2 - In The Beginning: The Origin Of Life

The first chapter dealt with a scientific search that had a very practical goal-ways of killing dangerous insects. When you solve the problem, there is no mistake about it; the insects die.

But there are also problems that are much more difficult to tackle; problems that are so complicated it is even hard to tell whether we are on the road to solving them, or just in a blind alley. Yet they are problems so important that man's curiosity forces him to tackle them anyway.

Consider the question: What is life?

There is no plain answer yet and some scientists wonder if there ever can be. Even the simplest form of life is composed of very complex substances that are involved in so many numerous complicated chemical changes that it is almost hopeless to try to follow them. What parts of those changes make up the real basis of life? Do any of them?

The problem is so enormous that it is like a continent that must be explored at different points. One group of explorers follows a huge river inland; another group may follow jungle trails elsewhere; while a third sets up a camel caravan into the desert.

In the same way, some biologists analyze the behaviour of living animals under various conditions; others study the structure of portions of tissue under microscopes while still others separate certain chemicals from tissue and work with them. All such work contributes in its own way to increasing knowledge concerning life and living things.

Enormous advances have indeed been made. The two greatest biological discoveries of the nineteenth century were 1) that all living creatures are constructed of cells, and 2) that life has slowly developed from simple creatures to more complex ones.

The first discovery is referred to as the "cell theory," the second as the "theory of evolution."

Both theories made the problem of life seem a little simpler. Cells are tiny droplets of living substance marked off from the rest of the world by a thin membrane. They are surprisingly alike no matter what creature they are found in. A liver cell from a fish and one from a dog are much more similar than the whole fish and dog are.

Perhaps if one could work out all the details of what makes individual cells alive, it would not be so difficult to go on and get an understanding about whole creatures.

Then, too, consider that there was a gradual development of complex organisms from simpler ones. In that case, it might well be that all creatures that exist today developed from the same very simple one that existed long ages ago. There would then be only one form of life, existing in many different varieties. If you understood what made a housefly alive, or even a germ, you ought then understand what makes a man alive.

But these nineteenth century theories also raised a new problem. The more people investigated cells and evolution, the more clear it became that all living creatures came from other living creatures; all cells came from other cells. New life, in other words, is only formed from old life. You, for example, are born of your parents.

Yet there must have been a time in the early history of the Earth when there was no life upon it. How, then, did life come to be? This is a crucial question, for if scientists knew how the first life was formed on a world that had no life on it, they might find they had taken a big step forward in understanding what life is.

Some nineteenth century scientists were aware of this question and understood its importance. Charles Darwin, the English biologist who first presented the theory of evolution to the world in its modern form, speculated on the subject. In a letter written to a friend in 1871, he wondered if the kind of complex chemicals that make up living creatures might not have been formed somewhere in a "warm little pond" where all the ingredients might be present.

If such complex compounds were formed nowadays, tiny living creatures existing in that pond would eat them up at once. In a world where there was no life, however, such compounds would remain and accumulate. In the end, they might perhaps come together in the proper way to form a very simple kind of life.

But how can one ever find out? No one can go back billions of years into the past to look at the Earth as it was before life was on it. Can one even be sure what conditions were like on such an Earth, what chemicals existed, how they would act?

So fascinating was the question of life's origin, however, that even if there was no real information, some scientists were willing to guess.

The twentieth century opened with a very dramatic guess that won lots of attention. The person making the guess was a well-known Swedish chemist, Svante August Arrhenius. In 1908, he published a book, Worlds in the Making, in which he considered some new discoveries that had recently been made.

It had just been shown that light actually exerted a push against anything it shone upon. This push was very small, but if the light were strong and an object were tiny, the light-push would be stronger than gravity and would drive the object away from the Sun.

The size of particles that could most easily be pushed by sunlight was just about the size of small cells. Suppose cells were blown, by air currents, into the thin atmosphere high above the Earth's surface. Could they then be caught by the push of sunlight and driven away from the Earth altogether? Could they then go wandering through space?

That might be so but wouldn't the cells then die after having been exposed to the vacuum of outer space?

Not necessarily. It had also been discovered that certain bacterial cells could go into a kind of suspended animation. If there was a shortage of food or water, they could form a thick wall about themselves. Within the wall, the bit of life contained in the cell could wait for years, if necessary, without food or water from the outside. They could withstand freezing cold or boiling heat. Then, when conditions had improved, the wall would break away and the bacterial cell could start to live actively once more.

Such walled cells in suspended animation are called "spores." Arrhenius argued that such spores, driven by the push of light, could wander through space for many years, perhaps for millions of years, without dying.

Eventually, such spores might strike some object. It might be some tiny asteroid or some other cold world without air or water. The spore would have to remain a spore forever, until even its patient spark of life winked out. Or it might strike a world so hot as to cause it to scorch to death.

But what if the spore struck a world with a warm, pleasant atmosphere and with oceans of water? Then it would unfold and begin to live actively. It would divide and redivide and form many cells like itself. Over long periods of time, these cells would grow more complicated. They would evolve and form many-celled creatures. In the end, the whole planet would become a home for millions of species of life.

Is that how life originated on Earth itself, perhaps? Once long ago; billions of years ago; did a spore from a far distant planet make its way into Earth's atmosphere? Did it fall into Earth's ocean and begin to grow? Is all the life on Earth, including you and I, the descendant of that little spore that found its way here?

It was a very attractive theory and many people were pleased with it, but alas, there were two things wrong with it.

In the first place, it wouldn't work. It was true that bacterial spores would survive many of the conditions of outer space, but not all. After Arrhenius' book had been published, astronomers began to learn more about what it was like in outer space. They learned more about the sun's radiation for instance.

The sun gives out not visible light alone, but all kinds of similar radiation that might be less energetic or more energetic than light itself.

It radiates infrared waves and radio waves, which are less energetic than ordinary light. It also radiates ultraviolet waves and x rays, which are more energetic than ordinary light. The more energetic radiation is dangerous to life.

Much of the energetic radiation is absorbed by Earth's atmosphere. None of the x rays and very little ultraviolet manage to make their way down to Earth's surface, under a blanket of air miles and miles thick. Even so, if we stand on the beach on a sunny summer day, enough ultraviolet light reaches us to penetrate the outer layers of the skin and to give us sunburn (if we are fair-skinned).

In outer space, the ultraviolet light and x rays are present in full force. They easily penetrate a spore wall and kill the spark of life inside.

If spores were drifting towards our solar system from other stars, they might strike the outermost planets without harm, but on Pluto or on Neptune they would find conditions too cold for development. As they drifted inward towards Earth, they would be coming into regions where sunlight was stronger and stronger. Long before they could actually reach our planet, the energetic radiation in sunlight would have killed them.

It would seem then that spores, giving rise to the kind of life we now have on Earth, couldn't possibly have reached Earth alive.

Then, too, another flaw in Arrhenius's theory is that it doesn't really answer the question of how life began. It just pushes the whole problem back in time. It says that life didn't begin on Earth but on some other planet far away and long ago and that it reached our world from that other planet. In that case, how did life start on that other planet? Did it reach that other planet from still another planet?

We can go back and back that way but we must admit that originally life must have started on some planet from nonliving materials. Now that is the question. How did life do that? And if life started somewhere from non-living materials, it might just as well have done so on Earth.

So don't let's worry about the possibility of life starting elsewhere and reaching Earth. Let us concentrate on asking how life might have started on earth itself from non-living materials.

Naturally, we ought to try to make the problem as simple as possible. We wouldn't expect non-living substances to come together and suddenly form a man, or even a mouse, or even a mosquito. It would seem reasonable that before any creature even as complicated as a mosquito was formed, single cells would have come into existence; little bits of life too small to be seen except under a microscope.

Creatures exist, even today, that are made up of just one cell. The amoeba is such a creature. Thousands of different species of one-celled plants and animals exist everywhere. There are also the bacteria, which are composed of single cells even smaller than those of the one-celled plants and animals.

But these cells are complicated, too; very complicated. They are surrounded by membranes made up of many thousands of complex molecules arranged in very intricate fashion. Inside that membrane are numerous small particles that have a delicately organized structure.

It seems hopeless to expect the chemicals in a non-living world to come together and suddenly form even as much as a modern bacterial cell. We must get down to things that are even simpler.

Every cell contains chemicals that don't seem to exist in the non-living world. When such chemicals are found among non-living surroundings, we can be sure that those surroundings were once alive, or that the substances were originally taken from living cells.

This seems to be so clear that early in the nineteenth century chemists began to speak of two kinds of substances. Chemicals that were associated with living creatures, or organisms, were called "organic." Those that were not were "inorganic."

Thus, wood and sugar are two very common organic substances. They are certainly not alive in themselves. You may be sitting in a wooden chair, and you can be sure that it is no more alive than if it were made of stone. However, that wood, as you know very well, was once part of a living tree.

Again, the sugar you put on your morning cereal is certainly not alive. Still, it was once part of a living sugar cane or sugar beet plant.

Salt and water, on the other hand, are inorganic substances. They are found in all living organisms, to be sure; your own tears, for instance, are nothing but a solution of salt in water. However, they are not found only in organisms and did not originate only in organisms. There is a whole ocean of salt water that we feel pretty sure existed in some form or other before life appeared on this planet.

(Beginning in the middle of the nineteenth century, chemists began to form new compounds that were not to be found in nature. They were very similar in many ways to organic compounds, though they were never found in living organisms or anywhere else outside the chemists' test tubes. These "synthetic" compounds were, nevertheless, lumped together with the organic group because of the similarity in properties.)

It would seem then we could simplify our problem. Instead of asking how life began out of non-living substances, we could begin by asking how organic substances came to be formed out of inorganic substances in the absence of life.

To answer that question, we ought to know in what way organic substances differ from inorganic ones.

Both organic and inorganic substances are made up of "molecules"; that is, of groups of atoms that cling together for long periods of time. Organic molecules are generally larger and more complicated than inorganic ones. Most inorganic molecules are composed of a couple of dozen atoms at most; sometimes only two or three atoms. Organic molecules, however, usually contain well over a dozen atoms and may, indeed, be made up of hundreds, thousands, or even millions of atoms.

When we ask how organic compounds may be formed from inorganic compounds, then, we are really asking how large and complicated molecules might be formed from small and simple ones.

Chemists know that to force small and simple molecules to join together to form large and complicated ones, energy must be added. This is no problem, really, for a very common source of a great deal of energy is sunlight, and in the early lifeless Earth, sunlight was certainly blazing down upon the ocean. We will come back to that later.

It is also true that the different kinds of atoms within molecules cannot change their nature under ordinary circumstances. The large organic molecules in living matter must be formed from small and simple molecules that contain the same kinds of atoms.

We must ask ourselves what kinds of atoms organic molecules contain.

There are over a hundred different kinds of atoms known today (each kind making up a separate "element"). Over eighty are found in reasonable quantities in the inorganic substances making up the Earth's crust. Only half a dozen of these elements, however, make up the bulk of the atoms in organic molecules.

The six types of atoms occurring most frequently in organic molecules are carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulphur. We can let each one be represented by its initial letter: C, H, O, N, P,. and S. The initial letters could also stand for a single atom of each element. C could be a carbon atom, H a hydrogen atom, and so on.

Of these elements, carbon is, in a way, the crucial one. Carbon atoms can combine with each other to form long chains, which can branch in complicated ways. They can also form single rings or groups of rings; or, for that matter, rings with chains attached. To the carbon atoms arranged in any of these ways, other atoms can be attached in different manners.

These complicated chains and rings of carbon atoms are found only in organic compounds, never in inorganic compounds. It is this which makes organic molecules larger and more complicated than inorganic ones.

Carbon atoms can be hooked together in so many ways, and can attach other atoms to themselves in so many ways that there is almost no end to the different variations. And each different variation is a different substance with different properties.

Hundreds of thousands of different organic compounds are known today. Every year many more organic compounds are discovered and there is no danger of ever running out of new ones. Uncounted trillions upon trillions of such compounds can exist.

This seems to make the problem of the origin of life more difficult again. If we are trying to find out how organic substances are formed from inorganic ones, and if there are uncounted trillions upon trillions of organic substances possible, how can we decide which organic substance ought to be formed and which were formed in the past.

Suppose, though, we can narrow down the choice. Not all organic compounds are equally vital to life. Some of them seem to be more central to the basic properties of life than others are.

All cells without exception, whether plant, animal, or bacterial, seem to be built about two kinds of substances that are more important than any others. These are "proteins" and "nucleic acids."

Even viruses can be included here. They are tiny objects, far smaller than even the smallest cells, yet they seem to be alive since they can invade cells and multiply there. They, too, contain proteins and nucleic acids. Some viruses, in fact, contain practically nothing else but proteins and nucleic acids.

Now we have narrowed the problem. We must not ask how organic compounds were built up out of inorganic ones, but how proteins and nucleic acids were built up out of them.

That still leaves matters complicated enough. Both proteins and nucleic acids are made up of very large molecules, often containing millions of atoms. It is too much to expect that small inorganic molecules would come together suddenly to form a complete molecule of protein or nucleic acid.

Let's look more closely at such giant molecules. Both proteins and nucleic acids are composed of simpler structures strung together like beads on a necklace. Both protein and nucleic acid molecules can be treated chemically in such a way that the string breaks and the individual "building blocks" separate. They can then be studied separately.

In the case of the protein molecule, the building blocks are called "amino acids." The molecule of each amino acid is built around a chain of three atoms, two of which are carbon and one nitrogen. We can write this chain as -C-C-N-.

There would be different atoms attached to each of these. The atoms attached to the carbon and nitrogen atoms at the end are always the same in all the amino acids obtained from proteins (with a minor exception we needn't worry about). The carbon atom in the middle, however, can have any of a number of different atom-groupings attached to it. If we call this atom-grouping R, then the amino acid would look like this:

Each different structure for R results in a slightly different amino acid. Altogether there are nineteen different amino acids that are found in almost every protein molecule. The simplest R consists of just a hydrogen atom. The rest all contain different numbers of carbon and hydrogen atoms, while some contain one or two oxygen atoms in addition, or one or two nitrogen atoms, or even one or two sulphur atoms. Individual amino acids are made up of from eleven to twenty-six atoms.

Although there are only nineteen different amino acids in most proteins, they can be put together in many different ways, each way making up a slightly different molecule. Even a middle-sized protein molecule is made up of several hundred of these amino acids and the number of different combinations is enormous.

Imagine yourself to be given several hundred beads of nineteen different colours and that you set to work to string them. You could make necklaces of many trillions of different colour combinations. In the same way, you could imagine protein molecules of many trillions of different amino acid combinations.

In thinking of the origin of life, then, you don't have to worry, just at first, about forming complicated protein molecules. That would come later. To begin with, it would be satisfying to know whether the amino acid building blocks could be formed and, if so, how.

The nucleic acids are both simpler and more complicated than the protein. Nucleic acid molecules are made up of fewer different kinds of building blocks but the individual building block is more complicated.

The huge nucleic acid molecule is made up of long chains of smaller compounds known as "nucleotides," each of which is made up of about three dozen atoms. These include carbon, hydrogen, oxygen, nitrogen, and phosphorus.

An individual nucleotide molecule is made up of three parts. First there is a one-ring or two-ring combination made up of carbon and nitrogen atoms. If there is only one ring, this portion is called a "pyrimidine"; two rings is a "purine."

The second portion is made up of a ring of carbon and oxygen atoms. This comes in two varieties. One is called "ribose"; the other, with one fewer oxygen atom, is "deoxyribose." Both these compounds belong to the class called sugars.

Finally, the third part is a small atom group containing a phosphorus atom. It is the "phosphate group."

We might picture a nucleotide as follows:

There are two kinds of nucleic acid molecules. One of them is built up of nucleotides that all contain ribose. This is, there fore, "ribosenucleic acid" or RNA. The other is built up of nucleotides that all contain deoxyribose; "deoxyribosenucleic acid" or DNA.

In both cases, individual nucleotides vary in the particular kind of purine or pyrimidine they contain. Both RNA and DNA are made up of chains of four different nucleotides. Even though there are only four different nucleotides, so many of them are present in each enormous nucleic acid molecule that they can be arranged in trillions upon trillions of different ways.

Now that we have decided we want to form amino acids and nucleotides out of inorganic compounds, we must ask out of what inorganic compounds we can expect them to be formed. We must have inorganic compounds, to start with, that contain the right atoms: carbon, hydrogen, oxygen, and the rest.

To begin with, there is the water molecule in the oceans. That is made up of two hydrogen atoms and an oxygen atom and it can therefore be written H₂O. Then there is the carbon dioxide of the air, which dissolves in the ocean water and which is made up of a carbon atom and two oxygen atoms, C0₂. Water and carbon dioxide can supply carbon, hydrogen, and oxygen, three of the necessary elements.

Also dissolved in ocean water are substances that are called nitrates, sulphates, and phosphates. They contain nitrogen atoms, sulphur atoms, and phosphorus atoms respectively. These substances all have certain properties in common with ordinary table salt and can be lumped together as "salts."

What we have to ask ourselves now is this: Is it possible that once long ago, when the world was young, water, carbon dioxide, and salts combined to form amino acids and nucleotides. If so, how was it done?

There are certain difficulties in this thought.

To begin with, in order for water, carbon dioxide, and salts to form amino acids and nucleotides, oxygen atoms must be discarded. There is much more oxygen in water, carbon dioxide, and salts, than there is in amino acids and nucleotides.

But Earth's atmosphere contains a great deal of oxygen. To discard oxygen, when oxygen is already all about, is very difficult. It is like trying to bail the water out of a boat that is resting on the lake bottom.

Secondly, it takes energy to build up amino acids and nucleotides out of simple inorganic molecules and the most likely source is sunlight. Just sunlight isn't enough, however. To get enough energy, you must use the very energetic portion of the sunlight; you must use ultraviolet waves.

But very little of the ultraviolet waves gets down to the surface of the Earth. The air absorbs most of it. When scientists studied the situation more closely it turned out that it was the oxygen in the air that produced the substance that absorbed the ultraviolet.

So oxygen was a double villain. It kept the ultraviolet away from the surface of the Earth and its presence made it very difficult to discard excess oxygen.

To be sure, the plant life that covers the land and fills the sea is carrying through just the sort of thing we are talking about and doing it right now. Plants absorb water, carbon dioxide, and salts and use the energy of sunlight to manufacture all sorts of complicated organic compounds out of them. In doing so, they discard oxygen and pour it into the atmosphere.

However, to do this, plants make use of visible light, not ultraviolet waves. Visible light (unlike ultraviolet waves) can penetrate the atmosphere easily, so that it is available for the plants to use. Visible light has considerably less energy than ultraviolet waves but the plants make use of it anyway.

You might wonder if this could not have happened on the early Earth. Suppose the energy of visible light had been used to build up the amino acids and nucleotides.

It doesn't seem likely, though, that it could have happened that way. The reason it happens now is that plants make use of a complicated chemical system that includes a substance known as "chlorophyll." Chlorophyll is an organic compound with a most complicated molecule that is formed only by living organisms.

In thinking of the early Earth, a planet without life on it, we must suppose that chlorophyll was absent. Without chlorophyll, the energy of visible light is not enough to form amino acids and nucleotides. The more energetic ultraviolet waves are necessary and that can't pass through our atmosphere.

We seem to be stuck.

But then, in the 1920s, an English biochemist, John Burdon Sanderson Haldane, suggested that oxygen had not always existed in Earth's atmosphere.

After all, plant life is always using up carbon dioxide and producing oxygen, as it forms organic substances from inorganic substances. Might it not be that all the oxygen that is now in the Earth's atmosphere is the result of plant action? Before there was life, and therefore before there were plants, might not the atmosphere have been made up of nitrogen and carbon dioxide, instead of nitrogen and oxygen, as today?

If that were the case, ultraviolet waves could get right down to the Earth's surface without being much absorbed. And, of course, oxygen could be discarded with much greater ease.

The suggestion turned the whole question in a new direction. It wasn't proper to ask how amino acids and nucleotides might be formed from small compounds that are now available under conditions as they exist now. Instead we must ask how amino acids and nucleotides might be formed from small compounds that would be available when the Earth was a young and lifeless planet under conditions as they existed then.

It became necessary to ask, then, what kind of an atmosphere and ocean the Earth had before life developed upon it.

That depends on what the universe is made up of, generally. In the nineteenth century, ways were worked out whereby the light from the stars could be analyzed to tell us what elements were to be found in those stars (and even in the space between the stars).

Gradually, during the early decades of the twentieth century, astronomers came more and more to the conclusion that by far the most common atoms in the universe were the two simplest: hydrogen and helium. In general, you can say that 90 percent of all the atoms in the universe are hydrogen and 9 percent are helium. All the other elements together make up only 1 percent or less. Of these other elements, the bulk was made up of carbon, nitrogen, oxygen, sulphur, phosphorus, neon, argon, silicon, and iron.

If that is so, then you might expect that when a planet forms out of the dust and gas that fills certain sections of space, it ought to be mostly hydrogen and helium. These are the gases that would make up most of the original atmosphere.

Helium atoms do not combine with any other atoms, but hydrogen atoms do. Because hydrogen atoms are present in such quantities, any type of atom that can combine with hydrogen will do so.

Each carbon atom combines with four hydrogen atoms to form "methane" (CH₄). Each nitrogen atom combines with three hydrogen atoms to form "ammonia" (NH₃). Each sulphur atom combines with two hydrogen atoms to form "hydrogen sulphide" (H₂S). And, of course, oxygen atoms combine with hydrogen to form water.

These hydrogen-containing compounds are all gases, or liquids that can easily be turned into gases, so they would all be found in the primitive atmosphere and ocean.

The silicon and iron atoms, together with those of various other fairly common elements such as sodium, potassium, calcium, and magnesium, don't form gases. They make up the solid core of the planet.

This sort of logic seems reasonable, for a large, cold planet like Jupiter was found, in 1932, to have just this sort of atmosphere. Its atmosphere is chiefly hydrogen and helium, and it contains large quantities of ammonia and methane.

Jupiter is a huge planet, however, with strong gravitation. Smaller planets like Earth, Venus, or Mars, have gravitation that is too weak to hold the very small and very nimble helium atoms or hydrogen molecules. (Each hydrogen molecule is made up of two hydrogen atoms, H₂.)

On Earth, therefore, we would expect the very early atmosphere to contain mostly ammonia, methane, hydrogen sulphide, and water vapour. Most of the water would go to make up the ocean and in that ocean would be dissolved ammonia and hydrogen sulphide. Methane is not very soluble but small quantities would be present in the ocean also.

If we began with such an atmosphere, would it stay like that forever? Perhaps not. Earth is fairly close to the sun and a great deal of ultraviolet waves strike the Earth's atmosphere. These ultraviolet waves are energetic enough to tear apart molecules of water vapour in the upper atmosphere and produce hydrogen and oxygen.

The hydrogen can't be held by Earth's gravity and drifts off into space, leaving the oxygen behind. (Oxygen forms molecules made up of two oxygen atoms each, 0₂, and these are heavy enough to be held by Earth's gravity.)

The oxygen does not remain free, however. It combines with the carbon and hydrogen atoms in methane to form carbon dioxide and water. It wouldn't combine with the nitrogen atoms of ammonia, but it would combine with the hydrogen to form water, leaving the nitrogen over to form molecules made up of two atoms each (N₂).

Little by little, as more and more water is broken apart by ultraviolet light, all the ammonia and methane in the atmosphere is converted to nitrogen and carbon dioxide. In fact, the planets Mars and Venus seem to have a nitrogen plus carbon dioxide atmosphere right now.

You might wonder, though, what could happen if all the ammonia and methane were converted to nitrogen and carbon dioxide and if water molecules continued to break up into hydrogen and oxygen. The oxygen would not have anything more to combine with. Perhaps it would gradually accumulate in the air.

This, however, would not happen. As free oxygen accumulates, the energy of sunlight turns some of it into a three-atom combination called "ozone" (O₃). This ozone absorbs the ultraviolet light of the sun and because the ozone layer forms about fifteen miles high in the atmosphere, the ultraviolet light is shielded from the regions of the atmosphere where water vapour exists.

No further water molecules can be broken up and the whole process comes to an end before oxygen can really fill the atmosphere. It is only later on when plants develop and make use of chlorophyll to tap the energy of visible light which can get through the ozone layer that the process begins again. After plants come on the scene, the atmosphere fills with oxygen.

So we have three atmospheres for Earth. The first, "Atmosphere I" was chiefly ammonia, methane, and water vapour, with an ocean containing much ammonia in solution. "Atmosphere II" was chiefly nitrogen, carbon dioxide, and water vapour, with an ocean containing much carbon dioxide in solution. Our present atmosphere "Atmosphere III," is chiefly nitrogen, oxygen, and water vapour, with an ocean in which only small quantities of gas are dissolved.

Atmosphere III formed only after life had developed, so life must have originated in the first place in either Atmosphere I or Atmosphere II (or possibly while Atmosphere I was changing into Atmosphere II) .

Haldane had speculated that life had originated in Atmosphere II, but a Russian biochemist, Alexander Ivanovich Oparin, thought otherwise.

In 1936, he published a book called The Origin of Life, which was translated into English in 1938. Oparin was the first to go into the problem of the origin of life in great detail, and he felt that life must have originated in Atmosphere I.

How was one to decide which was the correct answer? How about experiment? Suppose you were actually to start with a particular mixture of gases that represents an early atmosphere and add energy in the way it might have been added on the early Earth. Will more complicated compounds be formed out of simple ones? And if they are, will they be the kind of compounds that are found in living creatures?

The first scientist who actually tried the experiment was Melvin Calvin at the University of California.

In 1950, he began to work with a portion of Atmosphere 11- carbon dioxide and water vapour. The fact that he left out nitrogen meant that he couldn't possibly form nitrogen containing molecules, like amino acids and nucleotides. However, he was curious to see what he would get.

What he needed, to get anything at all, was a source of energy. He might have used ultraviolet waves, the most likely source on the early Earth, but he preferred not to.

Instead, he made use of the energy of certain kinds of atoms that were always exploding. They were "radioactive" atoms. The radioactive elements on Earth are very slowly breaking down so that every year there are very slightly less than the year before. Several billion years ago there must have been twice as much radioactivity in the Earth's crust as there is now. The energy of radioactivity could therefore have been important in forming life.

Since Melvin Calvin was engaged in experimental work that made use of radioactive substances, he had a good supply

of them to work with. He bombarded his gas mixture with flying particles released by radioactive atomic explosions. After a while, he tested the gas mixture and found that in addition to carbon dioxide and water, he had some very simple organic molecules in solution. He had, for instance, a molecule containing one carbon atom, two hydrogen atoms, and one oxygen atom (CH₂O), which was well known to chemists under the name of "formaldehyde." He also had formic acid, which has a second oxygen atom, and has a formula written HCOOH by chemists.

This was just a beginning but it showed a few important things. It showed that molecules could be made more complicated under early Earth conditions. For another the complicated molecules contained less oxygen than the original molecules, so that oxygen was being discarded.

In 1953 came an important turning point, something that was the key discovery in the search for the origin of life. It came in the laboratories of Harold Clayton Urey at the University of Chicago.

Urey was one of those who had tried to reason out the atmosphere of the early Earth, and, like Oparin, he felt it was in Atmosphere I that life might have got its start. He suggested to one of his students, Stanley Lloyd Miller, that he set up an experiment in which energy would be added to a sample of Atmosphere I. (At the time Miller was in his early twenties, working for his Ph.D. degree.)

Miller set up a mixture of ammonia, methane, and hydrogen in a large glass vessel. In another glass vessel, he boiled water. The steam that was formed passed up a tube and into the gas mixture. The gas mixture was pushed by the steam through another tube back into the boiling water. The second tube was kept cool so that the steam turned back into water before dripping back into the hot water.

The result was that a mixture of ammonia, methane, hydrogen, and water vapour was kept circulating through the system of vessels and tubes, driven by the boiling water. Miller made very certain that everything he used was completely sterile; that there were no bacteria or other cells in the water or in the gases. (If he formed complicated compounds he wanted to make sure they weren't formed by living cells.)

Next, energy had to be supplied. Urey and Miller reasoned that two likely sources of energy were ultraviolet light from the sun and electric sparks from lightning. (There may have been numerous thunderstorms in Earth's early days.)

Of the two, ultraviolet light is easily absorbed by glass and there was a problem as to how to get enough energy through the glass into the chemicals within. Miller therefore thought that as a first try he would use an electric spark like a small bolt of lightning. Through the gas in one portion of the system he therefore set up a continuing electric spark.

Now it was only necessary to wait.

Something was happening. The water and gases were colourless to begin with, but by the end of one day, the water had turned pink. As the days continued to pass, the colour grew darker and darker, till it was a deep red.

After a week, Miller was ready to see what he had formed in his water reservoir. Fortunately, he had at his disposal a new technique for separating and identifying tiny quantities of chemical substances. This is called "paper chromatography" and it had been first developed in 1944 by a group of English chemists.

Like Calvin, Miller found that the simple gas molecules had combined with each other to form more complicated molecules, discarding oxygen atoms.

Again like Calvin, Miller found that formic acid was an important product. He also found, however, that compounds had been formed which were similar to formic acid but were more complicated. These included acetic acid, glycolic acid,

and lactic acid, all substances that were intimately associated with life.

Miller had begun with a nitrogen-containing gas, ammonia, which Calvin had lacked. It is not surprising, therefore, that Miller ended up with some molecules that contained nitrogen as well as carbon, hydrogen, and oxygen. He found some hydrogen cyanide, for instance, which is made up of a carbon atom, a hydrogen atom, and a nitrogen atom in its molecule (HCN).

He also found urea, which has molecules made up of two nitrogen atoms, four hydrogen atoms, a carbon atom, and an oxygen atom (NH₂CONH₂).

Most important of all, though, Miller discovered among his products two of the nineteen amino acid building blocks that go to make up the various protein molecules. These were "glycine" and "alanine," the two simplest of all the amino acids, but also the two that appear most frequently in proteins.

With a single experiment, Miller seemed to have accomplished a great deal. In the first place, these compounds had formed quickly and in surprisingly large quantities. One-sixth of the methane with which he had started had gone into the formation of more complex organic compounds.

He had only worked for a week, and with just a small quantity of gas. How must it have been on the early Earth, with its warm ocean, full of ammonia, and with winds of methane blowing over it, all baking under the sun's ultraviolet radiation or being lashed by colossal lightning bolts for a billion years?

Millions of tons of these complex compounds must have been formed, so that the ocean became a kind of "warm soup." Secondly, the kind of organic molecules formed in Miller's experiment proved particularly interesting. Among the first compounds formed were simple amino acids, the building blocks of proteins. In fact, the path taken by the simple molecules as they grew more complex seemed pointed directly towards life. No molecules were formed that seemed to point in an unfamiliar direction.

Suppose that, as time went on, more and more complicated molecules were built up, always in the direction of compounds now involved with life and not in other directions. Gradually, bigger and bigger molecules would form as building blocks would join together. Finally, something like a real protein molecule and nucleic acid molecule would form and these would eventually associate with each other in a very simple kind of cell.

All this would take a lot of time, to be sure. But then, there was a whole ocean of chemicals to work with, and there was lots of time - a billion years, at least.

Miller's experiment was only a beginning, but it was an extremely hopeful beginning. When its results were announced, a number of biochemists (some of whom were already thinking and working in similar directions) began to experiment in this fashion.

In no time at all, Miller's work was confirmed; that is, other scientists tried the same experiment and got the same results. Indeed, Philip Hauge Abelson, working at the Carnegie Institution of Washington, tried a variety of experiments with different gases in different combinations.

It turned out that as long as he began with molecules that included atoms of carbon, hydrogen, oxygen, and nitrogen somewhere in their structure, he always found amino acids included among the substances formed. And they were always amino acids of the kind that served as protein building blocks.

Nor were electric discharges the only source of energy that would work. In 1959, two German scientists, Wilhelm Groth and H. von Weyssenhoff, tried ultraviolet waves and they also got amino acids.

It could be no accident. There was a great tendency for atoms to click together in such a way as to produce amino acids. Under the conditions that seemed to have prevailed on the early Earth, it appeared impossible not to form amino acids.

By 1968, every single amino acid important to protein structure had been formed in such experiments. The last to be formed were certain important sulphur-containing amino acids, according to a report from Pennsylvania State University and from George Williams University in Montreal.

Perhaps other important compounds also couldn't help but form. Perhaps they would just naturally come together to form the important large molecules of living tissue.

If that is so, life may be no "miracle." It couldn't help forming, any more than you can help dropping downward if you jump off a building. Any planet that is something like the Earth, with a nearby sun and a supply of water and an atmosphere full of hydrogen compounds, would then have to form life. The kinds of creatures that eventually evolved on other worlds would be widely different and might not resemble us any more than an octopus resembles a gorilla. But, the chances are, they would be built up of the same chemical building blocks as we.

More and more, scientists are beginning to think in this way, and they are beginning to speculate that life may be very common in the universe.

Of course, on planets that are quite different from Earth; much bigger and colder, like Jupiter, or much smaller and hotter, like Mercury, our kind of life could not form. On the other hand, other kinds of life, based on other types of chemistry, might be formed. We have no way of telling.

But we are getting ahead of ourselves. Miller's experiments were enough to start speculation of this sort, but it was still important to check matters. A couple of amino acids weren't enough. What about the nucleotides, which served as building blocks for nucleic acids? (Since the 1940s, biochemists have come to believe that nucleic acids are even more important than proteins.)

One could repeat Miller's experiment for longer and longer periods, hoping that more and more complicated molecules would be formed. However, as more and more kinds of compounds were formed, there would be less and less of each separate kind, and it would become more difficult to spot each one.

Possibly, one could start with bigger and bigger quantities of gases in the first place. Even so, the large number of complicated molecules that would be formed would confuse matters.

It occurred to some experimenters to begin not at the beginning of Miller's experiment, but at its end. For instance, one of the most simple products of Miller's experiment was hydrogen cyanide, HCN.

Suppose you assumed that this gas was formed in quantity in Earth's early ocean and then started with it. In that way you would begin partway along the road of development of life and carry it on further.

At the University of Houston, a Spanish-born biochemist, Juan Oro, tried just this in 1961. He found that not only amino acids were formed once he added HCN to the starting mixture, but individual amino acids were hooked together in short chains, in just the way in which they are hooked together in proteins.

Even more interesting was the fact that purines were formed, the double rings of carbon and nitrogen atoms that are found in nucleotides. A particular purine called "adenine" was obtained. This is found not only in nucleic acids but in other important compounds associated with life.

As the 1960s opened, then, imitations of the chemical environment of the early Earth were being made to produce not only the building blocks of the proteins, but the beginnings of the nucleotide building blocks of the nucleic acids.

It was just the beginnings in the latter case. The nucleotides contained not only purines but also the somewhat similar, but simpler, one-ringed compounds, the pyrimidines. Then there were the sugars, ribose and deoxyribose. And, of course, there was the phosphate group.

The experimenters bore on. All the necessary purines and pyrimidines were formed. The sugars proved particularly easy. Sugar molecules are made up of carbon, hydrogen, and oxygen atoms only. No nitrogen atoms are needed. That reminded one of Calvin's original experiment. Calvin had obtained formaldehyde (CH₂0) from carbon dioxide and water. What if one went a step farther and began with formaldehyde and water.

In 1962, Oro found that if he began with formaldehyde in water and let ultraviolet waves fall upon it, a variety of sugar molecules were formed, and among them were ribose and deoxyribose.

What next?

Purines and pyrimidines were formed. Ribose and deoxyribose were formed. Phosphate groups didn't have to be formed. They existed in solution in the ocean now, and very likely did then, in just the form they existed in inorganic molecules.

One researcher who drove onward was a Ceylon-born biochemist, Cyril Ponnamperuma, at Ames Research Center at Moffett Field, California. He had conducted experiments in which he had, as a beginning, formed various purines with and without hydrogen cyanide. He had formed them through the energy of beams of electrons (very light particles) as well as ultraviolet waves.

In 1963, he, along with Ruth Mariner and Carl Sagan, began a series of experiments in which he exposed a solution of adenine and ribose to ultraviolet waves. They hooked together in just the fashion they were hooked together in nucleotides. If the experimenters began with phosphate also present in the mixture, then the complete nucleotide was

formed. Indeed, by 1965, Ponnamperuma was able to announce that he had formed a double nucleotide, a molecule consisting of two nucleotides combined in just the fashion found in nucleic acids.

By the middle 1960s, then, it seemed clear to biochemists that the conditions on the early Earth were capable of leading to the formation of a wide variety of substances associated with life. These would certainly include the amino acids and nucleotides, those building blocks that go to make up the allimportant proteins and nucleic acids. Furthermore, these building blocks hook together under early conditions to make up the very chains out of which proteins and nucleic acids are formed.

All the raw materials for life were there on the early Earth, all the necessary chemicals. But life is more than just chemicals. There are all sorts of chemical changes going on in living organisms, and they must be taken into account. Atoms and groups of atoms are shifting here, shifting there, coming apart and reuniting in different ways.

Many of these changes won't take place unless energy is supplied. If we're dealing with the ocean, the energy is supplied by the sun's ultraviolet radiation, or in other ways. But what happens inside the tiny living creatures once they come into existence?

Actually, there are certain chemicals in living creatures which break up easily, releasing energy. Such chemicals make it possible for important chemical changes to take place that would not take place without them. Without such chemicals life as we know it would be impossible no matter how many proteins and nucleic acids built up in the early ocean.

Could it be that some of the energy of sunlight went into the production of these energy-rich compounds? In that case, everything necessary for life might really be supplied.

The best-known of the energy-rich compounds is one called "adenosine triphosphate," a name that is usually abbreviated as ATP. It resembles a nucleotide to which two additional phosphate groups (making three altogether) have been added.

If, then, adenine, ribose, and phosphate groups are exposed to ultraviolet waves and if they hook together to form a nucleotide containing one phosphate group, perhaps we can go farther. Perhaps longer irradiation or the use of more phosphate to begin with will cause them to hook together to form ATP, with three phosphate groups. Ponnamperuma tried, and it worked. ATP was formed.

In 1967 a type of molecule belonging to a class called "porphyrins" was synthesized from simpler substances by Ponnamperuma. Belonging to this class is the important chlorophyll molecule in green plants.

No one doubts now that all the necessary chemicals of life could have been produced in the oceans of the early Earth by chemical reactions under ultraviolet.

To be sure, the life that was formed at first was probably so simple that we might hesitate to call it life. Perhaps it consisted of a collection of just a few chemicals that could bring about certain changes that would keep the collection from breaking apart. Perhaps it would manage to bring about the formation of another collection like itself.

It may be that life isn't so clear-cut a thing that we can point a finger and say: Right here is something that was dead before and is now alive.

There may be a whole set of more and more complex systems developing over hundreds of millions of years. To begin with, the systems would be so simple that we couldn't admit they were alive. To end with, they would be so complex that we would have to admit they were indeed alive. But where, in between, would be the changeover point?

We couldn't tell. Maybe there is no definite changeover point. Chemical systems might just slowly become more and more "alive" and where they passed the key point, no one could say.

With all the successful production of compounds that followed the work of Calvin and Miller, there still remained the question of how cells were formed. The experimenters who formed compounds recognized that that question would have to be answered somehow.

No one type of compound is living, all by itself. Everything that seems living to us is a mixture of all sorts of substances which are kept close together by a membrane and which react with each other in a very complicated way.

There are viruses, to be sure, which are considered alive and which sometimes consist of a single nucleic acid molecule wrapped in a protein shell. Such viruses, however, don't really get to work in a truly living way till they can get inside some cell. In there, they make use of cell machinery.

Haldane, who had started the modern attack on the problem, wondered how cells might have formed. He pointed out that when oil is added to water, thin films of oil sometimes form bubbles in which tiny droplets of water are enclosed.

Some of the compounds formed by the energy of ultraviolet light are oily and won't mix with water. What if they were to form a little bubble and just happen to enclose a proper mixture of protein, nucleic acid, and other things? Today's cell membrane may be the development of that early oily film.

Oparin, the Russian biochemist, went into further detail. He showed that proteins in solution might sometimes gather together into droplets and form a kind of skin on the outside of those droplets.

The most eager experimenter in this direction, once Miller's work had opened up the problem, was Sidney W. Fox at the University of Miami. It seemed to him that the early Earth must have been a hot planet indeed. Volcanoes may have kept the dry land steaming and brought the ocean nearly to a boil.

Perhaps the energy of heat alone was sufficient to form complex compounds out of simple ones.

To test this, Fox began with a mixture of gases like that in Atmosphere I (the type that Oparin suggested and Miller used) and ran them through a hot tube. Sure enough, a variety of amino acids, at least a dozen, were formed. All the amino acids that were formed happened to be among those making up proteins. No amino acids were formed that were not found in proteins.

Fox went a step farther. In 1958, he took a bit of each of the various amino acids that are found in protein, mixed them together, and heated the mixture. He found that he had driven the amino acids together, higgledy-piggledy, into long chains which resembled the chains in protein molecules. Fox called these chains "proteinoids" (meaning "protein-like"). The likeness was a good one. Stomach juices, which digest ordinary protein, would also digest proteinoids. Bacteria, which would feed and grow on ordinary protein, would also feed and grow on proteinoids.

Most startling of all, when Fox dissolved the proteinoids in hot water and let the solution cool, he found that the proteinoids clumped together in little spheres about the size of small bacteria. Fox called these "microspheres."

These microspheres are not alive, but in some ways they behaved as cells do. They are surrounded by a kind of membrane. Then, by adding certain chemicals to the solution, Fox could make the microspheres swell or shrink, much as ordinary cells do. The microspheres can produce buds, which sometimes seem to grow larger and break off. Microspheres can divide in two or cling together in chains.

Not all scientists accept Fox's arguments, but what if, on the early Earth, more and more complicated substances were built up, turning the ocean into the "warm soup" we spoke of. What if these substances formed microspheres? Might it not be that, little by little, as the substances grew more complicated and the microspheres grew more elaborate, that eventually an almost-living cell would be formed? And after that, a fully living one?

Before life began, then, and before evolutionary changes in cells led to living creatures that were more and more complicated, there must first have been a period of "chemical evolution." In this period, the very simplest gases of the atmosphere and ocean gradually become more and more complicated until life and cells formed.

All these guesses about the origin of life, from Haldane on, are backed up by small experiments in the laboratory and by careful reasoning. Is it possible that we might find traces of what actually happened on the early Earth if we look deep into the Earth's crust.

We find out about ordinary evolution by studying fossils in the crust. These are the remains of ancient creatures, with their bones or shells turned to stone. From these stony remains we can tell what they looked like and how they must have lived.

Fossils have been found deep in layers of rock that must be 600 million years old. Before that we find hardly anything. Perhaps some great catastrophe wiped out the earlier record. Perhaps forms of life existed before then that were too simple to leave clear records.

Actually, in the 1960s discoveries were reported of traces left behind by microscopic one-cell creatures in rocks that are more than two billion years old. Prominent in such research is Elso Sterrenberg Barghoorn of Harvard. It is a good guess that there were simple forms of life on Earth at least as long as three billion years ago.

If we are interested in discovering traces of the period of chemical evolution, then, we must search for still older rocks. In them, we might hope to find chemicals that seem to be on the road to life.

But will chemicals remain unchanged in the Earth for billions of years? Can we actually find such traces if we look for them?

Certainly the important chemicals of life, the proteins and nucleic acids, are too complex to remain unchanged for long after the creature they were in dies and decomposes. In a very short time, it would seem, they must decompose and fall apart.

And yet, it turns out, sometimes they linger on, especially when they are in a particularly well-protected spot. Abelson, one of the people who experimented with early atmospheres, also worked with fossils. He reasoned that living bones and shell contain protein. Bones may be 50 percent protein. Clam shells have much less, but there is some. Once such bones and shells are buried deep in the Earth's crust, remaining there for millions of years while they turned to stone, it might be that some of the protein trapped between thin layers of mineral might survive.... Or at least they might break down to amino acids or short chains of amino acids that might survive.

Painstakingly, Abelson dissolved these ancient relics and analyzed the organic material he extracted. There were amino acids present all right, exactly the same amino acids that are present in proteins of living creatures. He found some even in a fossil fish which might have been 300 million years old.

Apparently, then, organic compounds last longer than one might think and Melvin Calvin began the search for "chemical fossils" in 1961.

In really old rocks, it is unlikely that the organic chemicals would remain entirely untouched. The less hardy portions would be chipped away. What would linger longest would be the chains and rings of carbon atoms, with hydrogen atoms attached. These compounds of carbon and hydrogen only are called "hydrocarbons."

Calvin has isolated hydrocarbons from ancient rocks as much as three billion years old. The hydrocarbons have molecules of a complicated structure that looked very much as though they could have originated from chemicals found in living plants.

J. William Schopf of Harvard, a student of Barghoorn, has gone even further. He has detected traces of 22 different amino acids in rocks more than three billion years old.

They are probably the remnants of primitive life. It is necessary now to probe farther back and find chemical remnants that precede life and show the route actually taken.

Very likely it will be the route worked out by chemists in their experiments, but possibly it won't be.

We must wait and see. And perhaps increasing knowledge of what went on in the days of Earth's youth will help us understand more about life now.

From: Twentieth Century Discovery by Isaac Azimov