Last universal ancestor

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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 3.9 to 4.1 billion years ago.

Contents 1 Misconceptions 2 The hypothesis 3 Criticism 4 See also

[edit] Misconceptions

The LUA is not hypothesised as:

1. the first living organism ever.
2. the most primitive possible living organism.
3. being alone.

[edit] The hypothesis

At the time when LUA was first hypothesized, construction of cladograms based upon genetic distance between all living cells indicated that there was an early split between the Archaeans and the remainder of life. This was inferred from the fact that, at that time, all known archaeans were highly resistant to environmental extremes such as high salinity, temperature or acidity, and led some to suggest that LUA evolved in areas like the deep ocean vents, where such extremes prevail today. However, since that time, archaeans have been discovered in less hostile environments and are now believed by many taxonomists to be more closely related to Eukaryotes than Bacteria, though this is still a matter of some contention.

It is possible that all of LUA's contemporaries would have since become extinct with only the LUA's genetic heritage living on to this day. However, this suggestion ignores the possibility of horizontal gene transfer. Carl Woese has proposed that there was no individual organism which could be considered a LUA, but rather that the genetic heritage of all modern organisms derives from an ancient community of organisms.[1]

[edit] Criticism

This concept assumes that all life is related. This may not be the case, as life may have originated and evolved more than once in different parts of the planet. In other words, self-replicating chemicals (such as RNA and DNA) may have eventually begun to form basic living units (ex: primitive cells) at around the same time in various locales around the Earth. Note that "around the same time" should be construed on the geological scale (ie: on the order of millions of years). Such a scenario, however, should provide an explanation for the remarkable chemical unity of all known lifeforms, which all share the same basic organisation: genetic information encoded in DNA, transcribed into RNA, through the effect of protein- and RNA-enzymes, then translated into proteins through highly conserved ribozymes, with ATP, NADH and others as energy currencies, etc. In particular, the universality of the genetic code must be accounted for.

Some people claim that life in extreme environments would have to evolve along a completely different path than life elsewhere on the surface of the Earth. While true, it has no bearing on whether or not there once existed a common ancestor to all life (which may have been an ancestor to the extremophiles, before they colonized their present locales). For example, it's possible that a common and very primitive ancestor first appeared on Earth in a very hospitable area, and then by replication slowly began to colonize various ecological niches. From that point on, evolutionary paths would have diverged as natural selection favored organisms with mutations specific to each locale.

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.

The Last Universal Ancestor hypothesis has since been refuted on many grounds^{[citation needed]}. For example, it was once thought that the genetic code was universal (see: universal genetic code). Back in the early 1970s, evolutionary biologists thought that a given piece of DNA specified the same protein subunit in every living thing, and that the genetic code was thus universal. Since this is something unlikely happen by chance, it was interpreted as evidence that every organism had inherited its genetic code from a single common ancestor, aka. the "Last Universal Ancestor." In 1979, however, exceptions to the code were found in mitochondria, the tiny energy factories inside cells. Biologists subsequently found exceptions in bacteria and in the nuclei of algae and single-celled animals. It is now clear that the genetic code is not the same in all living things, and that it does not provide powerful evidence that all living things evolved on a single tree of life^{[citation needed]}. Further support that there is no "Last Universal Ancestor" has been provided over the years by lateral gene transfer in both prokaryote and eukaryote single cell organisms. This is why phylogenetic trees cannot be rooted, why almost all phylogenetic trees have different branching structures, particularly near the base of the tree, and why many organisms have been found with codons and sections of their DNA sequence that are unrelated to any other species^{[citation needed]}.

[edit] See also

• Timeline of evolution
• Most recent common ancestor