The chances are that you are wearing a gold ring or some other adornment fashioned from a precious metal. Take a good look at it; the atoms in that piece of jewelry are older than the entire Earth. Then consider the iron in your blood, the calcium in your bones, the oxygen that you breathe—all those atoms are older than our home planet and all were forged inside a massive star. Understanding how this stardust has been processed into living organisms is one of the thorniest questions in science.
Trapped within the meteorites that fall to Earth is a wealth of clues to our origins as stardust. Planetary scientists have even found miniscule samples of stardust itself, which they call “pre-solar grains.” The tell-tale sign may be a silicon carbide crystal about a millionth of a meter across, or a nano-diamond containing just a hundred or so atoms. When meteorite material is analyzed, stardust stands out because of its unique mix of isotopes (elements of a particular mass)—the tiny crystals have remained intact since they were formed, either in an expanding cloud of supernova debris or in a red giant star’s tenuous atmosphere. The pre-solar grains grow in these regions because of the relatively slow flow of warm gas there, rather like soot forming in chimneys. The constituents of the grains allow scientists to work out what processes take place inside stars to produce the chemical elements of the world today.
All the elements we find on Earth, with the possible exception of hydrogen, were created by nucleosynthesis (nuclear fusion reactions) inside the cores of stars (see What Are Stars Made From?). Somehow, at sometime in the Earth’s history, the chemicals of stardust have grouped themselves together in such a way that living systems have evolved. This process could be said to be the ultimate mystery: how life formed.
Each step, from inorganic chemicals born in stars to living cells, involves crossing boundaries between the three traditional sciences of physics, chemistry and biology. Across each boundary, matter begins to behave differently and manifest unanticipated properties. For example, put enough subatomic particles together and they organize themselves into atoms based on the laws of physics. As soon as these atoms start to interact with one another, physics hands over to chemistry because the panoply of chemical interactions is difficult, if not impossible, to predict from the laws of physics. When a sufficiently complicated network of chemicals comes together, life spontaneously emerges and chemistry can no longer predict the rich variety of behaviors.
To tackle the problem of how life began on Earth, we need to go back to the final stages of the planet’s formation, when it was pummeled with asteroids and comets (see How Did the Earth Form?). This “late bombardment” began 4.6 billion years ago and lasted approximately 700 million years. It brought water and other volatile materials to the planets. (In this context, volatile describes chemicals that vaporize at relatively low temperatures, such as water, carbon dioxide, methane and ammonia.) All of the elements contained in the molecules of these volatile substances are from the so-called “CHNOPS” range of elements, upon which life on Earth is based: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. Of these, oxygen happens to be the most abundant element on Earth, making up nearly half the mass of our planet, with most of it bound into rocks rather than found in the atmosphere. Similarly phosphorus and sulfur are found in the rocks from which Earth is made. In other words, the late bombardment brought many of the vital ingredients for the subsequent development of life on Earth.
“Astronomy leads us to a unique event, a Universe which was created out of nothing, one with the very delicate balance needed to provide exactly the conditions required to permit life.”
ARNO PENZIAS 20TH CENTURY PHYSICIST
In 1871, Charles Darwin wrote a letter in which he described life’s origin as taking place in a “warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc. present, so that a protein compound was chemically formed ready to undergo still more complex changes.” Our current knowledge, however, suggests that this gentle scenario may be a long way from the truth.
The late bombardment created hellish conditions on the Earth. It melted the crust of the planet, threw molten rock into the atmosphere, and evaporated the fledgling oceans. Nothing could seem more opposed to the development of fragile biological molecules, yet intriguingly the evidence suggests that life began soon after the late bombardment began to tail off. There was no sudden end to the bombardment, but a gradual reduction of collisions over time, and by 3.9 billion years ago the impacts had dwindled so much that the late bombardment was effectively over. Scientists find the first evidence for life in rocks dating from just 100 million years after the bombardment stopped. The indication is an enrichment of the lightest stable carbon isotope, carbon-12, which life uses in preference to its heavier cousins because the lighter variety can pass more easily through cell membranes. Wherever living creatures have died, they tend to leave behind an enrichment of carbon-12. So, finding the isotope in those ancient rocks has been taken as a sign that microbes were present on Earth relatively soon after the fury of the late bombardment had ceased.
The first fossils are found in Earth rocks dating back 3.5 billion years ago, in western Australia; these are microfossils, which are the preserved remains of ancient bacteria, the prehistoric equivalent of pond scum. It may not be a particularly satisfying thought that those were the first life forms on Earth but it seems to be the hand nature has dealt us. The “cyanobacteria,” as they are called, are found as stromatolites, pillow-shaped communities of bacteria that grow rather like a coral reef. They are formed in shallow water as the cyanobacteria trap nutrient-rich sediments and become cemented into a colony.
So, whatever triggered life on Earth undeniably happened relatively shortly after our planet’s formation and the late bombardment subsided. One other fact seems certain. Today, the only way in which life is created is through biological reproduction; it is not spontaneously forming around us, which strongly suggests that the conditions life first formed under must have been utterly different from those that help it thrive today. Darwin recognized this as a problem for his “warm little pond” hypothesis, which blindly assumed that Earth’s environment had always been largely the same throughout its existence. He suggested that the chemical steps toward life are still being taken in the ponds of today, but that anything produced would be instantly palatable and so eaten before it could develop any further. Modern molecular analysis proves that this is not the case, but it does nevertheless suggest other possible routes to life.
In the 1950s, chemists Harold Urey and Stanley Miller conducted an experiment that tried to simulate the early Earth. They based their work on the hypothesis of Russian biochemist Alexander Ivanovich Oparin, who in the 1920s, had been the first to suggest that that there is no magical difference between living and nonliving matter, that the characteristics of life simply emerge from a sufficiently complex arrangement of matter. He also suggested, spurred on by the discovery of methane in the atmosphere of Jupiter, that methane and its volatile cousins, water and ammonia, were the chemical ingredients from which life formed.
Urey and Miller set out to test Oparin’s ideas. They filled a flask with the chemicals they believed existed in the early Earth’s atmosphere—chiefly methane and ammonia—and then applied electrical sparks to simulate lightning. As the electricity caused the gases to react together, longer molecules were formed, which dropped into a small pool of water at the bottom of the flask and formed a tarry substance. Upon analysis, this thick gunk was found to contain amino acids, which are the building blocks of proteins. It seemed, miraculously, as if the first step in the process toward life might have been found.
However, as other scientists built on this work, they came to the conclusion that Earth’s early atmosphere was more likely to have been composed principally of carbon dioxide. This was bad news because when the Miller-Urey experiment was re-run with a carbon dioxide atmosphere it was nowhere near as successful at producing amino acids. But just when scientists faced a dead end, a new clue landed in their laps—almost literally.
It was September 28, 1969, late morning in the quiet town of Murchison in Victoria, Australia. A burning fireball split the sky, broke into three and disappeared from view, leaving a smoke cloud hanging. Many fragments of the meteorite were soon found, totaling more than 100 kilograms, and were identified by visiting academics as a rare form of space rock known as a “carbonaceous chondrite.” Samples were rushed to NASA for analysis, where scientists were amazed to find more than 90 different amino acids within the meteorite material. This clearly indicated that amino acids were assembled in space and brought to Earth during the late bombardment.
However, it seems that the Earth exploited only a small fraction of these amino acids to create proteins—just 20 amino acids go together in different combinations to make up the millions of different proteins used by life on Earth. So the puzzle is how the multitude of 90 or more amino acids led to a functioning life form using just 20.
It is believed that all life today evolved from a single common ancestor, the first organism to form and presumably a very simple living thing. To understand how it came about, a good place to start investigating is amongst the smallest living things on Earth today: microbes. The bacterium E. coli is the “laboratory standard”; it is rod-shaped and just three millionths of a meter in length. It contains 4377 genes, as compared with the human genome which contains about 40,000. The genes hold the blueprints for the proteins needed to make the life form function. In the case of humans, genes control everything from musculature to eye color. The common thread between microbes, humans and all other forms of life on Earth, is that all genes are contained in molecules of DNA, deoxyribonucleic acid.
DNA is a long molecule, the backbone of which is a chain of carbon atoms. From this carbon spine hang the genes, each composed of a sequence of chemicals. There are two complementary strands that wrap around each other to form the famous double helix, locking the genes inside. But at specific times the strands can unwrap and make copies of themselves; this ability to replicate lies at the heart of all living things. Yet the replication is not a perfect process; errors creep into the genes when they are being copied and while this might seem like a bad thing, it is actually what drives evolution. Although the errors, known as “mutations,” mostly make the proteins behave less successfully, occasionally they improve their function and the life form flourishes, increasing the chances that the favorable mutation will be passed on. By charting mutations and by showing when they diverged from one another, biologists can build up a tree of life that shows how organisms are related. Human beings and other mammals lie near the top of the tree, above simpler animals such as reptiles and insects. Toward the base of the tree are the microbes such as E. coli, and sitting closer to the bottom of the tree than anything else is a group of microbes called the hyperthermophiles.
BLACK SMOKERS: SEAWATER IS HEATED BY VOLCANIC ACTIVITY, DISSOLVES CHEMICALS AND THEN GUSHES BACK UPWARD
Hyperthermophiles live on the sea floor in the scalding water surrounding volcanic vents; they are able to withstand temperatures up to 121 degrees Celsius and would actually die if placed in water at less than 90 degrees. They are a subset of a group called the extremophiles, which all live in conditions that would be detrimental to the vast majority of Earthly life. Some extremophiles like acidic conditions or highly salty ones; some like it hot, others like it very cold. They are the ultimate niche organisms. Each variety has evolved a highly unusual metabolism that can extract energy from its chosen extreme environment.
All animal life on Earth requires oxygen, yet various extremophiles can live off hydrogen, iron or many other chemicals that would spell certain death for most living things. Nowhere on Earth contains more readily available chemicals than the volcanic vents on the sea floor, where they are dissolved in the hot water gushing up into the ocean. This makes each volcanic vent a haven for extremophiles. Sea-floor volcanic vents are the underwater equivalent of geysers, such as Old Faithful in Yellowstone National Park. Often known as “black smokers” because the dissolved minerals condense into black clouds as soon as they hit the frigid water surrounding them, they were first found near the Galapagos Islands in 1977. Most surprising of all, the analysis to place their organisms on the tree of life revealed the hyperthermophiles to be the most ancient forms of life on the planet.
This could mean that black smokers are the actual sites for the origin of life; they certainly offer some advantages because the hyperthermophiles are the only known colonies on Earth that do not rely on sunlight for energy. If the Sun went out tomorrow, the communities around the black smokers would continue to thrive. Their energy comes from volcanic activity, driven by radioactivity within the Earth, and so they live quite independently of the Sun. This makes them immune to almost anything going on at the surface: ice ages or other climate catastrophes, even the last years of the late bombardment could have taken place without threatening them.
But there is one inconvenient fact that casts doubt on the black smokers as the site of life’s origin. The “tree of life” analysis shows that the hyperthermophiles are indeed ancient but cannot be the common ancestor of all life on Earth. The microbes that evolved into us split away before the hyperthermophiles developed; this is indicated by the fact that hyperthermophiles contain genes that we do not. So, life may have developed elsewhere and some form of it then migrated to the black smokers. We have not yet been able to discover the earliest life forms, either because they are extinct and have left no traces, or because we have yet to look in the right place. This has led some scientists to think that life’s original form may be even less complicated than a microbe.
In 1996, geologist Philippa Uwins discovered tiny growths on rock samples retrieved from oil wells. The rocks came from between 3 and 5 kilometers (1.8 to 3.1 miles) below the sea floor, and the growths looked strangely organic. They attracted a dye that binds to DNA, further hinting that they could indeed be living, and researchers began to wonder whether these could be the simplest forms of life on Earth.
They were dubbed “nanobes” because the smallest ones are just 20 billionths of a meter across. Even the largest are just one tenth the size of a microbe, and this presents a fundamental problem: there does not appear to be enough space within a nanobe to hold the DNA copying machinery usually found in a microbe. Unless nanobes contain some simpler kind of DNA-copying system, they would appear to lack the necessary mechanism to be alive. If the nanobes are confirmed to be living, they hint that life began deep inside the Earth. As the nanobes migrated upward, perhaps they evolved into the microbes we find around the black smokers.
Since nanobes were discovered, other minuscule “life forms” have been suggested but the question of whether such incredibly small things can truly be alive is a controversial area of study, with no firm conclusion yet reached. How will scientists know for sure that these tiny objects have taken the leap in complexity from nonliving to living matter, crossing the boundary from chemistry to biology? At present, science cannot answer this question because it lacks an incontrovertible definition for life.
The best route in trying to define life is to list a number of traits that it must have. For example, we could say that all living things must: 1—eat or take in some form of energy; 2—excrete what they do not use; 3—respond to their environment, usually by moving; 4—reproduce and pass on traits to their progeny; 5—be capable of having those traits change between generations. So far so good, but then the mule springs to mind. It is the offspring of a male donkey and a female horse, and in the vast majority of cases it is infertile. By strict application of our rules above, the mule fails points 4 and 5, yet it is undoubtedly alive. Now consider a virus: it needs to invade a living cell in order to hijack the copying mechanism and reproduce itself. So its status as a living organism is debatable.
Coming up with a strictly applicable definition probably requires a new way of looking at life. Think about how to define water: we might say “a clear, colorless liquid.” Unfortunately this will not do, because so is ammonia, to name just one noxious liquid that you would not want to confuse with water. The only way to pin down water precisely is to describe its chemical composition: H2O. Before we had knowledge of atoms and chemistry, it was impossible to define water—we just knew it when we tasted it. We are in the same situation with defining life. We know it when we see it, we can take a stab at describing it, but as yet we cannot define it.
A way may present itself through a branch of mathematics called “information theory.” Founded in 1948 at the dawn of the computer age, information theory seeks to quantify information and find the fundamental limits that govern its storage, processing and communication. Since DNA carries information in the form of genes, perhaps if we regard biology as a form of computation we may be able to define life mathematically. Imagine that a biological system is a computer: it takes information from the genes, rather like a computer reading a program from a hard drive. It similarly processes the information and creates an output. In the case of a living system, this output is expressed in the form of proteins. So, it is credible that the definition of life lies in the way our cells process the information content of our molecules. Certainly there is no active information processing taking place in a rock.
Work continues to investigate this analogy between living systems and computers, in an attempt to see if it can be transformed into a mathematical definition of life. Simultaneously, in laboratories researchers try to find out whether there are simpler molecules that can replicate themselves like DNA. If so, these could have held the genes of earlier, less complex forms of life. Further clues may come from space probes that are being sent to other planets and to comets to look for amino acids and other building blocks of life. Although we can say with absolute certainty that we are made of stardust, how that stardust then transformed itself into life remains a mystery.