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  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  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.
 Lake JA (2009). Evidence for an early prokaryotic endosymbiosis. Nature, 460 (7258), 967-71 PMID: 19693078
 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