24. Chemistry and Context for Life

Transcript: Early in the Earth’s history, the first half billion years, it was a violent and inhospitable place. The crust had just formed; volcanism was extreme and very active. There was much lava flowing, and the surface was barely solid, certainly not hospitable for life. The primary energy sources for plate tectonics were radioactive decay deep in the core and the cooling of the core with heat radiating outwards. Also in the first half billion years, there were many planetesimals left over from the formation of the planets and the Sun itself, and these deposited much material that was essential to life on Earth. For example, ices from early comets and meteors were deposited. The ocean probably condensed about a half a billion years after the Earth formed. Before that it was vapor, and the continents took hundreds of millions of years more to form and have only been in their current state in the last hundred and fifty million years.

Transcript: The Earth, the Sun, and all the planets were formed at the same time, 4.6 billion years ago. This time is estimated fairly accurately from radioactive decay in rocks from the Moon and the Earth added to the timescale for the collapse of the solar nebula. As the Sun formed, rocky debris and simple small particles built up by accretion in zones outside the Sun. In the inner region of the solar system, the early, luminous, young Sun drove out much of the hydrogen and helium, leaving rocky cinders which built over a period of fifty or sixty million years to the size of the terrestrial planets. The Moon formed early in the history of the Earth by a huge impact, probably a hundred million years after the formation. The Moon is essential in the dynamics of the Earth because it causes the tides, and it stabilizes the Earth in its orbit. In the first few hundred million years after the Earth’s formation there was much debris leftover from the formation of the solar system. These large planetesimals bombarded the Earth, and if life had formed in the first few hundred million years, some of them may have raised the temperature enough to sterilize the entire planet. The age dating of the Moon and the Earth is reliably dated to 4.6 billion years, but the estimates of the age of the first life are much less certain.

Transcript: Charles Darwin was born into a wealthy family in England in 1809. He had two famous grandfathers. One, Erasmus Darwin, was a noted scientist, and the other, Josiah Wedgwood, was one of the first entrepreneurs of the industrial revolution. Darwin enrolled in medical school, but it turned out he hated the sight of blood. He intended to become a minister, but then he took a job as a naturalist on the H.M.S. Beagle. At age twenty-two, Darwin embarked on the voyages that would change his view of biology and our view of life forever. Gathering data for five years, he made close observations of the species, especially on the Galapagos Island. His ideas were framed by ideas of Malthus about the competition of species in the natural environment. Together these turned into the idea of natural selection. He started to work on a manuscript. But he had thousands of pages of notes developed, and Darwin was actually afraid of the effect of his ideas on the society in England at the time. Then he received a manuscript from Alfred Russell Wallace who’d independently come up with the same idea. So in 1858, he hurried out his classic book The Origin of Species. Charles Darwin is buried in Westminster Abbey next to Isaac Newton.

Transcript: By the middle of the eighteenth century, scientists began to suspect that fossils represented the extinct ancestors of living species. The French naturalist Lamarck suggested that these species had failed to adapt to their environment and so had not endured. The question was why and how? What was the mechanism that caused some species to endure and others to become extinct? Lamarck believed that species passed on their traits directly to their progeny. This is not a viable mechanism. The true answer came from Charles Darwin working in the mid-nineteenth century. He came up with a compelling concept for the survival of species based on three premises. First, on the fact that there is competition for survival and resources among living species. Second, that there are individual genetic variations among members of a species. And these two facts combine to the third consequence, unequal reproductive success so that species that endure are culled according to their adaptation to their environment. This is the central idea of natural selection.

Transcript: Evolution is perhaps the essential attribute of life. It’s evolution that's allowed life on Earth to propagate, diversify, and endure for billions of years. The first ideas of evolution occur very early in the history of science at the time of the ancient Greek philosophers. In the sixth century B.C. Anaximander speculated that life arose in water and evolved from simple to more complex organisms. One century later, Empedocles even speculated that poorly adapted creatures would die and not persist. This is a precursor of the idea of natural selection. All of this speculation, however, was put on hold by the vast influence of Aristotle who held that species were fixed, did not evolve, and were independent of each other. Aristotle also got cosmology wrong too, but Aristotelian thought was so powerful that over the next millennium it became merged with Christian theology and the idea of static life that could not change or evolve. In theology this was called the chain of being.

Transcript: What is life? This simple question is amazingly difficult to answer. Even biologists cannot utterly agree on it. At one extreme, if we are too specific in our definition of life, we exclude the possibility that life might not be based, for example, on carbon chemistry, but if we are too general, we start to have a definition that doesn’t seem like life or chemistry at all. And we have to beware of induction, using a sample of only one, life on Earth. The general definition of life: that it’s something that has the ability to reproduce and evolve and utilize energy for its growth and development. Life creates order by utilizing energy. This is an essential attribute of life. Life has created versatile polymers that are used for reproduction, DNA, for the creation of structure, cellulose, and for utilizing energy, for example glycogen. Life is selective. It uses only a fraction of the elements in the periodic table, only a fraction of the possible amino acids that can be constructed, and a tiny fraction of the possible proteins. We should always remember that the molecular basis of life defined most broadly need not be DNA or RNA, need not be any molecule we are familiar with at all, and finally, we must be aware that the possibility exists that life elsewhere in the universe might not be based on carbon chemistry.

Transcript: Life on Earth formed in water. Earth is the water planet, and water is believed to be essential for life if life elsewhere is similar to that on Earth. Water is forty to ninety percent of the mass of all plants and animals. Even though we or our ancestors emerged from the oceans hundreds of millions of years ago, we are mostly made of water. Water in a cosmic sense is the most abundant liquid molecule in the universe, so it’s perhaps natural that water might be the basis of life. It's a solvent that facilitates chemical reactions which must have been important in the development of the first biology. It’s not the only possible solvent. At lower temperatures, for example, ethanol, which boils at minus eighty-nine degrees and freezes at minus a hundred and seventy-two degrees centigrade, could be used. Or at higher temperatures, phenol, which freezes at forty three degrees centigrade and boils at a hundred and eighty-two, but water is the best solvent. Being a solvent and having the properties that water does allows organisms to regulate their temperature. Water also absorbs ultraviolet radiation and so protects the cells from damage, and it has the unique property that it expands upon freezing so when life formed in the ponds or oceans of the early Earth, they did not freeze solid. Water is not unique as a liquid or as a solvent or as a basis for life, but its best in all of its properties for the formation of life.

Transcript: If life elsewhere in the universe is like life on Earth, then carbon will be essential for the creation of life. Hydrogen, carbon, nitrogen, and oxygen together form ninety-five percent of the mass of all life forms on Earth. Carbon, nitrogen, and oxygen are the most abundant products of stars, but carbon is special in its chemical properties. It has the unique ability to form multiple bonds with itself and with other elements. For example, the most abundant element in the universe, hydrogen, can only combine two ways with oxygen to form H2O and H2O2, can only combine two ways with nitrogen to form NH3 and N2H2, but it can combine thousands of ways with carbon to form chains as long as C90H84. The long carbon based chains of organic chemistry are essential to the coding of complexity in life. However, organic chemistry can only exist and operate in a narrow temperature range, so the robustness of life depends sensitively on the physical conditions. Silicon has been speculated to be another basis for life given its similar position in the periodic table However, silicon is ten times less abundant cosmically than carbon, and its chemical properties are not as good for the formation of long term, stable, complex molecules.

Transcript: Astronomers make the strong assumption that stars are essential for life. This is clear in the case of the Earth. The Sun is the energy source for all life on Earth. When you move, the energy that powers your muscles came from the food that you ate. If that food was meat, then those animals ate plants. Those plants got their energy from photosynthesis, and the source of that energy was the Sun. When we follow the food chain of life on Earth, it always points to the Sun. Solar energy is therefore essential for life on Earth, and we think life could not exist without it. But it’s worth bearing in mind that energy can be supplied in other ways. There could be gravitational energy from a collapsing object, thermal energy from compression caused by gravity, or even energy caused by tidal stretching. None of these mechanisms need thermonuclear fusion from a star to exist, so we should be open minded about the possibility somewhere in the universe of life that does not depend on stars.

Transcript: Astronomers are focusing their search for life in the universe on planets because it’s presumed that the surfaces of planets provide the best place for chemistry and biology to develop and interactions and complexity to emerge. However, this is an assumption also. Astronomers have found giant planets around extrasolar stars, so we know that planets are abundant in the universe. While we are finding giant planets, it’s not anticipated that life will easily form in the atmospheres of giant planets, and their surfaces are likely to be too hot and at too high a pressure for life as we know it to develop. So astronomers would like to be able to locate Earth-like planets or terrestrial planets around other stars, but we might speculate whether a planet is needed at all. At the very least, moons might be suitable sites for life, either in the inner or outer parts of solar systems, because moons can get there energy sources from tidal heating or if they are massive enough from their own internal gravity. The astronomer Fred Hoyle wrote a science fiction book called Dark Cloud where he speculated about life forming in the dense inner regions of an interstellar molecular cloud. Although nobody really believes that life could emerge in a region that's devoid of stars or planets and is just diffuse gas in space, we do not know for sure, so we should keep an open mind about the possible range of sites for life in the universe.

Transcript: We could learn something about the requirements of life by looking at the chemistry of organisms. Taking a human being as a typical example, we find that the primary constituents are carbon, nitrogen, oxygen, and hydrogen, but life depends in practice on the chemistry that involves twenty other trace elements. In the human body, oxygen accounts for sixty-five percent of the atoms, carbon eighteen percent, hydrogen ten percent, and nitrogen three percent. The hydrogen and oxygen combine to form water which is the largest single molecular constituent of the human body. The trace elements that follow are calcium at two percent, potassium at one percent, phosphorous at 0.4 percent, sulfur at 0.3 percent, sodium and chlorine at 0.2 percent. These are all trace minerals in the human body, but they are essential for life chemistry. Thus the four essential life elements are hydrogen, carbon, nitrogen, and oxygen.

Transcript: If we assume that life exists on the surface of planets, then the chemistry of planets is important. The Earth at its core consists of a large amount of heavy element, iron and nickel composite, but its crust is composed of other elements. Earth’s crust has its primary constituent oxygen at forty-six percent, silicon at twenty-eight percent. This is typical of the carbonates of sedimentary material and the silicates of most rock. The next most abundant elements are aluminum at eight percent, iron at six percent, calcium at four percent, sodium at 2.4 percent, magnesium at 2.3 percent, and potassium at 2.1 percent. Notice that of the essential elements for life, carbon, nitrogen, and oxygen, only one of these is well represented in the Earth’s crust. Metals tend to be heavily represented in the crust of planets.

Transcript: The chemistry of stellar material is not generally conducive to the formation of life. The two most abundant elements in the Sun and most other stars are hydrogen and helium. The abundance of hydrogen is seventy-five percent by mass and helium twenty-five percent by mass. Together, these two elements cannot make a complex chemistry. The next most abundant elements are oxygen at 0.01 percent, carbon at 0.009 percent, iron at 0.001 percent, silicon at 0.001 percent, nitrogen at 0.009 percent, and magnesium at 0.008 percent. Notice that carbon, nitrogen, and oxygen, the essential life elements, are trace elements in stellar material at abundances one part in a thousand compared to the most abundant elements. Other heavy elements are even less abundant in the Sun.

Transcript: Astronomers define the range of distances from a star within which life can exist as the habitable zone. By tradition, the habitable zone is declared in terms of the liquid state of water. The outer edge of the habitable zone corresponds to the distance from a star at which water becomes frozen, or ice. The inner distance of the habitable zone corresponds to the distance from a star at which water boils. This is purely an assumption and is of course only valid if life must have liquid water to exist. Low mass stars live a long time. There is plenty of time for life to develop, but their luminosities are very low. The luminosity of a star ten times less massive than the Sun is three thousand times less than that of the Sun, so its habitable zone is very close to the star and very slender. On the other hand, high mass stars live a short time, perhaps not long enough for life to develop, but their habitable zones are relatively large. So the volume of space over the whole galaxy, or even universe, in which life can exist is a trade-off between the large habitable zones of short-lived stars and the small habitable zones of long-lived stars.

Transcript: Stars are extremely important for life in several fundamental ways. First, stars are the cauldrons that create heavy elements. Fusion at the center of massive stars can create carbon, nitrogen, and oxygen that we believe are essential for our form of biology. Second, stars provide the energy source for life itself, and third, the process of star formation leaves a rocky residue which form planets which astronomers believe are the necessary sites for life. While those stars may be essential for life, its always worth reconsidering that assumption and speculating as to whether life could exist without the presence of a star.

Transcript: We think of chemistry as something that's defined on Earth, but it’s very important for the consideration of life in the universe that chemistry is universal. Astronomers have studied distant galaxies, distances of billions of lightyears, and discovered the same universal abundance of elements that they would find nearby in the Milky Way or the solar neighborhood. This is enormously important because it means that the raw ingredients for life exist throughout time and space in this huge universe filled with stars and galaxies. Of the universal abundance of elements, the most abundant elements, hydrogen and helium, were created in the big bang itself, and all other elements are essentially trace elements produced by stars and ejected into the interstellar medium. The fundamental life elements, carbon, nitrogen, and oxygen, have abundances of about 0.1 percent relative to hydrogen, the most abundant element. They are trace elements, but they are most abundant trace elements. Thereafter in the periodic table, the abundance declines steadily to elements like iron and nickel at about 0.01 percent abundance and then rapidly falling to metals like silver, tin at about a billionth of a percent solar abundance and gold at ten to the minus ten percent. The heavy element abundance of trace elements is important because a tiny concentration of these elements has nonetheless been sufficient to concentrate in planets and to aggregate into chemical and biological systems.

Transcript: The universe has not always been the way it is now. The early universe was smaller, hotter, and denser. In fact in the universe soon after the big bang, matter was too hot to exist in the normal, neutral form. Atoms and molecules did not exist, and even after this period, the only elements that could exist were hydrogen and the helium created in cosmic nucleosynthesis by the universe itself plus trace amounts of lithium and beryllium. Thus in the very early universe, there were essentially no heavy elements, so no possibility to form planets, and no possibility of carbon chemistry. The first stars formed a few hundred million years after the big bang. The time is still uncertain, and this epoch has not yet been directly observed by astronomers. However, as soon as the first stars and galaxies formed, heavy elements began to be produced in the cycle of star birth and death: fusion in the cores of massive stars, ejection of the elements in supernovae, and winds from massive stars into the interstellar medium where they subsequently could be part of a next generation of stars. Thus over cosmic time, the heavy element abundance in the universe has steadily increased, and in that sense life can become more likely as time progresses. But not all parts of the universe are created equal. Elliptical galaxies form early, have relatively low rates of star formation, and low heavy element abundances whereas spiral galaxies have active star formation and higher heavy element abundances. So we might imagine that spiral galaxies are more likely galactic sites for life to occur.

Transcript: The enormous size of the universe is an important consideration in the discussion of the possibility of life. The typical distance between stars near the Sun is a couple of lightyears. The size of the Milky Way, a typical galaxy, is a few tens of thousands of lightyears. The distance between typical galaxies in the universe is a few million lightyears, and the size of the observable universe is about ten billion lightyears. In our consideration of life, we have to understand the nature of time and space implied by the vastness of the universe. The distance between stars is too far for direct exploration by any space probe that we’ve developed so far. Therefore, we can never expect to return samples of life on nearby planets of other stars. We have to use remote sensing techniques, information from the electromagnetic spectrum. Even then, we are limited by the velocity of light. The light travel times, therefore, between nearby stars are a couple of years. The light travel time across the Milky Way is tens of thousands of years, and it would take twice that amount for a return signal to be communicated, say, between civilizations. The distance to nearby galaxies is such that it would take a return signal millions of years to be exchanged. Most distances in discussions in the subject of life in the universe involve only the Milky Way, but whatever we conclude about the Milky Way we must multiply up the possibilities by a factor of fifty billion representing the totality of galaxies and stars in the universe. astronomy astronomers

Transcript: Cosmology sets the context for the discussion of life in the universe. Our current view of the universe is that it is large, old, and cold. The universe is expanding from an initial hot, dense state, about eleven or twelve billion years ago, the big bang. The universe therefore is several times older than the Earth itself, and so life had billions of years to evolve and develop before the Earth even formed. The universe is huge and filled with matter in the form of stars and galaxies. The total number of galaxies is about fifty or sixty billion. Together they contain ten to the power twenty stars, a hundred billion billion stars, and this forms the total set of potential sites for life. So the universe has plenty of potential places that life could exist and has had plenty of time to evolve the complex chemical and biological processes needed for life.

Transcript: Surveys continue to show that the public believes that aliens are real and exist, perhaps even that the government has evidence of them that it’s hiding. This widespread public belief goes against the grain of science which relies on evidence to confirm a hypothesis. Carl Sagan has said, “Extraordinary claims require extraordinary evidence.” Claims that aliens exist and have already been found requires physical evidence, proof. A conspiracy theory will not suffice. Scientists should be cautious skeptics. Virtually every scientist working today does not believe in the existence of intelligent aliens that may have visited the Earth or visit the Earth even now. Scientists believe that the scientific study of life in the universe is possible but only if we apply the rules of the scientific method. Knowledge gained in this way must be public, and it must be testable.

Transcript: “Either we are alone in the universe, or we are not. Either way, the implications are staggering.” So said visionary and architect Buckminster Fuller. What he meant was the universe is huge and filled with planets and stars. In the multitude of stars and potential planets, if the Earth were the only one to harbor life, it would indeed be an extraordinary outcome, but logically it is possible. On the other hand, if the universe is filled with life forms of different kinds, where does that place us in our view of ourselves? The study of life in the universe starts with the study of the Earth itself. How did life emerge four billion years ago from simple chemical ingredients, and where did those chemical ingredients come from? Do planets exist out there in space around other stars, and what fraction of them have conditions suitable for complex chemistry and biology? What are the possible modes of biology that can evolve given suitable chemistry, and how many of those modes of biology lead to complexity? Because when we ask “Are we alone?” we’re really asking a more complicated question. “Are we alone?” implies the existence of intelligent life. Its possible that life may exist and be widespread but not be intelligent, purely microbial. We want to know the answer to the question either way, but the stakes are raised enormously if intelligent life exists elsewhere in the universe.

Transcript: Is there life in the Universe beyond the Earth? After centuries of speculation, scientists are finally developing the tools to answer this fundamental question about our place in the universe. The discovery of life beyond Earth would be perhaps the final step in the Copernican Revolution. Copernicus showed that the Earth is not the center of the universe, that the planets are other bodies like the Earth, that the Sun is the center of the solar system. Since then we have found that the Sun is just a typical star, and the Milky Way is just a typical galaxy. In a universe filled with billions of galaxies and many trillions of stars, how unlikely would it be that the Earth is the only place where life developed? This is speculation. Scientists need the methods of science to answer the question. The scientific method is on thin ice talking about life in the universe because we only have one example to study, life on Earth. The history of life on Earth can teach us lessons, but in the end we need data or evidence from beyond the Earth to answer the question. The study of life on the Earth is the subject of astrobiology. It’s an interdisciplinary pursuit occupying scientists in physics, astronomy, space science, biology, chemistry, and even sociology. Finally we may be on the verge of answering this question.

Transcript: Astronomers have learned much about the universe as a whole and now know its expansion rate, its size, its age, the density of matter and radiation, and the way in which structure formed. There are still deep mysteries regarding dark matter and dark energy. But the big bang model makes successful predictions going back to the first minute after the big bang, and it’s possible to meaningfully speculate about the first tiny fractions of a second. In their search for ultimate theories, modern cosmologists are the heirs to Democritus and Pythagoras, trying to find and understand the harmonies in nature.

Transcript: Cosmologists can trace the big bang back to a quantum seed. In the inflationary model, the physical universe, the totality of space-time, must be very much larger than the observable universe that we can see. Alan Guth, one of the founders of inflation, has said that the universe may be the ultimate free lunch because in fact the entire cosmic expansion can be created from a quantum fluctuation, from energy borrowed from the vacuum of space. Another idea is chaotic inflation which hypothesizes many regions of space-time, all quantum fluctuations, creating different physical properties. In the sea of space-time foam, many universes might emerge. Some would be stillborn. Others might be short-lived, and a few might take flight as ours has done. All the universes could have different laws of physics with the laws of physics in our universe just one representation. Chaotic inflation is pure speculation, but its an evocative idea to make it clear that our universe might not be the only one.

Transcript: As theorists in physics attempt to unify gravity with our ideas of matter, they’ve come up with a series of theories called superstring theories or brane theories, brane in this case being spelt b-r-a-n-e. In these theories, we replace the idea of particles with the idea of one dimensional strings. All the properties of particles result from the many modes of interaction, oscillation, vibration, and creation and annihilation of tiny little strings. These are microscopic entities far smaller than the nucleus of an atom. All of the properties of normal particles including their mass are derived quantities from the behavior of these superstrings. In many of these theories, theorists have to consider multi-dimensional space, beyond the three dimensions we are familiar with. Some of the theories have six dimensions, others have ten, even eleven. The theories are utterly speculative. The mathematics is fiendishly difficult, but there are many clever people working in this subject and time will tell whether they are headed towards the ultimate theory of matter.

Transcript: Given our knowledge of the fundamental forces of nature, cosmologists speculate about an early phase of the universe on an unimaginable iota of time after the big bang, ten to the minus forty three seconds after the big bang, when the forces of nature were all equal. This is called the Planck era or the Planck epoch after one of the founding figures of quantum mechanics. In this tiny instant of time after the big bang the temperature was ten to the power thirty-two Kelvin. The size of the universe was ten to the minus thirty-five meters, much smaller than a proton. The universe was a seething space time foam where there was no distinction between particles and energy or even between particles and the space they occupy. This represents the limit to our knowledge. With no current theory for the quantum nature of gravity, astronomers can only speculate what the universe was like at the instant of its creation.

Transcript: It's a premise of unified theories of nature that the world is governed by an underlying single superforce, but we live in a world where the symmetry is not obvious. Matter vastly dominates antimatter, and the four forces of nature have vastly different strengths and effects on our world. Symmetry in physical theories is only reached at the very highest energies, and as the energy falls the symmetry is broken. There are more mundane examples of the same effect. Think of water where the molecules are all randomly oriented, a symmetric situation, but in ice the particles must align in a particular situation. Or a magnet, where as the temperature is raised, the particular alignment of the magnetic domains becomes randomized. Or consider a set of pencils standing on their end which is a high energy configuration. As they fall, lowering their energy, they take up a particular set of orientations with no more symmetry. The example in the universe we live in is the asymmetry that results from the separation of the strong nuclear force from the weak nuclear force in the electromagnetic force as the universe cooled in the very early fazes of the expansion. The slight asymmetry gave the result of a slight excess of matter over antimatter. Without the broken symmetry, if matter and antimatter had been utterly equal at that epoch, they would have annihilated completely leaving a universe filled only with radiation. We should be thankful for the broken symmetry, because without it we would not exist.

Transcript: Unification is the idea that the four fundamental forces of nature are all manifestations of some simple unified super force. On the face of it, this seems unlikely. The four fundamental forces span a range in strength of a factor of ten to the power forty. Two are infinite range forces, and two are short-range within the atom. But physicists made progress in the 1970s when they showed that the electromagnetic and weak nuclear forces could indeed be unified at the energies reached by accelerators, and since then physicists have made theoretical progress on grand unified theories which unite the electroweak, or electromagnetic and weak nuclear forces, with the strong nuclear force. Observational verification of this might come in the next generation of accelerators, but it could also come from astronomical observations of the early universe. The ultimate goal in this game is to bring in the fourth force, the weak force of gravity, and unite the worlds of the quantum and cosmology. Quantum cosmology is an exceptionally difficult theoretical undertaking that is subject to the efforts of the world’s best physicists.