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137-423: In 1996, a group of scientists found features in the likeness of microscopic fossils of bacteria in the meteorite, suggesting that these organisms also originated on Mars. The claims immediately made headlines worldwide, culminating in U.S. president Bill Clinton giving a speech about the potential discovery. These claims were controversial from the beginning, and the wider scientific community ultimately rejected

274-403: A faunal assemblage , rather than an individual species — this allows greater precision as the time span in which all of the species in the assemblage existed together is narrower than the time spans of any of the members. Further, if only one species is present in a sample, it can mean either that (1) the strata were formed in the known fossil range of that organism; or (2) that the fossil range of

411-515: A "wet" Mars; ALH84001 is the only meteorite originating when Mars may have had liquid surface water. In October 2011, it was reported that isotopic analysis indicated that the carbonates in ALH84001 were precipitated at a temperature of 18 °C (64 °F) with water and carbon dioxide from the Martian atmosphere . The carbonate carbon and oxygen isotope ratios imply deposition of the carbonates from

548-416: A basic component of the development of a potential extraterrestrial technological civilization, as it is on Earth. Fossil fuels may likely be generated and used on such worlds as well. The abundance of chlorofluorocarbons in the atmosphere can also be a clear technosignature, considering their role in ozone depletion . Light pollution may be another technosignature, as multiple lights on the night side of

685-419: A basis for defining geologic periods , and then for faunal stages and zones. Species of microfossils such as acritarchs , chitinozoans , conodonts , dinoflagellate cysts, ostracods , pollen , spores and foraminiferans are amongst the many species have been identified as index fossils that are widely used in biostratigraphy . Different fossils work well for sediments of different ages. To work well,

822-479: A boom around 1,000  million years ago , increasing in abundance, diversity, size, complexity of shape, and especially size and number of spines. Their increasingly spiny forms in the last 1 billion years may indicate an increased need for defence against predation. Acritarchs may include the remains of a wide range of quite different kinds of organisms—ranging from the egg cases of small metazoans to resting cysts of many kinds of chlorophyta (green algae). It

959-647: A calcareous or chitinous bivalve-like shell. There are about 70,000 known species, 13,000 of which are extant . Ostracods are typically about 1 mm (0.039 in) in size, though they can range from 0.2 to 30 mm (0.008 to 1.181 in), with some species such as Gigantocypris being too large to be regarded as microfossils. Conodonts ( cone tooth in Greek) are tiny, extinct jawless fish that resemble eels. For many years, they were known only from tooth-like microfossils found in isolation and now called conodont elements. The evolution of mineralized tissues has been

1096-611: A catalog of microorganisms, with the way each one reacts to sunlight. The goal is to help with the search for similar organisms in exoplanets, as the starlight reflected by planets rich in such organisms would have a specific spectrum, unlike that of starlight reflected from lifeless planets. If Earth was studied from afar with this system, it would reveal a shade of green, as a result of the abundance of plants with photosynthesis. In August 2011, NASA studied meteorites found on Antarctica, finding adenine , guanine , hypoxanthine and xanthine . Adenine and guanine are components of DNA, and

1233-611: A gradually evaporating subsurface water body, probably a shallow aquifer meters or tens of meters below the surface. In April 2020, researchers reported discovering nitrogen-bearing organics in Allan Hills 84001. A later study in January 2022 concluded that ALH84001 did not contain Martian life; the discovered organic molecules were found to be associated with abiotic processes (i.e., "serpentinization and carbonation reactions that occurred during

1370-499: A length that can range from tens of micrometres to 100 micrometres, and spindle-like microfossils can be as long as 50 micrometres. Siliceous ooze is a type of biogenic pelagic sediment located on the deep ocean floor . Siliceous oozes are the least common of the deep sea sediments, and make up approximately 15% of the ocean floor. Oozes are defined as sediments which contain at least 30% skeletal remains of pelagic microorganisms. Siliceous oozes are largely composed of

1507-478: A living entity must have the capacity to replicate itself, the capacity to avoid damage/decay, and the capacity to acquire and process resources in support of the first two capacities. Life on Earth started with an RNA world and later evolved to its current form, where some of the RNA tasks were transferred to DNA and proteins . Extraterrestrial life may still be stuck using RNA, or evolve into other configurations. It

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1644-524: A math equation, none of its values were known at the time. Although some values may eventually be measured, others are based on social sciences and are not knowable by their very nature. This does not allow one to make noteworthy conclusions from the equation. Based on observations from the Hubble Space Telescope , there are nearly 2 trillion galaxies in the observable universe. It is estimated that at least ten per cent of all Sun-like stars have

1781-466: A mixed aromatic - aliphatic structure") that could be created naturally, and rapidly, by stars . It is still unclear if those compounds played a role in the creation of life on Earth, but Sun Kwok, of the University of Hong Kong, thinks so. "If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life." In August 2012, and in

1918-436: A narrower search for extraterrestrial intelligent life . Depending on the category of search, methods range from analysis of telescope and specimen data to radios used to detect and transmit communications. The concept of extraterrestrial life, and particularly extraterrestrial intelligence, has had a major cultural impact, especially extraterrestrials in fiction . Science fiction has communicated scientific ideas, imagined

2055-627: A planetary system formed by a star at the center, the Sun , and the objects that orbit it: other planets, moons, asteroids, and comets. The sun is part of the Milky Way , a galaxy . The Milky Way is part of the Local Group , a galaxy group that is in turn part of the Laniakea Supercluster . The universe is composed of all similar structures in existence. The immense distances between celestial objects

2192-462: A potential explanation to the Fermi paradox. If extraterrestrial life exists, it could range from simple microorganisms and multicellular organisms similar to animals or plants, to complex alien intelligences akin to humans . When scientists talk about extraterrestrial life, they consider all those types. Although it is possible that extraterrestrial life may have other configurations, scientists use

2329-1024: A puzzle for more than a century. It has been hypothesized that the first mechanism of chordate tissue mineralization began either in the oral skeleton of conodont or the dermal skeleton of early agnathans . Conodont elements are made of a phosphatic mineral, hydroxylapatite . The element array constituted a feeding apparatus that is radically different from the jaws of modern animals. They are now termed "conodont elements" to avoid confusion. The three forms of teeth (i.e., coniform cones, ramiform bars, and pectiniform platforms) probably performed different functions. For many years, conodonts were known only from enigmatic tooth-like microfossils (200 micrometres to 5 millimetres in length) which occur commonly, but not always in isolation, and were not associated with any other fossil. Conodonts are globally widespread in sediments.Their many forms are considered index fossils , fossils used to define and identify geological periods and date strata. Conodonts elements can be used to estimate

2466-507: A range of possibilities, and influenced public interest in and perspectives on extraterrestrial life. One shared space is the debate over the wisdom of attempting communication with extraterrestrial intelligence. Some encourage aggressive methods to try to contact intelligent extraterrestrial life. Others – citing the tendency of technologically advanced human societies to enslave or destroy less advanced societies – argue it may be dangerous to actively draw attention to Earth. Initially, after

2603-566: A reasonable scientific explanation for any gathered data. In 1961, astronomer and astrophysicist Frank Drake devised the Drake equation as a way to stimulate scientific dialogue at a meeting on the search for extraterrestrial intelligence (SETI). The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilisations in the Milky Way galaxy . The Drake equation is: where: and Drake's proposed estimates are as follows, but numbers on

2740-413: A reservoir of paleoclimate information, and are also commonly used by biostratigraphers to assist in the correlation of rock units. Microfossils are found in rocks and sediments as the microscopic remains of what were once life forms such as plants, animals, fungus, protists, bacteria and archaea. Terrestrial microfossils include pollen and spores . Marine microfossils found in marine sediments are

2877-443: A result has been conceived, even if hypothetical. Evolution requires life to be divided into individual organisms, and no alternative organisation has been satisfactorily proposed either. At the basic level, membranes define the limit of a cell, between it and its environment, while remaining partially open to exchange energy and resources with it. The evolution from simple cells to eukaryotes, and from them to multicellular lifeforms,

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3014-468: A scale. Life on Earth is quite ubiquitous across the planet and has adapted over time to almost all the available environments in it, extremophiles and the deep biosphere thrive at even the most hostile ones. As a result, it is inferred that life in other celestial bodies may be equally adaptive. However, the origin of life is unrelated to its ease of adaptation, and may have stricter requirements. A celestial body may not have any life on it, even if it

3151-470: A series of factors that range from the location in the galaxy and the configuration of the Solar System to local characteristics of the planet, and that it is unlikely that all such requirements are simultaneously met by another planet. The proponents of this hypothesis consider that very little evidence suggests the existence of extraterrestrial life, and that at this point it is just a desired result and not

3288-853: A sexual cycle but are resistant structures used for survival under unfavourable conditions. Chitinozoa are a taxon of flask -shaped, organic walled marine microfossils produced by an as yet unknown organism. Common from the Ordovician to Devonian periods (i.e. the mid-Paleozoic), the millimetre-scale organisms are abundant in almost all types of marine sediment across the globe. This wide distribution, and their rapid pace of evolution, makes them valuable biostratigraphic markers. Their bizarre form has made classification and ecological reconstruction difficult. Since their discovery in 1931, suggestions of protist , plant , and fungal affinities have all been entertained. The organisms have been better understood as improvements in microscopy facilitated

3425-563: A solvent other than water. The low temperatures required would add an extra problem, the difficulty to kickstart a process of abiogenesis to create life in the first place. Norman Horowitz , head of the Jet Propulsion Laboratory bioscience section for the Mariner and Viking missions from 1965 to 1976 considered that the great versatility of the carbon atom makes it the element most likely to provide solutions, even exotic solutions, to

3562-440: A star's light and heat, and so its lifeforms would not grow beyond a certain complexity. There is also research in assessing the capacity of life for developing intelligence. It has been suggested that this capacity arises with the number of potential niches a planet contains, and that the complexity of life itself is reflected in the information density of planetary environments, which in turn can be computed from its niches. It

3699-457: A starting point, and "will not explain many of the features described by us in ALH84001." According to McKay, a plausible inorganic model "must explain simultaneously all of the properties that we and others have suggested as possible biogenic properties of this meteorite." The rest of the scientific community disagreed with McKay. In January 2010, a team of scientists at Johnson Space Center , including McKay, argued that since their original paper

3836-651: A system of planets, i.e. there are 6.25 × 10 stars with planets orbiting them in the observable universe. Even if it is assumed that only one out of a billion of these stars has planets supporting life, there would be some 6.25 billion life-supporting planetary systems in the observable universe. A 2013 study based on results from the Kepler spacecraft estimated that the Milky Way contains at least as many planets as it does stars, resulting in 100–400 billion exoplanets. The apparent contradiction between high estimates of

3973-418: A tool for primitive life detection." Interpretation of morphology is notoriously subjective, and its use alone has led to numerous errors of interpretation. Features of ALH84001 that have been interpreted as suggesting the presence of microfossils include: The 2001 mystery-thriller novel Deception Point by Dan Brown , about a discovered meteorite that seems to prove the existence of extraterrestrial life,

4110-461: A world first, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde , in a distant star system. The molecule was found around the protostellar binary IRAS 16293-2422 , which is located 400 light years from Earth. Glycolaldehyde is needed to form ribonucleic acid , or RNA, which is similar in function to DNA. This finding suggests that complex organic molecules may form in stellar systems prior to

4247-416: Is a bioessential element and is efficiently recycled in the marine environment through the silica cycle . Distance from land masses, water depth and ocean fertility are all factors that affect the opal silica content in seawater and the presence of siliceous oozes. Phytoliths (Greek for plant stones ) are rigid, microscopic structures made of silica , found in some plant tissues and persisting after

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4384-585: Is a difficulty for the study of extraterrestrial life. So far, humans have only set foot on the Moon and sent robotic probes to other planets and moons in the Solar System. Although probes can withstand conditions that may be lethal to humans, the distances cause time delays: the New Horizons took nine years after launch to reach Pluto . No probe has ever reached extrasolar planetary systems. The Voyager 2 has left

4521-501: Is a form of calcium carbonate derived from planktonic organisms that accumulates on the sea floor . This can only occur if the ocean is shallower than the carbonate compensation depth . Below this depth, calcium carbonate begins to dissolve in the ocean, and only non-calcareous sediments are stable, such as siliceous ooze or pelagic red clay . Ostracods are widespread crustaceans, generally small, sometimes known as seed shrimps . They are flattened from side to side and protected with

4658-501: Is also conceivable that there are forms of life whose solvent is a liquid hydrocarbon , such as methane , ethane or propane . Another unknown aspect of potential extraterrestrial life would be the chemical elements that would compose it. Life on Earth is largely composed of carbon, but there could be other hypothetical types of biochemistry . A replacement for carbon would need to be able to create complex molecules, store information required for evolution, and be freely available in

4795-416: Is being carried out both directly and indirectly. As of September 2017 , 3,667 exoplanets in 2,747 systems have been identified , and other planets and moons in the Solar System hold the potential for hosting primitive life such as microorganisms . As of 8 February 2021, an updated status of studies considering the possible detection of lifeforms on Venus (via phosphine ) and Mars (via methane )

4932-403: Is common knowledge that the conditions on other planets in the solar system, in addition to the many galaxies outside of the Milky Way galaxy , are very harsh and seem to be too extreme to harbor any life. The environmental conditions on these planets can have intense UV radiation paired with extreme temperatures, lack of water, and much more that can lead to conditions that don't seem to favor

5069-485: Is complex and defined by several factors. Being in the habitable zone is not enough for a planet to be habitable, not even to actually have such liquid water. Venus is located in the habitable zone of the Solar System but does not have liquid water because of the conditions of its atmosphere. Jovian planets or gas giants are not considered habitable even if they orbit close enough to their stars as hot Jupiters , due to crushing atmospheric pressures. The actual distances for

5206-479: Is divided between classifying them as polychaetes and regarding it as unsafe to classify them as members of any broader grouping. In 2020, a new study showed the presence of Nephrozoan type guts , the oldest on record, supporting the bilaterian interpretation. Cloudinids are important in the history of animal evolution for two reasons. They are among the earliest and most abundant of the small shelly fossils with mineralized skeletons , and therefore feature in

5343-509: Is embodied in the Copernican principle , which states that Earth does not occupy a unique position in the Universe, and the mediocrity principle , which states that there is nothing special about life on Earth. Other authors consider instead that life in the cosmos, or at least multicellular life, may be actually rare. The Rare Earth hypothesis maintains that life on Earth is possible because of

5480-512: Is entirely artificial, it is not without merit, as the form taxa show traits similar to those of genuine taxa — for example the ' explosion ' in the Cambrian and the mass extinction at the end of the Permian . Acritarch diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion . Precambrian marine diversity was dominated by acritarchs. They underwent

5617-477: Is first found in the fossil record in the late Devonian period, but at that time it is indistinguishable from spores. It increases in abundance until the present day. A spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants , algae , fungi and protozoa . Bacterial spores are not part of

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5754-488: Is formed from, or contains a high proportion of, calcium carbonate in the form of calcite or aragonite . Calcareous sediments ( limestone ) are usually deposited in shallow water near land, since the carbonate is precipitated by marine organisms that need land-derived nutrients. Generally speaking, the farther from land sediments fall, the less calcareous they are. Some areas can have interbedded calcareous sediments due to storms, or changes in ocean currents. Calcareous ooze

5891-439: Is likely devoid of life. However, Venus is still of interest to astrobiologists, as it is a terrestrial planet that was likely similar to Earth in its early stages and developed in a different way. There is a greenhouse effect , the surface is the hottest in the Solar System, sulfuric acid clouds, all surface liquid water is lost, and it has a thick carbon-dioxide atmosphere with huge pressure. Comparing both helps to understand

6028-476: Is likely that most acritarch species from the Paleozoic represent various stages of the life cycle of algae that were ancestral to the dinoflagellates . The nature of the organisms associated with older acritarchs is generally not well understood, though many are probably related to unicellular marine algae . In theory, when the biological source (taxon) of an acritarch does become known, that particular microfossil

6165-444: Is no guarantee that it is transmitting radio communications in the direction of Earth. The length of time required for a signal to travel across space means that a potential answer may arrive decades or centuries after the initial message. The atmosphere of Earth is rich in nitrogen dioxide as a result of air pollution , which can be detectable. The natural abundance of carbon, which is also relatively reactive, makes it likely to be

6302-657: Is not guaranteed. The Cambrian explosion took place thousands of millions of years after the origin of life, and its causes are not fully known yet. On the other hand, the jump to multicellularity took place several times, which suggests that it could be a case of convergent evolution , and so likely to take place on other planets as well. Palaeontologist Simon Conway Morris considers that convergent evolution would lead to kingdoms similar to our plants and animals, and that many features are likely to develop in alien animals as well, such as bilateral symmetry , limbs , digestive systems and heads with sensory organs . Scientists from

6439-506: Is produced in huge quantities. There is an extensive fossil record of pollen grains, often disassociated from their parent plant. The discipline of palynology is devoted to the study of pollen, which can be used both for biostratigraphy and to gain information about the abundance and variety of plants alive — which can itself yield important information about paleoclimates. Also, pollen analysis has been widely used for reconstructing past changes in vegetation and their associated drivers. Pollen

6576-637: Is removed from the acritarchs and classified with its proper group. Acritarchs were most likely eukaryotes . While archaea, bacteria and cyanobacteria ( prokaryotes ) usually produce simple fossils of a very small size, eukaryotic unicellular fossils are usually larger and more complex, with external morphological projections and ornamentation such as spines and hairs that only eukaryotes can produce; as most acritarchs have external projections (e.g., hair, spines, thick cell membranes, etc.), they are predominantly eukaryotes, although simple eukaryote acritarchs also exist. Acritarchs are found in sedimentary rocks from

6713-466: Is that the chemical elements that make up life, such as carbon and water, are ubiquitous in the universe. The third is that the physical laws are universal, which means that the forces that would facilitate or prevent the existence of life would be the same ones as on Earth. According to this argument, made by scientists such as Carl Sagan and Stephen Hawking , it would be improbable for life not to exist somewhere else other than Earth. This argument

6850-421: Is that there are natural sources of such signals as well, such as gamma-ray bursts and supernovae, and the difference between a natural signal and an artificial one would be in its specific patterns. Astronomers intend to use artificial intelligence for this, as it can manage large amounts of data and is devoid of biases and preconceptions. Besides, even if there is an advanced extraterrestrial civilization, there

6987-447: Is unclear if our biochemistry is the most efficient one that could be generated, or which elements would follow a similar pattern. However, it is likely that, even if cells had a different composition to those from Earth, they would still have a cell membrane . Life on Earth jumped from prokaryotes to eukaryotes and from unicellular organisms to multicellular organisms through evolution . So far no alternative process to achieve such

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7124-534: Is unknown, Cloudinids comprise two genera: Cloudina itself is mineralized, whereas Conotubus is at best weakly mineralized, whilst sharing the same "funnel-in-funnel" construction. Cloudinids had a wide geographic range, reflected in the present distribution of localities in which their fossils are found, and are an abundant component of some deposits. Cloudina is usually found in association with microbial stromatolites , which are limited to shallow water, and it has been suggested that cloudinids lived embedded in

7261-474: The Big Bang the universe was too hot to allow life. 15 million years later , it cooled to temperate levels, but the elements that make up living things did not exist yet. The only freely available elements at that point were hydrogen and helium . Carbon and oxygen (and later, water ) would not appear until 50 million years later, created through stellar fusion. At that point, the difficulty for life to appear

7398-606: The Precambrian to the Holocene . They are most common in deposits of marine environments, but also occur in brackish water, fresh water and terrestrial sedimentary deposits. While every kingdom of life is represented in the microfossil record, the most abundant forms are protist skeletons or microbial cysts from the Chrysophyta , Pyrrhophyta , Sarcodina , acritarchs and chitinozoans , together with pollen and spores from

7535-750: The Rio Tinto river in Spain. NASA officials soon distanced NASA from the scientists' claims, and Stoker herself backed off from her initial assertions. In November 2011, NASA launched the Mars Science Laboratory that landed the Curiosity rover on Mars. It is designed to assess the past and present habitability on Mars using a variety of scientific instruments. The rover landed on Mars at Gale Crater in August 2012. A group of scientists at Cornell University started

7672-447: The Sun , from an exterior perspective, due to a layer of "fiery brightness" in the outer layer of the atmosphere. He theorised all extraterrestrial bodies could be inhabited by men, plants, and animals, including the Sun. Descartes wrote that there was no means to prove the stars were not inhabited by "intelligent creatures", but their existence was a matter of speculation. When considering

7809-603: The University of Hawaii at Manoa , presented an analysis of the origin of ALH84001 using data from the Mars Global Surveyor and 2001 Mars Odyssey spacecraft orbiting Mars. According to the analysis, Eos Chasma in the Valles Marineris canyon appears to be the source of the meteorite. The analysis was not conclusive, partly because it was limited to areas of Mars not obscured by dust. The theory holds that ALH84001

7946-403: The atmospheric composition and ecosystems hosted by extraterrestrial bodies, extraterrestrial life can seem more speculation than reality, due to the harsh conditions and disparate chemical composition of the atmospheres, when compared to the life-abundant Earth. However, there are many extreme and chemically harsh ecosystems on Earth that do support forms of life and are often hypothesized to be

8083-489: The formation of the Earth 4.54 billion years ago. Nonetheless, life may have started even earlier, at nearly 4.5 billion years ago, as claimed by some researchers. Index fossils , also known as guide fossils, indicator fossils or dating fossils, are the fossilized remains or traces of particular plants or animals that are characteristic of a particular span of geologic time or environment, and can be used to identify and date

8220-459: The hydrothermal vents found on the ocean floor are known to support many chemosynthetic processes which allow organisms to utilize energy through reduced chemical compounds that fix carbon. In return, these reactions will allow for organisms to live in relatively low oxygenated environments while maintaining enough energy to support themselves. The early Earth environment was reducing and therefore, these carbon fixing compounds were necessary for

8357-416: The microbial mats , growing new cones to avoid being buried by silt. However no specimens have been found embedded in mats, and their mode of life is still an unresolved question. The classification of the cloudinids has proved difficult: they were initially regarded as polychaete worms, and then as coral-like cnidarians on the basis of what look like buds on some specimens. Current scientific opinion

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8494-553: The vascular plants . A microfossil is a descriptive term applied to fossilized plants and animals whose size is just at or below the level at which the fossil can be analyzed by the naked eye. A commonly applied cutoff point between "micro" and "macro" fossils is 1 mm. Microfossils may either be complete (or near-complete) organisms in themselves (such as the marine plankters foraminifera and coccolithophores ) or component parts (such as small teeth or spores ) of larger animals or plants. Microfossils are of critical importance as

8631-772: The "father of modern oceanography", who proposed the term radiolarian ooze for the silica deposits of radiolarian shells brought to the surface during the Challenger expedition . A biogenic ooze is a pelagic sediment containing at least 30 per cent from the skeletal remains of marine organisms. The study of microfossils is called micropaleontology . In micropaleontology, what would otherwise be distinct categories are grouped together based solely on their size, including microscopic organisms and minute parts of larger organisms. Numerous sediments have microfossils, which serve as significant biostratigraphic , paleoenvironmental , and paleoceanographic markers. Their widespread presence around

8768-545: The 1984–85 season, by Roberta Score, Lab Manager of the Antarctic Meteorite Laboratory at the Johnson Space Center . ALH84001 is thought to be one of the oldest Martian meteorites, proposed to have crystallized from molten rock 4.091 billion years ago. Chemical analysis suggests that it originated on Mars when there was liquid water on the planet's surface. In September 2005, Vicky Hamilton, of

8905-648: The 350 described freshwater dinoflagellate species, and in about 10% of the known marine species. Dinocysts have a long geological record with geochemical markers suggest a presence that goes back to the Early Cambrian . Spicules are structural elements found in most sponges . They provide structural support and deter predators . The meshing of many spicules serves as the sponge's skeleton , providing structural support and defense against predators. Smaller, microscopic spicules can become microfossils, and are referred to as microscleres . Larger spicules visible to

9042-471: The Solar System at a speed of 50,000 kilometers per hour, if it headed towards the Alpha Centauri system, the closest one to Earth at 4.4 light years, it would reach it in 100,000 years. Under current technology such systems can only be studied by telescopes, which have limitations. It is estimated that dark matter has a larger amount of combined matter than stars and gas clouds, but as it plays no role on

9179-616: The University of Oxford analysed it from the perspective of evolutionary theory and wrote in a study in the International Journal of Astrobiology that aliens may be similar to humans. The planetary context would also have an influence: a planet with higher gravity would have smaller animals, and other types of stars can lead to non-green photosynthesizers . The amount of energy available would also affect biodiversity , as an ecosystem sustained by black smokers or hydrothermal vents would have less energy available than those sustained by

9316-729: The aqueous alteration of basalt rock by hydrothermal fluids") produced on the very early Mars 4 billion years ago instead. On August 6, 1996, a team of researchers led by NASA scientists including lead author David S. McKay announced that the meteorite may contain trace evidence of life from Mars . This was published as an article in Science a few days later. Under a scanning electron microscope , structures were visible that some scientists interpreted as fossils of bacteria -like lifeforms. The structures found on ALH84001 are 20–100 nanometres in diameter, similar in size to theoretical nanobacteria , but smaller than any cellular life known at

9453-410: The area surrounding these hydrothermal vents which can suggest that some form of life can be supported even in the harshest of environments like the other planets in the solar system. The aspects of these harsh environments that make them ideal for the origin of life on Earth, as well as the possibility of creation of life on other planets, is the chemical reactions forming spontaneously. For example,

9590-568: The atom speeds, either too fast or too slow, make it difficult for specific ones to meet and start chemical reactions. A liquid medium also allows the transport of nutrients and substances required for metabolism. Sufficient quantities of carbon and other elements, along with water, might enable the formation of living organisms on terrestrial planets with a chemical make-up and temperature range similar to that of Earth. Life based on ammonia rather than water has been suggested as an alternative, though this solvent appears less suitable than water. It

9727-500: The boundless immensity of space" in The City of God . Pre-modern writers typically assumed extraterrestrial "worlds" are inhabited by living beings. William Vorilong , in the 15th century, acknowledged the possibility Jesus could have visited extraterrestrial worlds to redeem their inhabitants. Nicholas of Cusa wrote in 1440 that Earth is "a brilliant star" like other celestial objects visible in space; which would appear similar to

9864-402: The cold temperatures would make such chemistry take place at a very slow pace. Water is rock-solid on the surface, but Titan does have a subsurface water ocean like several other moons. However, it is of such a great depth that it would be very difficult to access it for study. The science that searches and studies life in the universe, both on Earth and elsewhere, is called astrobiology . With

10001-418: The containing rocks. To be practical, index fossils must have a limited vertical time range, wide geographic distribution, and rapid evolutionary trends. Rock formations separated by great distances but containing the same index fossil species are thereby known to have both formed during the limited time that the species lived. Index fossils were originally used to define and identify geologic units, then became

10138-469: The creation or maintenance of extraterrestrial life. However, there has been much historical evidence that some of the earliest and most basic forms of life on Earth originated in some extreme environments that seem unlikely to have harbored life at least at one point in Earth's history. Fossil evidence as well as many historical theories backed up by years of research and studies have marked environments like hydrothermal vents or acidic hot springs as some of

10275-554: The debate about why such skeletons first appeared in the Late Ediacaran. The most widely supported answer is that their shells are a defense against predators, as some Cloudina specimens from China bear the marks of multiple attacks, which suggests they survived at least a few of them. The holes made by predators are approximately proportional to the size of the Cloudina specimens, and Sinotubulites fossils, which are often found in

10412-439: The decay of the plant. These plants take up silica from the soil, whereupon it is deposited within different intracellular and extracellular structures of the plant. Phytoliths come in varying shapes and sizes. The term "phytolith" is sometimes used to refer to all mineral secretions by plants, but more commonly refers to siliceous plant remains. The term calcareous can be applied to a fossil, sediment, or sedimentary rock which

10549-473: The discovery was announced, many immediately conjectured that the fossils were the first true evidence of extraterrestrial life—making headlines around the world, and even prompting President of the United States Bill Clinton to make a formal televised announcement to mark the event. McKay argued that likely microbial terrestrial contamination found in other Martian meteorites does not resemble

10686-413: The end, the star blows much of its content back into the stellar medium, where it would join clouds that would eventually become new generations of stars and planets. Many of those materials are the raw components of life on Earth. As this process takes place in all the universe, said materials are ubiquitous in the cosmos and not a rarity from the Solar System. Earth is a planet in the Solar System ,

10823-416: The existence of sapient life elsewhere in the universe. The science of extraterrestrial life is known as astrobiology . Speculation about the possibility of inhabited worlds beyond Earth dates back to antiquity. Early Christian writers discussed the idea of a "plurality of worlds" as proposed by earlier thinkers such as Democritus ; Augustine references Epicurus 's idea of innumerable worlds "throughout

10960-403: The first hominids appeared 60 million years ago. Life on other planets may have started, evolved, given birth to extraterrestrial intelligences, and perhaps even faced a planetary extinction event millions or even billions of years ago. The brief times of existence of Earth's species, when considered from a cosmic perspective, may suggest that extraterrestrial life may be equally fleeting under such

11097-462: The first places that life could have originated on Earth. These environments can be considered extreme when compared to the typical ecosystems that the majority of life on Earth now inhabit, as hydrothermal vents are scorching hot due to the magma escaping from the Earth's mantle and meeting the much colder oceanic water. Even in today's world, there can be a diverse population of bacteria found inhabiting

11234-410: The formation of planets, eventually arriving on young planets early in their formation. In December 2023, astronomers reported the first time discovery, in the plumes of Enceladus , moon of the planet Saturn , of hydrogen cyanide , a possible chemical essential for life as we know it, as well as other organic molecules , some of which are yet to be better identified and understood. According to

11371-457: The fossil record by these dinocysts, typically 15 to 100 micrometres in diameter, which accumulate in sediments as microfossils. Organic-walled dinocysts have resistant cell walls made out of dinosporin . There are also calcareous dinoflagellate cysts and siliceous dinoflagellate cysts . Dinocysts are produced by a proportion of dinoflagellates as a dormant , zygotic stage of their lifecycle. These dinocyst stages are known to occur in 84 of

11508-405: The fossils used must be widespread geographically, so that they can be found in many different places. They must also be short lived as a species, so that the period of time during which they could be incorporated in the sediment is relatively narrow. The longer lived the species, the poorer the stratigraphic precision, so fossils that evolve rapidly. Often biostratigraphic correlations are based on

11645-476: The genus Nummulites . In 2017, fossilized microorganisms , or microfossils, were discovered in hydrothermal vent precipitates in the Nuvvuagittuq Belt of Quebec, Canada that may be as old as 4.28 billion years old, the oldest record of life on Earth , suggesting "an almost instantaneous emergence of life" (in a geological time-scale), after ocean formation 4.41 billion years ago , and not long after

11782-451: The giant planets themselves are highly unlikely to have life, there is much hope to find it on moons orbiting these planets. Europa , from the Jovian system, has a subsurface ocean below a thick layer of ice. Ganymede and Callisto also have subsurface oceans, but life is less likely in them because water is sandwiched between layers of solid ice. Europa would have contact between the ocean and

11919-404: The habitable zones vary according to the type of star, and even the solar activity of each specific star influences the local habitability. The type of star also defines the time the habitable zone will exist, as its presence and limits will change along with the stars stellar evolution. The Big Bang took place 14 billion years ago, the Solar System was formed 4 and a half billion years ago, and

12056-571: The hierarchy of lifeforms from Earth for simplicity, as it is the only one known to exist. The first basic requirement for life is an environment with non-equilibrium thermodynamics , which means that the thermodynamic equilibrium must be broken by a source of energy. The traditional sources of energy in the cosmos are the stars, such as for life on Earth, which depends on the energy of the sun. However, there are other alternative energy sources, such as volcanoes , plate tectonics , and hydrothermal vents . There are ecosystems on Earth in deep areas of

12193-492: The hypothesis once all the unusual features in the meteorite had been explained without requiring life to be present. Despite there being no convincing evidence of Martian life, the initial paper and the enormous scientific and public attention caused by it are considered turning points in the history of the developing science of astrobiology . ALH 84001 was found on the Allan Hills Far Western Icefield during

12330-423: The late Ediacaran period about 550 million years ago, and became extinct at the base of the Cambrian . They formed small millimetre size conical fossils consisting of calcareous cones nested within one another; the appearance of the organism itself remains unknown. The name Cloudina honors Preston Cloud . Fossils consist of a series of stacked vase-like calcite tubes, whose original mineral composition

12467-464: The main evidence of polychaetes in the geological past, and the only way to restore the evolution of this important group of animals. Small size of scolecodonts, usually less than 1 mm, puts them into a microfossil category. They are common by-product of conodont, chitinozoan and acritarch samples, but sometimes they occur in the sediments where other fossils are very rare or absent. The cloudinids were an early metazoan family that lived in

12604-543: The medium. To create DNA , RNA , or a close analog, such an element should be able to bind its atoms with many others, creating complex and stable molecules. It should be able to create at least three covalent bonds: two for making long strings and at least a third to add new links and allow for diverse information. Only nine elements meet this requirement: boron , nitrogen , phosphorus , arsenic , antimony (three bonds), carbon , silicon , germanium and tin (four bonds). As for abundance, carbon, nitrogen, and silicon are

12741-449: The microscopic shapes in ALH84001. In particular, the shapes within ALH84001 look intergrown or embedded in the indigenous material, while likely contamination does not. While it has not yet conclusively been shown how the features in the meteorite were formed, similar features have been recreated in the lab without biological inputs by a team led by D.C. Golden. McKay says these results were obtained using unrealistically pure raw materials as

12878-566: The most abundant ones in the universe, far more than the others. On Earth's crust the most abundant of those elements is silicon, in the Hydrosphere it is carbon and in the atmosphere, it is carbon and nitrogen. Silicon, however, has disadvantages over carbon. The molecules formed with silicon atoms are less stable, and more vulnerable to acids, oxygen, and light. An ecosystem of silicon-based lifeforms would require very low temperatures, high atmospheric pressure , an atmosphere devoid of oxygen, and

13015-406: The most common microfossils. Everywhere in the oceans, microscopic protist organisms multiply prolifically, and many grow tiny skeletons which readily fossilise. These include foraminifera , dinoflagellates and radiolarians . Palaeontologists (geologists who study fossils) are interested in these microfossils because they can use them to determine how environments and climates have changed in

13152-495: The naked eye are called megascleres . Spicule can be calcareous , siliceous , or composed of spongin . They are found in a range of symmetry types. Sediments at the bottom of the ocean have two main origins, terrigenous and biogenous. Terrigenous sediments account for about 45% of the total marine sediment, and originate in the erosion of rocks on land, transported by rivers and land runoff, windborne dust, volcanoes, or grinding by glaciers. Biogenous sediments account for

13289-497: The native planet that may not be caused by natural causes. There are three main types of techno-signatures considered: interstellar communications , effects on the atmosphere, and planetary-sized structures such as Dyson spheres . Organizations such as the SETI Institute search the cosmos for potential forms of communication. They started with radio waves , and now search for laser pulses as well. The challenge for this search

13426-418: The ocean that do not receive sunlight, and take energy from black smokers instead. Magnetic fields and radioactivity have also been proposed as sources of energy, although they would be less efficient ones. Life on Earth requires water in a liquid state as a solvent in which biochemical reactions take place. It is highly unlikely that an abiogenesis process can start within a gaseous or solid medium:

13563-415: The order Eunicida - a diverse and abundant group of worms which has been inhabiting different marine environments in the past 500 million years. Composed of highly resistant organic substance, the scolecodonts are frequently found as fossils from the rocks as old as the late Cambrian . Since the worms themselves were soft-bodied and hence extremely rarely preserved in the fossil record, their jaws constitute

13700-527: The organic material kerogen as the cell breaks down after death. Kerogen is insoluble in mineral acids , bases , and organic solvents . Over time, it is mineralised into graphite or graphite-like carbon , or degrades into oil and gas hydrocarbons. There are three main types of cell morphologies. Though there is no established range of sizes for each type, spheroid microfossils can be as small as about 8  micrometres , filamentous microfossils have diameters typically less than 5 micrometres and have

13837-400: The organism was incompletely known, and the strata extend the known fossil range. If the fossil is easy to preserve and easy to identify, more precise time estimating of the stratigraphic layers is possible. Microfossils can be classified by their composition as: (a) siliceous , as in diatoms and radiolaria , (b) calcareous , as in coccoliths and foraminifera , (c) phosphatic , as in

13974-568: The origin of extraterrestrial life. The Solar System has a wide variety of planets, dwarf planets, and moons, and each one is studied for its potential to host life. Each one has its own specific conditions that may benefit or harm life. So far, the only lifeforms found are those from Earth. No extraterrestrial intelligence other than humans exists or has ever existed within the Solar System. Astrobiologist Mary Voytek points out that it would be unlikely to find large ecosystems, as they would have already been detected by now. The inner Solar System

14111-451: The origin of life on Earth. Hydrothermal vents , acidic hot springs, and volcanic lakes are examples of life forming under difficult circumstances, provide parallels to the extreme environments on other planets and support the possibility of extraterrestrial life. Since the mid-20th century, active research has taken place to look for signs of extraterrestrial life, encompassing searches for current and historic extraterrestrial life, and

14248-410: The other 55% of the total sediment, and originate in the skeletal remains of marine protists (single-celled plankton and benthos microorganisms). Much smaller amounts of precipitated minerals and meteoric dust can also be present. Ooze , in the context of a marine sediment, does not refer to the consistency of the sediment but to its biological origin. The term ooze was originally used by John Murray ,

14385-468: The others are used in other biological processes. The studies ruled out pollution of the meteorites on Earth, as those components would not be freely available the way they were found in the samples. This discovery suggests that several organic molecules that serve as building blocks of life may be generated within asteroids and comets. In October 2011, scientists reported that cosmic dust contains complex organic compounds ("amorphous organic solids with

14522-490: The past, and where oil and gas can be found today. Some microfossils are formed by colonial organisms such as Bryozoa (especially the Cheilostomata ), which have relatively large colonies but are classified by fine skeletal details of the small individuals of the colony. As another example, many fossil genera of Foraminifera , which are protists are known from shells (called tests ) that were as big as coins, such as

14659-547: The planetary core ceased to generate a magnetic field, solar winds removed the atmosphere and the planet became vulnerable to solar radiation. Ancient life-forms may still have left fossilised remains, and microbes may still survive deep underground. As mentioned, the gas giants and ice giants are unlikely to contain life. The most distant solar system bodies, found in the Kuiper Belt and outwards, are locked in permanent deep-freeze, but cannot be ruled out completely. Although

14796-511: The precise differences that lead to beneficial or harmful conditions for life. And despite the conditions against life on Venus , there are suspicions that microbial life-forms may still survive in high-altitude clouds. Mars is a cold and almost airless desert, inhospitable to life. However, recent studies revealed that water on Mars used to be quite abundant, forming rivers, lakes, and perhaps even oceans. Mars may have been habitable back then, and life on Mars may have been possible. But when

14933-896: The present back into the Archean . They are typically isolated from siliciclastic sedimentary rocks using hydrofluoric acid but are occasionally extracted from carbonate-rich rocks. They are excellent candidates for index fossils used for dating rock formations in the Paleozoic Era and when other fossils are not available. Because most acritarchs are thought to be marine (pre-Triassic), they are also useful for palaeoenvironmental interpretation. The Archean and earliest Proterozoic microfossils termed "acritarchs" may actually be prokaryotes. The earliest eukaryotic acritarchs known (as of 2020) are from between 1950 and 2150 million years ago. Recent application of atomic force microscopy , confocal microscopy , Raman spectroscopy , and other analytic techniques to

15070-452: The probability of the existence of extraterrestrial civilisations and the lack of evidence for such civilisations is known as the Fermi paradox . Dennis W. Sciama claimed that life's existence in the universe depends on various fundamental constants. Zhi-Wei Wang and Samuel L. Braunstein suggest that a random universe capable of supporting life is likely to be just barely able to do so, giving

15207-476: The probe to study ocean water. Still, Cassini detected complex organic molecules, salts, evidence of hydrothermal activity, hydrogen, and methane. Titan is the only celestial body in the Solar System besides Earth that has liquid bodies on the surface. It has rivers, lakes, and rain of hydrocarbons, methane, and ethane, and even a cycle similar to Earth's water cycle . This special context encourages speculations about lifeforms with different biochemistry, but

15344-438: The problems of survival of life on other planets. However, he also considered that the conditions found on Mars were incompatible with carbon based life. Even if extraterrestrial life is based on carbon and uses water as a solvent, like Earth life, it may still have a radically different biochemistry . Life is generally considered to be a product of natural selection . It has been proposed that to undergo natural selection

15481-433: The researchers, "these [newly discovered] compounds could potentially support extant microbial communities or drive complex organic synthesis leading to the origin of life ." Although most searches are focused on the biology of extraterrestrial life, an extraterrestrial intelligence capable enough to develop a civilization may be detectable by other means as well. Technology may generate technosignatures , effects on

15618-426: The right conditions on Earth were met, life started by a chemical process known as abiogenesis . Alternatively, life may have formed less frequently, then spread – by meteoroids , for example – between habitable planets in a process called panspermia . During most of its stellar evolution stars combine hydrogen nuclei to make helium nuclei by stellar fusion, and the comparatively lighter weight of helium allows

15755-405: The right side of the equation are agreed as speculative and open to substitution: 10,000 = 5 ⋅ 0.5 ⋅ 2 ⋅ 1 ⋅ 0.2 ⋅ 1 ⋅ 10,000 {\displaystyle 10{,}000=5\cdot 0.5\cdot 2\cdot 1\cdot 0.2\cdot 1\cdot 10{,}000} The Drake equation has proved controversial since, although it is written as

15892-422: The rocky surface, which helps the chemical reactions. It may be difficult to dig so deep in order to study those oceans, though. Enceladus , a tiny moon of Saturn with another subsurface ocean, may not need to be dug, as it releases water to space in eruption columns . The space probe Cassini flew inside one of these, but could not make a full study because NASA did not expect this phenomenon and did not equip

16029-432: The same beds, have so far shown no such holes. These two points suggest that predators attacked in a selective manner, and the evolutionary arms race which this indicates is commonly cited as a cause of the Cambrian explosion of animal diversity and complexity. Some dinoflagellates produce resting stages , called dinoflagellate cysts or dinocysts , as part of their lifecycles. Dinoflagellates are mainly represented in

16166-528: The same samples suggests that a non-biological reaction is a more likely hypothesis. In February 2005 NASA scientists reported they may have found some evidence of extraterrestrial life on Mars. The two scientists, Carol Stoker and Larry Lemke of NASA's Ames Research Center , based their claim on methane signatures found in Mars's atmosphere resembling the methane production of some forms of primitive life on Earth, as well as on their own study of primitive life near

16303-423: The seasons. Acritarchs , Greek for confused origins , are organic-walled microfossils, known from about 2,000  million years ago to the present. Acritarchs are not a specific biological taxon, but rather a group with uncertain or unknown affinities. Most commonly they are composed of thermally altered acid insoluble carbon compounds ( kerogen ). While the classification of acritarchs into form genera

16440-441: The silica based skeletons of microscopic marine organisms such as diatoms and radiolarians . Other components of siliceous oozes near continental margins may include terrestrially derived silica particles and sponge spicules. Siliceous oozes are composed of skeletons made from opal silica Si(O 2 ) , as opposed to calcareous oozes , which are made from skeletons of calcium carbonate organisms (i.e. coccolithophores ). Silica (Si)

16577-399: The star to release the extra energy. The process continues until the star uses all of its available fuel, with the speed of consumption being related to the size of the star. During its last stages, stars start combining helium nuclei to form carbon nuclei. The higher-sized stars can further combine carbon nuclei to create oxygen and silicon, oxygen into neon and sulfur, and so on until iron. In

16714-482: The stellar evolution of stars and planets, it is usually not taken into account by astrobiology. There is an area around a star, the circumstellar habitable zone or "Goldilocks zone", where water may be at the right temperature to exist in liquid form at a planetary surface. This area is neither too close to the star, where water would become steam, nor too far away, where water would be frozen as ice. However, although useful as an approximation, planetary habitability

16851-508: The study of Earth's life, the only known form of life, astrobiology seeks to study how life starts and evolves and the requirements for its continuous existence. This helps to determine what to look for when searching for life in other celestial bodies. This is a complex area of study, and uses the combined perspectives of several scientific disciplines, such as astronomy , biology , chemistry , geology , oceanography , and atmospheric sciences . The scientific search for extraterrestrial life

16988-438: The study of some vertebrates , or (d) organic , as in the pollen and spores studied in palynology . This division focuses on differences in the mineralogical and chemical composition of microfossil remains rather than on taxonomic or ecological distinctions. Pollen has an outer sheath, called a sporopollenin , which affords it some resistance to the rigours of the fossilisation process that destroy weaker objects. It

17125-432: The study of the ultrastructure, life history, and systematic affinities of mineralized, but originally organic-walled microfossils, have shown some acritarchs are fossilized microalgae . In the end, it may well be, as Moczydłowska et al. suggested in 2011, that many acritarchs will, in fact, turn out to be algae. Cells can be preserved in the rock record because their cell walls are made of proteins which convert to

17262-572: The study of their fine structure, and it has been suggested that they represent either the eggs or juvenile stage of a marine animal. However, recent research has suggested that they represent the test of a group of protists with uncertain affinities. The ecology of chitinozoa is also open to speculation; some may have floated in the water column, where others may have attached themselves to other organisms. Most species were particular about their living conditions, and tend to be most common in specific paleoenvironments. Their abundance also varied with

17399-539: The survival and possible origin of life on Earth . With the little amount of information that scientists have found regarding the atmosphere on other planets in the Milky Way galaxy and beyond, the atmospheres are most likely reducing or with very low oxygen levels, especially when compared with Earth's atmosphere. If there were the necessary elements and ions on these planets, the same carbon fixing, reduced chemical compounds occurring around hydrothermal vents could also occur on these planets' surfaces and possibly result in

17536-464: The temperatures rocks have been exposed to, which allows the thermal maturation levels of sedimentary rocks to be determined, which is important for hydrocarbon exploration . Conodont teeth are the earliest vertebrate teeth found in the fossil record, and some conodont teeth are the sharpest that have ever been recorded. Scolecodonts ( worm jaws in Latin) are tiny jaws of polychaete annelids of

17673-405: The time of their discovery. If the structures had been fossilized lifeforms, as was proposed by the so-called biogenic hypothesis of their formation, they would have been the first solid evidence of the existence of extraterrestrial life , aside from the chance of their origin being terrestrial contamination. The announcement of possible extraterrestrial life caused considerable controversy. When

17810-445: The unusual properties of the meteorite were eventually explained as the result of inorganic processes, the controversy over its discovery laid the groundwork for the development of astrobiology. An experiment on the two Viking Mars landers reported gas emissions from heated Martian soil samples that some scientists argue are consistent with the presence of living microorganisms. Lack of corroborating evidence from other experiments on

17947-672: The world and physical toughness makes microfossils important for biostratigraphy, while the manner in which they have reacted to environmental changes makes them helpful when reconstructing past environments. Extraterrestrial life Extraterrestrial life , or alien life (colloquially, alien ), is life that originates from another world rather than on Earth . No extraterrestrial life has yet been scientifically conclusively detected. Such life might range from simple forms such as prokaryotes to intelligent beings , possibly bringing forth civilizations that might be far more advanced than humans. The Drake equation speculates about

18084-464: Was blasted away from the surface of Mars by the impact of a meteor about 17 million years ago, and fell on Earth about 13,000 years ago. These dates were established by a variety of radiometric dating techniques, including samarium–neodymium (Sm–Nd), rubidium–strontium (Rb–Sr), potassium–argon (K–Ar), and carbon-14 dating. Other meteorites that have potential biological markings have generated less interest because they do not contain rock from

18221-417: Was habitable. It is unclear if life and intelligent life are ubiquitous in the cosmos or rare. The hypothesis of ubiquitous extraterrestrial life relies on three main ideas. The first one, the size of the universe allows for plenty of planets to have a similar habitability to Earth, and the age of the universe gives enough time for a long process analog to the history of Earth to happen there. The second

18358-428: Was inspired by ALH84001. Microfossil A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy . A fossil which can be studied with the naked eye or low-powered magnification, such as a hand lens, is referred to as a macrofossil . Microfossils are a common feature of the geological record , from

18495-410: Was not the temperature, but the scarcity of free heavy elements. Planetary systems emerged, and the first organic compounds may have formed in the protoplanetary disk of dust grains that would eventually create rocky planets like Earth. Although Earth was in a molten state after its birth and may have burned any organics that fell in it, it would have been more receptive once it cooled down. Once

18632-475: Was published in November 2009, the biogenic hypothesis has been further supported by the discovery of three times the original amount of fossil-like data, including more "biomorphs" (suspected Martian fossils), inside two additional Martian meteorites, as well as more evidence in other parts of the Allan Hills meteorite itself. However, the scientific consensus is that "morphology alone cannot be used unambiguously as

18769-399: Was reported. Scientists search for biosignatures within the Solar System by studying planetary surfaces and examining meteorites . Some claim to have identified evidence that microbial life has existed on Mars. In 1996, a controversial report stated that structures resembling nanobacteria were discovered in a meteorite, ALH84001 , formed of rock ejected from Mars . Although all

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