Darwinian threshold or Darwinian transition is a term introduced by

Carl Woese Carl Richard Woese ( ; July 15, 1928 – December 30, 2012) was an American microbiologist and biophysicist. Woese is famous for defining the Archaea (a new domain of life) in 1977 through a pioneering phylogenetic taxonomy of 16S ribosomal ...

to describe a transition period during the

evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

of the first cells when genetic transmission moves from a predominantly horizontal mode to a vertical mode. The process starts when the

ancestor An ancestor, also known as a forefather, fore-elder, or a forebear, is a parent or ( recursively) the parent of an antecedent (i.e., a grandparent, great-grandparent, great-great-grandparent and so forth). ''Ancestor'' is "any person from ...

s of the

Last Universal Common Ancestor 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. The cell had a lipid bilayer; it possessed the genetic code a ...

(the LUCA) are no longer primarily dependent on horizontal (or lateral) gene transfer (HGT) and become individual entities with vertical

heredity Heredity, also called inheritance or biological inheritance, is the passing on of traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cells or organisms acquire the genetic infor ...

upon which

natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable traits characteristic of a population over generation ...

is effective. After this transition, life is characterized by

genealogies Genealogy () is the study of families, family history, and the tracing of their lineages. Genealogists use oral interviews, historical records, genetic analysis, and other records to obtain information about a family and to demonstrate kins ...

that have a modern tree-like phylogeny. Darwinian_threshold

Before the Darwinian threshold

The Last Universal Common Ancestor is often considered to be an already complex organism with a

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

-based

genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...

, a complex informational flow and an efficient

metabolism Metabolism (, from ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the co ...

, but some authors, like

, 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. Carl Woese indicated that most likely there existed high

mutation rate In genetics, the mutation rate is the frequency of new mutations in a single gene, nucleotide sequence, or organism over time. Mutation rates are not constant and are not limited to a single type of mutation; there are many different types of mu ...

s and small genomes. Also present were small

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

s and larger imprecisely translated "statistical proteins". Entities in which

translation Translation is the communication of the semantics, meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The English la ...

had not yet developed to the point that proteins of the modern type could arise, have been termed “

progenote The first universal common ancestor (FUCA) is thought to be a non-cellular entity that was the earliest organism with a genetic code capable of biological translation of RNA molecules into peptides to produce proteins. Its descendants would in ...

s,” and the era during which these were the most advanced forms of life, the “progenote era”. These organisms or biological entities, these progenotes (or ribocytes), had

RNA Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...

as informational

molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

instead of DNA. RNA is capable of both

catalysis Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

and replication and could have been central to the origins of heredity and life itself. It has been proposed that the initial molecular events were carried out by

transfer RNA Transfer ribonucleic acid (tRNA), formerly referred to as soluble ribonucleic acid (sRNA), is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length (in eukaryotes). In a cell, it provides the physical link between the gene ...

s (tRNAs). It is hypothesized that structured tRNAs could have provided

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

s during a process called self-translation of a single extended tRNA strand. 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.

After the Darwinian threshold

Most scientists place the LUCA at the root of the

tree of life The tree of life is a fundamental archetype in many of the world's mythology, mythological, religion, religious, and philosophy, philosophical traditions. It is closely related to the concept of the sacred tree.Giovino, Mariana (2007). ''The ...

. From this root depart two

Prokaryotic A prokaryote (; less commonly spelled procaryote) is a single-celled organism whose cell lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Greek (), meaning 'before', and (), meaning 'nut' ...

Domains: the

Bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

and the

Archaea Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...

. 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

Eukaryote The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

s so that now there are three Domains of life. 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. Some authors maintain LUCA was a

mesophilic A mesophile is an organism that grows best in moderate temperature, neither too hot nor too cold, with an optimum growth range from . The optimum growth temperature for these organisms is 37 °C (about 99 °F). The term is mainly applied ...

eukaryote. According to these authors the Domains that derived from LUCA through a process of reductive evolution or "streamlining" were Prokaryotes; mesophilic and

thermophilic A thermophile is a type of extremophile that thrives at relatively high temperatures, between . Many thermophiles are archaea, though some of them are bacteria and fungi. Thermophilic eubacteria are suggested to have been among the earliest bact ...

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.

References

{{Reflist Origin of life Evolutionary biology Genetic genealogy Phylogenetics Hypothetical life forms Most recent common ancestors

Before the Darwinian threshold

After the Darwinian threshold

See also

References