Astrobiology_A Very Short Introduction

by David C. Catling

  A boring billion years ended by the advent of animal life

  The ocean and land adjusted to the change of the Great Oxidation, and by about 1.8 Ga, atmospheric oxygen had settled down and remained between 0.2 and 2 per cent for an amazingly long time. The evolution towards complex life was slow perhaps because anoxic waters that were often rich in dissolved iron or hydrogen sulphide underlay a moderately oxygenated surface ocean. Such anoxic conditions are toxic for complex life. In fact, evolution was so sluggish that the interval from 1.8 to 0.8 Ga is called the boring billion. According to one scientific paper, ‘never in the course of Earth’s history did so little happen to so much for so long’!

  Actually, several notable events did occur in the otherwise boring billion and just before it. Around 1.9 Ga, the first fossils of single organisms appear that are visible to the naked eye. These impressions of spiral coils of a few centimetres diameter (named Grypania) were probably seaweeds. In China, acritarchs, a type of fossil larger than 0.05 mm in size with organic-walls, occur at 1.8 Ga. Some are thought to have been algal cysts, which form when algae turn into inactive balls during dry periods. New lineages also emerged, including red algae at 1.2 Ga, which were likely capable of sexual reproduction. Today, we boil red algae descendants to make agar, which is used to thicken ice cream, and we use one type, nori, to wrap sushi. Because sex was discovered, the boring billion was not that boring.

  Starting around 750 Ma, and culminating after fits and starts about 580 Ma, atmospheric oxygen rose a second time to levels exceeding 3 per cent, and the deep sea became fully oxygenated. Animal biomarkers and tiny fossils (possibly sponges) date from around 630 Ma. But it’s only after 580 Ma that we find complex fossils from centimetres to metres in size. These are strange soft-bodied organisms without mouths or muscles that must have relied upon diffusion of nutrients through their skin. Some resemble pizzas while others look like plant-like fronds. However, the organisms lived too deeply in the sea to receive sunlight so they cannot have been plants. Collectively, they are called the Ediacaran biota because they were first clearly identified in the Ediacaran Hills in southern Australia. The Ediacarans lasted tens of millions of years before dying out.

  Then, the Cambrian Explosion occurred, which was the relatively rapid appearance of animal fossils with new body architectures in a 10- to 30-million-year interval after the beginning of the Cambrian Period at 541 Ma. Representatives of most modern groups of animals emerged, including many with hard skeletons. The Cambrian (541–488 Ma) comes directly after the end of Proterozoic Aeon, and is the first period of the Phanerozoic Aeon (541 Ma to present). Phanerozoic means ‘visible animals’ from the Greek phaneros (visible) and zoion (animal). One tiny Cambrian creature, Pikaia, was a sort of swimming worm, a few centimetres long. Pikaia was probably in a group from which all the vertebrates, including us, evolved.

  The second rise of oxygen allowed a diversity of animals to arise but its cause remains unsolved. Suggestions include organic carbon burial (and hence oxygen production) accelerated by several factors. These were new life on land that boosted the breakdown of surfaces and nutrient release, the evolution of marine zooplankton, and the enhanced production of clays that helped to bury organic matter. Alternatively, moderate levels of around 100 parts per million of atmospheric methane throughout the ‘boring billion’ might have promoted yet more hydrogen escape to space that oxidized the seafloor and ushered in an increase of oxygen after a second global redox titration.

  Around 400 Ma, a third rise of oxygen took place when vascular plants colonized the continents. Such plants have special tissues (such as wood) to transport water and minerals. They probably enhanced nutrient release (by breaking down land surfaces with roots) and organic burial. Afterwards, atmospheric oxygen levels remained within 10 per cent to 30 per cent, allowing the persistence of animals and eventually our own appearance.

  Snowball Earth or Waterbelt Earth?

  Curiously, worldwide glaciations are associated with both the Great Oxidation Event and the second rise of oxygen, like bookends encompassing the ‘boring billion’. Some scientists argue that at these times, the Earth’s oceans entirely iced over in a so-called Snowball Earth. This state was far more extreme than the ice ages that have occurred in the last few million years that have only extended down to mid-latitudes, not the tropics. For context, the Proterozoic aeon is divided into three eras, the Palaeoproterozoic (2.5–1.6 Ga), Mesoproterozoic (1.6–1.0 Ga), and Neoproterozoic (1.0–0.541 Ga). Geologists commonly speak of the ‘Palaeoproterozoic glaciations’ to describe three or four glaciations that occurred at the beginning (2.45–2.22 Ga) of the Palaeoproterozoic, while the ‘Neoproterozoic glaciations’ refers to two large glaciations (at 715 and 635 Ma) and one small one (at 582 Ma).

  The rock record provides evidence of these worldwide glaciations. The flow of ice sheets mixes stones, sand, and mud into a type of rock called tillite. Glaciers also leave parallel scratch marks called striations on underlying rocks. Furthermore, rocks drop out of icebergs or melting ice shelves and end up below as so-called dropstones that deform sedimentary beds. Together, tillites, glacial striations, and dropstones are dead giveaways that ice sheets were once present.

  Magnetic measurements made in the 1980s and 1990s also demonstrated that the glaciations extended into the tropics. A compass shows the Earth’s north–south magnetic field. Less obvious is the fact that the magnetic field lines are approximately perpendicular to the surface near the poles and parallel to the surface near the equator. Volcanic rocks contain iron minerals, such as magnetite (an iron oxide, Fe3O4), which become magnetized in the direction of the prevailing magnetic field when they cool. Thus, when the trapped or remnant magnetic field lines are parallel to ancient beds of rock, we know that the rocks formed in the tropics. This was discovered in rocks associated with Neoproterozoic and Paleoproterozoic glaciations.

  The idea that the entire Earth might freeze over if sea ice reached the tropics originated in the 1960s. A Russian climatologist, Mikhail Budyko, calculated that if polar ice caps extended farther equatorward than 30° latitude, the entire Earth should freeze. Ice reflects a lot of sunlight and so has a high albedo, which is the fraction (between 0 and 1) reflected. Today, the Earth’s albedo is about 0.3, meaning that 30 per cent of the sunlight is reflected to space. But sea ice reflects 50 per cent when bare or 70 per cent if covered in snow. A disastrous ice-albedo runaway occurs when high-albedo ice in the tropics makes the Earth absorb less sunlight and cool, which creates more ice that makes the Earth even colder, and so on. A frozen globe is created with an average temperature below –35°C and an ice thickness of 1.5 km at the equator and 3 km at the poles.

  Joe Kirschvink of the California Institute of Technology proposed how a Snowball Earth would melt, as well as the very phrase Snowball Earth. He suggested that volcanoes would punch through the ice and vent carbon dioxide into the atmosphere. Because there would be no rainfall or significant open ocean to dissolve carbon dioxide, it would build up over a few million years until an enhanced greenhouse effect melted the Snowball. Also, the flow of ice from thick polar ice to thin tropical ice would carry volcanic ash and windblown dust, so that the tropics would become darker, making the Snowball prone to melt. Then the ice-albedo runaway would run in reverse: less ice would make the Earth warmer, which would melt more ice, and so on.

  Layers of carbonate rock called cap carbonates that sit on top of the glacial deposits possibly attest to this aftermath of a Snowball Earth. The idea is that after the ice melted, large amounts of carbon dioxide were converted into cap carbonates. Proponents of the Snowball Earth hypothesis note that the ratio of carbon-12 to carbon-13 isotopes in the carbonate is similar to that in volcanic carbon dioxide.

  A central problem for the Snowball Earth hypothesis is how photosynthetic algae survived under the thick ice, given that they require sunlight. Lineages of green and red algae went straight through the Neoproterozoic snowballs unscathed. One suggestion is that heat
around volcanoes allowed some open water or thin, transparent ice. But others question whether the entire Earth really froze over. Some reasonably sophisticated computer simulations of the Earth’s ancient climate produce a tropical belt of open water—a Waterbelt Earth—because this region is surrounded by bare ice of moderate albedo, whereas the very reflective, snow-covered ice occurs only at high latitudes.

  Whatever happened, the cause of both the Neoproterozoic and Paleoproterozoic glaciations was probably a diminished greenhouse effect. Continents drift because of plate tectonics, and reconstructions have them bunched up in the tropics prior to the ancient snowball eras. Rainfall over tropical continents perhaps drew carbon dioxide down to lower levels than typical, poising the Earth for glaciation. Also, the coincidence of rises in oxygen with the glaciation eras suggests a trigger through methane, another greenhouse gas. Probably the oxygen increases were associated with rapid decreases in methane because high abundances of the two gases are incompatible—they react.

  For astrobiology, the lesson from Snowball Earth is that the biosphere on an Earth-like planet is resilient. Despite drastic climate swings, life on Earth survived and emerged after the Neoproterozoic into a new aeon of animals.

  On the occurrence of advanced life

  In astrobiology, we often wonder if animal-like life exists elsewhere in the galaxy. That possibility can perhaps be informed by evolution on Earth. To become complex, terrestrial life had to overcome at least two key hurdles. First, life had to acquire complex cells, which were needed for differentiation on the larger scale, i.e. cells with functional distinctions, such as liver versus brain cells. Large, three-dimensional multicellular animals and plants are made only of eukaryotic cells, which can develop into a much larger range of cell types compared to the cells of microbes (see Chapter 5).

  A second precursor for animals was having sufficient oxygen to generate lots of energy with an aerobic (oxygen-using) metabolism, such as our own. Abundant energy is needed to grow large and move. Anaerobic metabolism, which doesn’t use oxygen, produces about ten times less energy for the same food intake than aerobic metabolism. For extraterrestrial life, we might wonder if fluorine or chlorine—which are powerful oxidants—could be used in place of oxygen to generate high energy levels. The answer is no. Fluorine is so reactive that it explodes with organic matter, while chlorine dissolves to make harmful bleach. Oxygen, it appears, is the best elixir for complex life in the periodic table—reactive enough but not too aggressive.

  Oxygen not only needs to be present but also concentrated. The first aerobic multicellular life probably consisted of aggregates of cells produced when dividing cells failed to separate. Such agglomerations would have been limited in size by diffusion of oxygen to the innermost cell so that greater levels of external oxygen would have permitted larger cell groupings. At typical metabolic rates, a diffusion-constrained animal precursor of about a few millimetres’ dimension would need atmospheric oxygen concentrations exceeding 3 per cent. This corresponds to the oxygen increase around 580 Ma that ushered in the animals.

  Fossil algae show that eukaryotes existed long before animals, so the slow fuse to the Cambrian Explosion was probably the time it took to build up adequate oxygen. We can even define an oxygenation time as that required to reach oxygen levels sufficient for complex animal life. On Earth, it was 4 billion years after the planet formed.

  The oxygenation time on Earth-like exoplanets is uncertain given that we’re still trying to understand what set the timescale for the Earth. However, oxygen comes from liquid water. So Earth-like exoplanets with oceans have the potential to develop oxygen-rich atmospheres if water-splitting photosynthesis evolves. If the oxygenation time exceeds 12 billion years on a certain exoplanet because it’s endowed with material that reacts with oxygen, a Sun-like parent star would turn into a ‘red giant’ before the conditions for animal life became possible, and the planet would be doomed to possess nothing more complex than microbial slime. But if other exoplanets have short oxygenation times, complex life might occur faster than it took on Earth.

  Trends in evolution? The lesson of mass extinctions

  Once animals evolve, we might wonder if evolution has a trend towards greater complexity. Consider life on land. In land colonization, the first plant spores are found around 470 Ma, while fossils of substantial parts of plants appear about 425 Ma. Insects appear on land around 400 Ma. Then fish evolved into amphibians around 365 Ma and their descendants ultimately became the reptiles and mammals, including us. This is a trend for the biosphere to use more of the Earth’s resources. But a misconception is to think of evolution as a steady progression ending in us. The fossil record shows that species come and go. Also, mass extinctions intermittently prune the diversity of complex life. They are events when more than 25 per cent of families are lost, where ‘family’ is the biological tier above genus and species (see Chapter 5). Such mass extinctions refer to the non-microbial part of the biosphere, of course.

  Only around the Proterozoic–Phanerozoic transition is there a dramatic increase in diversity because at that time organisms acquired new body architectures and ways of living that persisted. In animals, a key innovation was the body cavity, the coelom (pronounced ‘see-lum’), which could be filled with fluid to provide rigidity and allow the concentration of forces from muscles. This facilitated self-propulsion, which was further improved with hard skeletons, at first external and then internal. A predator–prey evolutionary arms race likely contributed to the Cambrian Explosion of animal varieties. Plant diversity later increased with the development of vascular systems with rigid cells for transport and associated organs that eventually gave rise to trees.

  In the past 500 Ma, there have been five mass extinctions that killed over 50 per cent of species, with the two biggest at 251 Ma and 65 Ma, respectively. Excluding crocodilians, no land animal larger than the size of a domestic dog got through the largest event, separating the Permian (299–251 Ma) and Triassic (251–200 Ma) periods. This event took a massive toll in the oceans. For example, the trilobites—icons of the Cambrian seas—had been in decline but the Permian–Triassic extinction finished them off. The second largest event at 65 Ma wiped out the dinosaurs and separates the Cretaceous (145–65 Ma) and Paleogene (65–23 Ma) periods. (Some literature refers to this event as Cretaceous-Tertiary based on an older naming system.) The Permian–Triassic mass extinction destroyed as much as 95 per cent of marine species and 70–80 per cent of land animals, while the Cretaceous–Paleogene extinguished 65–75 per cent of all species.

  The cause of the Permian–Triassic mass extinction was apparently a chain of misfortunes generated by the Earth herself. The trigger seems to have been large-scale volcanism in Siberia, covering an area similar to Europe. Because coal deposits underlay this area, the volcanism pumped out huge amounts of coal-derived methane and carbon dioxide greenhouse gases, which warmed and acidified the ocean. Oxygen is less soluble in warm water, so an anoxic deep sea developed, which may have belched poisonous hydrogen sulphide to the surface. The combination of climatic warming, ocean acidification, and noxious gases extinguished more life than any other event in the Phanerozoic.

  The impact of an asteroid of about 10 kilometres diameter appears to have caused the Creataceous–Paleogene mass extinction. Calamitous consequences included temporary destruction of the ozone layer, worldwide wildfires, acid rain, and subsequent climatic cooling caused by sulphate particles injected into the stratosphere that reflected sunlight. The impact crater has even been found about a kilometre underground near the Mexican town of Chicxulub (pronounced ‘sheek-soo-loob’).

  Extinctions destroy previously successful lineages but they also provide opportunities for others. You are reading this book because of the Chicxulub impactor. The mammals became dominant once the dinosaurs were gone.

  On the other hand, mass extinction events mean that if a civilization develops, it can be wiped out. Impacts the size of that producing the Cretaceous–Paleog
ene extinction should occur every 100 million years or so on Earth, give or take a factor of a few. The implication is that civilizations on exoplanets could be short-lived compared with the age of the universe merely because of such random catastrophes. Thus, an Earth-like planet can remain fit for life, but civilizations are probably ephemeral. The latter has consequences in the astronomical search for extraterrestrial intelligence (Chapter 7).

  Chapter 5

  Life: a genome’s way of making more and fitter genomes

  Life on Earth: the view from above

  In Chapter 1, I presented a general definition of life, but in finding life elsewhere, it helps to know our one example of life in great detail. To this end, let’s start with a global perspective of terrestrial biology and work downward to cells and molecules.

  Imagine an interstellar traveller who arrives on the Earth and wants to know about our biology. Perhaps from her planet, she had deduced that life exists here because of Earth’s wet, anomalously oxygen-rich atmosphere, or she had picked up TV broadcasts from decades ago that somehow didn’t put her off visiting. Amazingly, she speaks English (as extraterrestrials always do in the movies) and by a stroke of luck her spacecraft lands in an English-speaking country! What would we tell her?

  At the global level, Earth’s biosphere is the sum of all living and dead organisms. Sometimes the term includes the non-living regions that life occupies. By quantifying the biosphere in billions of tonnes of carbon (1 tonne = 1,000 kg), we can identify its broad components. The biomass on land is around 2,000 billion tonnes of carbon of which 30–50 per cent is living and the rest is dead. In the ocean, only 0.1–0.2 per cent of about 1,000 billion tonnes of biomass carbon is alive. Forests are the reason that there’s so much more living biomass on land than in the oceans.