Sharov (2009, 2010) has marshaled considerable data, based on the analysis of genomic evolution, which he believes demonstrates that life must have arisen around 10 billion years ago; a view shared by others (e.g. Joseph and Schild 2010). A first impression of the paper by Dr Sharov (2010) is that it attempts to embrace an enormous sweep of knowledge under eight headings. Since each of these headings could form the basis of extensive reviews there is inevitably much relevant published material that is excluded from this report. As an example, mention should have been made of the more recent work of Wächtershäuser on autocatalytic carbon dioxide fixation (Wächtershäuser, 2000) in view of the comment on his earlier (1988) report on that topic.

Evolutionary biologists would have little argument that life evolved from a simple system to gradually gain complexity, but the rationale for supporting the dominance of gradualism over punctuated evolution is dubious, particularly the claim that "if we accept the arguments and evidence against gradualism, then we must assume that any rate of change is possible, and this assumption conflicts with reality in evolutionary reconstructions". I don’t believe this is so; both gradualism and punctuated equilibria can both be seen in phylogenetic constructions; it is a matter of scale and there is no argument that the overall evolutionary process is slow. The follow-up question then posed by Dr Sharov relating to the appearance of complex organisms on Earth as early as 3.5 billion years ago is made irrelevant by his subsequent conclusion that life did not originate on this planet.

The regression-plot of the size of non-redundant functional genome of major phylogenetic lineages against the time of their ‘origin’ (Sharov, 2010, Figure 2) to show that life originated perhaps ten billion years ago (bya) is beguiling but incorrect for the following reasons:

The origin of all life was the first cell and all life has the same depth of evolution; there is no reason to expect anything meaningful to come from the figure shown. Taking two of the points shown on the figure, those for fish and mammals which shared a common ancestor around 500 million years ago; of what relevance to the origin of life has been the faster development of non-redundant genes in the lineage leading to mammals? Use of the hypothesized time of ‘origin’ of Eukaryotes (which according to Sharov's Figure 2), occurred around 2 bya is particularly unfortunate.

Firstly we don’t have any certainty about the timing of the origin of eukaryotes. Comparative genomic analyses led Hedges (2002) and Feng et al., (1997) to conclude that the first Earthly multicellular eukaryote, had evolved by 2.7 bya, but the first proto-eukaryotes may have evolved around 4 bya (Hedges et al., 2001). There are also microfossils dated to 3.8 bya which have been interpreted as simplified eukaryotes (Plfug 1984). Others have cited this evidence to argue that eukaryotics did not evolve on Earth, but also have an ancestry which predates the origins of this planet (Joseph 2009; Joseph and Schild 2010).

Secondly, the development of eukaryotes was arguably the greatest leap in evolution since the origin of the first self-replicating cell; certainly not within the gradualism camp since it resulted from a series of rare cell-fusions of prokaryotic cells. Once again, we migh ask of what relevance to the timing of the origin of life was this serendipitous event? It is wrong to place the origin of the prokaryotic cell at 3.5 Gigayears ago. Although we have evidence of the existence of prokaryotes at that time we also have evidence that their phylogeny, on Earth, runs much deeper from 3.8 bya (Mojzsis, et al., 1996; Pflug, 1978; Rosing, 1999, Rosing and Frei, 2004) to 4.2 bya (Nemchin et al. 2008; O'Neil et al. 2008). Prokaryotic life appeared on Earth shortly after life became possible on this planet (Joseph and Schild 2010). It we accept the Sharov's (2010) contentions, they may well have existed 10 billion years ago.

Despite these criticisms I agree with Dr Sharov’s (2010) conclusion that life as we know it did not begin on Earth, but base this conclusion instead on the calibration of the ‘new phylogeny’ against time-markers provided by isotopic evidence (e.g. Shen et al., 2001), fossil evidence (e.g. Allwood et al. 2006, Noffke et al., 2006) and microbial biomarkers (e.g. Brocks et al., 1999). The existence of an effectively modern Last Universal Ancestor of life perhaps earlier than 3.5 Gigayears ago (Line, 2007) and the relatively meager evolution of novel biochemical pathways or organelles (most notably the ribosome) since the parting of the three Domains of life provides excellent reason to embrace a much deeper evolutionary depth than is possible on Earth.

Sharov's tentative predictions of the nature of life-forms we might discover on other planets or satellites of our solar system are curious given his earlier comments of the improbability of repeated independent origins of life. Sharov (2010) writes; "The similarity (of life-forms) may be even so deep that some extraterrestrial bacteria may fit well into the existing classification of Prokaryotes. Indeed, they may even possess the same genetic code, which would indicate the shared cosmic ancestry of all life in the Solar system." (my italics).

In my view all life-forms we might find in the outer solar system will share the genetic code familiar to us. Any experimentation with alternative genetic systems in primordial cells would have been snuffed out by the development of the RNA world (Gilbert, 1986), which in turn was snuffed out (with the possible exception of some RNA viruses) by the DNA world. The important question remaining is whether both Prokaryotes and Eukaryotes are represented among these life-forms.

The dramatic direct evidence for fossil cyanobacteria in carbonaceous chondrites with their imprints of sheaths, trichomes, heterocysts, akinetes and hormogonia (Hoover, 2006) has profound implications. It is curious that this report has not been more widely disseminated. Apart from providing direct evidence for Panspermia, the existence of fossilized cyanobacteria in these and other chondrites (Zhmur and Gerasimenko 1999; Zhmur et al., 1997) if confirmed as truly extraterrestrial in origin, would prove conclusively that photosynthesis and oxygenic respiration pre-dated life on Earth. Not only that, since cyanobacteria form a branch within domain Bacteria, it would also mean that the Last Universal Ancestor also pre-dated life on Earth a view shared by others (Anisimov 2010; Burchell 2010; Jose et al., 2010; Joseph and Schild, 2010; Wainwright 2010; Wickramasinghe et al., 2009). If the evidence for extraterrestrial cyanobacteria microfossils in carbonaceous chondrites is confirmed, then we are likely to find representatives of all three domains of life as we know them beneath the Martian surface, as well as on other moons and planets and sequence analysis will unite them with terrestrial counterparts at or near the base of each of the existing domains.