[Author's response: This is a useful remark indeed; we have taken it into account in the present version]. Nature.

The topic of LUCA's identity has evolved considerably in the last few years and we believe that a synthetic working hypothesis can be proposed that draws its strength from several advances (Fig. 2000, 10: 606-611. Charlier D, Severne Y, Zafarullah M, Glansdorff N: Turn-on of inactive genes by promoter recruitment in Escherichia coli: inverted repeats resulting in artificial divergent operons. Besides, there are archaeon symbionts multiplying in eukaryotic cells [105, 106] but no evidence was provided that they shed their isoprenoid membrane to replace it by a fatty acid one. There are two basically different types of metabolism -autotrophic and heterotrophic- and, consequently, two opposite views for the origin of the first living cells; when the generation of metabolic energy is taken into account, these views are however not as irreconcilable as used to be considered [115]. If the phyletic distribution of nuclear-like structures is patchy, then assuming a homologous relationship between these forms appears little parsimonious, knowing that losing the nucleus is apparently not so common (at least for a eukaryote).

Paradoxically, although the authors are strong critics of the overwhelming-LGT theory, they have adopted the viewpoint of a communal LUCA which, historically, has been proposed partly to take into account this theory (see Woese [Woese CR. However Glansdorff, Xu & Labedan's model diverges considerably from other published scenarios in several important ways. 1991, 173: 3907-3910. This article was reviewed by Anthony Poole, Patrick Forterre, and Nicolas Galtier. 2nd edition, second issue, page 466, The Complete Work of Charles Darwin Online, Zoonomia; or The laws of organic life: in three parts (Volume 1) (1818), "The physiology and habitat of the last universal common ancestor", "[Luca: the last universal common ancestor]", "The Last Universal Common Ancestor: emergence, constitution and genetic legacy of an elusive forerunner", "Oldest fossil found: Meet your microbial mom", "Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in the, "The Very First Animal Appeared Amid an Explosion of DNA", "Reconstruction of the ancestral metazoan genome reveals an increase in genomic novelty", "All Species Evolved From Single Cell, Study Finds", "Primal Eukaryogenesis: On the Communal Nature of Precellular States, Ancestral to Modern Life", "Prediction 1.3: Consilience of independent phylogenies", "On universal common ancestry, sequence similarity, and phylogenetic structure: The sins of P-values and the virtues of Bayesian evidence", 29+ Evidences for Macroevolution: The Scientific Case for Common Descent, https://en.wikipedia.org/w/index.php?title=Common_descent&oldid=983416184, Short description is different from Wikidata, Articles with unsourced statements from March 2018, Creative Commons Attribution-ShareAlike License, This page was last edited on 14 October 2020, at 03:08.
10.1016/j.ceb.2003.11.005. Edited by: Neidhardt DCFC. (101 × 81.6 cm). They are many clear-cut evidences for such transfers. Both enzymes could have been present in the LUCA community; alternatively, under strong selective pressure, G1PDH could have been "borrowed" by HGT from a bacterium or recruited as a novel enzyme from a GDH.

We have seen above that LUCA may be understood as a diverse community of already metabolically and genetically sophisticated organisms. Mol Biol Evol.

2B).

Spliceosomal introns are found only in eukaryotes.

Moreover, paralogies may go unnoticed if gene duplicates remain cryptic as the result of inactivating mutations [178]. 2007, 29: 611-614.

Finally, Lifson's refutation of S. Kauffman's model (ref 211) that has been carried over in the literature appears to us inconclusive, something that had to be stressed as well].
In fact, our early analysis (ref 29, 172, 173) pointed the way to the systematic identification of paralogous gene copies in the LUCA community; this needs to be followed up rather than ignored]. 2006, 312: 1011-1014. Beiko RG, Harlow TJ, Ragan M: Highways of gene sharing in prokaryotes. Koonin EV, Makarova KS, Aravind L: Horizontal gene transfer in prokaryotes:quantification and classification. Comparable observations have been made for adaptation to cold [123, 124]. Science. 10.1016/j.femsle.2004.11.034.

10.1073/pnas.97.8.4112.

The paradox is only apparent, however, because isoprenoid lipids can remain in a crystalline liquid state over a very broad range, from about 0°C to above 100°C [122]. The section 'Eukaryote gene promoters...' is, on the other hand, so short as to be uninformative. We are happy to note that you have no objection to the concept of a phenotypically diversified LUCA community containing phagocytic organisms (see our answer to Dr Poole regarding Devos' model for the emergence of the nucleus and the endocytotic apparatus)]. Indeed, whereas Archaea are characterized by ether-isoprenoids lipids which are suitable for life at high temperature and other stressing conditions [122, 146] extreme thermophilic Bacteria that grow at temperatures above 65–70°C possess various types of membrane lipids that could represent convergent adaptations to life at high temperature (see early observations by Forterre [53] and [48, 49]) whereas membranes of psychrophilic, mesophilic or moderate-thermophilic Bacteria appear to respond to different temperature ranges by variations in the degree of lipid saturation and the nature of side chains [122, 147]. 10.1038/451632a. In its present state, and whatever the exact branching order of the three Domains, the universal tree of life, with many deep-branching chemoorganotrophic types of microorganisms, is not adverse to the notion of a heterotrophic LUCA (or to the presence of heterotrophs in a LUCA community of diverse metabolic types). The authors declare that they have no competing interests.