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If I understand correctly, the concept of the LUCA (last universal common ancestor) is based on the hypothesis that archaea and bacteria share common ancestry.

In the realm of mathematics, the same discoveries have often been made more than once, in different places. Sometimes (like with calculus) these independent discoveries were made almost simultaneously. Sometimes the discoveries appear to be totally unrelated (Hipparchus affirmative compound propositions before 100 BC, and David Hough's work on inserting parentheses in 1944).

If "life" started on Earth in some kind of primordial soup, it is conceivable that it started not once but many times, in different places and at different times.

How strong is the evidence that bacteria and archaea do not represent quite separate discoveries of "life", which (because of the nature of the chemical substances that they use) happen to share a number of features, such as DNA?

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    $\begingroup$ Multicellular, or metazoan, life may have evolved more than once, because sponges (Porifera) have a life-cycle and body plan unlike any other plants or animals (although this notion my be outdated). Check out the evolution of the aminoacyl-tRNA synthetases. Some authors have argued that the fact that there are two discrete families, and that each family can charge tRNAs for the major classes of amino acids as evidence that this capacity, to prepare a limiting substrate for protein synthesis evolved at least twice. $\endgroup$ – mdperry Aug 2 '15 at 21:46
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It is quite likely (but impossible to study) that the first stages of life originated more than once. However, the evidence that all currently existing life originated from the same population is very strong.

On the most basic level, many of the basic building blocks of life, RNA, DNA, amino acids are chiral, which means they come in multiple forms, but for the basic processes in nature, always the same one is chosen. For example, the DNA double-helix is almost always right-handed in nature, see also here. If there were multiple origins of archaea and bacteria, we would expect some to pick another form, if not other molecules. In addition, many crucial proteins such as DNA- and RNA- polymerases are very similar between all species, the probability that they would have originated independently is simply minuscule.

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  • $\begingroup$ Isn't the right-handed nature of DNA the result of energy requirements ? Wouldn't any new life form tend to adopt the configuration that requires lower energy levels? Is there no evidence of genetic transfer between archaea and bacteria that could account for the sharing of proteins? Might not the choice of molecules have been dictated by what was available in the original "soup"? $\endgroup$ – James Newton Aug 2 '15 at 18:07
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    $\begingroup$ 1. (energy) you could build left-handed DNA with the same energy levels as right-handed DNA using L-Ribose (since the sugar is also chiral). 2. there is likely genetic transfer, but at this point you might want to think which parts of an organism you would consider ancestral, 3. at least for amino acids, the version not occuring in proteins is used for other things. $\endgroup$ – bpeter Aug 2 '15 at 18:43
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    $\begingroup$ The universal nature of the genetic code strongly supports the hypothesis that unicellular life evolved once on the planet (or that the organisms that used this genetic code won out). $\endgroup$ – mdperry Aug 2 '15 at 21:49
  • $\begingroup$ If life did originate multiple times, it'd be quite a coincidence if it ended up with the same specific amino acids, nucleotides, sugars, genetic code and on top of that also the same chirality (for basic processes). $\endgroup$ – Inhibitor Aug 4 '15 at 11:39
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The genetic code of the tree of life is actively studied. So far, all the living beings have conformed to LUCA model. They are all RNA and DNA dependent, and no other life forms are known. All the RNA and DNA life forms share a tree like inheritance pattern with more simple organisms.

Sometimes there is doubt about wether mushrooms are more closely related to animals or plants.

Sometimes it is debated wether Prions are life forms or not. They are proteins that can reproduce by tricking human cells to making them and then transmit themselves, but they have not developed into anything else than a protein.

Some Virii have a special relationship with DNA, in that they just hyjack and eat DNA using RNA tricks, but their code is basically a lego block of other DNA they have hyjacked, they have fascinating evolution mechanism.

Here is the Tree Of Life, for the moment all genetic testing has found a tree structure shared by common acenstors, in all organsims, hence a LUCA: tree of life

here you can see it in detail, so far 50 000 species are on there, from the same philae and DNA -tested, and soon they will have more species, like 200k or something. huge pdf poster: http://www.timetree.org/public/data/poster/timetree24x32.pdf

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