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Astronomy 2021A/B

Chapter 6 6.1 Searching for Life’s Orgins For example, a 3.5-billion-year-old fossil tells us that life on Earth arose before 3.5 billion years ago. Because the geological record is incomplete and because we may not yet have discovered the oldest intact fossils, we do not know exactly how long life has existed on Earth. 3 Lines of Fossil Evidence Stromatolites- from the Greek word for “rock beds” -Characterized by a distinctive layered structure, in size, shape, and interior structure, ancient stromatolites look virtually identical to sections of mats formed today by colonies of microbes sometimes called “living stromatolites” -Living stromatolites contain layers of sediment intermixed with different types of microbes, microbes near the top generate energy through photosynthesis those beneath use organic compounds left as waste products by the photosynthetic microbes. -There is some controversy about the biological origin of stromatolites, because geological processes of sedimentation can mimic their layering. -If we are correct in concluding that stromatolites tell us that photosynthetic life already existed some 3.5 billion years ago, then we can infer that more primitive life must have existed even earlier, and that the origin of life itself substantially predates this. Microfossils- Individual fossilized cells, Finding ancient microscopic fossils, or microfossils, is quite challenging, both because rocks become increasingly rare with age and because the oldest rocks have been altered by geological processes in ways that tend to destroy microfossils within them. -It can be quite difficult to determine whether an interesting-looking microscopic structure is biological or mineral in origin (causes scientific controversy) -Microfossils in rocks dating to between 2.7 and 3.0 billion years old show more conclusive evidence for life: They contain particular molecules (such as a variety of hydrocarbons) that almost certainly indicate biological origin. (as opposed to those found from 3.5 billion years ago which have more questionable biological origin, northwesternAustralia,Africa) Isotopic Evidence- The third line of evidence for an early origin of life comes from isotopic analysis of some of the most ancient rocks on Earth. Living organisms can change the ratios of isotopes from their background, nonliving values (living organisms—and fossils of living organisms—always show a slightly lower fraction of carbon-13 atoms than that found in inorganic material) -On an island off the coast of Greenland, this lower carbon-13 ratio has been found in rocks that are more than 3.85 billion years old -The rocks are metamorphic, meaning they have been transformed substantially by high pressure or heat, which would explain why no intact microfossils remain within them. For these rocks to have contained life, we must presume that they were sedimentary before they under- went the metamorphic transformation; if they were volcanic (igneous) rocks, then it is much more difficult to see how they could have been home to or preserved evidence of living microbes. IMPLICATIONS OF THE EARLY ORIGIN OF LIFE -By itself, this early origin of life proves nothing about life elsewhere, since it is always possible that Earth was the lucky beneficiary of a highly improbable event. However, if we assume that what happened here would be typical of what might happen elsewhere, then the early origin of life is profoundly important: It suggests that we could expect life to also arise rapidly on any other world with similar conditions. Because we expect many other worlds to have conditions similar to those that prevailed on the young Earth, this idea gives us reason to think that life might be quite common in the universe. MAPPING EVOLUTIONARY RELATIONSHIPS -Because living species have evolved from common ancestors, the base sequence in the DNA of living organisms provides a sort of map of the genetic changes that have occurred through time. By comparing the genomes of different organisms, we should be able to reconstruct the evolutionary history of much of life on Earth. -Over millions and billions of years, continuing evolution led to new species with DNA molecules increasingly different from the DNA of the common ancestor. But, always, the new molecules were built by changes to the older ones so that, in principle, the changes are traceable in the precise base sequences of living organisms. -Two species with very similar DNA sequences probably diverged relatively recently in evolutionary history, while two species with very different DNA sequences probably diverged much longer ago. -These types of DNA sequence comparisons are what have enabled biologists to map out the relationships shown in the tree of life. That is, DNA studies tell us that life can be divided into the three domains (bacteria, archaea, and eukarya) -For example, the fact that animals and plants represent two branches that split off in about the same place from other eukarya tells us that all animals and plants are quite similar genetically, at least in comparison to organisms on most other branches. Moreover, organisms on branches located closer to the “root” of the tree must contain DNA that is evolutionarily older, suggesting that they more closely resemble the organisms that lived early in Earth’s history. WHERE DID LIFE BEGIN? -It seems unlikely that life could have arisen on the land surface. The early atmosphere contained practically no molecular oxygen, so our planet could not have had a protective layer of ozone -Before the ozone layer existed, any surface life would have been exposed to high levels of this radiation. While we can’t rule out the possibility that life might have arisen in such an environment—some organisms today (such as D. radiodurans) can survive high-radiation conditions—the environment would have been much more hospitable under water (because water also absorbs ultraviolet light) or in rocks beneath the surface. -One such possibility, first suggested by Darwin, is shallow ponds. Organic compounds may have formed spontaneously in such ponds. Once the compounds formed, tides or cycles of wetting and evaporation could have increased their concentration near the pond edges, spurring reactions that might have led to life. -However, while these factors suggest ponds could have been a good location for an origin of life, the shallow water would not have offered much protection against solar ultraviolet radiation. -Abetter possibility might be deep-sea or underground environments, which would have been protected from high-energy radiation. Deep-sea volcanic vents offer plenty of chemical energy to fuel reactions that might have led to life, and chemical energy is also available underground in reactions between water and minerals in rock. -Moreover, even if life first arose in ponds at the surface, the impacts of the late heavy bombardment probably would have allowed the survival only of life that had migrated to deep-sea or underground environments. For that reason, it now seems likely that the common ancestor of all life on Earth today evolved from organisms that lived near deep-sea vents or underground, even if the first origin of life occurred elsewhere. 6.2 The Origin of Life -Over the past few decades, laboratory experiments have given us insights into the chemical processes that likely occurred on the early Earth. While these experiments have not yet told us precisely how life first arose—and it’s possible that they never will—we’ll see in this section that they give us good reason to think that life may have started through natural, chemical processes. -The laboratory experiments generally try to re-create the chemical conditions that should have prevailed on the early Earth, an assumption that makes sense if life originated here. However, it is conceivable that life migrated to Earth from another world -Life today is based on the chemistry of organic molecules, making it logical to assume that the first life was somehow assembled from organic molecules produced by chemical reactions on the early Earth. Such reactions do not occur naturally today, because Earth’s oxygen- rich atmosphere prevents complex organic molecules from forming readily outside living cells. -The oxygen in our atmosphere is a product of life, produced by photosynthesis, which means it could not have been present before life arose THE MILLER–UREY EXPERIMENT -As early as the 1920s, some scientists recognized that Earth’s early atmosphere should have been oxygen- free, and they hypothesized that sunlight-fueled chemical reactions could have led to the spontaneous creation of organic molecules. -This hypothesis was put to the test in the 1950s in a famous experiment credited to Stanley Miller and Harold Urey, now known as the Miller–Urey experiment -The original Miller–Urey experiment used small glass flasks to simulate chemical conditions that scientists thought represented those on the early Earth. -One flask was partially filled with water to represent the sea and heated to produce water vapor. Gaseous methane and ammonia were added and mixed with the water vapor to represent the atmosphere. These gases flowed into a second flask, where electric sparks pro- vided energy for chemical reactions. Below this flask, the gas was cooled so that it could condense to represent rain and then was cycled back into the water flask. -The water soon began to turn a murky brown, and a chemical analysis (performed after letting the experiment run for a week) showed that it contained many amino acids and other organic molecules. -We now know that the methane and ammonia mixture in the original Miller–Urey experiment was not representative of Earth’s early atmosphere. Nevertheless, the experiment demonstrated that, at least under some conditions, the building blocks of life form naturally and abundantly. OTHER SOURCES OF ORGANIC MOLECULES -The first of these potential “other” sources are chemical reactions near deep-sea vents.As these undersea volcanoes heat the surrounding water, a variety of chemical reactions can occur between the water and the minerals. These chemical reactions would have occurred spontaneously in the conditions thought to have prevailed in the early oceans, and they should have resulted in the production of the same types of organic molecules thought to have been necessary for the origin of life. -The other additional source of organic molecules may have been material from space.Analysis of meteorites shows that they often contain organic molecules, including complex molecules such as amino acids. -Apparently, organic molecules can form under the conditions present in interplanetary space and can survive the plunge to Earth. -Moreover, recent research shows that ultraviolet light from the young Sun could also have produced some of the building blocks of life. It would do this by causing chemical reactions to occur on dust grains orbiting the Sun as part of the solar nebula. This dust, laden with organic molecules, could have “rained down” on the young Earth. -The many impacts of the heavy bombardment could have brought additional organic material from asteroids and comets. The heat and pressure generated by the impacts may have further facilitated the production of organic molecules in Earth’s atmosphere and oceans. -It’s likely that all three sources of organic molecules—chemical reactions near the ocean surface, chemical reactions near deep-sea vents, and material from space— played a role in shaping the chemistry of the early Earth. THE TRANSITION FROM CHEMISTRY TO BIOLOGY -There must have been at least a few intermediate steps—each involving a chemical pathway with a relatively high probability of occurring—that eased the transition from chemistry to biology. - Early life must also have had a self-replicating molecule, but it probably was not DNA: Double- stranded DNA seems far too complex, and its replication far too inter- twined with RNA and proteins, to have been the genetic material of the first living organisms. (more likely the step between is RNA) RNA WORLD -RNA is much simpler than DNA because it has only one strand rather than two and its backbone structure requires fewer steps in its manufacture. - In modern organisms, neither DNA nor RNA can replicate itself. Both require the help of enzymes - These enzymes are proteins that are made from genetic instructions contained in DNA and carried out with the help of RNA. -Dilemma is that RNA cannot replicate without enzymes, and the enzymes cannot be made without RNA. -Away around this dilemma was discovered in the early 1980s by Thomas Cech and his colleagues at the University of Colorado, Boulder. They found that RNA can catalyze biochemical reactions in much the same way as enzymes -We now know that RNA molecules play this type of catalytic role in many cellular functions, and we call such RNA catalysts ribozymes -Follow-up work has shown that some RNA molecules can at least partially catalyze their own replication. These discoveries have led biologists to envision that modern, DNA-based life may have arisen from an earlier RNA world, in which RNA molecules served both as genes and as chemical catalysts for copying and expressing those genes. - Experiments show that several types of inorganic minerals can facilitate the self- assembly of complex, organic molecules. Minerals of the type that geologists call clay* may have been especially important. Clay is extremely common on Earth and in the oceans, where it forms through simple weathering of silicate minerals; indeed, the oldest zircon grains suggest the widespread abundance of clays more than 4.4 billion years ago, so we expect clay to have been common at the time of the origin of life. More- over, clay minerals contain layers of molecules to which other molecules, including organic molecules, can adhere. When organic molecules stick to the clay in this way, the mineral surface structure can force them into such close proximity that they react with one another to form longer chains. - RNA strand only 5 bases long that can act as a ribozyme Possible explanation for the origin of life: 1. Through some combination of atmospheric chemistry, chemistry near deep-sea vents, and molecules brought to Earth from space, the early Earth had at least localized areas with significant amounts of organic molecules that could serve as building blocks for more complex organic molecules. 2. More complex molecules, including short strands of RNA, grew from the organic building blocks, probably with the aid of reactions catalyzed by clay minerals. The minerals also helped catalyze the production of microscopic pre-cells in which RNA and other organic chemicals became enclosed. 3. The concentration of RNA molecules within pre-cells facilitated reactions that eventually led to self-replicating RNA, at which point molecular natural selection favored the spread of those RNA molecules that replicated most accurately and efficiently. 4. Natural selection among the RNA molecules in pre-cells gradually led to an increase in complexity, until eventually some of these structures became true living organisms. 5. DNA evolved from RNA, and its advantages made it the preferred hereditary molecule. Natural selection continued, enabling organisms to adapt to a great many environmental niches on planet Earth. THE POSSIBILITY OF MIGRATION The idea that life could travel through space to land on Earth, some- times called panspermia, The presence of organic molecules in meteorites and comets tells us that the building blocks of life can survive in the space environment, and we’ve already discussed some Earth microbes that are capable of surviving at least moderate periods of time in space Scientists have identified and cataloged more than 30,000 meteorites They have found about three dozen with compositions that clearly suggest that they came from Mars, even more have been found from the Moon Examination of these meteorites suggests that over time the inner planets have exchanged many tons of rock. In a sense, Earth, Venus, and Mars have been “sneezing” on each other for billions of years, offering the possibility of microscopic life hitchhiking between worlds on one of the meteorites. For a living microbe to reach earth after such a journey it would have to survive 3 lethal events: The impact that blasts it off the surface of its home world, the time it spends in the harsh environment of interplanetary space, and the fiery plunge through our atmosphere Examination of Martian meteorites suggests that neither the first nor last of these events poses insurmountable obstacles, suggesting that microbes inside these rocks could survive both the initial impact and the later fall to Earth A few meteorites are likely to be launched into orbits that cause them to crash to Earth during one of their first few trips around the Sun. For example, calculations suggest that about 1 in 10,000 meteorites may travel from Mars to Earth in a decade or less. Because experiments in Earth orbit have already shown that some terrestrial microbes can survive at least 6 years in space, it seems quite reasonable to imagine microbes from Mars arriving safely on Earth. Considerations almost certainly rule out the possibility of migration from other star systems. Under the best of circumstances, meteorites from planets around other stars would spend millions or billions of years in space before reaching Earth; any living organisms would almost surely be killed by exposure to cosmic rays during this time, or simply die because of desiccation—the lack of water. REASONS TO CONSIDER MIGRATION The key question probably is not whether life could migrate through space but whether we have any reason to suppose it originated elsewhere rather than right here on Earth. Today, most ideas about migrating life fall into one of two broad categories. The first broad idea suggests that life does not form as easily as we have imagined, at least under the conditions present on the early Earth. In this view, the only explanation for life on Earth (other than invoking the supernatural) would be migration from elsewhere. The second broad idea suggests that life forms so easily that we should expect to find life originating on any planet with suitable conditions. In that case, the origin of life in our solar system would have occurred on whichever planet got those conditions first; for example, if the very early Venus or very early Mars had suitable conditions for life before Earth, life from one of those worlds might have migrated to Earth and taken hold on our planet as soon as conditions allowed. IMPLICATIONS OF MIGRATION TO THE SEARCH FOR LIFE BEYOND EARTH While ideas about microbes migrating to Earth are speculative, it seems a near- certainty that microbes from Earth have many times made the journey to Mercury, the Moon, Venus, and Mars. After all, Earth has suffered plenty of impacts large enough to blast rock into space during its long history, offering plenty of opportunity for hitchhiking microbes. Thus, if it were possible for Earth life to survive on any of these other worlds, we should actually expect to find it there. We can almost certainly rule out the possibility of survival on the Moon and Mercury, and probably on Venus as well. Mars, however, may well have habitats that could provide at least temporary refuge to terrestrial microbes, and Mars may have been globally habitable in the distant past. The likelihood of such interplanetary migration raises at least two important issues in astrobiology. First, if we someday find life on Mars, we will have to wonder if it is native or if it arrived there from Earth. Second, the possibility of life migrating among the planets raises the question of whether we could ever distinguish between an indigenous origin of life on Earth and an origin based on migration from elsewhere. The only way we may ever be confident that life elsewhere is not transplanted Earth life will be if its biochemistry is too different from that of terrestrial life to allow for a common ancestor. It is possible that Venus once had oceans and a habitable climate in which life might have arisen. However, Venus now is so hot that any fossil record would almost certainly have been destroyed by the heat and subsequent geological activity. One way or another, life arose on Earth quite soon after conditions first allowed it, and even if life migrated here from another world, we have good reason to think that it evolved naturally, through chemical processes that favor the creation of complex, organic molecules and the subsequent molecular evolution of self- replicating molecules. 6.3 The Evolution of Life Regardless of how or where it originated, life on Earth has been evolving throughout the 4 billion or so years Careful studies of the geological record provide the key data with which we attempt to re-create the evolutionary time scale, while genome comparisons offer data that help us map relationships among species. EARLY MICROBIAL EVOLUTION The earliest organisms must have been quite simple, but they undoubtedly had at least a few enzymes and a rudimentary metabolism. Their cells probably looked somewhat like those of the simplest modern bacteria or archaea, lacking cell nuclei and other complex structures that we find in eukarya. Moreover, because the atmosphere at that time was essentially oxygen-free, all early life must have been anaerobic, meaning that it did not require molecular oxygen; by contrast, we are aerobic organisms, because we cannot survive with- out molecular oxygen. Both photosynthesis and the ability to digest other organisms probably evolved much later, so we expect that the first microorganisms were chemoautotrophs organisms that obtained their carbon from carbon dioxide dissolved in the oceans and their energy from chemical reactions involving inorganic chemicals. Natural selection probably caused rapid diversification among the early life-forms. Modern DNA replication involves a variety of enzymes that help keep the mutation rate low. Early organisms, with a much more limited set of enzymes, probably experienced many more errors in DNA copying. Because more errors mean a higher mutation rate, evolution would have been rapid among early microbes Fossil evidence supports the idea of rapid diversification. Recall that stromatolites suggest the presence of organisms that obtained energy by Photosynthesis some 3.5 billion years ago, and some of the oldest micro- fossils also resemble modern photosynthetic organisms. Because photo- synthesis is a complex metabolic pathway, its early emergence indicates that early evolution was rapid. Photosynthesis developed through multiple steps: First, some organisms developed light-absorbing pigments that absorb light energy-these evolved to enabled the cell to make use of the absorbed solar energy The rise of oxygen created a crisis for life, because oxygen attacks the bonds of organic molecules. Many species of microbes probably went extinct Some avoided these effects because they lived in (or migrated to) underground locations where the oxygen did not reach them. We still find many anaerobic microbes in such locales today, living in soil or deeper underground in rocks. Others survived because the oxygen content of the atmosphere rose gradually, allowing them time to evolve new metabolic processes and protective mechanisms that enabled them to thrive rather than die in the presence of oxygen. Plants and animals, including us, still use the metabolic processes that evolved in response to the “oxygen crisis” faced by living organisms some 2 billion or more years ago. THE EVOLUTION OF EUKARYA The evolution of eukarya was the crucial first step in our own eventual evolution; we are, after all, members of this domain. Even single-celled eukaryotes exhibit much more diversity in cellular structure than exists among bacteria or archaea, and multi-celled eukaryotes enjoy diversity far beyond that. Because more variations are possible on complex structures than on simple ones. When did eukarya arise? Despite the fact that modern eukarya have more complex cellular structures than bacteria and archaea, genome studies do not suggest any substantial differences in the evolutionary ages of the three domains. That is, it is quite likely that members of all three domains—bacteria, archaea, and eukarya— split from a common ancestor early in Earth’s history. The oldest known fossils that clearly show cell nuclei date to about 2.1 billion years ago. Modern, complex eukarya probably evolved through a combination of at least two major adaptations that arose in their simpler ancestors. First, some early species of eukarya may have developed specialized in folding’s of their membranes that compartmentalized certain cell functions, ultimately leading to the creation of a cell nucleus. Second, some relatively large ancestral host cells absorbed small bacteria within them, creating a symbiotic relationship in which both the invading organisms and the host organisms benefited from living together Evidence for symbiosis comes from two structures in eukarya which appear to be cells within cells (mitochondria). Organs within a cell help produce energy (by making molecules ofATP and chloroplasts, structures in plant cells that produce energy by photosynthesis). Both also have their own DNA and reproduce themselves within their eukaryotic homes. Moreover, sequencing of the DNA in mitochondria and chloroplasts clearly groups them with domain bacteria, rather than with eukarya, making it a near-certainty that they originated as free-living bacteria THE CAMBRIAN EXPLOSION We have seen that life on Earth existed at least 3.5 billion years ago (and perhaps hundreds of millions of years before that), and all three domains of life were well established by at least 2.1 billion years ago. However, the fossil record tells us that all this life remained microscopic until much later. The earliest fossil evidence for complex, multicellular organisms—all of which are eukarya—dates to only about 1.2 billion years ago. Thus, microbes had our planet to themselves for more than 2 billion years after the origin of life. Even today, the total biomass of microbes far exceeds that of multicellular organisms like fungi, plants, and animals The fossil record suggests that animal evolution progressed slowly at first, with relatively little change seen between fossils from 1.2 billion years ago and those from a half-billion years later. But then something quite dramatic happened. In the broadest sense, biologists classify animals according to their basic “body plans.”Animals are grouped by body plan into what biologists call phyla Mammals and reptiles both belong to the phylum Chordata which represents animals with internal skeletons. Insects, crabs, and spiders belong to the phylum Arthropoda, which represents animals with body features such as jointed legs, an external skeleton, and segmented body parts. Remarkably, nearly all of these different body plans make their first known appearance in the geological record during a period spanning only about 40 million years— less than about 1% of Earth’s history. This remarkable flowering of animal diversity appears to have begun about 542 million years ago, which corresponds to the start of the Cambrian period, called the Cambrian explosion. Presents us with two questions: Why did the Cambrian explosion occur so suddenly, at least in geological terms, yet so long after the origin of eukaryotes, and why hasn’t any similar diversification happened since? No one knows the answers to these questions, but we can identify at least four possible contributing factors st 1 Oxygen level in our atmosphere may have been well below its present level until the time of the Cambrian explosion (change in animal life a result of oxygen reaching a critical level for the survival of larger more energy-intensive forms) 2 Evolution of genetic complexity, as eukaryotes evolved they developed more genetic variation in their DNA, opening more possibilities for further variation (great diversity within short time). rd 3 Climate change (series of snowball Earth episodes ending prior to the Cambrian explosion) 4 Absence of efficient predators, once predators were efficient and widespread, it would have been much more difficult for entirely new body forms to find an available environmental niche. THE COLONIZATION OF LAND Because most early microfossils are found in sediments that were originally deposited in the oceans, it’s difficult to know when life first migrated onto land. Plenty of such locations are available on land—including underground and anyplace where water can pool under a shelter of over- hanging rock—so it is hard to imagine reasons why microbial life would not have taken hold on land quite early. For larger organisms, surviving on land was more difficult than surviving in the oceans, primarily because it required evolving a means of obtaining water and mineral nutrients without simply absorbing them from surroundings. The timing of the development of the ozone layer may also have played a role in the late colonization of land. Fossil evidence shows that plants (and perhaps fungi as well) were the first large organisms to develop the means to live on the land. The colonization of land by plants appears to have begun about 475 million years ago. DNA evidence suggests that plants evolved from a type of algae. Because such locales occasionally dry up, natural selection would have favored adaptations, such as thick cell walls, that allowed the algae to survive during periods of dryness. Cell walls would have given the organisms structure that would have helped them survive on land. Once plants moved onto the land, it was only a matter of time until animals followed them out of the water. Within about 75 million years, amphibians and insects were eating land plants. By the beginning of the Carboniferous period, about 360 million years ago, vast forests and abundant insects thrived around the world THE ORIGIN OFATMOSPHERIC OXYGEN Molecular oxygen is a highly reactive gas that would disappear from the atmosphere in just a few million years if it were not continually resupplied by life. Fire, rust, and the discoloration of freshly cut fruits and vegetables are everyday examples of oxidation reactions—chemical reactions that remove oxygen from the atmosphere Before oxygen-breathing organisms evolved, oxidation reactions involved primarily volcanic gases, dissolved iron in the oceans, and surface minerals (especially those containing iron) that could react with oxygen. Remarkably, it seems that we owe our oxygen atmosphere to microscopic bacteria sometimes called “blue-green algae” but more technically known as cyanobacteria (atleast 2.7 billion yr ago, it took 2 billion years for oxygen to build to present levels) TIMING OF THE OXYGEN RISE Careful study of rock chemistry offers even more clues. Cyanobacteria split water and release oxygen in photosynthesis and are thought to have been responsible for the rise of oxygen in Earth’s atmosphere. Studies of rocks that are between about 2 and 3 billion years old, especially rocks of a type called banded iron formations, show that the atmosphere during that time contained less than 1% of the amount of oxygen it contains today. Very low oxygen in atmosphere until 2.35 billion years ago (great oxidation event began to build up the atmosphere) If we are correct in assuming that cyanobacteria began to produce oxygen at least 2.7 billion years ago—or at least 350 million years before the great oxidation event at 2.35 billion years ago—what took so long? Our best guess is that nonbiological processes, such as oxidation of surface rock and ocean minerals, were at first able to remove oxygen from the atmosphere as rapidly as the cyanobacteria could make it; only after the rock and ocean minerals were saturated with oxygen could the atmospheric buildup begin. IMPLICATIONS FOR LIFE ELSEWHERE Our study of the origin of life gives us reason to think that life might be common on worlds with conditions like those of the early Earth. But the fact that it took so long for oxygen to build up in the atmosphere on Earth should make us wonder about the likelihood of getting oxygen-breathing life on other worlds. Could Earth have been “lucky” to get conditions that allowed the buildup of oxygen? If so, perhaps life on most other worlds would never evolve past microscopic forms; life might then be common, but advanced or intelligent life quite rare Alternatively, maybe Earth was “unlucky” in having conditions that prevented the oxygen buildup for so long. In that case, other worlds might have complex plants and animals by the time they are just 1 to 2 billion years old, instead of having to wait until they are 4 billion years old. We will therefore continue our study with the assumption that Earth has been “typical,” until and unless we learn otherwise. 6.4 Impacts and Extinctions Reptiles evolved from amphibians; by about 245 million years ago, dinosaurs and mammals followed. But the fossil record shows that the path was not smooth. In particular, there is evidence for a number of striking transitions in the nature of living organisms. The most famous of these defines the boundary between the Cretaceous and Tertiary periods, which dates to about 65 million years ago. THE K–T BOUNDARY LAYER The thin layer of sediments that marks the Cretaceous–Tertiary boundary, called the K–T boundary for short (the K comes from the German word for “Cretaceous,” Kreide), is unusually rich in iridium, an element that is rare on Earth’s surface (because Earth’s iridium sank to the core when our planet underwent differentiation but common in meteorites). They calculated that it would have taken an asteroid about 10–15 kilometers in diameter to deposit the iridium distributed worldwide in the K–T boundary layer. Biological devastation (meteorite) occurred 65 million years ago. EVIDENCE FOR THE IMPACT Besides being unusually rich in iridium, this layer contains four other unusual features: (1) High abundances of several other metals, including osmium, gold, and platinum (2) Grains of “shocked quartz,” quartz crystals with a distinctive structure that indicates they experienced the high-temperature and high-pressure conditions of an impact (3) Spherical rock “droplets” of a type known to form when drops of molten rock cool and solidifies in the air (4) Soot (at some sites) that appears to have been produced by widespread forest fires. In addition to the evidence within the sediments, scientists have identified a large, buried impact crater that appears to match the age of the sediment layer. The crater, about 200 kilometers across, is located on the coast of Mexico’s Yucatán Peninsula, about half on land and half underwater If the impact was indeed the cause of the mass extinction, here’s how it probably happened: On that fateful day some 65 million years ago, the asteroid or comet slammed into Mexico with the force of a hundred million hydrogen bombs. It apparently hit at an angle, sending a shower of red-hot debris across the continent of NorthAmerica.Ahuge tsunami sloshed more than 1000 kilometers inland. Much of NorthAmerican life may have been wiped out almost immediately. Not long after, the hot debris raining around the rest of the world-ignited fires that killed many other living organisms. Indeed, the entire sky may have been bright enough to roast most life on land. Dust and smoke remained in the atmosphere for weeks or months, blocking sunlight and causing temperatures to fall as if Earth were experiencing a global and extremely harsh winter. The reduced sunlight would have stopped photosynthesis for up to a year, killing large numbers of species throughout the food chain. Perhaps the most astonishing fact is not that up to 75% of all plant and animal species died but that some 25% survived.Among the survivors were a few small mammals. These mammals may have survived in part because they lived in underground burrows and managed to store enough food to outlast the global winter that immediately followed the impact. For 180 million years, dinosaurs had diversified into a great many species large and small, while most mammals (which had arisen at almost the same time as the dinosaurs) had remained small and rodent like. With the dinosaurs gone, mammals became the new animal kings of the planet. Over the next 65 million years, the small mammals rapidly evolved into an assortment of much larger mammals—ultimately including us. The primary problem is that identifying the extinction of a species requires finding its last occurrence in the fossil record, which means we can be misled if we’ve yet to find a more recent occurrence of the species. OTHER MEANS OF MASS EXTERMINATION High levels of volcanic activity can lead to climate change and high extinction rates (sick earth hypothesis) Mutation rate, copying errors within the cells, can be caused by external factors (ultraviolet light, solar wind) Supernovae, death by cosmic rays or gamma ray bursts Chapter 7 When we discuss habitability, we generally mean an environment in which life of some kind might survive, not necessarily human life. Where can we expect to find building blocks of life? Life on Earth uses about 25 of the 92 naturally occurring chemical elements, although just four of these elements—oxygen, carbon, hydrogen, and nitrogen— make up about 96% of the mass of living organisms. Thus, a first requirement might be the presence of most or all of the elements used by life. Moreover, the elements oxygen, carbon, and nitrogen—arguably the most crucial elements for life—are the third-, fourth-, and sixth-most-abundant elements in the universe, respectively The nebular theory of solar system formation gives reason to expect the elements of life to be common on other worlds. The first step, condensationaffects only the heavier elements or hydrogen compounds containing heavy elements, because pure hydrogen and helium always remain gaseous.As long as condensation and accretion can occur, we expect the resulting worlds to contain the elements needed for life. Life on Earth is carbon based and we have good reason to think that life elsewhere would be carbon based as well. A somewhat stricter requirement is the presence of these elements in molecules that can be used as ready-made building blocks for life, just as the early Earth probably had at least moderate abundances of amino acids and other complex molecules. Recall that Earth’s organic molecules likely came from some combination of three sources: chemical reactions in the atmosphere, chemical reactions near deep-sea vents in the oceans, and molecules brought to Earth from space. Thus, we should concentrate on worlds that have either an atmosphere or a surface or subsurface liquid medium, such as water, or both. The first two sources can occur only on worlds with atmospheres or oceans, respectively. But the third source should have brought similar molecules to nearly all worlds in our solar system. Studies of meteorites and comets suggest that organic molecules are widespread among both asteroids and comets. Every world should have received at least some organic molecules However, organic molecules tend to be destroyed by solar radiation on surfaces unprotected by atmospheres. Molecules probably cannot react with each other unless some kind of liquid or gas is available to move them about. Where can we expect to find energy for life? In addition to a source of molecular building blocks, life requires an energy source to fuel metabolism. Some organisms get energy directly from sunlight through photosynthesis. Others get energy by consuming organic molecules (for example, by eating photosynthetic organisms) or through chemical reactions with inorganic compounds of iron, sulfur, or hydrogen. The energy available in sunlight decreases with the square of the distance from the Sun, example twice as far from the sun as earth = 1/4 the amount of energy Does life need liquid water? 3 vital roles for life on eIt dissolves organic molecules, making them available for chemical reactions within cells; it allows for the transport of chemicals into and out of cells; and it is involved directly in many of the metabolic reactions that occur in cells. But can other liquids do this? No one knows whether other liquids could support life in the absence of liquid water Potential Candidates:Ammonia, Methane and Ethane ADVANTAGES OF WATER 3 Advantages: Water remains liquid over a wider and higher range of temperatures (can stay liquid through changes in weather or climate, higher range facilitates chemical reactions, faster moving molecules=faster chemical reactions) Typically, the rate of a given chemical reaction doubles with each 10°C increase in temperature. Life using these alternative liquids would probably have a much slower metabolism than life on Earth. SecondAdvantage- they way water freezes, water is the only liquid that is less dense as a solid than a liquid, allows ice to float and ice to insulate water below This is important to long term climate stability (ice ages, ice prevents all liquid from turning to ice, as if ice was more dense than water it would sink) ThirdAdvantage- the way electrical charge is distributed within water molecules the electrons tend to be distributed in a way that makes one side have a net positive charge and the other side have a net negative charge This charge separation affects the way in which water dissolves other substances. Living cells have membranes that do not dissolve in water, so the membranes effectively protect the interior contents of cells. If we place living cells in liquid ethane, methane, or ammonia—molecules with less charge separa
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