PLYLOGENIC TREEE
The origin of the living cell have been marked by various stories and rumours but the plylogenic tree does depict the real origin of the first cell which was called progenote.
The fact that genes can move between distant
branches of the tree of life even at low probabilities raises challenges to
scientists who are trying to reconstruct evolution from studying genes and gene
sequences in different organisms, because horizontal gene transfer (HGT) effectively
scrambles the information on which biologist are relying to reconstruct a
phylogeny of organisms - that is their evolutionary history and relationships.
Furthermore, the challenges that are raised by HGT are most awkward for the
ambitious (but extremely interesting) reconstruction of the earliest events in
evolution - that is the early branches of the tree of life, because over long
time intervals and with large numbers of organisms many low probability HGT
events are certain to have actually occurred.
A three domain tree
of life showing the separation of Bacteria, Archaea, and Eukaryote domains. The
three main early branches of the tree of life have been intensively studied by microbiologists because the
first organisms were microorganisms. Microbiologists (led by Carl Woese) have introduced the term domain for the
three main branches of this tree, where domain is a phylogenetic term very similar in meaning to biological
kingdom. To reconstruct this tree of life, the sequence of a particular genes
encoding the small subunit of ribosomal
RNA (SSU rRNA, 16s rRNA) have proven very useful, and the tree shown to the
left relies heavily on information from this single gene. Eukaryotes are all organisms with a well defined nucleus, and this domain comprises protists, fungi, and all organisms in the animal and plant kingdoms, including humans
- The most common gene used for constructing phylogenetic relationships in microrganisms is the small subunit ribosomal RNA gene, as its sequences tend to be conserved among members with close phylogenetic distances, yet it is variable enough that differences can be measured . The SSU rRNA as a measure of evolutionary distances was pioneered by Carl Woese when formulating the first modern "tree of life", and his results led him to propose the Archaea as a third domain of life.) However, recently it has been argued that SSU rRNA genes can also be horizontally transferred. ] Although this may be rare, this possibility is forcing scrutiny of the validity of phylogenetic trees based on SSU rRNAs. Recent discoveries of 'rampant' HGT in microorganisms, and the detection of horizontal movement of even genes for the small subunit of ribosomal RNA have forced biologists to question the accuracy of at least the early branches in the tree shown on the left, and even question the validity of trees as useful models of how early evolution occurs.Sequence comparisons suggest recent horizontal transfer of many genes among diverse species including across the boundaries of phylogenetic "domains". Thus determining the phylogenetic history of a species can not be done conclusively by determining evolutionary trees for single genes." [] HGT is thus a potential confounding factor in inferring phylogenetic trees from the sequence of one gene. For example, if two distantly related bacteria have exchanged a gene, a phylogenetic tree including those species will show them to be closely related even though most other genes have diverged substantially. For this reason it is important to use other information to infer phylogenies, such as the presence or absence of genes, or, more commonly, to include as wide a range of genes for analysis as possible. vita vivenda
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