Darwinian threshold

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Darwinian threshold or Darwinian transition is a term introduced by Carl Woese to describe a transition period during the evolution of the first cells when genetic transmission moves from a predominantly horizontal mode to a vertical mode. [1] [2] The process starts when the ancestors of the Last Universal Common Ancestor (the LUCA) become refractory to horizontal (or lateral) gene transfer (HGT) and become individual entities with vertical heredity upon which natural selection is effective. After this transition, life is characterized by genealogies that have a modern tree-like phylogeny. [3]

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Darwinian threshold: the transition period during the evolution of the first cells when genetic transmission moves from a predominantly horizontal mode to a vertical mode Darwinian threshold.jpg
Darwinian threshold: the transition period during the evolution of the first cells when genetic transmission moves from a predominantly horizontal mode to a vertical mode

Before the Darwinian threshold

The Last Universal Common Ancestor is often considered to be an already complex organism with a DNA-based genome, a complex informational flow and an efficient metabolism, but some authors, like Carl Woese, believe instead that the LUCA was not a discrete entity but rather a diverse community of cells that survived and evolved as a biological unit. [1]

Carl Woese indicated that most likely there existed high mutation rates and small genomes. Also present were small proteins and larger imprecisely translated "statistical proteins". Entities in which translation had not yet developed to the point that proteins of the modern type could arise, have been termed “progenotes,” and the era during which these were the most advanced forms of life, the “progenote era”. [1]

These organisms or biological entities, these progenotes (or ribocytes), had RNA as informational molecule instead of DNA. [4] RNA is capable of both catalysis and replication and could have been central to the origins of heredity and life itself. [5] It has been proposed that the initial molecular events were carried out by transfer RNAs (tRNAs). It is hypothesized that structured tRNAs could have provided amino acids during a process called self-translation of a single extended tRNA strand. [4]

Compartmentalization with membranes was not yet completed and translation of proteins was not precise. Not every progenote had its own metabolism; different metabolic steps were present in different progenotes. Therefore, it is assumed that there existed a community of sub-systems that started to cooperate collectively and culminated in the LUCA. [4]

After the Darwinian threshold

Most scientists place the LUCA at the root of the tree of life. From this root depart two Prokaryotic Domains: the Bacteria and the Archaea. Just after this first split, one of the branches, going towards the Archaea, splits again and gives rise to a third branch which is that of the Eukaryotes so that now there are three Domains of life. [6] Carl Woese thought that even during the era around the origin of the LUCA, the root and the first branches were very blurred since the cells were not very well defined yet and HGT was still quite important. [1] Some authors maintain LUCA was a mesophilic eukaryote. [7] According to these authors the Domains that derived from LUCA through a process of reductive evolution or "streamlining" were Prokaryotes; mesophilic and thermophilic Bacteria and thermophilic Archaea. The term "prokaryote" should therefore be abandoned, since it suggests that "prokaryotes" preceded "eukaryotes" in their evolution from LUCA towards complexity. [7] [6]

See also

Related Research Articles

<span class="mw-page-title-main">Carl Woese</span> American microbiologist (1928–2012)

Carl Woese was an American microbiologist and biophysicist. Woese is famous for defining the Archaea in 1977 through a pioneering phylogenetic taxonomy of 16S ribosomal RNA, a technique that has revolutionized microbiology. He also originated the RNA world hypothesis in 1967, although not by that name. Woese held the Stanley O. Ikenberry Chair and was professor of microbiology at the University of Illinois Urbana–Champaign.

In biology, a kingdom is the second highest taxonomic rank, just below domain. Kingdoms are divided into smaller groups called phyla.

<span class="mw-page-title-main">Symbiogenesis</span> Evolutionary theory holding that eukaryotic organelles evolved through symbiosis with prokaryotes

Symbiogenesis is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possibly other organelles of eukaryotic cells are descended from formerly free-living prokaryotes taken one inside the other in endosymbiosis. Mitochondria appear to be phylogenetically related to Rickettsiales bacteria, while chloroplasts are thought to be related to cyanobacteria.

In biological taxonomy, a domain, also dominion, superkingdom, realm, or empire, is the highest taxonomic rank of all organisms taken together. It was introduced in the three-domain system of taxonomy devised by Carl Woese, Otto Kandler and Mark Wheelis in 1990.

<span class="mw-page-title-main">Three-domain system</span> Hypothesis for classification of life

The three-domain system is a taxonomic classification system that groups all cellular life into three domains, namely Archaea, Bacteria and Eukarya, introduced by Carl Woese, Otto Kandler and Mark Wheelis in 1990. The key difference from earlier classifications such as the two-empire system and the five-kingdom classification is the splitting of Archaea from Bacteria as completely different organisms. It has been challenged by the two-domain system that divides organisms into Bacteria and Archaea only, as Eukaryotes are considered as a clade of Archaea.

<span class="mw-page-title-main">Horizontal gene transfer</span> Type of nonhereditary genetic change

Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the evolution of many organisms. HGT is influencing scientific understanding of higher-order evolution while more significantly shifting perspectives on bacterial evolution.

<span class="mw-page-title-main">Last universal common ancestor</span> Most recent common ancestor of all current life on Earth

The last universal common ancestor (LUCA) is the hypothesized common ancestral cell from which the three domains of life, the Bacteria, the Archaea, and the Eukarya originated. It is suggested to have been a "cellular organism that had a lipid bilayer and used DNA, RNA, and protein". The LUCA has also been defined as "a hypothetical organism ancestral to all three domains". The LUCA is the point or stage at which the three domains of life diverged from preexisting forms of life. The nature of this point or stage of divergence remains a topic of research.

<span class="mw-page-title-main">Neomura</span>

Neomura is a proposed clade of biological life composed of the two domains Archaea and Eukaryota, coined by Thomas Cavalier-Smith in 2002. Its name reflects the hypothesis that both archaea and eukaryotes evolved out of the domain Bacteria, and one of the major changes was the replacement of the bacterial peptidoglycan cell walls with other glycoproteins.

<span class="mw-page-title-main">Prokaryote</span> Unicellular organism lacking a membrane-bound nucleus

A prokaryote is a single-cell organism whose cell lacks a nucleus and other membrane-bound organelles. The word prokaryote comes from the Ancient Greek πρό 'before' and κάρυον 'nut, kernel'. In the two-empire system arising from the work of Édouard Chatton, prokaryotes were classified within the empire Prokaryota. But in the three-domain system, based upon molecular analysis, prokaryotes are divided into two domains: Bacteria and Archaea. Organisms with nuclei are placed in a third domain, Eukaryota.

<span class="mw-page-title-main">Archaea</span> Domain of single-celled organisms

Archaea is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria, but this term has fallen out of use.

<span class="mw-page-title-main">Horizontal gene transfer in evolution</span> Evolutionary consequences of transfer of genetic material between organisms of different taxa

Horizontal gene transfer (HGT) refers to the transfer of genes between distant branches on the tree of life. In evolution, it can scramble the information needed to reconstruct the phylogeny of organisms, how they are related to one another.

<span class="mw-page-title-main">Eukaryote</span> Domain of life whose cells have nuclei

The eukaryotes constitute the domain of Eukarya or Eukaryota, organisms whose cells have a membrane-bound nucleus. All animals, plants, fungi, and many unicellular organisms are eukaryotes. They constitute a major group of life forms alongside the two groups of prokaryotes: the Bacteria and the Archaea. Eukaryotes represent a small minority of the number of organisms, but given their generally much larger size, their collective global biomass is much larger than that of prokaryotes.

Evolution of cells refers to the evolutionary origin and subsequent evolutionary development of cells. Cells first emerged at least 3.8 billion years ago approximately 750 million years after Earth was formed.

<span class="mw-page-title-main">Bacterial taxonomy</span> Rank based classification of bacteria

Bacterial taxonomy is subfield of taxonomy devoted to the classification of bacteria specimens into taxonomic ranks.

<span class="mw-page-title-main">Eocyte hypothesis</span> Hypothesis in evolutionary biology

The eocyte hypothesis in evolutionary biology proposes that the eukaryotes originated from a group of prokaryotes called eocytes. After his team at the University of California, Los Angeles discovered eocytes in 1984, James A. Lake formulated the hypothesis as "eocyte tree" that proposed eukaryotes as part of archaea. Lake hypothesised the tree of life as having only two primary branches: prokaryotes, which include Bacteria and Archaea, and karyotes, that comprise Eukaryotes and eocytes. Parts of this early hypothesis were revived in a newer two-domain system of biological classification which named the primary domains as Archaea and Bacteria.

Microbial phylogenetics is the study of the manner in which various groups of microorganisms are genetically related. This helps to trace their evolution. To study these relationships biologists rely on comparative genomics, as physiology and comparative anatomy are not possible methods.

<span class="mw-page-title-main">Lokiarchaeota</span> Phylum of archaea

Lokiarchaeota is a proposed phylum of the Archaea. The phylum includes all members of the group previously named Deep Sea Archaeal Group, also known as Marine Benthic Group B. Lokiarchaeota is part of the superphylum Asgard containing the phyla: Lokiarchaeota, Thorarchaeota, Odinarchaeota, Heimdallarchaeota, and Helarchaeota. A phylogenetic analysis disclosed a monophyletic grouping of the Lokiarchaeota with the eukaryotes. The analysis revealed several genes with cell membrane-related functions. The presence of such genes support the hypothesis of an archaeal host for the emergence of the eukaryotes; the eocyte-like scenarios.

<span class="mw-page-title-main">Eukaryogenesis</span> Process of forming the first eukaryotic cell

Eukaryogenesis, the process which created the eukaryotic cell and lineage, is a milestone in the evolution of life, since eukaryotes include all complex cells and almost all multicellular organisms. The process is widely agreed to have involved symbiogenesis, in which archaea and bacteria came together to create the first eukaryotic common ancestor (FECA). This cell had a new level of complexity and capability, with a nucleus, at least one centriole and cilium, facultatively aerobic mitochondria, sex, a dormant cyst with a cell wall of chitin and/or cellulose and peroxisomes. It evolved into a population of single-celled organisms that included the last eukaryotic common ancestor (LECA), gaining capabilities along the way, though the sequence of the steps involved has been disputed, and may not have started with symbiogenesis. In turn, the LECA gave rise to the eukaryotes' crown group, containing the ancestors of animals, fungi, plants, and a diverse range of single-celled organisms.

<span class="mw-page-title-main">Two-domain system</span> Biological classification system

The two-domain system is a biological classification by which all organisms in the tree of life are classified into two big domains, Bacteria and Archaea. It emerged from development of knowledge of archaea diversity and challenges to the widely accepted three-domain system that defines life into Bacteria, Archaea, and Eukarya. It was preceded by the eocyte hypothesis of James A. Lake in the 1980s, which was largely superseded by the three-domain system, due to evidence at the time. Better understanding of archaea, especially of their roles in the origin of eukaryotes through symbiogenesis with bacteria, led to the revival of the eocyte hypothesis in the 2000s. The two-domain system became more widely accepted after the discovery of a large group (superphylum) of archaea called Asgard in 2017, which evidence suggests to be the evolutionary root of eukaryotes, implying that eukaryotes are members of the domain Archaea.

The first universal common ancestor (FUCA) is a proposed non-cellular entity that is the earliest ancestor of the last universal common ancestor (LUCA) and its descendants, including every modern cell. FUCA would also be the ancestor of ancient sister lineages of LUCA, none of which have modern descendants.

References

  1. 1 2 3 4 Woese, C. (1998-06-09). "The universal ancestor". Proceedings of the National Academy of Sciences of the United States of America. 95 (12): 6854–6859. Bibcode:1998PNAS...95.6854W. doi: 10.1073/pnas.95.12.6854 . ISSN   0027-8424. PMC   22660 . PMID   9618502.
  2. ↑ Woese, Carl R. (2002-06-25). "On the evolution of cells". Proceedings of the National Academy of Sciences of the United States of America. 99 (13): 8742–8747. Bibcode:2002PNAS...99.8742W. doi: 10.1073/pnas.132266999 . ISSN   0027-8424. PMC   124369 . PMID   12077305.
  3. ↑ Arnoldt, Hinrich; Strogatz, Steven H.; Timme, Marc (2015-11-13). "Toward the Darwinian transition: Switching between distributed and speciated states in a simple model of early life". Physical Review E. 92 (5): 052909. arXiv: 1501.05073 . Bibcode:2015PhRvE..92e2909A. doi:10.1103/PhysRevE.92.052909. PMID   26651764. S2CID   204906567.
  4. 1 2 3 José, Marco V.; Rêgo, Thais Gaudêncio; Farias, Sávio Torres de (2015-12-03). "A proposal of the proteome before the last universal common ancestor (LUCA)". International Journal of Astrobiology. 15 (1): 27–31. doi: 10.1017/S1473550415000464 . ISSN   1473-5504.
  5. ↑ West, Timothy; Sojo, Victor; Pomiankowski, Andrew; Lane, Nick (2017-12-05). "The origin of heredity in protocells". Philosophical Transactions of the Royal Society B: Biological Sciences. 372 (1735): 20160419. doi:10.1098/rstb.2016.0419. ISSN   0962-8436. PMC   5665807 . PMID   29061892.
  6. 1 2 Patrick., Forterre (2007). Microbes de l'enfer. Paris: Belin--pour la Science. ISBN   9782701144252. OCLC   228784853.
  7. 1 2 Glansdorff, Nicolas; Xu, Ying; Labedan, Bernard (2008-07-09). "The Last Universal Common Ancestor: emergence, constitution and genetic legacy of an elusive forerunner". Biology Direct. 3 (1): 29. doi: 10.1186/1745-6150-3-29 . ISSN   1745-6150. PMC   2478661 . PMID   18613974.
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