Eukaryogenesis

I.b-1.) The Last Universal Common Ancestor (LUCA)

While the progenitor of the eukarya is now traditionally nested within the archaea, Carl Woese (1998) suggests that the Last Universal Common Ancestor (LUCA) of the three domains may antedate individuation into discrete lifeforms separated by cell membranes, and that the LUCA may have actually been a community of proto-cells with extremely primitive, porous membranes — called "progenotes" (Woese, 1998) — through which, due to pervasive lateral gene transfer (LGT) (Woese, 2002), genetic material was exchanged and propogated itself more-or-less freely. The picture this evokes is one in which, prior to the “Darwininian threshold" (Woese, 2002), the primary, smallest, most irreducible unit of life was less like a modern cell than like a modern chromosome.

Other authors (e.g. Martin & Russell, 2003; Koonin, 2014) have taken up modifications of Woese's theory which agree that the common ancestor of the Archaea and the Bacteria was a pre-organismal, pre-Darwinian community, but which take this further in hypothesizing that the last common ancestral state (LUCAS) was pre-cellular, and which reassert the importance of Endosymbiotic Theory in understanding the origin of the eukarya. If true, this would mean that the bacteria, whence the mitochondria evolved, and the archaea, a lineage of which went on to become the eukarya, represent two separate evolutions of "true" (discrete, cellular) life. The consequences of this are that (1) the divergence of the two prokaryotic domains from a common ancestor is inextricably interwoven with the origins of life, and (2) that eukaryogenesis represents both a novel convergence of two independently evolved forms of life and a genetic reconvergence of hereditary material that had been separated since before the origin of "true" life.

This view is not universally accepted, with some authors going so far as to adopt literary bravados which (wrongfully) assert that this idea has been abandoned due to common membrane-encoding genes shared by all three domains (e.g., Forterre, 2014), alternate explanations for these shared genes, such as lateral gene transfer (LGT), or even (plausibly, considering the small size and streamlined organization of prokaryotic genomes) convergenet evolution, have not been ruled out.

I.b-1A.) The Terrestrial Origin of Life

Currently there are two primary approaches to the scientific investigation into the origins of life: the replicator-first approach and metabolism-first approach (Anet, 2004). These approaches differ conceptually in hypothesizing either an autocatalytic string of covalently bonded informational molecules at least functionally akin to DNA, or a sequence of chemical reactions a among set of noncovalently bonded molecules that was autocatalytic as whole and thus functionally akin to metabolism (von Meijenfeldt, 2013), as the essential origin of life.

Here it should be noted that the designations "replicator-first" or "metabolism-first" may apply either literally or figuratively. An approach which places primary emphasis on replication over metabolism, for example, may be regarded as a "replicator-first" approach even if it is not literally suggesting that replicators antedate metabolic systems, and vice versa. These are designations of emphasis, not necessarily descriptors of sequence.

(Delaye & Lazcano, 2005)

I.b-1Aα.) Replicator-First Approach

The most crucial entity in and most basal unit of evolution is the replicator (Zachar, 2010).

Since the 1982 discovery of ribozymes (Kruger et al.) proved that RNA had catalytic properties, and could therefore “act both as information carrier and as catalyst” (Alberts et al., 2002), resolved the DNA-protein “chicken & egg” paradox by demonstrating that RNA could act as both “chicken” and “egg” (Bernhardt, 2012; Sankaran, 2013), the replicator-first approach has quite naturally come to be dominated by the “RNA World” hypothesis (von Meijenfeldt, 2013) and this has only been reinforced by the now much-expanded catalytic repertoire of RNA and the import thereof with regard to key cellular reactions (Doudna & Cech, 2002) “which can be viewed as molecular fossils of an earlier world,” meaning we never fully transitioned out of the RNA World (Alberts et al., 2002). Evidence indicating that the common ancestor of the Archaea and the Bacteria possessed an RNA-based genome (Leipe & al., 1999) and only after divergence each separately acquired DNA from DNA viruses, in which DNA first arose (Leipe & al., 1999; Forterre, 2002), has further reinforced this hypothesis.

(Achbergerová & Nahálka, 2011)

(Fegley & Lewis, 1980)

"messy RNA" (Szostak, 2017)

I.a-1Aβ.) Metabolism-First Approaches

Emphasizing ecosystems over discrete organisms and metabolism over reproduction, Eric Smith et al. have characterized the emergence of Earth's biosphere as a sort of geochemical "discharge" resulting from a build-up of energy between the hydrosphere and the lithosphere (Smith, 2007; 2015).

(Smith &al., 2004)

(Copley &al., 2007)

(Morowitz &al., 2008)

Water is necessary to fold proteins (McLain, 2017)

first cells (Szostak, 2017)

(Hazen, 2005)

(Hazen, 2006)

(Hazen & Sverjensky, 2010)

(Hazen & Deamer, 2006)

(Hazen, 2006)

(Deamer &al., 2002)

(Deamer & Monnard, 2006)

I.b-1Aγ.) Problems with a Terrestrial Origin

While the "plausibility" criterion for a given chemical pathway suggested as being involved in prebiotic synthesis generally requires that the chemical environment has to be one which could have plausibly been present on the early Earth, such a criterion ignores that the early Earth isn't the only place that life could have formed. Such is an a priori assumption completely unsupported, and one might go so far as to say contradicted, by all experimental and observational data thus far gathered. The Earth is but one planet among hundreds known and among billions thought to exist in our galaxy alone. Furthermore, there is no particular reason to hypothesize that the generation of life is something which must occur on a planet at all! Especially when one considers how early in the geological record life first began to appear on Earth, other environments such as other Solar planets, exoplanets, planetesimals, asteroids, comets, moons, protoplanetary disks, protostellar nebulae and the stelliferous nebulae (star-forming regions of giant molecular clouds) to which they belong, as well as any interactions between such systems, must all be considered as potential environments for the various stages of prebiotic synthesis, and "plausibility" determined only when a particular chemical pathway has been demonstrated to be likely (or not) in one of such environments. An early-Earth-based "plausibility" criterion is, frankly, an unnatural constraint which ignores the fact that in our universe, no planet is an island.

Many stages of prebiotic synthesis wouldn't have worked out in hydrothermal vent scanrios or on a hot early Earth.

The hydrothermal vent model, though seemingly ruled out, would give us a mathematically elegant and thermodynamically sensible sequence of OGT≥80°C, OGT=65±15°C, OGT≤50°C and a cellular LUCA model gives us a progressive OGT≈0°C, OGT≤50°C, OGT=65±15°C thermotolerance sequence before a general decline to OGT≤50°C with a few modern lineages OGT=65±15°C or even OGT≥80°C, while a precellular LUCA model merely gives us an OGT≤50°C; OGT≥50°C threshold separating the cryophilic-to-mesophilic nucleobases, RNA, LUCA, and protocells, and the thermophilic-to-hyperthermophilic first true lifeforms, followed eventually by diversification into various levels of thermotolerance.

(Szostak, 2012; part 2)

(Sleep, 2010)

Cool Early Earth model.

Here, however, we run into another problem. Water is highly corrosive, and would've been deadly to the first RNA-based replicators (Barras, 2014)

(Ricardo &al., 2004)

(Benner &al., 2012)

I.b-1B.) The Possibility of Panspermia

The hardiness of bacteria features in many lithopanspermia models, the most conservative of which are interplanetary lithopanspermia models, most relavent to we Earthlings in the hypothetical case of Mars-Earth interplanetary lithopanspermia, in which life found its way to Earth from our neighbor, Mars.

We know that bacteria could have been ejected into space by planetary impacts (Stoffler et al. 2006), survive the subsequent rapid acceleration (Mastrapaa et al. 2001), survive exposure to vacuum and high radiation (Saffary &al., 2002; Bucker & Horneck, 1970; Horneck, 1971; Nicholson, &al. 2005), thrive and grow in asteroidal and meteoritic interiors (Mautner 2002), survive re-entry into a planetary atmosphere (blah) and subsequent impact (blah),

Deinococcus radiodurans, first isolated in Corvallis, Oregon, in 1956 (Anderson, &al.)

I.a-1Bα.) Interplanetary Lithopanspermia

Radioresistance in bacteria. Mars fertile for life before Earth.

(Hoch & Losick, 1997)

(Fajardo-Cavazos &al., 1997)

(Matti &al., 2009)

(Clark, 2001)

(Loupkin, 2006)

“So, back to the period of heavy bombardment and with computer simulations you can, you can model what happens when an impact hits a planetary surface. And it's not much different from if you sprinkle Cheerios on a bed [...] and then you smack the surface of the bed, there's a— a sort of a recoil in effect and Cheerios pop upwards. It turns out Mars may have been wet — we've known at some point it had water — and fertile for life before Earth, and at this period of heavy bombardment if it had started life — surely it would've been simple life, as we've no reason to think otherwise — we've learned bacteria can be quite hardy, as I'm sure you know, so we imagine a bacterial stowaway in the nooks and crannies in one of these rocks that are cast back into space. In fact if you do the calculation, there's hundreds of tons of Mars rocks that should that should have fallen to Earth by now, over the history of the Solar system. Maybe one of those rocks carried life from Mars to Earth, seeding life on Earth. My great disappointment would be going to Mars and finding Mars life based on DNA. That it would not've been a separate experiment in life.” — Neil Degrasse Tyson to Richard Dawkins (Dawkins & Tyson, 2016, 6:05-7:24 minutes in)

I.a-1Bβ.) Interstellar Lithopanspermia

Microbe-containing impact ejecta from Earth may be subsequently pulverized by collisions into micron-sized particles that, though large enough to contain colonies of microbial life and shield them from UV radiation, would be small enough to be carried out to the Kuiper Belt via Solar winds eventually to be deposited in the protoplanetary discs of future planetary systems as the Solar system moves through interstellar dust clouds in its course through the Milky Way (Wallis, 2003; Napier, 2003), seeding the Milky Way in just a few billion years, which, considering any Earth-like planet could hypothetically have done the same, makes it highly unlikely that Earth Herself was not a benefactor of such a process (Napier, 2003).

V. Conclusions

V.a. Summary

V.a-1.) Timeline of Events

— formation of the Milky Way

— formation of the Giant Molecular Cloud (GMC), 10⁶-10⁸ M_☉ (Montmerle, 2006), containing complex organic molecules (Ehrenfreund & Charnley, 2000)

— formation of a star-forming region similar to the Orion Nebula
"in a large star-forming region that produced massive stars, possibly similar to the Orion Nebula.[16][17] Studies of the structure of the Kuiper belt and of anomalous materials within it suggest that the Sun formed within a cluster of between 1,000 and 10,000 stars with a diameter of between 6.5 and 19.5 light years and a collective mass of 3,000 M_☉. This cluster began to break apart between 135 million and 535 million years after formation.[18][19]"

— fragmentation of the GMC, first into fragments 1 parsec in diameter and then into "cloudlets" 0.01-0.1 parsecs or 2k-20k AU in diameter and around 1 M_☉ (Montmerle, 2006).

— formation of the Pre-Solar Nebula, a fragment of the GMC about 1 M_☉ (Montmerle, 2006).

4.6 GYA — beginning of the Stellar era (first million years of Solar evolution), the Disk era (first 10 million years of Solar evolution), and the Telluric era (first 100 million years of Solar evolution): formation of the Sun in a stellar cluster via accretion of a circumstellar disk fed by a progressively diminishing circumstellar envelope (Montmerle, 2006), possibly incited by a nearby supernova (Montmerle, 2006; Williams, 2009).

4.599 GYA — Stellar era ends, 99 million years prior to the end of the Telluric era and 90 million years prior to the end of the Disk era (Montmerle, 2006).

4.59 GYA — Disk era ends, 90 million years prior to the end of the Telluric era (Montmerle, 2006).

4.5682 GYA — oldest solid material in the Solar system.

4.55 GYA — formation of the Earth

4.53 GYA — formation of the Moon

4.5 GYA — Telluric era ends. Noachian period begins on Mars.

"Several simulations of our young Sun interacting with close-passing stars over the first 100 million years of its life produce anomalous orbits observed in the outer Solar System, such as detached objects.[20]"

(Brasser &al. 2013)

(Brasser &al. 2017)

(Lykawka & Ito, 2013)

4.4 GYA — "Cool Early Earth" begins

(Valley &al., 2002)

4.3 billion years ago — earliest evidence of liquid water

Planet migration. Nice model. Quaker belt? Kuiper belt? Ice falls inward, planets move outward. Jupiter orbits twice for every one Saturn orbit (2:1 resonance).

(Walsh & Morbidelli, 2010)

(Gomes &al., 2010)

4.1 billion years ago — onset of The Late Heavy Bombardment (LHB), or lunar cataclysm; earliest evidence of biogenic carbon (Bell et al. 2015)..

4 billion years ago — "Cool Early Earth" ends, plate tectonics

3.8 billion years ago — end of The Late Heavy Bombardment (LHB), or lunar cataclysm.

3.5 billion years ago — Noachian period ends on Mars; establishment of the geomagnetic field on Earth, protecting the atmosphere from being swept away by Solar winds.

2.45 billion years ago — the Great Oxygenation Event.

References