Last universal ancestor

This topic is sometimes abbreviated as LUA. To find other uses, please go to Lua.

Last universal ancestor (LUA), is the hypothetical latest living organism from which all currently living organisms descend. As such, it is the most recent common ancestor of the set of all currently living organisms. Also LCA (last common ancestor) or LUCA (last universal common ancestor). It is estimated to have lived some 3.5 billion years ago.

Misconceptions

The LUA is not hypothesised as:

the first living organism ever.
the most primitive possible living organism.
being alone.

The hypothesis

Construction of cladograms, based upon genetic distance between all living cells, shows that there was a very early split between the Archaeobacteria that are highly resistant to environmental extremes of great salinity, high temperature or high acidity, and the remainder of life. This has led some to suggest that LUA evolved in areas like the deep ocean vents, where such extremes prevail today.

All its contemporaries would have since become extinct with only the LUA's genetic heritage living on to this day. Carl Woese proposed that our pre-LUA genetic heritage derives from a community of organisms, rather than an individual.^*

Horizontal gene transfer

Horizontal gene transfer is a potential confounding factor in inferring phylogenetic trees based on the sequence of one gene. For example, given two distantly related bacteria that have exchanged a gene, a phylogenetic tree including those species will show them to be closely related because that gene is the same, even though most other genes have substantially diverged. For this reason, it is often ideal to use other information to infer robust phylogenies, such as the presence or absence of genes, or, more commonly, to include as wide a range of genes for phylogenetic analysis as possible.

For example, the most common gene to be used for constructing phylogenetic relationships in prokaryotes is the 16s rRNA gene, since its sequences tend to be conserved among members with close phylogenetic distances, but variable enough that differences can be measured. However, in recent years it has also been argued that 16s rRNA genes can also be horizontally transferred. Although this may be infrequent, validity of 16s rRNA-constructed phylogenetic trees must be reevaluated.

Biologist Gogarten suggests "the original metaphor of a tree no longer fits the data from recent genome research" therefore "biologists use the metaphor of a mosaic to describe the different histories combined in individual genomes and use ^*

"Using single genes as phylogenetic markers, it is difficult to trace organismal phylogeny in the presence of HGT gene transfer. Combining the simple coalescence model of cladogenesis with rare HGT gene transfer events suggest there was no single last common ancestor that contained all of the genes ancestral to those shared among the three domains of life. Each contemporary molecule has its own history and traces back to an individual molecule cenancestor. However, these molecular ancestors were likely to be present in different organisms at different times." ^*

Uprooting the Tree of Life by W. Ford Doolitte (Scientific American, February 2000, pp 72-77) contains a discussion of the Last Universal Common Ancestor, and the problems that arose with respect to that concept when one considers horizontal gene transfer. The article covers a wide area - the endosymbiont hypothesis for eukaryotes, the use of small subunit ribosomal RNA (SSU rRNA) as a measure of evolutionary distances (this was the field Carl Woese worked in when formulating the first modern "tree of life", and his research results with SSU rRNA led him to propose the Archaea as a third domain of life) and other relevant topics. Indeed, it was while examining the new three-domain view of life that horizontal gene transfer arose as a complicating issue: Archaeoglobus fulgidus is cited in the article (p.76) as being an anomaly with respect to a phylogenetic tree based upon the encoding for the enzyme HMGCoA reductase - the organism in question is a definite Archaean, with all the cell lipids and transcription machinery that are expected of an Archaean, but whose HMGCoA genes are actually of bacterial origin.

Again on p.76, the article continues with:

"The weight of evidence still supports the likelihood that mitochondria in eukaryotes derived from alpha-proteobacterial cells and that chloroplasts came from ingested cyanobacteria, but it is no longer safe to assume that those were the only lateral gene transfers that occurred after the first eukaryotes arose. Only in later, multicellular eukaryotes do we know of definite restrictions on horizontal gene exchange, such as the advent of separated (and protected) germ cells."

The article continues with:

"If there had never been any lateral gene transfer, all these individual gene trees would have the same topology (the same branching order), and the ancestral genes at the root of each tree would have all been present in the last universal common ancestor, a single ancient cell. But extensive transfer means that neither is the case: gene trees will differ (although many will have regions of similar topology) and there would never have been a single cell that could be called the last universal common ancestor.

"As Woese has written, 'the ancestor cannot have been a particular organism, a single organismal lineage. It was communal, a loosely knit, diverse conglomeration of primitive cells that evolved as a unit, and it eventually developed to a stage where it broke into several distinct communities, which in their turn became the three primary lines of descent (bacteria, archaea and eukaryotes)' In other words, early cells, each having relatively few genes, differed in many ways. By swapping genes freely, they shared various of their talents with their contemporaries. Eventually this collection of eclectic and changeable cells coalesced into the three basic domains known today. These domains become recognisable because much (though by no means all) of the gene transfer that occurs these days goes on within domains."

Gourt Home::Gourt :: Last universal ancestor

Misconceptions

The hypothesis

Horizontal gene transfer

See also