Environmental Adaptation from the Origin of Life to the Last Universal Common Ancestor

Radiation exposure would significantly challenge the development of an early nucleotide-based system because radiation damage harms the stability and fidelity of genetic material.

“Because radiation damage has strong negative consequences for the stability and fidelity of genetic material (Ganesan and Hanawalt 2016;Kladwang et al. 2012), radiation exposure would present a significant challenge in the development of an early nucleotide-based system.”

• cites Ganesan and Hanawalt 2016 sourceNot checked yet

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• cites Kladwang et al. 2012Not checked yet

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Photoreactivation repair mechanisms suggest that a lineage is exposed to ultraviolet radiation.

“Therefore, the presence of cellular repair mechanisms specific for pyrimidine dimer repair, also known as photoreactivation (PHR), is an indication that an organismal lineage experiences exposure to ultraviolet radiation.”

The authors interpret molecular biological features as indicating an environmental transition from a cold, radiation-shielded origin to a mesophilic LUCA living at the surface.

“Using this lens, we interpret several molecular biological features as indicating an environmental transition between a cold, radiation-shielded origin of life and a mesophilic, surface-dwelling LUCA.”

Low-temperature scenarios for life's origin and early evolution are plausible given the environment.

“These observations suggest that low-temperature scenarios for the origin and early evolution of life are environmentally plausible, while the hot-start model presents challenges for the stability and accumulation of prebiotically significant compounds (Islas et al. 2003).”

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Shielded environments that could plausibly be in fairly close contact with unshielded environments should be preferred because of the importance of UV in prebiotic chemistry.

“Radiation shielding can be effectively provided by a water column, ice cover, sediment, organic material or other barriers (Cockell 1998), but the importance of UV in prebiotic chemistry suggests that shielded environments which could plausibly be in fairly close contact with unshielded environments should be preferred.”

Early diversification and the emergence of ecology before LUCA may have served as an important pre-adaptation for life's persistence on a changing planet.

“Early diversification and the emergence of ecology before LUCA could be an important pre-adaptation for life's persistence on a changing planet.”

The functional range of RNA molecules observed today probably limits the environments where early life could have emerged.

“Therefore, the observed biofunctional range of RNA molecules today likely constrains the environments in which early life could have emerged.”

• AssumptionRests on an assumption: RNA functions best within a particular range of conditions such as temperature, pH, and radiation.

A hot-start model faces difficulties with the stability and buildup of prebiotic compounds.

“These observations suggest that low-temperature scenarios for the origin and early evolution of life are environmentally plausible, while the hot-start model presents challenges for the stability and accumulation of prebiotically significant compounds (Islas et al. 2003).”

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Organisms needed a portable envelope containing their genetic material and metabolism to disperse from their original environment and diversify.

“In order to diversify across environments and move away from their environment of origin, organisms would need to be able to contain their genetic information and metabolisms within a portable envelope.”

A transition from a pre-RNA system resistant to UV to an RNA system sensitive to UV would seem selectively disadvantageous.

“However, moving from a UV-resilient pre-RNA system to a UV-sensitive RNA system would appear to be selectively disadvantageous.”

DNA and RNA are damaged by both ultraviolet and ionizing radiation.

“Both UV and ionizing radiation damage DNA and RNA.”

• cites Ganesan and Hanawalt 2016 sourceNot checked yet

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• cites Kladwang et al. 2012Not checked yet

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Only UV radiation causes pyrimidine dimerization.

“One type of radiation damage, pyrimidine dimerization, is exclusively caused by UV radiation (Ravanat and Douki 2016).”

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UV is thought to have played a significant role in prebiotic synthesis, including the synthesis of uracil and cytosine.

“Despite UV's potentially deleterious effects on life, UV is thought to have played a significant role in prebiotic synthesis (eg., Ranjan and Sasselov 2016;Beckstead et al. 2016;Sutherland 2017), and plays a key role in the prebiotic synthesis of uracil and cytosine achieved by Powner et al. (2009), an important achievement in understanding the prebiotic emergence of RNA.”

• cites Ranjan and Sasselov 2016 sourceNot checked yet

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• cites Beckstead et al. 2016 sourceNot checked yet

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• cites Sutherland 2017 sourceNot checked yet

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Water columns, ice, sediment, organic material, and other barriers can effectively shield against radiation.

“Radiation shielding can be effectively provided by a water column, ice cover, sediment, organic material or other barriers (Cockell 1998), but the importance of UV in prebiotic chemistry suggests that shielded environments which could plausibly be in fairly close contact with unshielded environments should be preferred.”

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Key amino acids form most easily in cold conditions during prebiotic synthesis.

“Experiments by Miller and colleagues (Bada et al. 1994;Levy et al. 2000) showed that prebiotic synthesis of key amino acids occur most readily under cold conditions.”

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• cites Levy et al. 2000 sourceNot checked yet

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Life diversified into new environments between its origin and LUCA, unlike its original habitat.

“These observations suggest that between the origin of life and LUCA, life diversified into environments unlike the one in which it originated.”

Physiological constraints on early RNA-based life may have been more stringent than those observed for individual RNA systems today.

“Since life in an RNA world is inferred to rely on RNA for so many additional functions, such as replication, catalysis, and perhaps even structure (Bernhardt 2012), physiological constraints may have been even more stringent than what is observed for individual RNA systems today.”

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The Archean Earth likely received more short-wavelength UV radiation.

“However, it has been observed that wavelengths below 290 nm are most damaging to DNA, and that the Archean Earth would have experienced a higher flux of shorter-wavelength UV radiation though water and atmospheric carbon dioxide may have efficiently blocked wavelengths much below 200 nm (Cockell 1998;Ranjan and Sasselov 2016).”

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• cites Ranjan and Sasselov 2016 sourceNot checked yet

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A faint young sun could have made early Earth's surface cold enough for substantial ocean ice.

“However, with a faint young sun, the surface of the early Earth may have been cold enough for significant ocean ice to form (Kienert et al. 2012;Feulner 2012), and the environment of the early Earth may have been more temperate than the traditional model suggests (Hren et al. 2009;Vincent et al. 2004).”

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• cites Feulner 2012Not checked yet

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A protected environment that would allow the RNA world to emerge despite substantial environmental flux at Earth's surface should be preferred as a potential habitat for life's origin.

“Therefore, a protected environment that would permit the RNA world to emerge in the presence of substantial environmental flux at the Earth's surface should be preferred as a potential habitat for life's origin.”

• AssumptionThe authors state this as an assumption: UV levels high enough for substantial prebiotic synthesis would also harm RNA.

The earliest amino acids used by life fit a temperate origin, not a hot one.

“Work on the addition of amino acids to the canonical repertoire suggest that the first amino acids used by life are also consistent with a temperate, rather than hot, origin of life (Brooks et al. 2002;Zeldovich et al. 2007;Fournier and Gogarten 2010).”

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• cites Fournier and Gogarten 2010 sourceNot checked yet

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Radiation damage can cause mutations or transcription errors with significant deleterious effects at the cellular level.

“This damage can result in mutations or transcription errors that have significant, deleterious consequences for organisms at the cellular level.”

Early Earth's environment might have been more temperate than traditionally thought.

“However, with a faint young sun, the surface of the early Earth may have been cold enough for significant ocean ice to form (Kienert et al. 2012;Feulner 2012), and the environment of the early Earth may have been more temperate than the traditional model suggests (Hren et al. 2009;Vincent et al. 2004).”

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• cites Vincent et al. 2004 sourceNot checked yet

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The complexity of DNA excision repair in modern E. coli makes its emergence in the early RNA world implausible.

“This complexity makes their emergence in the early RNA world implausible.”

If RNA was a component of life from its origin, then limits on RNA stability and function also constrain the environments where early life could have originated.

“If RNA were a component of life from its origin, limits on the stability and function of RNA are also limits on the environments in which early life could have originated, especially since RNA-based repair mechanisms or stabilizing RNA modifications could only have developed after establishment of RNA as the biomolecule of inheritance and expression.”

• AssumptionRests on an assumption: RNA functions best within a particular range of conditions such as temperature, pH, and radiation.

Inferences from the functional history of highly conserved ancient protein families cannot constrain early life's ecological history because these families operate largely independently of environmental context.

“Because these very ancient protein families are highly functionally conserved, and relate to fundamental processes that operate largely independently of the specific environmental context of organismal systems, inferences from their functional history cannot constrain the ecological history of early life.”

Without a known abiotic mechanism or catalyst to repair radiation damage, RNA emerging in an environment with significant radiation flux seems unlikely.

“Without a known abiotic mechanism or catalyst to repair radiation damage, any scenario wherein RNA emerged in an environment with significant radiation flux seems unlikely.”

DNA is most damaged by wavelengths shorter than 290 nm.

“However, it has been observed that wavelengths below 290 nm are most damaging to DNA, and that the Archean Earth would have experienced a higher flux of shorter-wavelength UV radiation though water and atmospheric carbon dioxide may have efficiently blocked wavelengths much below 200 nm (Cockell 1998;Ranjan and Sasselov 2016).”

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• cites Ranjan and Sasselov 2016 sourceNot checked yet

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The LUCA lineage contained photolyase.

“Phylogenetic analyses indicate that photolyase was present in the lineage of the Last Universal Common Ancestor (LUCA) and that the observed paraphyletic distribution across the three domains is due to widespread gene loss (Heijde et al. 2010;Kanai et al. 1997).”

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Without evidence for a pre-RNA system or additional transitions, the uniformitarian hypothesis of a primordial RNA-based earliest life remains the most compelling.

“In either case, without evidence for such a pre-RNA system or such additional transitions, the simper, uniformitarian hypothesis of a primordial RNA-based earliest life remains the most compelling (Bernhardt 2012).”

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The young Sun emitted more of its energy in the ultraviolet spectrum than today.

“UV radiation decreases during the lifetime of a star, and the young Sun would have emitted more of its energy in the ultraviolet spectrum than today (Feulner 2012).”

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The early emergence of cellularity is necessary for environmental dispersal and diversification during the transition from life's origin to LUCA.

“Considering the importance of compartmentalization and motility, we propose that the early emergence of cellularity is required for environmental dispersal and diversification during these transitions.”

Above 0°C, nucleotides hydrolyze quickly, making their accumulation hard in a hot-start scenario.

“The rapid rates of hydrolysis of nucleotides at temperatures above 0 degrees C would make accumulation of these nucleotides difficult in a hot-start scenario (Kua and Bada 2011;Levy and Miller 1998;Kawamura et al. 2005).”

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• cites Levy and Miller 1998 sourceNot checked yet

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• cites Kawamura et al. 2005 sourceNot checked yet

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Higher temperatures severely restrict RNA folding.

“RNA folding, which is required for RNA to function, is strongly limited at higher temperatures (Moulton et al. 2000).”

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Archaea and Bacteria use unrelated pathways to synthesize chemically distinct membrane phospholipids.

“Archaea and Bacteria use unrelated pathways to synthesize chemically distinct membrane phospholipids; therefore, it has been proposed that LUCA did not use phospholipids in its membrane, and may have used mineral structure as a means of compartmentation or been totally acellular (Koonin and Martin 2005;Koga et al. 1998).”

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Compartmentation existed at the time of LUCA.

“Darwinian evolution at the organismal scale requires that both lineage and heredity-a specific line of evidence strongly suggesting compartmentation at the time of LUCA (Lombard et al. 2012;Ranea et al. 2006).”

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• cites Ranea et al. 2006 sourceNot checked yet

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The evidence for a colder start is compelling.

“While the temperature conditions at the origin of life remain controversial, the evidence for a colder start is compelling.”

ATP synthase complexes need an enclosed lipid bilayer to work.

“ATP synthase protein complexes utilize a proton or sodium gradient across a membrane, requiring an enclosed lipid bilayer-i.e., cellularity-to function.”

Genome size estimates for LUCA based on comparative genomics may be best understood as lower-bound estimates because many genes present in LUCA could have been lost later.

“Since many genes present in LUCA could have been subsequently lost in descendant lineages, these genome size estimates based on comparative genomics may be best understood as lower-bound estimates.”

The complexity and comprehensiveness of LUCA's genome indicates that Darwinian evolution was in effect at an organismal scale.

“The complexity and comprehensiveness of LUCA's genome indicates that Darwinian evolution was in effect at an organismal scale (Lombard et al. 2012).”

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Fatty acid, fatty alcohol, or fatty-acid glycerol ester vesicles remain stable between 0 and 100°C.

“Vesicles composed of fatty acids, fatty alcohols or fattyacid glycerol esters are stable from 0 to 100 degrees C (Mansy and Szostak 2008).”

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A Domain level takeover event is at least biophysically plausible.

“Consistent with this, recent work suggests that a Bhybrid^membrane of both bacterial-and archaeal-type lipids is stable and could have been used by LUCA, and thus a Domain level takeover event is at least biophysically plausible (Shimada and Yamagishi 2011).”

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LUCA had nearly all the essential functional systems present in modern organisms.

“functional analysis of these superfamilies indicates that LUCA had nearly all the essential functional systems present in modern organisms (Ranea et al. 2006).”

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Concentration changes can drive primitive homeostatic behavior.

“Concentration changes can also drive similar primitive homeostatic behavior (Engelhart et al. 2016).”

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At colder temperatures, better stabilization and less degradation of ribozymes offset slower polymerization, allowing very long molecules to form.

“While colder temperatures would slow down RNA polymerization rates, the enhanced stabilization and reduced degradation of the ribozyme counteract this effect, permitting synthesis of remarkably long molecules (Attwater et al. 2013;Attwater et al. 2010).”

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Vesicle stability from 0 to 100°C might let temperature shifts drive cellular processes.

“Stability across these ranges could permit temperature fluctuations to drive cellular processes, with template-copying chemistry enhanced at lower temperatures and strand-separation and nutrient uptake enhanced at higher temperatures (Mansy and Szostak 2008).”

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Before membrane stability limited them, early organisms could live in or spread to environments with varied temperatures.

“This broad range of temperature resilience would also mean that early organisms could inhabit and/or disperse into environments with a wide range of temperatures before membrane stability became a limiting factor.”

Because vesicles are broadly thermostable, nucleic acid properties may be more informative than membrane properties about the temperature of the early RNA world.

“Because of this broad thermostability range, the properties of nucleic acids, rather than of membranes, may be more informative about the temperature of the early RNA world.”

Cellular membranes must have existed when ATP synthase complexes first arose.

“Therefore, enclosed cellular membranes must have existed at the time of the origin of ATP synthase complexes.”

Temperature-driven processes could allow early cells to complete a primitive cycle of growth, strand separation, template replication, and division without dedicated cellular mechanisms.

“Through these temperature-driven processes, early cells could complete a primitive Bcell cycle^of growth, strand separation, template replication, and division without requiring dedicated cellular mechanisms.”

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A fairly complex protein synthesis system existed before LUCA.

“This indicates a fairly complex protein synthesis system existed pre-LUCA (Hoeppner et al. 2012).”

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There is good evidence that LUCA's genome was small but functionally fairly comprehensive.

“However, there is good evidence that LUCA's genome was small but functionally fairly comprehensive (Mirkin et al. 2003;Ranea et al. 2006;Koonin 2003).”

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• cites Ranea et al. 2006 sourceNot checked yet

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Several RNA families have patterns of vertical inheritance consistent with their origin before LUCA.

“Comparative analysis of RNA families indicates that, while most RNA families are domain-specific, several RNA families have patterns of vertical inheritance consistent with their origin pre-LUCA.”

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A short experimental ribozyme derived from a longer parent showed a lower optimal temperature.

“Also, the first ribozymes were likely short; an experimental short ribozyme evolved from a longer parent ribozyme was observed to have a lower temperature optimum (Akoopie and Müller 2016).”

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LUCA did not possess reverse gyrase.

“This would mean that LUCA did not possess reverse gyrase.”

Reconstructions of LUCA rRNA have shown a G + C content consistent with mesophily.

“Reconstructions of LUCA rRNA have shown a G + C content consistent with mesophily (Galtier et al. 1999;Groussin et al. 2013).”

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As RNA transcription fidelity improved, longer and more effective ribozymes, which would have more elaborate, stabilizing secondary and tertiary structures, could have emerged.

“As RNA transcription fidelity improved, longer and more effective ribozymes, which would have more elaborate, stabilizing secondary and tertiary structures, could have emerged.”

The pre-LUCA histories of some gene families can be examined.

“However, the pre-LUCA histories of some gene families can be examined (eg., Mirkin et al. 2003;Ranea et al. 2006;Gogarten and Taiz 1992;Fournier et al. 2015a;Fournier et al. 2015b).”

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• cites Ranea et al. 2006 sourceNot checked yet

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• cites Gogarten and Taiz 1992Not checked yet

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Reverse gyrase appears to have evolved in Archaea and spread into Bacteria through horizontal gene transfer.

“This scenario is further borne out by the observation that reverse gyrase appears to have evolved in Archaea and spread into Bacteria through horizontal gene transfer (HGT) (Brochier-Armanet and Forterre 2006).”

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The IVYWREL content of LUCA proteins is consistent with an optimal growth temperature below 50 degrees C, indicating mesophily.

“The IVYWREL content of LUCA proteins, based on phylogenetic analysis, is consistent with an optimal growth temperature below 50 degrees C, indicating a mesophily.”

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G + C nucleotides are present more frequently in the rRNA of thermophiles and hyperthermophiles than in the rRNA of mesophiles.

“Due to their additional thermostability, G + C nucleotides are present more frequently in the rRNA of thermophiles and hyperthermophiles than in the rRNA of mesophiles (Galtier et al. 1999;Groussin et al. 2013).”

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Reverse gyrase is critical to hyperthermophilic life.

“Reverse gyrase is critical to hyperthermophilic life (Forterre 1996;Forterre 2002;Kikuchi and Asai 1984;Heine and Chandra 2009).”

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• cites Kikuchi and Asai 1984 sourceNot checked yet

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Reverse gyrase is a topoisomerase unique to thermophiles and hyperthermophiles.

“Reverse gyrase is a topoisomerase unique to thermophiles and hyperthermophiles (Forterre 1996;Forterre 2002) that converts closed, circular DNA to a positively supercoiled form (Kikuchi and Asai 1984).”

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Modern hydrothermal vent systems typically last between 1 and 10,000 years.

“Modern hydrothermal vent systems typically last between 1 and 10,000 years (Lombard et al. 2012).”

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The inferred environmentally distinct states of a primordial RNA world and LUCA constitute an additional challenge to the mineral compartmentation scenario, because they imply environmental dispersal and diversification requiring motility not possible in highly localized mineral compartments.

“We propose that the inferred environmentally distinct states of a primordial RNA world and LUCA constitute an additional challenge to this scenario, as these infer environmental dispersal and diversification, necessitating motility that is not possible in highly localized mineral compartments dependent on a direct hydrothermal source for both compartmentation and energy.”

The abundance of amino acids I, V, Y, W, R, E, and L (IVYWREL) within protein sequences correlates strongly with an organism's optimal growth temperature.

“Abundance of the amino acids I, V, Y, W, R, E, and L (denoted as IVYWREL) within protein sequences correlates strongly with an organism's optimal growth temperature (Zeldovich et al. 2007).”

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Reverse gyrase includes two distinct regions, homologous to a topoisomerase and a helicase, respectively.

“The evolutionary history of reverse gyrase is well constrained because of its structure: it includes two distinct regions, homologous to a topoisomerase and a helicase, respectively (Forterre 1996;Forterre et al. 1995).”

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The thermostability of vesicles across a range of temperatures indicates that membrane stability would not limit environmental dispersal.

“Therefore, environmental adaptation could have been an important force even in life's earliest history; the thermostability of vesicles across a range of temperatures indicate that membrane stability would not limit environmental dispersal.”

If life before LUCA was not free-living and was intrinsic to mineral structures within hydrothermal systems, all pre-LUCA evolution would have been tied to a specific site and thus would have had to occur in a relatively short time.

“If life pre-LUCA was not free-living and was intrinsic to mineral structures within hydrothermal systems, all pre-LUCA evolution would have been tied to a specific site, and thus would have had to occur in a relatively short time (Lombard et al. 2012).”

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Reverse gyrase and hyperthermophily are closely linked.

“While one experiment suggested that the thermophile Thermococcus kodakarensis can survive without reverse gyrase, growth in mutant strains was significantly inhibited and not observed at temperatures >90 degrees C (growth is observed in wild-type strains at up to 100 degrees C) (Atomi et al. 2004), supporting the conclusion that reverse gyrase and hyperthermophily are closely linked.”

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Psychrophiles have several specific adaptations to low temperatures that have apparently occurred independently across lineages.

“Psychrophiles have several specific adaptations to low temperatures that have apparently occurred independently across lineages.”

Double-stranded DNA is not more thermally stable than double-stranded RNA.

“It is unclear how increasing temperature could have driven this transition, as double-stranded DNA is not more thermally stable than double-stranded RNA (Wienken et al. 2011).”

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Several compelling lines of evidence make the case for LUCA's mesophily rather than thermophily or hyperthermophily.

“Several compelling lines of evidence make the case for LUCA's mesophily, rather than thermophily or hyperthermophily.”

The simplest explanation is most consistent with a mesophilic LUCA.

“While the same caveats as above apply-perhaps there was a primitive form of reverse gyrase or LUCA was a hyperthermophile that did not require reverse gyrase-the simplest explanation is also the most consistent with a mesophilic LUCA.”

Organisms that used pre-existing helicases and topoisomerases did not use reverse gyrase.

“Therefore, the organisms that used these pre-existing helicases and topoisomerases, by necessity, did not use reverse gyrase.”

This differential would provide selective advantage for being metabolically active at higher temperatures.

“This differential would provide selective advantage for being metabolically active at higher temperatures.”

Environmental adaptation could have been an important force even in life's earliest history.

“Therefore, environmental adaptation could have been an important force even in life's earliest history; the thermostability of vesicles across a range of temperatures indicate that membrane stability would not limit environmental dispersal.”

There may have been little or no cryosphere during the Paleocene-Eocene Thermal Maximum (PETM) 55.5 million years ago.

“For example, there may have been little or no cryosphere to speak of during the Paleocene-Eocene Thermal Maximum (PETM) 55.5 million years ago (Sluijs et al. 2006) and extant psychrophile clades may all have diverged after this event.”

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No major microbial classes have psychrophily as a shared character.

“Yet there are no major microbial classes that have psychrophily as a shared character.”

The fusion scenario requires that helicases and topoisomerases had already diversified to some degree prior to the development of reverse gyrase.

“This scenario requires that helicases and topoisomerases had already diversified to some degree prior to the development of reverse gyrase.”

It is unclear how increasing temperature could have driven the RNA to DNA transition.

“It is unclear how increasing temperature could have driven this transition, as double-stranded DNA is not more thermally stable than double-stranded RNA (Wienken et al. 2011).”

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Rapid diversification together with complex communities may be a significant pre-adaptation for life's persistence over the long term within a dynamic planetary system.

“Rapid diversification and complex communities might be an important pre-adaptation for life's long-term persistence as part of a dynamic planetary system, in part because more diverse communities are more resilient to environmental change (Reusch et al. 2005;Hughes et al. 2008;Peterson et al. 1998).”

Martian life or evidence of Martian life may now exist only within refugia environments unlike the one in which Martian life originated.

“It would also follow that Martian life or evidence of Martian life may now exist only within refugia environments unlike the one in which Martian life originated.”

Europa's irradiated, oxidized surface products are placed into contact with its liquid inner ocean through processes operating on geologically short timescales such as diapirism, cryovolcanism, fracturing, and reservoir collapse.

“Studies of Europa, an icy moon of Jupiter, have suggested that products from its irradiated, oxidized surface are placed into contact with its liquid inner ocean through processes operating on geologically short timescales such as diapirism, cryovolcanism, fracturing, and reservoir collapse (Brown and Hand 2013;Sparks et al. 2017).”

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Early diversification may be a crucial pre-adaptation for life's persistence on a changing planet.

“Early diversification could be a vital pre-adaptation for life's persistence on a changing planet.”

This approach can also serve, at least in part, to explain the acquisition of complex biological characters that are firmly established by the time of LUCA.

“It can also serve, at least in part, to explain the acquisition of complex biological characters that are firmly established by the time of LUCA.”

This scenario reconciles bottom-up observations (such as experimental prebiotic chemistry) with top-down observations (such as phylogenetics) that support conflicting accounts of life's origins, provided it is restricted to a single primordial location.

“Such a scenario reconciles sets of bottom-up (e.g., experimental prebiotic chemistry) and top-down (e.g., phylogenetics) observations that support conflicting narratives of life's origins, if constrained to a singular primordial location.”

By the time of LUCA, organisms had diversified into environments that included moderate-temperature surface locations.

“By the time of LUCA, organisms had diversified into environments including surface locations of moderate temperature.”

Life probably diversified, between its origin and LUCA, into environments different from the one in which it arose.

“Between the origin of life and LUCA, life likely diversified into environments unlike the one where it emerged.”

This schema agrees with ecosystems emerging very early as an important force in life's history.

“This schema is consistent with a very early emergence of ecosystems as an important force in life's history.”

Life's capacity to diversify and adapt to novel environments arose very early in its history.

“Life's ability to diversify and to adapt to new environments, which we suggest emerged very early in its history, has permitted the continual reworking of function and physiology for ~4 Ga on a changing planet.”

Mars may have been habitable for hundreds of millions of years early in its geological history.

“Recent findings suggest that Mars may have been habitable for hundreds of millions of years early in its geological history (Grotzinger et al. 2014).”

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The early appearance of cellularization would have enabled this diversification.

“This diversification would have been facilitated by the early emergence of cellularization.”

Some genes present at the time of LUCA but not present within the genome of LUCA were likely transferred to the ancestors of extant groups by lineages that existed alongside LUCA but became extinct more recently.

“Some genes, present at the time of LUCA but not present within the genome of LUCA, were likely transferred to the ancestors of extant groups by lineages that existed alongside LUCA but became extinct more recently (Fournier et al. 2015b).”

• cites Fournier et al. 2015bNot checked yet

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If early Earth environments were widespread, varied and connected, the earliest environmental adaptations plausibly arose from life spreading into neighbouring unoccupied environments along gradients of light, temperature, nutrients or other factors.

“If environments on the early Earth were widespread, varied, and linked, it seems likely that first environmental adaptations could have been the result of life's expansion into adjacent and unoccupied environments along light, temperature, nutrient, or other gradients.”

• AssumptionRests on an assumption: Environments on the early Earth were widespread, varied and connected.

Communities with greater diversity are more resilient to environmental change.

“in part because more diverse communities are more resilient to environmental change (Reusch et al. 2005;Hughes et al. 2008;Peterson et al. 1998).”

• cites Reusch et al. 2005 sourceNot checked yet

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• cites Hughes et al. 2008 sourceNot checked yet

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• cites Peterson et al. 1998 sourceNot checked yet

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An actualistic approach suggests that environmental dispersal and adaptation are consistent with the observed disjunction between LUCA and the origin of life.

“An actualistic approach-one that prefers explanations centered on processes that have been observed in modern systems-suggests that environmental dispersal and adaptation are consistent with the observed disjunction.”

Europa's dynamic, icy landscape and interior may provide a model for the processes that can assemble and concentrate prebiotic chemicals relevant to the origin of life.

“Europa's dynamic, icy landscape and interior may provide a model for the processes that can assemble and concentrate prebiotic chemicals relevant to the origin of life.”

Given the range of possible habitats on the early Earth and the evidence for ancient lineages living alongside LUCA and its proximal descendants, such diversification seems probable.

“Given the range of possible habitats on the early Earth and the evidence for ancient lineages living alongside LUCA and its proximal descendants, such diversification seems probable.”

It seems likely that population bottlenecks and adaptive radiations occurred following major innovations between the origin of life and LUCA.

“It seems likely that population bottlenecks and adaptive radiations occurred following major innovations between the origin of life and LUCA.”

That capacity has allowed function and physiology to be continually reworked over roughly 4 billion years on a changing planet.

“Life's ability to diversify and to adapt to new environments, which we suggest emerged very early in its history, has permitted the continual reworking of function and physiology for ~4 Ga on a changing planet.”

The early Earth could have provided a range of environmental niches stable for millions of years.

“Recent work suggesting that the early Earth was more temperate (Valley et al. 2002;Hren et al. 2009), and the flux of impactors smaller and less damaging than previously thought (Boehnke and Harrison 2016;Abramov and Mojzsis 2009) predicts that, as a whole, the early Earth could have provided a range of environmental niches stable for millions of years.”

• cites Valley et al. 2002 sourceNot checked yet

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• cites Hren et al. 2009 sourceNot checked yet

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• cites Boehnke and Harrison 2016 sourceNot checked yet

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The model calls for production and accumulation of prebiotically significant molecules in separate environments.

“The model calls for production and accumulation of prebiotically significant molecules in separate environments.”

This scenario supports viewing LUCA as one population of cells among many cellular or protocellular systems that probably coexisted across diverse ecologies.

“This scenario also supports LUCA's identity as one population of cells among many cellular or protocellular systems that likely coexisted across diverse ecologies.”

Better understanding the impact of environmental limitations on the origin and early evolution of life, such as UV flux, can help determine the habitability of planetary bodies outside our solar system, such as planets around stars with high UV fluxes.

“Better understanding the impact of environmental limitations on the origin and early evolution of life, such as UV flux, can help determine the habitability of planetary bodies that are even outside our own solar system, such as planets around stars with high UV fluxes, e.g., the TRAPPIST-1 system (Gillon et al. 2017;O'Malley-James and Kaltenegger 2017).”

• cites Gillon et al. 2017 sourceNot checked yet

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• cites O'Malley-James and Kaltenegger 2017Not checked yet

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