Summary of Our Improbable Universe
Chapter 6: Muck Makes Microbes
The accumulation of the material of the Earth out of the solar nebula took half a billion years. During that time the Earth's surface was repeatedly subjected to the bombardment of meteor showers. The last major shower liquefied the surface of the Earth and the Moon. The huge craters on the Moon witness its violence. At that time, four billion years ago, both worlds must have been sterilized. Despite this, within a mere few hundred million years, complex single cell bacteria evolved. The imprints of their spherical cells have been found in ancient rocks. The mystery of how these early life forms might have evolved out of the complex material reality that the Big Bang produced is explored in this chapter. The fact that it happened so rapidly indicates that it is a very common occurrence in this universe. Therefore, in a general sense, the history of the evolution of life on Earth is a history of its evolution in the universe as a whole.
The basic molecules of life, such as sugars, fats, and the building blocks of proteins called amino acids, are very common in this universe. They have been found in many meteorites. They can be made by exposing simple mixtures of gases to a number of naturally occurring energy sources. The heat of volcanoes, the ultra-violet light of the sun, and the plasma discharge of lightening are all capable of generating these molecules from the kinds of gases that are found today in the outer reaches of our solar system. These gases and some of these complex molecules have also been found in the giant molecular clouds from which stars condense. Therefore the raw materials for our kind of life exist in great abundance throughout the universe.
The abundance of environments in which life can evolve and the rapidity with which it evolved in our solar system combine to support the assertion that this universe is a Superfertile Universe. Life is a very common occurrence here. There are many implications to this statement. It implies that we have neighbors within a few hundred years travel time using nuclear technology. It implies that some of these neighbors are billions of years more advanced than ourselves. It also allows for the deistic hypothesis that the universe was deliberately designed to produce life. Presumably a creator that was interested in providing the conditions for life would not do a shoddy job by creating a slightly fertile universe. The Anthropic hypothesis only requires that the universe be marginally capable of evolving life. These observations and speculations are explored in more detail in later chapters.
The evidence that the universe is Superfertile is abundant. But the details of how life evolved here are a mystery. There is no fossil record of the prebiotic environment. Molecules are too small to leave imprints in rocks. Scientists are forced to make educated guesses that are based on test tube simulations of prebiotic chemical processes. The best hypotheses of our time are that the self replicating genetic molecules of life first formed spontaneously from a soup of chemicals some where in the early solar system. Molecules such as DNA, RNA, and PNA (peptide nucleic acid) spontaneously self assemble in mixtures that contain nucleic acid molecules. The place where this occurred may not have been here on Earth. The first stepping stone in the evolution of biological life may have been located anywhere in our solar system. Many meteorites from the surface of Mars and the Moon have already been found on the ice sheets of Antarctica. They were blasted from these bodies billions of years ago by the impact of large meteors. It is only a matter of time before pieces of other members of our solar system are found. Pieces of neighboring solar systems are undoubtedly here as well. So of these may have transported hitch hiking bacteria spores.
Once self replicating molecules evolved, the prebiotic soup was quickly consumed. The resulting environmental stress forced the hoarding of resources within cell walls. Naturally occurring spherical enclosures form from fat molecules. They can even be found in meteorites. The virus like early forms of life, that were based on strands of nucleic acids, only had to move in and set up shop. This is what modern viruses do to the complex cells that they infect. Life could leave a fossil record once this step was made. The oldest such fossils are three and a half billion years old. From their shape it is clear that life was already quite advanced at this early date.
The depletion of the prebiotic soup, by greedy free floating strands of nucleic molecules, not only motivated the evolution of the cell wall. After this stepping stone on the path to complex life was passed it became advantageous to be a producer of food as well as a consumer of it. The food that a cell produced would benefit the producer and not its competitors. Many complex chemical path ways evolved to exploit the geochemical energy sources of the infant Earth. But as the Earth cooled these sources became more limited. They are found only around hot springs where the Earth's crust is breached by molten rock from the interior. The most primitive bacteria alive today live only in these places.
One of the most important biological stepping stones was the evolution of photosynthesis. Solar energy is ten thousand times more abundant than geothermal energy. With this new source of energy life could really take off. But after a billion years the amount of oxygen in the atmosphere built up to a poisonous concentration. All of the life forms of the day were severely challenged by global oxygen pollution. Some retreated into deep muds where the oxygen could not penetrate. Others evolved the ability to use the oxygen to get ten times more energy out of burning sugar than was previously possible using only fermentation. These bacteria became the way of the future. The invention of the oxidation of sugars to get energy was yet another stepping stone on the path to intelligent life.
The ancestor organisms of advanced life forms adapted to the lack of food and the poisonous atmosphere in a unique way. Bacteria, that had made these adaptations, were taken inside of its cell wall. The chloroplasts, that make food for our plant cells, were free swimming photosynthetic bacteria billions of years ago. The mitochondria, that provide energy for plants and animals by oxidizing sugar, were also free swimming bacteria. Our ancestral cells became communities of organisms that lived in what is known as a symbiotic relationship.
The power of communal action was apparent even in the biological environment that existed billions of years ago. But the advantages of community are often countered by fratricidal competition. Our ancestral cells may have evolved male and female sexes in order to stem internal mitochondrial competition as a result of the sexual exchange of genetic material. From the very beginning of life, pieces of genetic material were exchanged between cells in a primitive version of sexuality. The benefits a combining separately evolved traits motivated this activity. But if mitochondria were exchanged as well then they would compete with each other. Therefore the few mitochondria that the male does not donate to the female's egg are destroyed soon after fertilization. Only the DNA from the nucleus of the cell is transferred. The evolution of this restricted version of safe sex, over a billion years ago, laid the ground work for the accelerated evolution of our type of cell. The invention of the sexes was therefore yet another major stepping stone to our self reflective existence.
The human body is based on the chemistry of about thirty thousand different proteins. The evolution of each of these proteins occurred in thousands of minor steps. Our genetic variety shows that there are multiple paths to the same goal. Similarly the major mile stones in the evolution of biology on Earth, that are highlighted above, are not the only conceivable way for intelligent life to have evolved. But if the process had stalled at any of these steps, then it would not have produced us in time to counter the run away green house effect that lurks a billion years in the future. If each step took twenty five percent longer to happen then the gradual overheating of the Sun would sterilize the planet before an intelligent care taker could fix the problem.