Origin of terrestrial life: the in situ hypothesis. Astrobiology Lecture 3. Origin of life. Chronology of the origin of terrestrial life

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1 Origin of terrestrial life: the in situ hypothesis Astrobiology Lecture 3 To constrain the chronology and physico-chemical conditions of abiogenesis, we assume that terrestrial life originated on Earth ( in situ ) Strictly speaking, this is only a working hypothesis According to a few authors, terrestrial life originated outside Earth and was somehow transported to Earth ( panspermia hypothesis) At present time, there is no evidence of life being delivered on Earth - We do have evidence of organic material delivered on Earth SISSA, Academic Year Giovanni Vladilo (INAF-OATs) We do not consider here the panspermia hypothesis - By assuming the existence of panspermia we would shift the problem of the origin of life to an unknown epoch with unknown ambient conditions 1 3 Origin of life The origin of life is a central topic of astrobiology To predict if life can originate in remote astronomical environments we must understand the origin of (terrestrial) life The habitability of an environment does not guarantee the presence of life because the requirements for the origin of life could be different and tighter than the requiremens of habitability The scientific approach The origin of life is assumed to be the result of as a sequence of spontaneous processes that leads to the formation of the first living cells starting from non-biological chemical compounds Abiogenesis The transition from the abiotic world to life is called abiogenesis Chronology of the origin of terrestrial life We can set temporal limits on the epoch of life formation by dating: habitability boundary: time scales of the processes of Earth formation and early development of habitability conditions biosignature boundary: the oldest evidence of life found in the terrestrial crust By comparing the chronology of these events, we can estimate: the epoch of life formation the time interval available for life formation after the onset of habitability conditions 2 4

2 Age of formation of the Earth The age of formation of the Solar System can be dated with accuracy from the analyis of meteorites Date of the oldest objects in the Solar System: 4.57 x 10 9 yr After the formation of the Solar System the Earth and Moon formed in less than 10 8 yr The oldest terrestrial rocks From Gargaud et al. (2012) 5 7 The oldest terrestrial rocks It is extremely difficult to find terrestrial rocks with ages close to the epoch of Earth formation and early evolution This makes very hard dating the origin of life The main reason for this difficulty is tectonic activity, which is constantly recycling the Earth s crust The Late heavy bombardment (LHB) The impact craters on the bodies of the inner Solar System indicate a long history of impacts, starting from the epoch of Solar System formation The analysis of Moon impact craters indicates the existence of an episode with a large number of heavy impacts well after the formation of the Solar System The frequency and intensity drastically decays between 4.1 e 3.7 x 10 9 Ga The energy of the strongest impacts was sufficient to evaporate a present-day ocean As a result of the tectonics, the oldest, well preserved crust material has ages of about Ga Older material exists, with ages of Ga, but is sparse and quite altered Notwithstanding, zircon minerals with ages up to 4.4 Ga have been found, incorporated in younger strata 6 8

3 The Late heavy bombardment (LHB) Oldest evidence for life on Earth Implications of astrobiological interest - The LHB must have taken place also on Earth, even if we do not have direct geological evidences - Evidence for the existence of the LHB starts to accumulate also from other bodies of the Solar System (not just the Moon) - The LHB may have delivered water and organic material on Earth at a late stage - The cumulative effect of the impacts may have made impossible for Earth to be habitable until the end of the LHB The oldest, tentative, evidence are dated at about 3.8 Ga Sedimentary rocks in the south-east of Greenland (Isua, Akilia) Based on the isotopic ratio 12 C/ 13 C The oldest, more convincing, evidence are dated at about Ga Greenstone belts in Australia (Pilbara) and South-Africa (Barbeton) - Isotopic ratios - Microfossils - Stromatolites: sedimentary layers suggesting the presence of diffuse life in shallow water, close to the litoral 9 11 Searching for the oldest traces of life on Earth Summary of chronology relevant for studies of the origin of life on Earth Different types of experimental techniques are used to search for traces of ancient life in the oldest terrestrial rocks Study of isotopic ratios that can be altered biologically Example: 12 C/ 13 C Morphological evidences of microscopic forms of life Microfossils can be preserved thanks to the mineralization of organic matter of biological origin Geological layers of biological origin Examples: sedimentary layers similar to present-day stromatolites These methods only offer indirect evidences Results should be taken with caution However, convincing evidence can be obtained by the combination of different methods 10 12

4 Conclusions: temporal constraints on the origin of life on Earth If we consider the temporal window between the end of the LHB and the oldest, tentative evidence of life, the origin of life should have taken place around Ga, on a relatively short time scale (~10 8 yr) If we take the more robust evidence for the oldest trace of life, the origin of life should have taken place between 3.5 and 3.9 Ga, on a time scale of a few hundred millon years Even if we consider the more robust evidences, life must have originated before 3.5 Ga, when it was already widespread These time scales can be relaxed by several hundred millon years if we assume that life originated before the end of the LHB We cannot exclude that life emerged before the LHB The early atmosphere of the Earth The primary atmosphere of the Earth must have been lost This is deduced from the low abundances of rare gases ( 20 Ne, 36 Ar, 84 Kr) in the present-day atmosphere, compared to the cosmic abundances of the same elements The expected composition of the secondary atmosphere of the early Earth depends on the formation models of our planet Slow formation models (probably not realistic) The Earth s interior is cold and rich of volatiles Volatiles from the interior are gradually heated and released to the atmosphere These volcanic emissions produce a reducing atmosphere (rich of hydrogen), with a high content of H 2, CH 4, and NH 3 Fast formation models ( millon years; more realistic) Because of the impacts with accreting planetesimals, the interior is hot and does not retain significant amounts of volatiles By the end of the accretion process, the atmosphere is weakly reducing, being dominated by CO 2 e N 2 with traces of CO and H Properties of the Earth at the epoch of the origin of life The early climate of the Earth: the Faint Young Sun paradox The physico/chemical conditions of the early Earth set the reference frame for studying which chemical pathways may have lead to the origin of life We mention a few aspects of the early Earth conditions relevant to the origin of life: Early atmospheric composition of the Earth Early climate of the Earth Origin of Earth s oceans The standard model of evolution of the Sun indicates that the solar luminosity at the epoch of the origin of life was about 25% fainter than today With a lower level of insolation, models of Earth climate indicate that the Earth should have been completely frozen Assuming an intensity of the greenhouse effect similar to the present-day one We know that this was not the case, since there are evidences of liquid water at the same epoch of Earth s history This contradiction is known as the faint young Sun paradox Te Effective temperature of the Earth Ts Mean surface temperature of the Earth The shaded region indicates the greenhouse effect 14 16

5 Possible solutions of the Faint Young Sun paradox Most commonly adopted explanation: Larger efficiency of the greenhouse effect Atmosphere rich in CO 2 and/or CH 4 However, this solutions faces some limitations since the possible amount of CO 2 is limited by geochemical constraints The possibility that water has been delivered on Earth by impacts of minor bodies (asteroids and comets) is tested with studies of the isotopic ratio D/H The oceanic D/H ratio is compared with measurements performed in meteorites and comets So far, asteroids appear to be favoured, whereas comets have a significantly higher D/H ratio Testing the origin of Earth s water The origin of Earth s water Understanding the origin of Earth s water is best done within the context of the standard model of accretion of terrestrial planets According to this model, the terrestrial planets accreted from a swarm of planetesimals and planetary embryos Therefore, Earth s volatiles, including water, are likely to have been derived from their planetesimal precursors and their chondritic building blocks Specifically, the most likely sources of Earth s volatiles could have been the outer asteroid belt, the giant planet regions, and the Kuiper belt - (Morbidelli et al. 2000) Gases in the solar nebula were probably not an important source of volatiles - the extreme solar wind associated with the T-Tauri phase of stellar evolution is likely to have blown the solar gas away Studies on the origin of life Fields of research related to the studies of the origin of life Prebiotic chemistry (synthesis of precursors of biomolecules) Origin of homochirality Emergence of replicative and metabolic functions Search for the least evolved living organisms Two types of approaches are used: bottom-up trying to build-up complex biological molecules in laboratory, starting from non biological constituents top-down trying to cast light on the characteristics of the least evolved forms of life, proceeding backwards in evolution 18 20

6 Prebiotic chemistry Search for plausible chemical pathways of synthesis of the molecular building blocks of biological macromolecules One of the goals of prebiotic chemistry is to understand which organic molecules are the most likely to initiate these chemical pathways Possible scenarios for the synthesis of prebiotic material: In space On Earth Both scenarios are taken in consideration in studies of the origin of life Prebiotic material delivered on Earth by meteorites Meteorites are representative of the epoch of planetary formation Some of the meteorites collected on Earth show evidence of relatively complex organic material One of the most interesting cases is the Murchison meteorite (Australia, 1969) where evidence have been found of aminoacids and nucleobasis The non-terrestrial origin of these organics compounds is confirmed by several tests: Out of the 74 aminoacids found, only 11 are protein aminoacids The aminoacids appear in a near racemic mixtures (both L- and D- types), at variance with protein aminoacids A slight eccess of the L enantiomer has been found, the same enantiomer of biological aminoacids Synthesis of prebiotic material in space Material delivered on Earth by comets The primitive Earth is likely to have been enriched by organic material delivered by meteorites of asteroidal and cometary origin - complex organic material delivered from space may have played a role in prebiotic chemistry - the synthesis of organic molecules may have started in the molecular cloud from which the protosolar nebula originated - additional chemical processing may have taken place during the stages of planetary formation, during the delivery on Earth, and on the Earth s surface Indirect evidence supporting the delivery of complex organics in the past is found from the study of meteorites recently arrived on Earth and of space observations of comets Studies of molecular clouds and protoplanetary disks indicate that relatively complex organic molecules are indeed formed in space Also comets may have delivered material on the primitive Earth the early flux of comets was likely to be higher in the early stages of evolution of the Solar System analysis of present-day comets that still preserve their original composition can be used to trace the history of material in comets several studies confirm that comets do possess volatiles and organic material data from the Rosetta mission suggests that - comets did deliver xenon on the Earth, but only a small fraction of water - comets do have complex organic material 22 24

7 Rosetta mission: organics in comet 67 P/C-G D/H higher than in terrestrial oceans D/H ~ in H 2 O Altwegg et al., Science, 2015 In situ mass spectrometry of cometary volatiles: discovered a large number of organics, many of them for the first time in a comet Ammonia, Methylamine, Ethylamine Benzene, Toluene, Xylene, Benzoic acid, Naphthalene Methane, Ethane, Propane, Butane, Pentane, Hexane, Heptane Methanol, Ethanol, Propanol, Butanol, Pentanol Acetylene, HCN, CH3CN, Formaldehyde Hydrogensulfide, Carbonylsulfide, Sulfur dioxyde, Carbon disulfide, Thioformaldehyde Glycine The Urey-Miller experiment The Urey-Miller experiment proved that aminoacids can spontaneously form in simulated conditions of the early Earth (electric discharges, oceans) starting from very simple molecules (H 2, H 2 O, CH 4, NH 3 ) The reducing power of the early earth atmosphere was probably overestimated Recent versions of the Urey-Miller experiment adopt a weakly reducing atmosphere, in agreement with the current expectations for the early Earth s atmosphere The experiment is still able to produce aminoacids, albeit with a much lower efficiency Laboratory studies of prebiotic chemistry Laboratory experiments are a fundamental tool for studies of prebiotic chemistry They aim at reproducing the physico-chemical conditions conducive to prebiotic chemistry in space and in the primitive Earth Early developments of prebiotic chemistry After the formation of aminoacids, experiments of prebiotic chemistry aimed at producing the bases of nucleic acids The first succesful experiments, performed by Joan Oró, managed to produce adenine, in addition to amino acids, using hydrogen cyanide (HCN) as a precursor The first, historical, experiment of prebiotic chemistry on Earth was performed by Urey & Miller in 1953 Later on, also guanine was produced, always starting from HCN However, the formation of pyrimidines (uracil, thymin and cytosin) from the same chemical pathways was not possible In addition, the nucleic bases produced were highly unstable, posing a problem for the viability of subsequent prebiotic steps 26 28

8 Prebiotic chemistry with formamide Interstellar observations show that formamide (HCONH 2 ) is ubiquitous in the Universe Formamide can be produced by the reaction of water and hydrogen cyanide (HCN) Formamide presents several advantages compared to HCN Formamide has a boiling point of 210 o C, higher than the water boiling point Therefore, formamide can be easily become concentrated through the evaporation of water The concentration of HCN is difficult because HCN is in gaseous form at ambient temperature and pressure Steps of prebiotic chemistry leading to the biopolymers The ambient physico-chemical requirements may change in different steps Precursors Long polymers synthesis Ribozymes polymerization Monomers Short polymers Activated monomers 29 RNA Prebiotic chemistry with formamide PROTEINS METABOLISM Formamide is potentially involved in all relevant steps of prebiotic chemistry. Succesful experiments exist for most steps of prebiotic chemistry. However, experiments in a single pot are able to perform only one, or a few, steps at a time. Origin of molecular replication and metabolism Conceptual chicken-egg problem In present-day cells, nucleic acids and proteins are responsible for replication and metabolic functions, respectively The formation of each one of these two types of macromolecules requires the previous existence of the other one The synthesis of nucleic acids is catalyzed by proteins (enzymes) The synthesis of proteins requires the instructions stored in the nucleic acids Who came first? Proteins or nucleic acids? Replication/genetic or metabolic functions? Different approaches have been adopted to tackle this problem Old approach: Metabolism first or genes first Present-day approach: search for macromolecules that show both properties 32

9 The RNA world Present-day, main stream theory in studies on life s origin Introduced by Walter Gilbert (1986) after the discovery of ribozymes RNA molecules with catalytic properties According to this theory, the genetic system is the first to emerge, but with self-catalytic properties Present-day ribozymes would be a sort of molecular fossiles of an ancient RNA world Present-day DNA-world would have emerged at a later stage because of its advantages DNA provides greater genetic stability The lack of an oxygen atom in the sugar (deoxyribose instead of ribose) makes DNA less reactive than RNA The DNA world has an extremely greater flexibility due to the introduction of proteins specialized in a large variety of metabolic functions Replication and molecular evolution Imperfect replication and chemical selection are supposed to be the ingredients of evolution that has lead to the molecular machinery that we see today In a broad sense, molecular replication and chemical selection is an extension of the concept of Darwinian evolution which, strictly speaking, takes place only after the first living organisms are born Darwinian evolution works a posteriori, in the sense that it favours the most suitable variations for a given function that already exists Molecular replication is probably the key function for the initial selection Life as a kinetic state of matter Addy Pross Casting light on the first living cells Example of the kinetic power of self replication Comparison between normal and self-catalytic reactions start with 1 molecule of catalyst X assume reaction rate 1µs in both cases Time required to build up a mole of products (6 x ) Normal case: 20 billon years Self-catalytic case: 79 µs The kinetic control of chemical reactions could be the key for understanding the origin of life (in chemistry, the term kinetics is related to the rate of chemical reactions) see literature by Addy Pross A, B: reactants X: catalyst Top-down approach From the study of present-day living organisms, we try to characterize the properties of the first terrestrial organisms proceeding backwards in evolution One of the methods being employed is the comparison of genetic sequences of present-day living organisms Thanks to this comparative analysis, we can trace backwards the evolution at the molecular level The results are visualized in the phylogenetic tree, where the distances between different species are proportional to the differences found in the genetic sequences 34 36

10 Genetic sequences and classification of organisms The techniques of molecular biology allow us to classify organisms on the basis of their genetic sequences, rather than on their morphology or phenotype (composite of observable traits and behaviour of organisms) The classification based on genetic sequences has revolutionized our understanding of unicellular organisms The classification based on genetic sequences has lead to distinguish three different types of unicellular organisms: archaea, eubacteria and eukaryotes Archaea have been discovered through genetic classification The gap between the RNA world and the LUCA The root of the phylogenetic tree is not representative of the oldest living cell Other forms of life, extinct in the course of the evolution, must have preceded the LUCA This early form of life is sometimes called the progenote The early life could have been a collection of somewhat different cells, rather than a single type of cell Detailed analysis suggests that early life was mesophilic, rather than thermophilic The phylogenetic tree of life Horizontal gene transfer (also called Lateral Gene Transfer) Bacteria can exchange genetic material not only during their reproduction ( vertical gene transfer, VGT) but also via direct exchange from one cell to another ( horizontal gene transfer, HGT) The existence of HGT complicates the reconstruction of the philogenetic tree, which is based on the VGT scenario HGT must have played an essential role in the early stages of life, providing a simple mechanism to exchange genetic material before more complex mechanisms of vertical transmission were set in place LUCA = Last Universal Common Ancestor of present-day living organisms, also called Cenancestor Close to the root of the tree, we find thermophilic Archaea and Bacteria 38 40

11 Heterotrophic versus autotrophic origin of life Heterotrophic hypothesis The first organisms harvest organic material and energy from prebiotic molecules that are already present in the environment The molecular ingredients could have been delivered on Earth from space or could also have been synthesized on the primitive Earth This hypothesis does not require a specific enviromental niche Life evolution Autotrophic hypothesis The first organisms extract energy and synthesise organic material from the abiotic world The early life forms would have emerged in the proximity of redox or ph gradients, using the harvested energy to feed biosynthesis reactions. These processes require extremely reactive chemical environments. This scenario can only take place in specific thermodynamical niches 43 The cradles of life Deep sea hydrothermal vents Provides the presence of inorganic compartments, versatile catalysis and sources of organic matter, in the framework of the autotrophic hypothesis An origin in the oceans, often considered in the past, poses the problems of the containment of the reactions in an open water environment The presence of Na salts, typical of the oceans, would hinder the formation of biological membranes (Natochin 2010) Anoxic geothermal fields In line with the heterotrophic hypothesis, supported by geochemical data and phylogenomic analysis (Mulkidjanian et al. 2012) Geothermal fields are conducive to condensation reactions and enable the involvement of solar light as an energy source and as a selector factor of stable nucleotides Geothermal vapour is enriched in phosphorus compounds that could be essential for the emergence of the first RNA-like oligomers Life evolution General considerations - There is no distinction between the last stages of life origin and the first stages of life evolution - The present-time living species are a small fraction of the total number of (extinct) species appeared in the course of evolution After abiogenesis, life spreads on the planet and starts to influence the environment and its physical conditions - The co-evolution of life and its environment should be considered together - Evolution of the biosphere 42 44

12 Mainly based on: Methods of life-evolution studies Analysis of geological strata that include traces of past life The strata can be dated accurately by means of radiodating techniques From the geochemical study of the strata we can: find traces of past biological activity even in the absence of macroscopic fossil records deduce the physico/chemical conditions of the environment that hosted the fossil forms of life Phylogenetic analysis Provides evidence of the evolution at the molecular level Relative (but not absolute) dating can be obtained Development of photosynthesis Photosynthesis Energy source not limited in time and available on all the planet surface Greater possibility of life expansion First photosynthetic systems already present around the mid archean Mostly anoxygenic systems - in bacteria, but not in archaea Green bacteria, purple bacteria (sulfur and non-sulfur types) Oxygenic photosynthesis was surely present at 2.9 Ga, perhaps even much earlier Cyanobacteria carry out oxygenic photosynthesis, up to the present time The great oxidation event Important steps in the evolution of terrestrial life after the emergence of fully-developed cells with DNA-proteins machinery enclosed in biological membranes The oxygen produced by photosynthesis is initially consumed by oxidation of the minerals present on the Earth surface For a long period of time the level of atmospheric oxygen does not increase Between 2.5 and 2.0 Ga there is a sudden rise of the atmospheric oxygen From ~1% PAL (Present Atmospheric Level), to ~10% circa 1.5 Ga 46 48

13 Emergence of eukaryotic cells Appearance of multicellular organisms From prokaryotic (archaea and bacteria) to eukaryotic cells Eukaryotic cells have a much higher level of internal organization, featuring organelles with specific functional properties The oldest robust evidence of eukaryotes are dated at ~ Ga Likely to be present even before Multicellular organisms are characterized by a coordinated network of cells that, despite sharing the same genetic information, are highly specialized and carry out different functions Multicellular life probably emerged as a response to environmental conditions Unicellular organisms are not able to exploit all the potential resources offered by the environment Multicellular life emerged several times on Earth Animals, plants and most fungi have emerged through independent evolutionary pathways prokaryotes: 1 5 μm eukaryotes: μm 49 Emergence of eukaryotic cells 51 Appearance of multicellular organisms Multicellular life appears only after the emergence of eukaryotic cells Prokaryotic cells only give rise to unicellular organisms The organelles are reminiscent of bacteria and their presence is interpreted as the result of a phenomenon of endosymbiosis Examples: chloroplasts reminiscent of cyanobacteria (photosynthesis) mitochondria reminiscent of purple bacteria (ATP production) Multicellular organisms probably appear around Ga The increase of the oxygen level must have played an important role in the development of eukaryotes and multicellular organisms Oxygen metabolism is more efficient than anaerobic metabolism 50 52

14 Evolution of macroscopic organisms Macroscopic organisms appear at ~ 0.6 Ga (Ediacaran) About 3 billon years after the origin of life The Cambrian period features a fast development of all present-day species Starting at 540 Ma ( Cambrian explosion ) Major extinctions appear in the geological record (red arrows) At intervals of the order of 10 8 years, but without a defined frequency Extinctions lie at the border between geological periods The mechanisms of evolution Natural selection Individuals of a given species with genes best suited to adapt to a specific environmental change have better chances to transmit their genes to the following generations The accumulation of the modified genetic pool in the course of generations leads to the origin of new species At variance with non-scientific teleological interpretations, Darwinian evolution works a posteriori, favouring the most suitable variations for a given function that already exists Genetic variation The capability of accumulating variations in the genetic pool is one of the key ingredients of evolution Variations of the genetic pool can be obtained through vertical and horizontal gene transfer; also mutations can provide a source of genetic variability, even though they tend to be destructive Last steps of life evolution Emergence of the homeothermy Most animals and plants are poikilotherms, i.e. they have little control of their internal temperature and are extremely sensitive to variations of ambient conditions Part of the animal kingdom developed the homeothermy, i.e. the capability of stabilizing the internal temperature in presence of (moderate) variations of ambient conditions Brain development Neural connections in the animal kingdom gradually developed functions of central control and brains Brains are extremely sensitive to temperature variations and are only present in homeotherms Self-conscious organisms A few millon years ago, about 3.5 billon years after the origin of life, self-conscious organisms emerged To our knowledge, this transition has occurred only once on Earth 54