The last common ancestor

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. 

As for the first living cells (the first common ancestor) we know even less, but again we are not completely in the dark. The evidence that we do have tells us that they came from the ribotype world, and therefore their genomes were made almost completely of RNAs. This means that during the transition from first to last common ancestor, the cells substituted RNA with DNA in their genes, probably by using enzymes that were very similar to reverse transcriptases. Traces of this substitution, in fact, seem to have survived, because many modern enzymes that produce DNA (the DNA polymerases) are still capable of functioning as reverse transcriptases (Poole et al., 1998). 

In addition to changing the genome s nucleic acids, it is possible that other modifications took place during the evolution from first to last common ancestor, but for the moment we know virtually nothing about these developments. The characteristics of the last common ancestor are therefore highly hypothetical, and yet many have already decided that they were bacterial features. Such a conclusion has 

In reality, the (relative) simplicity of bacteria can be best explained by the idea that it was the result of a streamlining process, just as modern computers have been obtained by a simplification of progenitors that were bulkier, heavier and slower. The properties that we should attribute to the common ancestor, in other words, are not necessarily the most simple, but rather the most primitive, and with this criterion we can obtain at least three interesting conclusions. 

One of the main bacterial features is the fact that DNA transcription is immediately followed by translation, to the extent that in most cases protein synthesis starts on primary transcripts that are still attached to DNA. The result is that there is neither the time nor the space for a modification of the transcripts. In the ribotype world, on the other hand, the first nucleic acids were mostly random molecules, and the first systems were necessarily full of statistical RNAs. It is likely therefore that some kind of screening had to be made before protein synthesis, which means that primitive translation was taking place some time after primitive transcription. A system that contains both useful and useless RNAs is more primitive than a system in which all RNAs are useful, and so it is likely that in the common ancestor transcription was separated from translation. 

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Although, on an evolutionary time scale, this was recent the last common ancestor of vertebrates and arthropods existed some 600 million years ago (Jamroz et al., 1993)... 

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. 

Cardol P, Matagne RF, Remade C (2002) Impad of mutations affeding ND mitochondria-encoded subunits on the adivity and assembly of complex I in Chlamydomonas. Implication for the strudural organization of the enzyme. J Mol Biol 319 1211-1221 Castresana J, Moreira D (1999) Respiratory chains in the last common ancestor of living organisms. J Mol Evol 49 453-460... 

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. 

Despite their antiquity (perhaps 2.5 billion years old), the resurrected ancestral elongation factors are inferred for bacteria that lived long after the origin of life. Nevertheless, the notion that early life lived at high temperatures, in water, and at nearly neutral pH is consistent with available data. For example, a recent reconstruction of the phylogenetic tree of life based on 31 common gene families supports the notion that the last common ancestor lived at high temperatures.65... 

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. 

Delaye L, Becerra A, Lazcano A. The last common ancestor what s in a name Orig. Life Evol. Biosph. 2005 35 537-554. 

The inter-aeon boundary between the Hadean and Archean is presently not defined (Nisbet, 1991). There are various options (i) the date of the first life on Earth (ii) the date of the last common ancestor (iii) a round number, such as exactly 4 Ga—4,000,000,000 years ago (iv) the oldest record of a terrestrial rock ( 4 Ga ago) (v) the oldest record of a terrestrial mineral crystal (—4.3-4.4 Ga ago). 

The last common ancestor is more accessible to geology and molecular biology than the first ancestor. Though not less controversial than the first ancestor, it is at least the subject of testable hypotheses. 

The last common ancestor is the notional cell, or population of cells, from which all modem living cells are descended (Woese, 1999). One definition of the Hadean/Archean boundary is the date of the last common ancestor. This last ancestor would have been a DNA-based organism, already complex, with many of the so-called housekeeping proteins that are broadly common to nearly all modern types of cell. Note however, that vimses, especially RNA vhuses, may (or may not) be separately descended from an earlier ancestor. 

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. 

Conceivably, if the last common ancestor were mesophile, the majority of bacteria (except perhaps planctomycetes (Brochier and Philippe, 2002)) may descend from an early mesophile prokaryote, perhaps via a genetically streamlined descendant that occupied a hyperthermophile setting. Archea too may descend from the last common ancestor via a streamlined cell that had evolved to inhabit hyperthermophile settings. In contrast, the Eucarya may be directly descended from a mesophile, as may the planctomycetes. 

A possible geological scenario for this process may be that the last common ancestor lived on the... 

Whatever the setting of the last common ancestor, there are many aspects of modern cells that have a possible or likely hyperthermophile origin. To possess such a heritage, it is not necessary that a cell s primary ancestral line once occupied a hyperthermophile habitat. There has been much genetic exchange between organisms both within lines and even massively between domains (Figure 6(b)). 

Today, metals are scavenged from water by extremely sophisticated biochemical processes (Morel and Price, 2003). Thus, seawater can have very low ambient levels of metal ions. Early Archean seawater would likely have been much richer in trace metals. But given that early organisms presumably had very unsophisticated processes for capturing metals, even in seawater rich in metal it would have been difficult to access the metal. Perhaps the earliest distribution of organisms was very restricted, with few cells living away from locations such as volcanoes that had readily accessible metals. Only the evolution of effective metal-gaining siderophores would have allowed the spread of life. There is thus reason to believe that, even if the last common ancestor was not hyperthermophile but lived... 

Replication of the last common ancestor would lead to mutation in turn, mutation would create accidental pre-adaptation to life in diverse new habitats. Whatever the habitat of the last common ancestor, the spread of life across the more accessible other locations on Earth was probably rapid, when compared to a geological timescale. 

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