I was really surprised, due to my lack of knowledge in too many areas of life sciences, that our history, the history of all species, from bacterias to humans, it is not like a tree. Usually we think about evolution as a tree, where some species have developed and give rise to a new, improved one (and I seay improved because Darwin’s view of evolution states that non-improved species should be eliminated in the process, which is fairly logic).
I am not familiar with the representation of evolution in dendograms and some other types. But a recent paper in Nature [1] teached me that, in fact, the evolution of the species cannot be represented as a tree. The right representation should be a ring. And I am far from understand the reasoning behind that statement, but it sounds right. When you have several species in a tree, and two of them get togheter, the two branches guided by that species are joined, closing that branches and creating a new one. Now, think about the origins of life. Surely not so many species should exists at that time; so, a few species are creating a very small tree of life. So, if two species from that “primordial” tree are joined, you create a ring, where the “joining point” is specifically the event in which the two species got together. But, the question remains: what events can account for such a joining? James Lakes states that an early prokaryotic endosymbiosis, between a clostridum and an actinobacterium, accounts for such event and created a plaform for the evolution of the eukaryotic life.
Some years ago, Lake presented an article in Nature [2] explaining that a genome fusion was the event responsible for the rise of the eukaryotic life. I remembered a discussion in my class, when I was an undergraduate student, regarding the issue that some eukaryotic genes were related to one prokaryotic lineage, and some others were related to another lineage. The analysis of Lake and Rivera showed that a ring structure can easily explain that discrepancy (or, in other words, the hypothesis that a genome fusion event between two prokaryotic species explain the rise of eukaryotes): informational genes of eukaryotes are derived from Archaea, and operational genes are derived from Bacteria.
It seems a good start. These two works provides interesting views about evolution of eukaryotes. But some questions remains, and are proposed by Lake itself. For example, can really the endosymbiosis be the only mechanisms explaining the ring structure? Maybe it is easier to have just two species joining that several species exchanging genes and, by an unusual mechanism, evolving until create a new organism harboring informational and operational genes from two specific lineages. But, why we cannot observe such endosymbiotic organisms? Lake presents the example of a prokaryotic consortium, Chlorochromatium aggregatum, as the only near example to an endosymbiosis between prokaryotic species. Could we be able to create an exclusively prokaryotic endosymbiosis in vitro? That would be interesting. If we are consistent with the notion of “don’t believe in what you can’t see”, we should be willing to explore others explanations for the rise of eukaryotic life.
[1] Lake JA (2009). Evidence for an early prokaryotic endosymbiosis. Nature, 460 (7258), 967-71 PMID: 19693078
[2] Rivera MC, & Lake JA (2004). The ring of life provides evidence for a genome fusion origin of eukaryotes. Nature, 431 (7005), 152-5 PMID: 15356622
Actually, Lake’s analysis is kind of flawed… as you pointed out, there’s not many examples of prokaryotic endosymbiosis in prokaryotes; only one that I know of that is permament and more or less confirmed. There’s also a -few- bacterial endoparasites of other bacteria, but again it’s quite unclear how it works (sorry don’t have the reference with my right now, but I think if you search for “bacterial endosymbionts of prokaryotes’ you may find something… actually, type in this:
“Bacterial Endosymbionts in Prokaryotes”
D Corsaro, D Venditti – Complex Intracellular Structures in Prokaryotes, 2006 – books.google.com
(URL is kind of ugly and can’t be bothered to html right now…)
The obsession with endosymbiosis and LGT are a little bit overboard, and actually the more sane (and less vocal) thinkers on the subject don’t really buy into this kind of stuff. A quite plausible explanation for the archaeal-like features of eukarya is Cavalier-Smith’s Neomuran Hypothesis (see Cavalier-Smith 2009 “Predation and eukaryote cell origins: A coevolutionary perspective”):
Basically, initially most eubacteria double membraned, then single membraned forms arise by loss of outer membrane (Lake’s like fuckup there is incredible: he designated the outermost membrane as being outer regardless of evolutionary origin, and then that left him rather confused: the ‘outer’ membrane of single membraned bacteria is actually chemically related to the ‘inner’ membrane of double membraned ones!!! No need for his ridiculous endosymbiosis story)
So these ‘posibacteria’ as TC-S calls single membraned ones had membrane + murein wall left. Under specific conditions, the murein wall was lost, and this led to two solutions: a new type of wall as in archaea, and a cytoskeletal system in proto-eukarya, leading to phagocytosis, nuclei and beyond. Together, those form the Neomura. Perhaps some of the archaeal features in eukaryotes (or eukaryotic features in archaea, if you will) were developed among the early Neomura prior to the segregation of archaea and eukarya, eg. histones, etc. TC-S proposes some of those were adaptations to the extreme environment they may have found themselves in (subsequently, archaea were able to carve themselves a niche as extremophiles, and eukarya became bacteriovores due to their ability to phagocytose…)
My brief and ill-worded comment here does no justice to Cavalier-Smith’s elaborate hypothesis, so you should check it out if interested. I do intend to blog about it properly once I come back from vacation…
Anyway, just my two cents, hope you didn’t mind too much!
Should get back to work…
Cheers,
-Psi-