Last Eukaryotic Common Ancestor

Margulis L, Chapman M, Guerrero R, Hall J (2006) The last eukaryotic common ancestor (LECA) Acquisition of cytoskeletal motility from aerotolerant spirochetes in the Proterozoic Eon. Proc Natl Acad Sci USA 103 13080-13085... 

In contrast, Penny and Poole (1999) suggested that the last universal common ancestor may have been a mesophile with many features of the eukaryote genome, and the first distinct eukaryote may thus also have been mesophile—a distal inhabitant of a hydrothermal system, or a planktonic form. 

The common root represents LUCA, the Last Universal Common Ancestor of bacteria, archaea and eukaryotes. Notice that the Archaea are intermediate between the Bacteria and Eucarya, as inferred from many of their detailed morphological and biochemical properties, as well as their ribosomal RNA sequences. The boxed dates indicate the minimum age of selected branches, based on fossil evidence and biochemical fingerprints, such as the characteristic membrane steroids found by Jochen Brocks and his colleagues in the shales underlying the Hammersley iron formation in Australia. Adapted with permission from Andrew Knoll and Science. 

The family of eukaryotic Ras-like small GTPases may be divided into subfamilies, namely those of ARF, Rab, Ran, Ras, Rho, and Sar (ARF, RAB, RHO, RAS, RHO, SAR), which all contain representatives from fungi, plants, and metazoa. Consequently, these subfamilies and their cellular functions are likely to have emerged early in eukaryotic history. This implies that the last common ancestor of fungi, plants, and metazoa possessed vesicular transport (ARF and Sar), membrane trafficking (Rab), nuclear transport (Ran), signal transduction (Ras), and regulation of the actin cytoskeleton (Rho) functions. 

Ever since Woese and Fox (1977) suggested that the last common ancestor of all life was a precellular incompetent progenote and Van Valen and Maiorana (1980) suggested that eukaryotes evolved from archaebacteria there has been confusion over this issue. Woese and Fox s never remotely tenable idea of the cenancestor as a simple precellular entity has been adequately... 

High yield of ATP from aerobic respiration made possible the development of typically eukaryotic features (Vellai et al. 1998). It is suggested that the last common ancestor of all eukaryotes was an aerobically respiring organism capable of complete oxidation of carbohydrates to carbon dioxide and water. Some unicellular eukaryotes have either retained or secondarily acquired the ability for anaerobic respiration and hydrogen-evolving fermentation, which has allowed their adaptation to life under microaerophilic or anaerobic conditions. 

The experimental data that prove the existence of the three primary kingdoms do not tell us much about the last common ancestor, but we can still say that such a progenitor must have existed, because all cells of the three kingdoms have the same genetic code, the same metabolic currency based on ATP, and roughly 50% of bacterial genes have homologues in eukaryotes. 

The last common ancestor did not have the impressive structures that we usually associate with eukaryotes - it did not have a nucleus, a cytoskeleton, mitochondria, chloroplasts, mitosis, meiosis or sexuality - and yet it did already have the basic features that deep down characterise the eukaryotic cell. Despite the lack of a nucleus, in short, the last common ancestor was not a bacterium, because it did not have the functional features that are specific of bacteria. 

In this view, the eukaryotes may well preserve some very primitive characteristics that are not seen in prokaryotes. Glansdorff (2000) reappraised claims for lateral gene transfer and concluded that the extent of transfer was overemphasized moreover, Glansdorff inferred that the last common ancestor was probably nonthermophilic and perhaps a protoeukaryote, from which the thermophilic archaea may have been the first divergent branch. 

The molecular evidence for the descent of the eukaryotes (Hartman and Fedorov, 2002) is deeply controversial (see Section 8.01.6.3). Standard models (Woese, 1987) suggest an ancestral hne among the Archea, with massive transfers and symbioses from the bacteria. The standard model (e.g., see summaries in Pace (1997) Nisbet and Fowler, 1996a,b) is that early archaea and bacteria diverged from a hyperthermophile last common ancestor. Then, a sequence of symbiotic events took place between a stem-cell line, among the archaea, that developed partnerships with symbiotic purple and cyanobacteria, either in separate events, or in a single moment of fusion. This produced the eukaryote cell, with the mitochondria derived (Bui et al., 1996 from within the a-proteobacteria such... 

Figure 11 Standard and alternative models of eukaryote evolution (see Figure 7) (with thanks to J. Fuerst). Alternative model assumes a last common ancestor that was mesophile, and that the divergence of the planctomycetes...

However, if the eukaryote-like view of the last common ancestor (Forterre and Philippe, 1999 Glansdorff, 2000) is correct, then the sequence of events may have been greatly different. In this view, an ancient common ancestor may have been a fairly complex organism, living in mesophile conditions, possibly some hundreds of meters distal to a shallow hydro-thermal system, in water between, say, 35-45 °C (blood-temperature, optimal for DNA-based life), perhaps in water with pH around 7 or more alkaline (to account for the cytochromes). 

At some stage in this hypothesis came the key acquisition by the ancestor of the modern Eucarya of the mitochondrion. Possibly (i) this was very early—perhaps not so much an acquisition as a primitive characteristic alternately (ii) it may have been a later product of a symbiosis between a mesophile eukaryote stem-organism directly descended from a mesophile eukaryote-like last common ancestor and a proteobacterium that had evolved from a line that had passed through a hyperthermophile bottleneck or, (iii) it could have been a later product of a symbiosis between two organisms that had both been through a hyperthermophile stage, an achaea-like host and a proteobacterial symbiont. 

As for the antiquity of the Eucarya there is no consensus. Those who support a eukaryote-like last common ancestor, of course, propose that the eukaryotes date to the very start of the Archean and end of the Hadean. It is not improbable to those who consider that the Eucarya were the last domain to appear, that Eukaryotes first evolved in the Archean aeon. There is, however, little support in the rock record for the hypothesis of a very late origin of both the archaea and eukaryotes, proposed by Cavaher-Smith (2002), especially as the evidence for early methanogens is strong (Grassineau et al., 2002 Rye and Holland, 2000). However, a proterozoic origin of the eukaryotes is not yet excluded, as the sterols found by Brocks et al. (1994) could be of prokaryote origin. 

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