Primal ancestor

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Modern representation of the relationships of all recent organisms in the form of a so-called cladogram (to this extent also called “Tree of Life”). The primal ancestor (LUCA) is at the root of this cladogram. The position of the most recent common stem form of plants, animals, fungi and others is also recorded. a. Euglens and Leishmanias (LECA; see below )

The primal ancestor (English Last Universal Ancestor , LUA , or Last Universal Common / Cellular Ancestor , LUCA ) is the - hypothetical - last common ancestral form of all today's ( recent ) cellular organisms . It is postulated by the so-called monophyletic theories of ancestry , which assume that all living beings on earth, plants , animals , fungi and unicellular forms , can be traced back to a single "primitive" archetype, while all other contemporary, likely existing on the early earth, similar primitive forms have not left any descendants in the recent living world or in recent geological and evolutionary history. It is assumed in these hypotheses that this original form lived at least 3.5 billion years ago, i.e. in the late Hadean or early Archean .

background

For the ancestors, according to the thrift principle ( Occam's razor ), it is usually assumed that they had those characteristics of today's living organisms that they have in common: a cell structure , DNA , the (largely identical) genetic code as well as mRNA , tRNA and a ribosomal mediated Translation . Even DNA repair mechanisms show similarities (homologies) across all cellular organisms and some viruses : the family of RecA recombinases with RecA in bacteria, Rad51 and DMC1 in eukaryotes, RadA in archaea and UvsX in bacteriophage T4 , a bacterial virus .

Since the organization of DNA in bacteria differs fundamentally from that in archaea and complex cells ( eukaryotes ), the thesis has recently been advanced that cellular LUCA still belonged to the RNA world . The storage of genetic information in DNA is then seen as a skill that was initially 'invented' by retroviruses and which cellular organisms have acquired several times through the transmission of such viruses . This results in the bacteria on the one hand, and the archaea and eukaryotes on the other. The basic structure of the ribosomes as protein factories and the genetic code, on the other hand, agree so well in all cellular organisms that the LUCA must already have it.

In addition, it is possible that the ancestor was thermophilic . This is, however, increasingly doubted because this hypothesis was based on comparative sequence analyzes of rRNA ( ribosomal RNA ), which give questionable results in thermophilic organisms. It appears that its cell wall components contained murein , a common cell wall component of modern bacteria .

Notes on possible fallacies:

  1. The Last Universal Ancestor is not a specific individual, but a taxon with the rank of a species and thus a population .
  2. This primordial population does not necessarily represent the first existing (cellular) organisms.
  3. Consequently, it was not necessarily the "most primitive" possible forms of life.

The genesis and development of the genetic code and the ribosomes would then have taken place in the RNA world before LUCA. Before, only ribozymes were available as biocatalysts , then real (protein) enzymes (two phases of the RNA world). Modern ribosomes consist of both ribosomal RNA ( rRNA ) and (auxiliary) proteins. If the term Progenot is understood to mean an organism that is still in the process of developing the relationship between genotype and phenotype , then this organism would have to be located in the first phase, i.e. before the LUCA, which is presumably to be located in the second phase. The term protocell or protobiont is sometimes understood similarly: RNA is replicated in a replicating vesicle (see also microsphere ).

A 2010 statistical study found that life is very likely to descend from a single common ancestor. A single common ancestor is 10 2860 times more likely than several. All of his contemporaries have since died out; only the genetic inheritance would have survived to this day.

In contrast, Carl Woese propagated that our genetic pre-LUCA inheritance came from a large number of organisms than from just one species ( biofilm theory , horizontal [lateral] gene transfer ). The ability of primitive living beings for horizontal gene transfer could mean that instead of a single universal ancestor, a community ( gene pool ) of primitive unicellular organisms appears. Since these were in constant gene exchange with one another ('Common ancestral community'), from today's perspective they could appear as a kind of unit (species). This gene exchange could also have been mediated by viruses.

The structure of dendrograms (tree diagrams), based on the genetic distance between all existing cell types, shows a relatively early split between archaea, which are highly resistant to extreme living conditions ( extremophile ), and the other life forms. This has led to some suspicions that the ancestor might have evolved in such extreme ecosystems , such as the deep sea ridge .

Two or three domains

New investigation techniques have changed the picture significantly since around 2015. The previous family trees were based on the comparison of homologous DNA sequences mainly from the smaller subunit of ribosomal RNA . The methods used for the amplification , especially the primers used , now turned out to be unsuitable for a number of unusual prokaryotes. Until then, these organisms had escaped the attention of biologists because they could not be cultivated with the standard microbiological techniques developed over decades, and most of them still cannot be cultivated to this day. Many of them have a very small genome, they lack the synthesis and metabolic pathways essential for life processes, which is why it is assumed that they may only survive as symbionts or parasites , together with other organisms. Since the formal description of prokaryote taxa is tied to cultivation, the newly found lines persist on the status of a Candidatus . But in fact many taxa, due to their divergence with the rank of were tribes must be classified (phyla) found, in which not a member of cultivable included. Their existence is revealed by analytical methods in which genomes in the environment are analyzed directly without detour via cultivation and then assigned to individual lines / species (called metagenomics ) or in which the genome of a single, isolated cell is amplified and sequenced.

One such group, the “ Lokiarchaeota ” belonging to the archaea , unexpectedly turned out to be closely related to the eukaryotes after genetic analysis. Then further sister groups of the "Lokiarchaeota" were found (" Thorarchaeota ", " Odinarchaeota ", " Heimdallarchaeota "); all together are grouped together to form the "Asgard" super group.

A new family tree of life established with the help of the new techniques in 2016, which is based on more than 3000 fully analyzed genomes, found not only a new large group of bacteria, which (with one exception) only contains previously uncultivable bacterial strains. Another result was that the origin of the eukaryotes lies within the archaea and is not, as previously assumed, independent of them. After that, only two (instead of three) domains of life would exist, because the eukaryotes would be reduced to a line within the archaea. However, this had been suggested earlier in the form of the eocyte hypothesis , but was previously considered to be in contradiction to the findings. Now the Asgard supergroup of archaea would come into question as such a starting point for eukaryotes.

A further support for this dichotomy of living beings comes from the knowledge that the DNA replication systems of bacteria on the one hand and archaea and eukaryotes on the other are so different that - in contrast to the ribosomes and the genetic code - for this no common origin ( homology ) can be assumed. It is possible that the cellular LUCA (like many viruses ) still had an RNA genome, i.e. it was a (hypothetical) ribocyte .

The same applies to proteins considered in terms of DNA repair mechanisms. While the proteins from the RecA recombinase family have homologies across all cellular organisms (and some viruses) as mentioned above, there are homologies among eukaryotes and archaea ( Rad54 , Mre11 , and Rad50 ) for another repair protein .

In connection with these discoveries, other previous views on the systematic value of fundamental differences, such as the structure of lipids in the cell membrane or the origin of gram-positive bacteria, were also questioned. However, since the complex basic structure of the ATP synthase is common to all living things (with the exception of a few purely fermenting microbes and primitive multicellular animals such as some myxozoa and corset animals ), the LUCA should already have a basic version of it. Since the ATP synthase is a membrane-bound protein, the LUCA should already have membranes (in some form), so that an at least primitive cellular organism can be assumed in it, for example with a semipermeable membrane envelope (instead of an impermeable one, as is the case with today's cells Membrane envelope and active transport mechanisms). The analysis of the protein families of the different subunits of membrane-bound F- and A- / V-type ATPases allows conclusions to be drawn about the detailed origin and development of these 'motor' enzymes. This further supports the assumption that LUCA was already a cellular organism with at least a primitive form of membranes, and that the eukaryotes evolved from a branch of the archaea.

Another support of the assumption of a common origin of all cellular organisms could be from the comparative stöchiometrichen analysis of fundamental metabolic pathways ( English metabolic pathways ) of model organisms arise. At MIT, Jean-Benoît Lalanne, Gene-Wei Li and colleagues found a remarkable agreement in a publication in March 2018 on a corresponding study on various bacteria and on baker's yeast . The evolutionary forces behind this highly conserved stoichiometry initially remained in the dark.

The results obtained so far are still provisional due to their novelty, they partly contradict the knowledge obtained using traditional methods and therefore still need to be fully examined. However, they suggest the possibility that some prokaryotes with a very small genome may not go back to the reduction of complex, organized ancestors, but that they could be relictary lines that could only survive in the modern world as symbionts.

Urviruses

In contrast to the cellular organisms (biota) there are apparently different primordial viruses (viral LCAs). Although extremely different viruses also have homologous envelope proteins, there are twenty or more fundamentally different such proteins that are not related to one another, which speaks against a common ancestor of all viruses. Viruses seem to have arisen in a period of strong horizontal gene transfer through reduction from primitive cells that had a segmented RNA genome (ribocytes, RNA world), not from a “modern” counterpart. In accordance with the RNA world and ribocyte hypothesis, the original viruses must have been RNA viruses before they discovered the DNA used to store the genetic information.

Urkaryotes, archezoans and LECA

The last common ancestor of the eukaryotes is sometimes referred to as Last Eukaryotic Common Ancestor , LECA for short , in analogy to the Last Universal Common Ancestor .

All eukaryotes examined so far have, in addition to a cell nucleus separated from the cytoplasm by a nuclear membrane , mitochondria or organelles ( hydrogenosome , mitosome, etc.), which evidently descend from mitochondria or have a common ancestor with them. At least genes of mitochondrial origin could be detected everywhere in the cell nucleus DNA as a result of a lateral gene transfer from an earlier mitochondrion to the nucleus. Therefore LECA should already have a nucleus and mitochondrial-like organelles (with their own mtDNA ).

If the acquisition of the cell nucleus preceded that of the mitochondria , there must have previously been amitochondrial organisms with a cell nucleus that acquired the mitochondria through endosymbiosis in the course of their evolution (see endosymbiotic theory ). Carl Woese and George Fox led in 1977 amoeboid for these hypothetical forms of "primitive" predatory protozoans the notion Urkaryoten (Engl. Urkaryotes ) and Thomas Cavalier-Smith led for recent, primitive amitochondriale protozoa, which he regarded as descendants of these Urkaryoten, the group name Archezoa a. However, there was increasing evidence that the Archezoa are secondary amitochondrial real eukaryotes, i.e. descendants of LECA.

As an alternative to the original karyote hypothesis, the acquisition of mitochondria by archaea ( hydrogen hypothesis ) before or simultaneously with the cell nucleus, which may be of viral origin (among the NCLDVs ), is being discussed .

Archaea and Bacteria - LACA and LBCA

By analogy, the last common ancestor of the archaea can be called LACA , that of the bacteria as LCBA . (sometimes also LBCA).

literature

  • Nicolas Glansdorff, Ying Xu, Bernard Labedan: The Last Universal Common Ancestor: emergence, constitution and genetic legacy of an elusive forerunner. Biology Direct 2008, 3:29.

Web links

Individual evidence

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