Deep-Sea Hydrothermal Vent and Impact-Generated Hydrothermal Vent Systems: Insights into the Origin of Life

Abstract

Studies of deep-sea hydrothermal vent systems have generated a spectrum of hypotheses concerning the origin of life on Earth. The present paper integrates recent literature surrounding three separate hydrothermal vent systems (Lost City in the mid-Atlantic, Guaymas Basin in the Gulf of California, and 9°50′ N on the East Pacific Rise) to provide biological, chemical, and geophysical support for these origin-of-life hypotheses. Comparisons between deep-sea hydrothermal vents and impact-generated hydrothermal vent systems may provide further insights into the origin of life. Impact-generated hydrothermal vent systems may have cradled early life. A comprehensive review of studies conducted at Lonar Lake, the Haughton impact structure, and the Chicxulub impact crater provide evidence of long-term hydrothermal activity conducive to the formation of early life, as well as potentially unique DNA structures found in sediment samples—opening the discussion for further investigations into the possible origin (or origins) of life both on Earth and other planetary bodies.

1. Introduction

Life’s origin on Earth is a highly debated topic—one that often encompasses more than science. While there is very little definitive evidence about how the first biological molecules came to be, one hypothesis centers around the shocking discovery of diverse and densely populated communities surrounding deep-sea hydrothermal vents on the Galapagos Rift in 1977 [1,2,3]. The understanding of trophic life on Earth revolves around photosynthesis, where photosynthetic organisms convert inorganic energy into organic matter and thus comprise the base of all food webs. However, with the mean depth of the Galapagos Rift valley being 2450 m below the water’s surface, the then-newly discovered vent ecosystems fell deep below the euphotic zone (200 m), lacking access to sunlight [3,4]. The combination of the lack of sunlight, high biomass, multiple trophic levels, and high levels of hydrogen sulfide (H₂S) pointed to chemosynthesis as another energy-generating metabolic pathway used by autotrophs at the base of the food web [5,6,7]. Coupled with the extreme temperatures present in hydrothermal vent systems, it is hypothesized that prebiotic compounds and ‘pre-cells’ could have formed, creating necessary precursors for the origin of life [8].

Analysis of deep-sea hydrothermal vents and their potential role in the Earth’s origin of life has undergone continuous speculation, yet it remains a consistent hypothesis in the race to understand how living cells arose from the extreme conditions of our young planet. However, despite the popularity of these hypotheses, the possible role of impact-generated hydrothermal vent systems is not as common a topic and has only recently been proposed by several groups of researchers as a possible origin-of-life scenario [9,10,11,12]. Meteors and meteorites may have brought chemical elements and compounds required for life to Earth, and impacts would have exposed buried elements/compounds already present just below the surface. The hot crater could have facilitated the formation of organic molecules at high yield, which may have condensed into the precursors of biomolecules as the heat energy of the impact eventually dispersed and the crater cooled down. Subsequent wetting of the crater could have promoted the emergence of the first cells. Previously, it was believed that the oldest impact still recorded by Earth’s geological record was the Yarrabubba impact structure of Western Australia, which was dated at 2229 ± 5 Ma or 2.23 Ga—roughly 2.23 billion years ago [13]. However, recent studies of the Antarctic Creek Member (ACM) metasedimentary layer, located in the East Pilbara Terrane of Western Australia, uncovered an impact dated to 3.47 Ga [14]. This impact falls within the Archaean time period of early Earth and is an essential foothold for the concept that large-scale impacts during the Archaean period created the cratons that went on to form continental plates. While not all impacts affected Earth in the same manner, some impacts created hydrothermal vent systems in the central rise of the ensuing impact crater. These impact-generated hydrothermal vent environments may have provided ideal conditions for early microbial life (clay substrates, serpentinization, and high-temperature water-based solvents) and possibly even the origin of life [15].

This paper exists as an analysis of several notable deep-sea and impact-generated hydrothermal vents, comparing the different hydrothermal systems to show how origin-of-life hypotheses could apply to both. By evaluating the credibility of origin-of-life hypotheses at both deep-sea and impact-generated hydrothermal vent systems on Earth, we also stand to glean more information about the possibility of life on other planetary bodies.

2. Origin-of-Life Hypotheses

It is estimated that young Earth was habitable as early as 4.5 Ga, but with the Late Heavy Bombardment continuing its assault on the inner solar system until 3.8 Ga, it is hard to determine the exact time Earth would have been ready to cradle life [16,17]. Dating the earliest known microbial fossils found in western Australia’s Early Archean Apex Basalt beds would put life’s birthday somewhere around 3.5 Ga—but those microfossils are not simple organisms, nor were there only a few to look at; rather, the discovery of eleven structurally complex microbial taxa, dated to ~3.5 Ga, was enough to suggest they had been around for a while [18,19]. A brief investigation into the Greenland Isua Supracrustal Belt uncovered possible evidence of biologic activity as old as 3.8 Ga, with isotopically light carbon incorporated in the apatite grains of the banded-iron formation, suggesting the presence of biological processes—yet without hard proof, this possible pre-3.5 Ga biological activity is debated [20].

Several key discoveries in the 1800s laid the groundwork for modern origin-of-life hypotheses, including but not limited to the discovery of cells and cell theory [21,22], Darwin’s theory of evolution [23], disproof of the concept of spontaneous generation [24], and the discovery of “anorgoxydantism”/chemosynthesis [25,26]. Pasteur’s work disproving spontaneous generation in the 1860s forced origin-of-life hypotheses to shift away from the idea that life simply emerged from nothing and instead towards the concept that living cells arose from events or geochemical processes on early Earth. Many hypotheses turned towards abiogenesis, a gradual, chemical progression in which the first simple living cells originated from non-living molecules and compounds over long periods of time. Oparin and Haldane’s “primordial soup” hypothesis became the springboard of several prominent and popular origin of life hypotheses [27,28]. The Miller–Urey experiments and subsequent supporting publications for the Oparin–Haldane hypothesis further fanned the flames in the rush to discover the true scientific origin of life [29,30].

Thousands of hours of study and hundreds of publications led to the development of numerous hypotheses on how living organisms could have been formed from the unique mixture of elements, chemical compounds, and physical conditions of early Earth. The discovery of DNA and RNA led to the RNA world [31,32,33,34,35,36,37,38,39] hypothesis, which centered around the thought that RNA—an essential biomolecule used to transmit genetic information from DNA during the process of protein synthesis—existed as the first molecule of life and became more complex, eventually forming early life [40]. These hypotheses prompted further concepts that invoked different forms/builders of the RNA molecule as the base of life, including the ribonucleoprotein (RNP) world [41,42], peptide nucleic acid (PNA) world [43,44], and threose nucleic acid (TNA) world [45] hypotheses. The lipid world hypothesis offered a different approach, one that did not center around genetic material but rather simple lipids self-assembling to form organizing compartments like vesicles [46,47,48,49]. Due to the self-assembling nature of lipids, it seemed possible that life could have been born as small lipid vesicles. A bridge between the two above hypotheses is the lipid-RNA model, which suggests that early lipid structures self-assembled protocells that protected early versions of genetic molecules [46,47,48,49]. In these models, the formation of biomolecules is the driver for life’s growth. In comparison, surface metabolism or metabolism-first/autocatalytic set hypotheses posed that perhaps biological metabolisms were created first and then encapsulated, leading to a primitive “cell” or “pre-cell” [50,51,52,53,54,55,56,57,58]. These hypotheses put chemical reactions and energy generation before genetic information storage, saying that chemical pathways could have occurred first due to interactions between different free compounds and solids. It is suggested that self-sustaining chemical networks could have existed that worked as a system, where molecules catalyzed each other’s formation and acted as an interconnected web leading to more complicated formations [59,60,61,62,63]. Elements like iron or sulfur [64], compounds like CO₂ or HCN [65,66,67,68], or solids like clays and minerals offered possible chemical building blocks for RNA, lipid, and metabolism-based scenarios.

The origin of life can be viewed in many ways, through many different disciplines, yet the role chemistry plays in the development of life cannot be understated. For RNA-first models, the biological molecules that make up living DNA, RNA, or other forms of genetic material had to be built on something, and those polymers responsible had to come together via chemical reactions to create larger structures. In autocatalyzing or metabolism models, simple compounds interacting with each other created patterns and gradients needed to kickstart cycles of chemical reactions. The transfer of electrons or chemical changes caused by ionization or formed bonds can serve as “power” to start synthetic pathways from abiotic and inorganic material, unintentionally playing on the chemical tendency towards valence stability and leading to the creation of prebiotic molecules [69].

3. Deep-Sea Hydrothermal Vent Systems

Deep-sea hydrothermal vents were discovered on 12 February 1977 by researchers aboard the Woods Hole Oceanographic Institution vessel Knorr, who were investigating the deep-sea areas of the Galapagos Rift [1,3]. The Galapagos Rift Zone, which exists as an offshoot of the East Pacific Rise, had been previously recorded by heat probes to have unusual heating patterns. The existence of deep-sea circulation at this site and the East Pacific Rise had been proposed years earlier, as early as 1973, but in situ investigation revealed biological activity that was unexpected due to the depth of the rift valleys [70,71]. In the five vent areas probed during the submersible Alvin’s dives, each had significant biological evidence of ecosystems with multiple trophic levels—clams, mussels, worms, fish, tubeworms, limpets, crabs, “dandelions”, and even a lone purple octopus. Four of these five vent areas were active (“Clambake I”, “Dandelion Patch”, “Oyster Bed”, and “the Garden of Eden”), with the fifth vent (“Clambake II”) inactive and surrounded by dead and dissolving clam and mussel shells [1]. Further investigations into plate boundary areas revealed similar formations—chimney structures created by mineral depositions, cracks in the seabed where diffuse flow escapes, and pillowy lava rocks. As of 2009, 245 hydrothermal vents have been confirmed, and an additional 276 have been inferred based on location data [72]. Hydrothermal vents can be found in shallow or deep waters, but deep-sea hydrothermal vents (>200 m) differ from shallow-water vents (<200 m) by lower species richness, the development of higher/more vertical vent structures, a higher role of vents in community structure and composition, and the importance of symbiotic relationships in the maintenance of the food web. In addition, deep-sea vents have a higher ratio of vent obligate taxa and a larger endemic species ratio in comparison to shallow-water vents; meaning, at greater depths, there comes a reliance on these vents that is not necessary with shallow-water vents [73,74,75,76].

Hydrothermal vents located on spreading centers, back-arc basins, and volcanic seamounts use heat and thermal energy from magma residing in magma chambers or brought up closer to the Earth’s crust by tectonic activity [3,77,78]. Seawater penetrates the seafloor through fissures and cracks in the ocean crust. When it reaches temperatures around 50 °C, it begins to react with basaltic rock, altering several types of olivine, basaltic glass, and plagioclase feldspar. As the seawater seeps deeper, rising in temperature as it progresses closer to the heat source, it can go through several geochemical reactions that have been recorded in deep-sea samples, drill cores, and ophiolites—including clay precipitation via Mg-smectite or Mg-chlorite reactions, anhydrite precipitation following the leeching of Ca from basalt rock, reduction of seawater sulfate, reaction with ferrous Fe minerals, etc. [79,80,81]. The resulting fluid gets heated to extreme temperatures around 400 °C yet remains in liquid form due to the high pressure of the deep ocean. This superheated water, now less dense, begins to rise through spaces in the rock, leeching minerals, metals, and embedded elements—like sulfur, copper, iron, magnesium, zinc, gold, aluminum, manganese, and silicon—from the oceanic crust as it goes. When the superheated hydrothermal fluid mixes with ambient, near-freezing seawater, rapid chemical reactions cause the minerals to precipitate, creating mineral-rich deposits on the seafloor or large chimney structures [8,81,82,83,84]. These formations, called polymetallic sulfide deposits or seafloor massive sulfides, are extremely rich in valuable resources, including but not limited to iron, zinc, gold, silver, cobalt, and copper [8,85]. Numerous debates in recent years have sprung up since the discovery of the material depositions that make up deep-sea hydrothermal vents, as seabed mining seeks to capitalize on these mineral hotbeds despite the risk of destroying vent ecosystems [85,86,87].

There are several types of vents and vent systems, which create different and unique chemical and biological environments. Black smoker chimneys can release vent fluids around 350 °C, getting their name from the black smoke-looking sulfide particles that billow from the vent openings. The composition of these vents sparked several hypotheses surrounding prebiotic processes and was also the location at which the possible archaeal ancestor of the eukaryote, the superphylum Asgard archaea, was found [73,88,89,90]. These black smokers, and the diffuse flow vents that branch from them, provide the ideal environment for hyperthermophiles and chemosynthetic autotrophs, as the environmental gradients of pH, redox potential, chemical composition, chemical concentration, and temperature create several habitual ranges for microbes to thrive [90,91,92]. The highest temperature that can be withstood by life has been set at 122 °C with a pressure of 40 MPa by Methanopyrus kandleri, found in the vents at the Kairei hydrothermal field, Central Indian Ridge [93,94], followed by Strain 121/Geogemma barossii at 121 °C (near the Endeavor segment of the Juan de Fuca Ridge, Northeast Pacific Ocean) [95] and Pyrolobus fumarii at 113 °C (Trans-Atlantic Geotraverse (TAG) vent system, Mid-Atlantic Ridge) [96]. Potential biosignatures found in black smoker vents at the TAG hydrothermal site on the Mid-Atlantic Ridge support the idea that these high-temperature-favoring microbes may even influence the biogeochemical makeup of black smoker vents, perhaps laying the path for geological formation over time at these sites [97]. The porous surface of black smoker chimneys, as well as cracks in the seabed surrounding vent systems, form areas of diffuse flow, which have temperatures that range between ~2 °C and 100 °C—creating habitable areas that have a high chemical composition without the intense heat [86,98,99,100]. A majority of the biomass found at hydrothermal vents is found in or around diffuse flow vents, rather than inside or in direct contact with the superheated hydrothermal fluid that comes from black smokers [101].

Snowblower hydrothermal vents were first witnessed and recorded during the 1991 investigation of the East Pacific Rise between 9°45′ and 52′ North [102]. Using the submersible Alvin, researchers arrived during an eruption of the mid-ocean ridge vent sites they had originally been planning to study, and in some locations found themselves in a ‘blizzard’ of white particulates up to 50 m above the bottom. These particulates also formed mats along the cracks and crevasses of the seafloor, below the seabed surface, and for thousands of square miles around the ridge. This white floc was made up of filamentous, fast-growing chemosynthetic bacteria that seemed to thrive on the volatile-rich eruption/post-eruption hydrothermal fluids, and the resulting bacterial mats laid the groundwork for the multi-level deep-sea vent ecosystem that followed [101,102,103,104]. Later tests of cultures taken from diffuse vents post-eruption at the CoAxial segment of the Juan de Fuca Ridge (Northeastern Pacific Ocean) in 1993 found both thermophiles and hyperthermophiles, which confirmed the findings from the 9°50′ North investigation [105]. As time progressed past eruption, samples taken along a three-year timeline showed decreased abundances of the thermophilic/hyperthermophilic microbes in accordance with the decrease in temperature and hydrogen sulfide (H₂S) post-eruption—suggesting that the microbes bloom in the presence of high-sulfide hydrothermal fluid available at large quantities following an eruption, but dwindle as the H₂S decreases, as they are consumed, or as they get continuously displaced by plumes and deep-sea currents [102,103,104,105,106,107].

Though vent environments and structures vary based on location, rock composition, ocean composition, and biodiversity, chemosynthetic organisms remain the base of these food webs [75,108,109,110,111]. Hydrothermal fluids found at deep-sea hydrothermal vents may contain numerous potential electron acceptors, with the primary being oxygen, nitrate, sulfate, elemental sulfur, and ferric iron [112,113]. This variety of chemical energy sources, coupled with the different geophysical conditions and gradients found at deep-sea hydrothermal systems, creates a range of different habitats for microbes to proliferate [114]. Carbon fixation [115], methanogenesis [116,117,118], hydrogen-oxygenation [112,119], sulfur oxidation/sulfate reduction [120,121,122,123], ammonium oxidization/nitrate reduction [124], nitrogen fixation [125,126,127], and iron oxidation/iron reduction [128,129] are several of the (but not all) pathways recorded in deep-sea hydrothermal microbes [114,130,131,132]. The sheer scope of the chemoautotrophic pathways and processes recorded at deep-sea hydrothermal vents worldwide, coupled with the harsh and unique, chemically charged, and biologically abundant environments of deep-sea hydrothermal vents, contributed to the speculation that life may have originated there. Finding such a diversity of chemosynthetic microbes inhabiting these volatile environments inspired the idea that some of the oldest microfossils, found in Precambrian rock, may have been chemosynthetic organisms from hydrothermal vents [133]. Since then, the initial hypothesis proposed by Baross and Hoffman in their 1985 publication has changed and morphed with each new discovery [8,134,135]. Regardless, deep-sea hydrothermal vents have remained a popular contender as an origin-of-life location.

3.1. 9°50′ North, East Pacific Rise

The eruption of the 9°50′ North segment of the East Pacific Rise in 1991 did more than showcase the existence of snowblower vents—it led to the first documentation of the process of succession at a spreading center hydrothermal vent system [101,102]. Lava flows destroyed several vent communities, such as the notable “Tubeworm Barbecue,” while also forming new structures as the high-temperature basaltic lava rapidly cooled and shattered, leaving behind cracks, fissures, and holes in the seabed. Snowblower vents and dense microbial mats sprung up from these low-temperature, diffuse vent areas surrounding the eruption area, and elevated levels of hydrogen sulfide and other such chemical drivers created environments conducive for large populations of chemosynthetic microbes [102].

Following the mass proliferation of microbial chemoautotrophs, vent fauna such as Vestimentifera tubeworms, T. jerichonana, and R. pachyptila, were the first to colonize outside the diffuse flow vents [104]. Adult vestimentiferan tubeworms have no digestive system and instead rely on a symbiotic relationship with chemoendosymbiontic bacteria cultured in trophosome tissue for the nutrients they need to continue functioning [136,137]. Compared to other siboglinids, vestimentiferans display increased tolerance for hydrogen sulfide, which is a necessity for living at many deep-sea vents, as well as an increased growth in hydrothermal systems and a thick chitinous tube [138,139]. Recent studies into the sugar synthesis pathways of vestimentiferans revealed that, unlike most fish or mammals, vestimentiferans of the Siboglinidae family did not possess the sugar-synthesizing pathways for glucose. The tested vestimentiferans P. echinospica, L. luymesi, A. ivanovi, O. alvinae, and R. pachyptila lacked the glucose-6-phosphatase (G6Pase) enzyme required for glucose synthesis, yet instead were found to have trehalose-6-phosphate synthase/phosphatase (TPSP)—the enzyme used in the process of trehaloneogenesis [140]. In comparison, non-symbiotic Polychaeta C. teleta, D. gyrociliatus, O. fusiformis, and H. elegans tested positive for G6Pase, but lacked TPSP. To summarize, the symbiotic Polychaeta vestimentifera performed trehaloneogenesis, but the non-symbiotic Polychaeta performed gluconeogenesis [139]. This investigation suggests that it was evolutionarily beneficial for symbiotic vestimentiferan tubeworms to go through trehaloneogenesis. One hypothesis posits that trehalose provides an advantage for tubeworms in deep-sea hydrothermal vent environments, as trehalose has been observed to contribute to heat resistance in the presence of H₂S in maize [141]. In the highly competitive environment of the deep sea, access to food is the difference between life and death for all organisms, including microbes. As tubeworms cluster around diffuse flow vents, those on the inside get first access to the H₂S that feeds their chemoendosymbiotic bacteria—and those on the outside face starvation. However, getting too close to a vent opening can also spell danger in the form of extremely heated hydrothermal fluid. In R. pachyptila, temperature influences growth the most, as temperature has a direct effect on both the metabolite and their symbiotic bacteria’s chemosynthesis [138,142]. Thus, it would stand that heat resistance, especially in the presence of H₂S, would be biologically beneficial for deep-sea hydrothermal vent-inhabiting tubeworms—though more information is needed to confirm this theory.

3.2. Guaymas Basin, Gulf of California

The Gulf of California, located between the Baja California peninsula and Mexico, connects to the East Pacific Rise spreading center and creates the Guaymas Basin rift basin. Of the two axial troughs of Guaymas Basin, the southern trough was believed to be the only site of ongoing hydrothermal activity until the discovery of an active hydrothermal mound by the northern trough in 2015 [143,144,145]. Guaymas Basin hydrothermal activity is found along chimneys, mounds, and mats—creating a diverse hydrothermal circulation system that affects the biological and geochemical processes of the basin [146,147]. The formation of the basin and its complex layout cause steep geochemical and thermal gradients, which affect the habitable zones for the diverse microbes and vent-obligate flora/fauna found there [148,149,150,151,152]. In addition, Guaymas Basin experiences increased levels of sedimentation from both hydrothermal activity and terrestrial influence, which creates a deep, organically rich sedimentation and a unique environment for bacteria [143,146,153]. Both the chimneys and hydrothermal sediment mounds of the Guaymas Basin have been observed to host thick microbial mats dominated by the bacteria of genus Beggiatoa and Thiomargarita, family Beggiatoaceae [154,155,156]. Microbes of the family Beggiatoaceae are large, filamentous bacteria that oxidize sulfur. Their existence on both the hot chimney walls and cooler sediment led to investigations into their enzymatic activity, with some species of the larger-celled species being mesophiles and the narrower-celled species either being thermophiles or mesophiles with thermotolerant characteristics [154,156]. In addition to Beggiatoaceae, several sulfur-reducing bacteria, including Thermodesulfobacterium hydrogeniphilum sp. [157] and sulfur-reducing archaea like Archaeoglobus fulgidus [158], have been found in Guaymas Basin on chimneys and sediment beds. In addition to the sulfur distributed by hydrothermal activity, microbes in Guaymas Basin also have access to methane via off-axial seeps [146,159]. Methane seeps are a common alternative chemical energy source for deep-sea microbes and can be found around the world. Hydrocarbons, originating from deep-buried sedimented organic matter left to degrade, like methane, propane, and butane, escape upwards and become available for biological processes [160,161,162,163]. Anaerobic oxidation of methane with sulfate contributes to controlling the emission of these gases, with the process itself relying heavily on microbial archaea, specifically anaerobic methane-oxidizing (ANME) archaea, and sulfur-reducing bacteria [160,163]. Methane seeps are cold, as these sites do not require hydrothermal activity—but several of the methane seeps formed on the hydrothermal sediments in the Guaymas Basin are warmer, creating unique areas of biological productivity [163,164,165]. The higher temperature of the sediment promotes colonization by thermophilic and thermotolerant methanogens and faster growth rates and sets the microbial structure of Guaymas Basin seeps apart from the microbial communities at other cold methane seeps [166,167,168,169]. Across the different habitats created by the thermal, geological, and chemical ranges found at Guaymas Basin, lineages of sulfate-reducing, methane-oxidizing, or hydrocarbon-oxidizing microbial communities have been found to remain constant within areas of specific temperature ranges, which is credited to be the key to success for survival in the extreme variability of hydrothermal environs in Guaymas Basin [151,165,169]. The continued survival and success of the microbial communities of Guaymas Basin persist due to the dynamic biodiversity found there [170].

3.3. Lost City, Mid-Atlantic Ridge

The Lost City alkaline hydrothermal vent field represents a different type of hydrothermal vent, as well as an alternative origin-of-life theory. The off-axis vent field is located around the seabed mountain Atlantis Massif at 30° N and ~15 km west of the Mid-Atlantic Ridge, and was discovered in 2000 [171,172]. Most of the hydrothermal processes of Lost City occur at a singular ~70 m tall by ~100 m wide carbonate structure called “Poseidon”, which reaches temperatures between 40 °C and 110 °C [173,174]. Unlike most other hydrothermal vent sites and black smoker chimneys, which use volcanic activity as their heat source, Lost City’s thermal energy comes from the process of serpentinization [175,176]. Serpentinization is the exothermic alteration of ultramafic rock, which is a type of rock that contains low concentrations of silica and is composed mainly of iron- and magnesium-rich minerals [177,178]. In the basement of Lost City, peridotite (olivine) serpentinization reactions at ~200 °C create hydrothermal fluids that proceed to diffuse through veins and fissures in the existing structure. This warm and nutrient-rich fluid, which has a pH of ~9, interacts with carbonate in the surrounding cold seawater and precipitates into carbonate deposits [172,175,176]. Recent studies of 87Sr/86Sr, stable C, O, and clumped isotopes indicate that calcium carbonate and brucite form first, in the higher-temperature hydrothermal fluid-dominated waters of the interior chimney, whereas aragonite dominates in the exterior or seawater-dominating vents [172,179]. Older chimneys contain mostly calcite and magnesium, and higher concentrations of trace metals [180].

Serpentinization creates alkaline conditions, promoting an increased molecular hydrogen (H₂) concentration and increased methane and formate [181,182]. Sulfur has also been found in measurable quantities in Lost City vent fluid [183]. Lost City supports a diverse range of microbial life due to temperature and chemical variations throughout the structure [184]. Methane-metabolizing Methanosarcinales (Lost City Methanosarcinales [LCMS]) comprises most of Lost City’s archaea, but related ANME-1 and Methanomicrobiales archaea have been found [184,185,186]. In comparison to archaea, a high bacterial diversity was found at Lost City, including Gamma-, Alpha-, Beta-, and Epsilon-proteobacteria, Actinobacteria, Chloroflexus, Desulfotomaculum, Firmicutes, Methylobacter, Methylophaga, Nitrospira, Planctomycetes, Thiomicrospira, Vibrio, and more [187].

The diverse chemolithoautotrophic community at Lost City, and its estimated long life of ~30,000 years, has led many to speculate that serpentinization at alkaline hydrothermal vents may have played a role in the origin of life on the young Earth [176,188,189]. Serpentinization’s ability to generate redox chemical pathways, catalysts, and gradients provides the chemically derived free energy required for the formation of life [190,191,192,193,194]. It is very possible that alkaline vents existed during the Hadean eon (4.6–4.0 Ga), and if they did, they may have been a cradle for life [193,194].

4. Impact-Generated Hydrothermal Vents

In contrast to deep-sea hydrothermal vents, impact-generated hydrothermal vents use the heat generated from the impact of a meteor or meteorite. When there is a large-scale impact, heat is generated from the shock of the impact, as well as from the energy of the collision [195]. The amount of heat generated by the impact, as well as the size of the impact crater, would depend upon the size of the meteor or meteorite and the gravity of the target. Shockwaves created by the impact, moving outwards and down, evacuate the material in the target area. In larger impacts, gravitational effects cause more complex craters, with a central rise forming in the center of the crater as a portion of the melted materials rushes ‘backwards’ to fill the gap recently vacated (Figure 1). In some cases, with a large enough impact, deep, buried ultramafic rock, like basalt and peridotite, can be exhumed. This phase, immediately post-impact, is called the thermobaric phase [15,196]. The heat stored in the crater, especially in the central rise, will dissipate slowly; this creates areas of local, longstanding thermal energy, which can lead to hydrothermal circulation in the event that liquid is introduced to the crater [15,197].

Immediately after the impact, the surrounding area is effectively sterilized as several physical and biological process cycles are displaced. Over time, ranging from days to years depending upon the available source, an influx of water into the crater may form a crater lake and a hydrothermal vent system—this is considered the hydrothermal phase (Figure 2) [15].

Heat stored in the surrounding impact melt rock, geothermal gradients, and shockwave energy inside the central uplift power the hydrothermal vent system. As in deep-sea hydrothermal vents, chemicals are leeched from surrounding rock, creating patterns of deposition along the crater lake floor, which serve as records of these hydrothermal systems facilitating geochemical (and possibly biogeochemical) reactions [15]. Modeling of impact-generated hydrothermal systems has shown significant temporal longevity—enough to establish biological life [15]. In addition to creating a generally long-standing hydrothermal hotspot, the possibility for unearthed ultramafic rock to react with water also creates the possibility for serpentine to be produced. Serpentine plays a significant role in the release of hydrogen via the serpentinization process, and with hydrogen proving to be such a vital molecule in several metabolic pathways, many hypotheses consider it a key component for life’s origin (see Section 3.3. Lost City, Mid-Atlantic Ridge) [53,194,198,199].

4.1. The Haughton Impact Structure, Canada

Several successful impact-generated hydrothermal vent systems have been recorded in Earth’s geological history [196]. One of the largest impact structures on Earth that remains untouched by human activity is the Haughton Impact Structure in the Canadian Arctic, found on Devon Island. The Haughton Impact Structure has been aged to 31.04 ± 0.37 million years old (~23 mya) and has a dome diameter of 23 km [196,200]. In addition to having a central uplift structure characteristic of impact-generated hydrothermal vent systems, evaluation of the area’s geology shows veining within the impact melt breccias and hydrothermal structures/depositions [201,202]. The layering of the deposits gives insight into the Haughton Structure’s activity and temperature during its active years. Quartz and calcite formed first, denoted by being the oldest carbon-dated rocks, as well as the deepest layer, when the vent was young and estimated to be about 200 °C. Other minerals that were recovered at the site were celestite, barite, and fluorite. Then, between 200 °C and 100–120 °C, circulation around the rim led to the additional deposition of marcasite and pyrite. As the crater cooled further to <90 °C, deposition patterns returned to quartz and calcite. Patterns and thickness of depositions indicate that it took several thousand years after the impact for the crater to cool below 50 °C and for the hydrothermal system to dissipate. This timeframe was likely the result of the location of the impact and the size of the meteorite. The crater’s position in the Canadian Arctic means that it was in a relatively biologically barren and stable environment. Despite the decreased temperature from being closer to the poles, the magnitude of the heat given off by the impact and stored inside the resulting structure would have kept the crater lake from freezing or dissipating. The crater lake that appears in the deposition patterns of hydrous minerals in the Haughton Impact structure was 14 km in diameter at its largest and had underground waterways that formed in breccia cracks and veins [201,202,203].

4.2. Lonar Lake, India

Another impact-generated hydrothermal vent system on Earth is Lonar Lake, located in Maharashtra, India. This impact crater is one of two known impacts to have occurred on volcanic basalt target rock. The Lonar Lake impact occurred ~50,000 years ago, with a relatively small meteorite impact resulting in a 1.8 km diameter crater [196,204]. Though hydrothermal activity has since ceased, the location and relatively young age allow for in-depth analysis of core samples—specifically microbial core samples. Core samples were taken from hydrothermal deposition material, and microbial DNA was extracted and amplified via PCR with archaea and bacteria primers. The results showed 44 unique phylotypes within domain Bacteria, with the majority being Firmicutes (34%) and Proteobacteria (29.5%), as well as 13 unique phylotypes within domain Archaea—5 Crenarchaeota and 8 methanogenic Euryarchaeota [205]. Deeper samples taken at Lonar Lake show a similar pattern of microbial activity, with Proteobacteria (30%), Actinobacteria (24%), and Firmicutes (11%) dominating the sample [206]. The presence of Proteobacteria in both studies gives credence to the concept that life could possibly be generated in one of its many considered base forms—as Proteobacteria—in impact-generated hydrothermal vent systems. However, the timing of the Lonar Lake impact brings skepticism to the possibility that these unique phylotypes were formed due to the impact-generated hydrothermal vent system itself. The impact occurred in a period where Earth had already been colonized by all forms of life, meaning the sample site may have been contaminated. Around 50,000 years ago, Earth was in its Pleistocene period. During this period, early humans already existed, as well as many branches of bacteria and archaea [207]. As the current understandings of the DNA samples found at Lonar Lake are limited, there is no certainty that the microbial DNA fragments originated there, and the possibility of contamination due to already existing life is high—however, given the samples’ status as structures unique to Lonar Lake’s core samples, the possibility that these microbial fragments could have been created in the Lonar Lake impact-generated hydrothermal vent system cannot be overlooked.

4.3. Chicxulub Impact Crater, Gulf of Mexico

Compared to discussions of the origin of life, the Chicxulub impact crater marks one of the impact-centered extinctions that have been recorded over the course of Earth’s history [208]. This ~180 km diameter crater, found in the Gulf of Mexico, has been credited with the Cretaceous extinction event of ~65.5 million years ago [209,210]. The Chicxulub crater was found in 1978 by geophysicists during a magnetic survey for potential oil drilling locations, but was not reported officially until 1981 [211]. Just a year earlier, in June 1980, a paper had been published suggesting that the Cretaceous-Tertiary extinctions could have been caused by extraterrestrial impact [212]. The rationale behind this hypothesis came from a pattern of iridium and other platinum group elements found in deep-sea limestone recovered in Italy, Denmark, and New Zealand. This geological data pointed towards an asteroid impact capable of causing a global catastrophe and ejecting massive quantities of pulverized debris [212]. A decade later, in 1991, the Chicxulub crater was proposed to have not only originated via impact, but also the specific impact responsible for the Cretaceous-Tertiary extinctions [213]. Further investigation has confirmed these hypotheses to be true [210,214]. The magnitude of the crater suggests an asteroid size of 7–19 km in diameter that struck at a steep, 60–45° angle to the horizon [215,216]. The Chicxulub impact crater is currently listed as the third-largest impact crater found on Earth, behind the Vredefort crater (South Africa, ~300 km diameter) and the Sudbury crater (Canada, ~200 km diameter), though size estimations vary [217].

The impact struck shallow water, generating ~130 m of melt rock and marine breccia [218,219] and sending ejecta 80,000–50,000 km³ in volume [220] up to a paleodistance of 16,549 km [221]. Tsunamis generated by the impact have been calculated to have been 30,000 times more energetic than modern-day earthquake-generated tsunamis, with waves over 10 m high, which devastated local and distant land masses, as well as the seabed in the impact area [219,222,223,224,225]. The magnitude of the impact also created static stress hundreds of kilometers away, triggering geodynamic changes in the mantle and the Earth’s crust [226,227]. Wildfires, generated by the impact and by reentry of hypervelocity ejecta, left behind a heavy layer of soot found in clay deposits around the globe [228,229]. Gases, soot, and dust, including an estimated ~325 Gt sulfur and ~425 Gt CO₂, released after or during the impact created a blanketing and darkening effect—and a combination of the low light levels and prolonged cooling by at least 26 °C, with ocean acidification due to increased atmosphere CO₂, would have decimated photosynthesizing primary producers, contributing to the Cretaceous extinction [205,219,230,231].

The conditions created in the Chicxulub crater after the initial impact and the downward collapse of the crater ring would have been ideal for impact-generated hydrothermal vent formation [218,232,233,234]. A deep basement uplift created a 2–25 km center uplift in which the lower crust rests ~5–10 km below the seabed [232,235]. In addition, the impact breccias and melt rocks were hot and porous, creating places for returning seawater to seep down and interact with the uplifted crust [219,233]. Hydrothermal fluids, observed in precipitated minerals and alteration patterns, were iron- and sulfur-rich, creating ideal homes for iron, sulfate, and sulfur-reducing microorganisms [236,237,238,239]. Based on analyses of the cores taken from the Yaxcopoil-1 borehole, the composition of Chicxulub hydrothermal fluid would have resembled an iron oxide hydrothermal system [240]. Hydrothermal longevity inside the Chicxulub impact crater has been estimated to have lasted ~2 million years [236,238,239,240]. The multiple possible chemical metabolic pathways and prolonged staying ability for the hydrothermal processes at the Chicxulub impact crater suggest there would have been time and cause for biological colonization by thermophiles or hyperthermophiles [236,241,242].

5. Discussion

Since the publication of the Oparin–Haldane hypothesis and the Miller–Urey experimental proofs that followed, researchers have raced to find the mechanism responsible for generating living organisms from physical and biogeochemical processes. Several hypotheses have cropped up since the 1950s, and while some have been disproved, many have only evolved and been strengthened since. Proceeding to the modern day, hydrothermal vents remain a strong contender for providing the ideal conditions for early biotic progression. Though the focus of this hypothesis remains firmly in deep-sea hydrothermal and alkaline hydrothermal vent systems, impact-generated hydrothermal systems, recorded in Earth’s geological record, offer an additional possibility.

When considering deep-sea environments, there are several ongoing hypotheses and ideas as to what contributed to the beginnings of prebiotic pathways and metabolisms, as well as the creation and proliferation of early protocells, with a few such models being serpentinization and the chemistry of formamide, the RNA world, redox reactions, and the acetyl-CoA (Wood–Ljungdahl) pathway of CO₂ fixation [15,243,244,245,246,247]. This has been supported by various experiments, modeled to simulate seafloor hydrothermal systems in primordial settings, which have successfully synthesized organic prebiotic molecules—though most have concluded that, while synthesis was successful, the quantity and size of the products were too small to have contributed significantly towards more complex structures [248,249,250,251,252,253]. These experimental investigations have answered several questions regarding the building blocks of biomolecules, such as which molecules and compounds would have/could have existed in hydrothermal or volcanic settings. Some such experimental conclusions detail how it was possible for CO to be reduced into CH₃OH and other reaction intermediates at high temperature and pressure [250,254], while others discuss how α-hydroxy and α-amino acids/amino acids could have formed as products of carbon fixation using metal precipitates [255] and how peptides could be formed in through interactions between metals, CO, and sulfur catalysts under hydrothermal geochemical conditions [256]. These experiments use conditions that would have been present at Hadean hydrothermal vents—especially those with high concentrations of reactive metals and elements like nickel, iron, or sulfur. It is important to note that the above conclusions of these individual studies fail to consider the event of multiple chemical and physical synthetic pathways occurring simultaneously. Early Earth held thousands of organic and inorganic compounds, most overlapping in place and time [257,258]. With the success of so many experiments with various types of prebiotic molecules or early metabolic pathways, the hypothesis that life could have originated at hydrothermal vents may not be as far-fetched as it once seemed. In addition, advancements in science and engineering have led to several recent discoveries that support the possibility of unique biogeochemical processes around deep-sea vent systems [259,260,261,262,263].

As scientific innovation and our understanding of the formation of biological precursors advanced, researchers have turned their attention towards impacts as life’s possible origin environment. As previously mentioned, Earth underwent extreme impacts between the Hadean and Archean Eons, in an event dubbed the Late Heavy Bombardment (LHB)—a time interval where many asteroids, comets, and meteors collided with the planetary bodies of the inner Solar System [264]. The effect these collisions had on the young Earth is up for debate, but it is believed life either began prior to or during the LHB period based on microfossil and isotope records—though the exact time is still unknown [20,265,266]. With the intense impacts on Earth’s surface, geochemical reactions during and following impacts may have been the key to biological beginnings—specifically in the production of simple sugars and biologically available phosphate. Experiments with hydrogen cyanide (HCN), which is believed to have been present in the atmosphere of young Earth [267], show that through a Kiliani–Fischer-type synthesis, simple sugars (glycolaldehyde and glyceraldehyde) can be produced [10,268,269]. These simple carbon-based sugar structures are used in ribonucleotide assembly and are the precursors to the amino acids glycine (Gly), alanine (Ala), serine (Ser), and threonine (Thr). Furthermore, the formation of glyceraldehyde’s more stable isomer, dihydroxyacetone, is favored at higher temperatures—which, when reduced with H₂S, can create acetone and glycerol [270]. The phosphorylation of glycerol creates the lipid precursor glycerol-1-phosphate [10]. Different reactions with acetone (and the following acetylene) lead to the precursors of valine (Val), leucine (Leu), proline (Pro), arginine (Arg), asparagine (Asn), aspartic acid (Asp), glutamine (Gln), and glutamic acid (Glu) [10]. In addition to reactions with chemicals hypothesized to exist in Hadean Earth, samples from meteorite specimens have shown to have catalytic effects that lead to the formation of nucleobases, carboxylic acids, amino acids, and a few other miscellaneous chemical compounds. These meteorite samples came from 12 different specimens that represent several different types of meteors—specifically the iron, stony iron, chondrite, and achondrite classes. These samples were added to the thermal condensation reaction of the compound formamide (NH₂CHO) and resulted in the synthesis of organic, prebiotic molecules. The experiment resulted in the formation of 13 nucleobases/heterocycles, 17 carboxylic acids, 12 amino acids, and 5 miscellaneous products, including urea and guanidine [271]. It is important to consider that the introduction of water into post-impact crater lakes creates an aqueous solution, useful for facilitating chain chemical reactions—making impact-generated hydrothermal vents a very likely location for the origin of life. With experimental data showing direct conversions of chemical compounds into biological precursors and biological molecules, it is becoming increasingly possible for impact-generated systems to have been life’s early cradle.

5.1. Comparing Deep-Sea Hydrothermal Vents to Impact-Generated Hydrothermal Systems in the Scope of Origin-of-Life Hypotheses

Though both deep-sea and impact-generated hydrothermal vent systems share similar biogeochemical processes, there are a few distinct differences between the two. The most pressing difference is time; the longevity of a hydrothermal vent system is tied to its source of heat. For many deep-sea hydrothermal vent systems, the heat source is volcanic in nature; from spreading centers like the Guaymas Basin or volcanic structures like Loihi Seamount, the ‘fuel’ for these deep-sea vents is tied to volcanic and tectonic activity [72,272]. In some locations, such as the Mid-Atlantic Ridge, this activity is constant—if extremely slow. In other locations, vent activity is sporadically spaced. Some deep-sea vents become inactive, causing the devastation of the food chain as the base of the food web, the chemosynthetic organisms that feed on H₂S from hydrothermal fluids, die out. Most chimneys cycle through active and inactive stages with the flow of tectonic activity, while others stay inactive for good. Vent structure can change rapidly, both temporally and spatially, which leads to variation in geology as well as in the biological vent community [273]. In comparison, impact-generated hydrothermal vent systems feed on heat specifically generated in the impact itself, with no way of renewing said energy as it is used. Once the impact-generated heat dissipates, the hydrothermal circulation ends, and the lake and surrounding area are left to return to pre-impact values. The local climate and biological succession will adapt to the lack of heat input by the hydrothermal system, though traces of evolutionary markers will remain [15]. This issue of longevity creates one of the biggest differences between the two hydrothermal vent systems, with impact-generated systems cooling off on a much quicker scale than many deep-sea systems, but neither can be ruled out from the possibility of contributing to early life. Larger crater systems can generate enough thermal energy with the magnitude of their impact to last hundreds or thousands of years, which is reflected in the geological records left behind, with hydrological depositions creating a history of hydrothermal activity that persisted for centuries. For the Haughton Structure specifically, geological patterns allow modeling to estimate the structure held ideal microbial temperatures for over 50,000 years—long enough for pre-cells or early metabolic pathways to be established [201,202,203,274]. As previously mentioned, impact-generated hydrothermal systems can remain active until they run out of the heat energy generated by the initial impact, with bigger impacts generating and storing more heat energy. The existence of craters at such substantial sizes means there were likely active near-surface impact-generated hydrothermal systems that could have persisted for thousands or hundreds of thousands of years—sufficient time for biological molecules to form [15]. While these systems eventually cooled, they serve as possible “cradle” environments for the earliest forms of life due to their extreme temperature gradients, increased free chemical availability driven by high-temperature hydrothermal fluid interacting with preexisting geological formations, and abundance of water to facilitate reactions in shallow, near-surface locations.

5.2. Current Debate Favoring Impact-Generated Hydrothermal Systems over Deep-Sea Vents

In several current origin-of-life studies, the conditions required for the beginnings of life have been summarized to the following: a source of energy, a supply of biologically needed nutrients and major elements, reduced gases required for biosynthetic reactions, diverse conditions that allow for the creation of biochemical molecules and polymers, and a non-toxic low-salt aqueous solution to facilitate reactions (

Table 1. The nine requirements for the origin of life as described by Maruyama et al. (2019), adapted from Table 2 [275].

Environmental Factors	Deep-Sea Hydrothermal Vent System	Impact-Generated Hydrothermal Vent System
Energy source	Yes	Yes
Supply of nutrients	Yes a	Yes
Supply of elements	Yes	Yes
Supply and usable concentration of reducing gas	Yes	Yes
Cycling (dry/wet, phase change)	Yes b	Yes
Low salinity (Na-poor) water	No c	Yes
Non-toxic water environment	No d	Yes
Diverse conditions in environment	Yes	Yes

Conditionals: (^a) phosphate was most likely too low in quantity in Hadean oceans for deep-sea hydrothermal vent environments to use in origin of life hypotheses, but still possible [289]; (^b) supercritical CO₂ can act as an enriching agent for nitrogen gas, as well as a proponent to the water paradox [251,282,283,284]; (^c) gradients in deep-sea vent environments may create areas of low salinity water, but this is uncertain; (^d) deep-sea vent environments are considered toxic due to high H₂S and Na+/K+ ratio, both of which would exist in chemical gradients at vent environments [275].

) [275]. One current hypothesis that accounts for all the previous criteria that has gained popularity in recent years is the wet-dry cycling scenario. This hypothesis began with Darwin, who suggested that a “warm little pond” with essential ingredients could have created life [276]. Water, a compound necessary for life, became an issue for these hypotheses when it was revealed that some bonds in biopolymers could undergo hydrolysis or hydrolytic deamination when left in water without the protection of biochemical cycles and molecules that most likely were not available on early Earth—this created a “water paradox” where water was both essential to life but detrimental to biomolecules [277,278,279]. This water problem, coupled with the abundance of salts and the steep pH gradients found in deep-sea vent systems, became one of the biggest challenges to the idea that life could have originated at deep-sea hydrothermal vents [275,280,281]. There have been recent strives in the supercritical CO₂ hypotheses to contrast the water paradox, which has kept deep-sea hydrothermal vents in the running as possible origin-of-life locations. Supercritical CO₂ refers to CO₂ that exists above its critical temperature and pressure, allowing it to behave as both a liquid and a gas, and it can be found bubbling at hydrothermal vents. Not only does supercritical CO₂ offer a way around the water paradox by providing a separate liquid phase, non-liquid water environment at deep-sea vents, but it also aids in the enrichment of N₂ and the formation of organic compounds like amino acids and methanethiol [251,282,283,284]. Another way to combat the water paradox was with wet–dry/freeze–thaw interactions, which were applied to land-based hydrothermal systems like volcanic hot springs. These hypotheses posited that physical cycling, caused by periods of evaporation or freezing, could remove the excess water from the equation, allowing fragile biopolymers a chance to stabilize while also facilitating the formation of vesicles [285,286,287,288]. Prominent scientists behind this scenario cite geysers or hot springs as ideal locations for such interactions—due to their intense pH and temperature gradients, high chemical concentrations created in geochemical reactions, and the aqueous nature of these usually volcanic geological formations [285,288]. However, such conditions could also be applied to the impact-generated hydrothermal systems in shallow or cycling crater lakes.

Using the aforementioned requirements for the origin of life, impact-generated hydrothermal vent systems using wet–dry cycling constitute a very likely environment for the formation of early life. The heat energy and radial chemistry created by the impact act as an energy source for long periods of time. Gases found in the atmosphere can contribute to chemical reactions, while diverse geological conditions and low water activity due to water–vapor phase changes at high temperatures can create physical conditions like wet–dry cycling, which can contribute to the formation of biomolecules and polymers. Crater lakes can occur in low-salt, freshwater environments using rain or groundwater, though these lakes would be subject to change in response to the environment of the local area. The impact of meteorites and meteors is well-documented in Earth’s geological history as a means by which elements and minerals can be freed from rocks to participate in chemical interactions, and as previously stated, the hydrothermal system created by these impacts has enough longevity to support prebiotic formation [271,290,291,292,293,294]. Using the same requirements for the origin of life, deep-sea hydrothermal vents remain a possible location for the origin of life, but perhaps not as well as impact-generated systems. The energy source for deep-sea vent systems exists in the heat of volcanic, tectonic, or chemical activity below the vent itself, and it is the interaction between water heated by this source and the local rock that creates available nutrients and major elements (iron, nickel, and sulfur) and oxidized (for instance, supercritical CO₂) and reduced gases. Diverse conditions are generated by steep pH, temperature, and chemical gradients, which form areas where biochemicals and polymers can form, and the presence of supercritical CO₂ offers a solution to the water paradox in terms of wet–dry cycling. The water at deep-sea hydrothermal vents is assumed to be much saltier than at impact-generated crater lakes, which are often considered freshwater and non-toxic. In addition, though the water is often considered toxic for most life forms at deep-sea vents due to the high H₂S content and high Na+/K+ ratio, the presence of modern local flora and fauna that have adapted to survive and even thrive in such concentrations suggests it is possible for early life or early organisms to do the same. Deep-sea hydrothermal vent systems have been thoroughly investigated both in situ and via lab experimentation, which has kept the hypothesis of life originating at alkaline hydrothermal vents alive as time and science progressed—and now, impact-generated hydrothermal systems have recently become a plausible complementary scenario due to recent works. However, when comparing impact-generated hydrothermal systems with deep-sea hydrothermal vents under these requirements, it is the impact-generated system that seems to be a better fit.

5.3. Application Beyond Earth

Beyond the scope of life on Earth, there is also the possibility that similar processes can/could have occurred on other terrestrial bodies. For icy moons and ocean worlds like Europa and Enceladus, evidence—uncovered by probes, long-range cameras, and other spacefaring missions—of material plumes suggests underwater hydrothermal activity [295,296]. For these planets, suspected deep-sea hydrothermal systems created by seafloor heat flux and gravitational influences may act as the primary modes of heat transfer between the planet’s surfaces and inner layers [297,298,299,300,301]. In these cases, the same origin-of-life hypotheses used for Earth’s deep-sea hydrothermal vent systems can be applied, creating a way to not only predict how life may form on these planetary bodies but also catch a glimpse of what a young Earth may have looked like [302,303]. On places like Mars, where the presence of water is less apparent than on icy/ocean planetary bodies like Europa, impact-generated hydrothermal systems may play an important role in origin-of-life hypotheses. Patterns of impact melt breccias and hydrothermal deposition in Martian craters show that impact-generated hydrothermal systems on Noachian-era Mars did occur [304,305,306,307,308]. Analysis of core samples revealed that Noachian Mars could have hosted conditions conducive to prebiotic or biotic activity, which was bolstered by findings in the Allan Hills 84001 Meteorite. This meteorite, found in the Antarctic in 1984, was dated to Noachian era Mars and was found to contain magnetite (Fe₃O₄) crystal formations that had morphological, physical, and chemical similarities to particles generated by aquatic magnetotactic bacteria that exist on Earth [309,310]. Further research into impact-generated hydrothermal systems on Mars is needed and currently ongoing, as findings indicative of life originating in impact-generated hydrothermal systems would validate the application of this hypothesis elsewhere, such as Earth and other terrestrial planets.

6. Conclusions

Though knowing the exact pathway in which life was coaxed from primordial materials may stay beyond our reach, several hypotheses established in experimental evidence, geological records, and chemobiological knowledge have provided the basis for understanding a potential route from which the first biological building blocks originated. As science progresses, the possibility that deep-sea hydrothermal vents carried the seeds of prebiotic life, either through hyperthermophiles in volcanic focus-flow vents or through serpentinization and the Wood–Ljungdahl CO₂ fixation pathway at alkaline vents, becomes increasingly credible. In comparison, we are only just beginning to turn this same logic towards impact-generated hydrothermal vents. Based on the geologic record of large-scale impact-generated vent systems existing on a young Earth, and the evidence that unique DNA structures were found in the sediment of an impact-generated hydrothermal vent system, it could be proposed that biotic life could be generated by the heat energy and chemical compositions of impact-generated hydrothermal vents. The possible role of deep-sea and impact-generated hydrothermal vent systems cannot be understated for future investigations of how life could have formed—both on Earth and beyond.