic hypothesis accounts for the apparent lack of primitive amitochondrial eukaryotes that could be direct descendants of the archaezoa among the known eukaryotes.

Scientists discovered viruses at the end of the nineteenth century as ultramicroscopic parasites of plants and animals, which passed through filters that held back bacteria. By the middle of the twentieth century, it became clear that viruses can replicate only within cells. However, the actual prominence of viruses in the biosphere and their role in the evolution of life were not revealed until the advances of metagenomics allowed for the massive sequencing of genes and genomes in environmental samples without the isolation of individual organisms.

Viruses turn out to be the dominant biological entities on Earth. In the ocean, for example, viral particles outnumber cells by an order of magnitude Suttle Viruses are also dominant in terms of genetic variety. Indeed, the greatest number of unique genes without detectable homologs in other genomes is found in viral genomes Kristensen et al.

In contrast with cellular life-forms — which all employ the same, classic strategy of DNA replication, transcription, and translation — viruses possess diverse genetic cycles. Viruses employ nearly all imaginable strategies of genome replication and expression: Some viruses have single-stranded or double-stranded RNA genomes that do not involve DNA in their replication, some have RNA genomes that use DNA as a replication intermediate, and some have genomes that are either single-stranded or double-stranded DNA molecules.

How do viral genomes compare to those of cellular life-forms? In comparison to cellular life-forms, viruses possess small genomes, ranging in size from between about 1, and 1,, nucleotides. Viruses typically lack many of the genes that are universal among the three domains of cellular life — in particular, genes for translation system components.

However, a small core of viral "hallmark genes" have been discovered that are missing in cellular life-forms. These genes encode proteins essential for virus reproduction e. These hallmark genes are shared by an extremely diverse group of viruses with different replication strategies, although none of the genes is strictly universal among viruses. The discovery of the hallmark genes reveals the evolutionary unity of the viral empire Koonin et al.

Finally, viruses and related mobile genetic elements that lack capsids e. These selfish genetic elements are major agents of gene transfer. The genomes of many eukaryotes, particularly animals and plants, consist in large part of inactivated remnants of such elements up to 80 percent of the genome in plants. Biologists sometimes debate whether viruses should be considered living organisms. However, the debates seem to be largely issues of semantics.

Comparative genomics and metagenomics have transformed our understanding of the genetic universe. New discoveries have revealed the previously unrealized prominence of the viral world.

This second biological empire seems to be even more vast and diverse than the empire of cellular life-forms. A second key transformation in our understanding is that a complex network of treelike and netlike routes better explains evolution than does a single TOL. Even under this new network perspective, the three domains of cellular life — Bacteria, Archaea, and Eukarya — remain objectively distinct.

Although these domains are distinct, the eukaryotes are archaebacterial chimeras, which evolved as a result of, or at least under the strong influence of, an endosymbiotic event that gave rise to the mitochondria. Despite all the recent advances of evolutionary genomics, we still have to answer the most fundamental questions: How did cells evolve in the first place, what caused the fundamental differences between the two prokaryotic domains Archaea and Bacteria , and what triggered the emergence of the complex organization of the eukaryotic cell?

Brown, J. Archaea and the prokaryote-to-eukaryote transition. Microbiology and Molecular Biology Reviews 61 , — Doolittle, W. Pattern pluralism and the Tree of Life hypothesis.