Origin Story, page 8
Biosphere
CHAPTER 4
Life: Threshold 5
Life and Information: A New Type of Complexity
I spent the afternoon musing on Life. If you come to think of it, what a queer thing Life is! So unlike anything else, don’t you know, if you see what I mean.
—P. G. WODEHOUSE, MY MAN JEEVES
What lies at the heart of every living thing is not a fire, not warm breath, not a “spark of life.” It is information, words, instructions.… If you want to understand life, don’t think about vibrant, throbbing gels and oozes, think about information technology.
—RICHARD DAWKINS, THE BLIND WATCHMAKER
Life as we know it arose from exotic chemistry in the element-rich environments of the young planet Earth almost four billion years ago. If life exists elsewhere, it might look so strange that we wouldn’t recognize it. But on planet Earth, life is built from billions of intricate molecular nanomachines. They work together inside protective bubblelike structures we consider the building blocks of life—the basic structural, functional, and biological units of all known living organisms. These protected bubbles are called cells, from the Latin cella, meaning “small room.” Cells are the smallest units of life that can replicate independently. They survive by tapping delicate flows of nutrients and free energy from their surroundings.
Life has had a colossal impact on our planet because living organisms make copies of themselves that can multiply, spread, proliferate, and diversify. Over four billion years, a colossal army of living organisms has transformed Earth and created the biosphere: a thin layer at the planet’s surface made up of living organisms and everything shaped or altered or left behind by living organisms.
The spooky thing about life is that, though the inside of each cell looks like pandemonium—a sort of mud-wrestling contest involving a million molecules—whole cells give the impression of acting with purpose. Something inside each cell seems to drive it, as if it were working its way through a to-do list. The to-do list is simple: (1) stay alive despite entropy and unpredictable surroundings; and (2) make copies of myself that can do the same thing. And so on from cell to cell, and generation to generation. Here, in the seeking out of some outcomes and the avoidance of others, are the origins of desire, caring, purpose, ethics, even love. Perhaps even the beginnings of meaning, if that means the ability to discriminate between the significance of different events and signs. What is the meaning of this great white shark cruising behind me?
The appearance (or, perhaps, illusion) of purposefulness is new. It is not a feature of the other complex entities we have seen so far. Would it mean anything to say that stars have a purpose? Or planets, or rocks? Or even the universe? Not really, at least not within the conventions of the modern origin story. But living things are different. They don’t accept entropy’s rules passively; instead, like stubborn children, they push back and try to negotiate. They don’t just lock structures in place, like protons or electrons. They don’t live off stores of energy, like stars, which munch their way through a larder of protons that was well stocked at their birth and then fall apart when the larder is empty. Living organisms constantly seek out new flows of energy from their environments in order to maintain themselves in a state that is complex but unstable. This is not the behavior of rocks; it is that of a bird on the wing. Living organisms stay airborne (thermodynamically speaking) by taking in free energy to drive the elaborate chemistry that rearranges atoms and molecules in the patterns needed to keep them alive. When they can no longer pay entropy’s energy taxes, they crash.
Energy and life! In Australia, I remember watching my own children transform the energy in Vegemite sandwiches into the violent energy of motion as they roared around the garden. We can even measure the rate at which free energy (perhaps from a Vegemite sandwich) flows as it is transformed into talking energy, running energy, and, eventually, heat energy, with entropy increasing at each step. The average human takes in about 2,500 calories each day, about 10.5 million joules (a measure of work or energy; a calorie represents about 4,184 joules). Divide this by the 86,400 seconds in a day, and an individual mobilizes about 120 joules every second. This is the “power rating” of a human being: 120 watts, just slightly greater than the power rating of many traditional lightbulbs.1
Life, with its never-ending attempts to push back against entropy, represents a new type and level of complexity. Complexity theorists sometimes describe entities at this level as complex adaptive systems. Unlike the complex physical systems we have seen so far, the components of which behave in ways that can usually be predicted from the universe’s basic operating rules, the components in complex adaptive systems seem to have a will of their own. They appear to follow additional rules that are harder to detect. Indeed, complex adaptive systems, such as bacteria, your dog, or multinational companies, act as if every component is an agent with a will of its own, so each component is constantly adjusting to the behavior of many other components. And that yields extremely complex and unpredictable behaviors.2
In using the word agent, I have smuggled in a new idea that will become increasingly important: the idea of information. If agents react to other agents, they are reacting to information about what is happening around them, including information about what other agents are doing. If we imagine information as a character in our modern origin story, we should think of it as working undercover or in disguise, manipulating events but staying out of the spotlight. Energy causes change, so you can usually see it at work, but information directs change, often from the shadows. As Seth Lloyd puts it: “To do anything requires energy. To specify what is done requires information.”3
In its most general form, information consists of rules that affect outcomes by limiting possibilities. One of the most famous definitions of information is “a difference which makes a difference.”4 Rules determine which changes out of all conceivable options are actually possible at a given time and place, and that makes a difference. Information begins with the laws of physics, the basic operating system of our universe. The laws of physics steer change down particular pathways, like the pathways by which gravity created the first stars. Information in this very general sense limits what is possible, so it reduces randomness. This is why more information seems to mean less entropy, less potential for the disorder that entropy loves. This is universal information: the rules built into every smidgen of matter and energy. No one needed to tell gravity what to do; it just got on with the job.
In colloquial usage, though, the term information means more than rules. It means rules that are read by some person or agent or thing—in fact, by some complex adaptive system. This sort of information arises because many important rules are not universal. Like the laws of human societies, they change from place to place and moment to moment. As the universe evolved, new environments appeared, such as deep space, galactic dust clouds, and the surfaces of rocky planets. These environments had their own local rules that were not universal. Local rules have to be read or decoded or studied, just as you might want to learn which side of the road locals drive on before visiting Mongolia (the right, by the way).
Complex adaptive systems can survive only in very specific environments, so they need to be able to read or decode local information as well as the universal rules. And that’s new. All forms of life require mechanisms to interpret local information (such as the presence of different chemicals or local temperatures and acidity levels) so they can respond appropriately (Should I hug it or eat it or run?). The philosopher Daniel Dennett writes: “Animals are not just herbivores or carnivores. They are… informavores.”5 In fact, all living organisms are informavores. They all consume information, and the mechanisms they use for reading and responding to local information—whether they are eyes and tentacles or muscles and brains—account for much of the complexity of living organisms.
Local environments are unstable, so living organisms must constantly monitor their internal and external environments to detect significant changes. And as organisms increase in complexity, they need more and more information, because more complex structures have more moving parts and more links between their parts. The bacterium E. coli, which is probably flourishing in your intestines as you read this, allocates about 5 percent of its molecular resources to movement and perception, but in your body, most organs are devoted, directly or indirectly, to perception or motion, from brains to eyes to nerve tissues and muscles.6 Modern science is at the extreme end of a vast spectrum of information-gathering-and-analyzing systems that begin with the simple sensors of the earliest single-celled organisms.
Entropy, of course, keeps a beady eye on all of this. If more complexity means more information, then when you increase complexity and information, you are reducing entropy and its accompanying uncertainty or disorder. And entropy will notice. Entropy is rubbing its hands at the thought of the energy taxes and fees it can levy as complexity and information increase.7 Indeed, some have argued that entropy actually likes the idea of life (and may encourage it to appear in many parts of the universe), because life degrades free energy so much more efficiently than nonlife.
Explaining the origins of life on Earth and trying to figure out if something similar might have emerged elsewhere in our universe are among the most difficult problems facing modern science. At the moment, we know of only one planet with life. Astrobiologists are searching for life elsewhere through the Search for Extraterrestrial Intelligence (SETI) program, which began in 1960, but so far they have found none. For now, we are confined to studying the origins of life on Earth. Even that is extraordinarily difficult, as it means trying to determine what was happening on our planet almost four billion years ago, when Earth was very different.
Defining Life
Having only one sample makes it difficult even to know what life is. What distinguishes life from nonlife? Life is as hard to define as complexity or information, and there seems to be a murky border zone between life and nonlife.
Most modern definitions of life on Earth would include the following five features:
1. Living organisms consist of cells enclosed by semipermeable membranes.
2. They have a metabolism, mechanisms that tap and use flows of free energy from their surroundings so they can rearrange atoms and molecules into the complex and dynamic structures they need to survive.
3. They can adjust to changing environments by homeostasis, using information about their internal and external environments and mechanisms that allow them to react.
4. They can reproduce by using genetic information to make almost exact copies of themselves.
5. But the copies differ in minute ways from the parents, so, over many generations, the features of living organisms slowly change as they evolve and adapt to changing environments.
Let’s take each of these features in turn.
All living things on Earth consist of cells. Each cell contains millions of complex molecules that react with one another many times every second as they push their way through a watery, salty chemical sludge full of proteins in the gooey realm known as the cytoplasm. The cytoplasm is bounded by a sort of chemical fence, the cell membrane, that controls what comes in and goes out. Like the walls of a medieval city, the membrane has gates and guards that decide which molecular travelers can enter and when. Cells really are like cities. In a book on cells, Peter Hoffmann writes:
There is a library (the nucleus, which contains the genetic material), power plants (mitochondria), highways (microtubules and actin filaments), trucks (kinesin and dynein), garbage disposals (lysosomes), city walls (membranes), post offices (Golgi apparatus), and many other structures fulfilling vital functions. All of these functions are performed by molecular machines.8
All living organisms depend on carefully managed flows of free energy. Stop the flow, and they die, like a besieged city starved into submission. But if the flow is too violent, they will also die, like a city under aerial bombardment. So flows of energy need to be managed with great delicacy. Usually, cells take in and use energy in tiny doses, electron by electron or proton by proton. Though small enough not to be disruptive, these flows are large enough to provide the activation energies needed to drive lots of interesting chemistry. Etymologically, the word metabolism comes from the word meaning “change.” It’s a reminder that cells never stand still. Like birds in flight, they use flows of energy to keep adjusting to ever-changing environments.
Living organisms must constantly monitor and adjust to changes in their environments. This constant adjustment is known as preserving homeostasis. To maintain some sort of equilibrium in changing surroundings, cells must continually access, download, and decode information about their internal and external environments, decide on the best response, and then respond. The word homeostasis means “standing still,” which is the opposite of “change.” But it makes sense if you think of standing still in the never-ending molecular hurricane of the cell’s environment.
Impressive as these abilities are, they would be of little interest if living organisms appeared and vanished like spray on an ocean wave. And that may be what has happened on some planets around some stars, and perhaps even early in Earth’s history. But today on planet Earth, living organisms don’t just stand up in the hurricane of change and entropy. They also make copies of themselves, so that when particular cells fall down (and eventually they will all fall down), others can take their place. Reproduction is the ability to make viable copies of cells. Reproduction means that the template for making an organism (its genome, in modern terminology) can survive even after individuals have died. Like an instruction manual, the genome stores information about the proteins needed to build a copy of the parent as well as some basic assembly rules. Today, most of this information is stored in molecules of DNA. But early in the history of life on Earth, it was probably stored in RNA, a molecular cousin to DNA that still does a lot of heavy lifting inside cells.
Though the templates are more or less immortal, the copying process is not perfect. This is good news, because it means the templates can slowly change as a result of tiny copying errors, and that is the key to adaptation and evolution. Tiny genetic changes give life its remarkable resilience because they allow species to adapt to their environments by randomly creating slightly different templates. As environments change, so, too, do the rules that determine which templates will survive and which will perish.
This is the mechanism Charles Darwin described as natural selection. Natural selection is a fundamental idea in modern biology because it is an extraordinarily powerful driver of increasing complexity. Natural selection filters out some genetic possibilities, allowing only those compatible with local rules. So natural selection is a ratchet, like the fundamental laws of physics, because it locks nonrandom patterns in place. But in the biological realm, it is the local rules of particular environments, not the universal rules of physics, that determine what survives. And the biological rules are much more persnickety. Don’t expect a giraffe to survive underwater.
Like the mechanisms that generated the universe’s first structures, natural selection links necessity and chance. Variation provides multiple possibilities; natural selection uses local rules to pick out those that will work under local conditions. Here is how Darwin put it in The Origin of Species:
Can it… be thought improbable [that] variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations? If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection.9
Darwin’s idea, when linked to a modern understanding of genetics and heredity, explains life’s creativity, its ability over many generations to explore possibilities, tap new energy flows, and construct new types of structures. It explains how, in the biological realm, structures of staggering complexity can emerge through repetitive algorithmic processes as they are filtered out from myriad variations, step by step and generation by generation, over millions and billions of years.
The idea of natural selection shocked Darwin’s contemporaries, because it seemed to do away with the need for a creator god.10 And that idea was fundamental to the Christian origin story that most people accepted in Victorian England. Even Darwin was worried, and his wife, Emma, feared she and Charles would end up in different places in the afterlife. But the mechanism Darwin described really does seem to be fundamental to the history of life. Let finches breed on one of the Galápagos Islands that Darwin visited in his youth. If this island’s trees produce nuts with tough shells, over time those finches with beaks that can crack the shells most efficiently will survive better and produce more offspring than others. Wait a few generations, and you will find all the finches on this island have this type of beak. Over time, as some individuals are selected by “nature” (in fact, by the rules of the local environment), a new species will eventually emerge. Here, as Darwin showed, is the basic mechanism of biological evolution. This is Darwin’s complexity ratchet; this is how life builds more and more complex things, step by step.
The Goldilocks Conditions for Life
How did life first sputter into motion somewhere in the rich and varied Goldilocks environments of the young Earth?11
What Darwin did not know was that mechanisms similar to natural selection, in which random changes are filtered out by local rules, can also work in rough-and-ready ways in a world without life. Where there are complex mixtures of chemicals and plenty of free energy, molecules can arise that encourage the formation of other molecules and eventually create the molecules the reaction started out with. This is an autocatalytic cycle, a reaction whose components enable, or catalyze, the production of other components of the cycle, including its original ingredients, so the cycle can repeat itself. Fire up one of these cycles, and it will produce its components in larger and larger quantities as it extracts more and more food energy until it starts starving other, less successful reactions. The cycle may even modify itself slightly if new types of food appear. This is beginning to look like the survival of the most successful chemical reaction. So here we already have something a bit lifelike, something that can persist and reproduce by tapping energy from its surroundings. “Before we can have competent reproducers,” writes Daniel Dennett, “we have to have competent persisters, structures with enough stability to hang around long enough to pick up revisions.”12 This idea of chemical evolution will help us explain, at least in general terms, how the preconditions for life emerged on the young Earth.
CHAPTER 4
Life: Threshold 5
Life and Information: A New Type of Complexity
I spent the afternoon musing on Life. If you come to think of it, what a queer thing Life is! So unlike anything else, don’t you know, if you see what I mean.
—P. G. WODEHOUSE, MY MAN JEEVES
What lies at the heart of every living thing is not a fire, not warm breath, not a “spark of life.” It is information, words, instructions.… If you want to understand life, don’t think about vibrant, throbbing gels and oozes, think about information technology.
—RICHARD DAWKINS, THE BLIND WATCHMAKER
Life as we know it arose from exotic chemistry in the element-rich environments of the young planet Earth almost four billion years ago. If life exists elsewhere, it might look so strange that we wouldn’t recognize it. But on planet Earth, life is built from billions of intricate molecular nanomachines. They work together inside protective bubblelike structures we consider the building blocks of life—the basic structural, functional, and biological units of all known living organisms. These protected bubbles are called cells, from the Latin cella, meaning “small room.” Cells are the smallest units of life that can replicate independently. They survive by tapping delicate flows of nutrients and free energy from their surroundings.
Life has had a colossal impact on our planet because living organisms make copies of themselves that can multiply, spread, proliferate, and diversify. Over four billion years, a colossal army of living organisms has transformed Earth and created the biosphere: a thin layer at the planet’s surface made up of living organisms and everything shaped or altered or left behind by living organisms.
The spooky thing about life is that, though the inside of each cell looks like pandemonium—a sort of mud-wrestling contest involving a million molecules—whole cells give the impression of acting with purpose. Something inside each cell seems to drive it, as if it were working its way through a to-do list. The to-do list is simple: (1) stay alive despite entropy and unpredictable surroundings; and (2) make copies of myself that can do the same thing. And so on from cell to cell, and generation to generation. Here, in the seeking out of some outcomes and the avoidance of others, are the origins of desire, caring, purpose, ethics, even love. Perhaps even the beginnings of meaning, if that means the ability to discriminate between the significance of different events and signs. What is the meaning of this great white shark cruising behind me?
The appearance (or, perhaps, illusion) of purposefulness is new. It is not a feature of the other complex entities we have seen so far. Would it mean anything to say that stars have a purpose? Or planets, or rocks? Or even the universe? Not really, at least not within the conventions of the modern origin story. But living things are different. They don’t accept entropy’s rules passively; instead, like stubborn children, they push back and try to negotiate. They don’t just lock structures in place, like protons or electrons. They don’t live off stores of energy, like stars, which munch their way through a larder of protons that was well stocked at their birth and then fall apart when the larder is empty. Living organisms constantly seek out new flows of energy from their environments in order to maintain themselves in a state that is complex but unstable. This is not the behavior of rocks; it is that of a bird on the wing. Living organisms stay airborne (thermodynamically speaking) by taking in free energy to drive the elaborate chemistry that rearranges atoms and molecules in the patterns needed to keep them alive. When they can no longer pay entropy’s energy taxes, they crash.
Energy and life! In Australia, I remember watching my own children transform the energy in Vegemite sandwiches into the violent energy of motion as they roared around the garden. We can even measure the rate at which free energy (perhaps from a Vegemite sandwich) flows as it is transformed into talking energy, running energy, and, eventually, heat energy, with entropy increasing at each step. The average human takes in about 2,500 calories each day, about 10.5 million joules (a measure of work or energy; a calorie represents about 4,184 joules). Divide this by the 86,400 seconds in a day, and an individual mobilizes about 120 joules every second. This is the “power rating” of a human being: 120 watts, just slightly greater than the power rating of many traditional lightbulbs.1
Life, with its never-ending attempts to push back against entropy, represents a new type and level of complexity. Complexity theorists sometimes describe entities at this level as complex adaptive systems. Unlike the complex physical systems we have seen so far, the components of which behave in ways that can usually be predicted from the universe’s basic operating rules, the components in complex adaptive systems seem to have a will of their own. They appear to follow additional rules that are harder to detect. Indeed, complex adaptive systems, such as bacteria, your dog, or multinational companies, act as if every component is an agent with a will of its own, so each component is constantly adjusting to the behavior of many other components. And that yields extremely complex and unpredictable behaviors.2
In using the word agent, I have smuggled in a new idea that will become increasingly important: the idea of information. If agents react to other agents, they are reacting to information about what is happening around them, including information about what other agents are doing. If we imagine information as a character in our modern origin story, we should think of it as working undercover or in disguise, manipulating events but staying out of the spotlight. Energy causes change, so you can usually see it at work, but information directs change, often from the shadows. As Seth Lloyd puts it: “To do anything requires energy. To specify what is done requires information.”3
In its most general form, information consists of rules that affect outcomes by limiting possibilities. One of the most famous definitions of information is “a difference which makes a difference.”4 Rules determine which changes out of all conceivable options are actually possible at a given time and place, and that makes a difference. Information begins with the laws of physics, the basic operating system of our universe. The laws of physics steer change down particular pathways, like the pathways by which gravity created the first stars. Information in this very general sense limits what is possible, so it reduces randomness. This is why more information seems to mean less entropy, less potential for the disorder that entropy loves. This is universal information: the rules built into every smidgen of matter and energy. No one needed to tell gravity what to do; it just got on with the job.
In colloquial usage, though, the term information means more than rules. It means rules that are read by some person or agent or thing—in fact, by some complex adaptive system. This sort of information arises because many important rules are not universal. Like the laws of human societies, they change from place to place and moment to moment. As the universe evolved, new environments appeared, such as deep space, galactic dust clouds, and the surfaces of rocky planets. These environments had their own local rules that were not universal. Local rules have to be read or decoded or studied, just as you might want to learn which side of the road locals drive on before visiting Mongolia (the right, by the way).
Complex adaptive systems can survive only in very specific environments, so they need to be able to read or decode local information as well as the universal rules. And that’s new. All forms of life require mechanisms to interpret local information (such as the presence of different chemicals or local temperatures and acidity levels) so they can respond appropriately (Should I hug it or eat it or run?). The philosopher Daniel Dennett writes: “Animals are not just herbivores or carnivores. They are… informavores.”5 In fact, all living organisms are informavores. They all consume information, and the mechanisms they use for reading and responding to local information—whether they are eyes and tentacles or muscles and brains—account for much of the complexity of living organisms.
Local environments are unstable, so living organisms must constantly monitor their internal and external environments to detect significant changes. And as organisms increase in complexity, they need more and more information, because more complex structures have more moving parts and more links between their parts. The bacterium E. coli, which is probably flourishing in your intestines as you read this, allocates about 5 percent of its molecular resources to movement and perception, but in your body, most organs are devoted, directly or indirectly, to perception or motion, from brains to eyes to nerve tissues and muscles.6 Modern science is at the extreme end of a vast spectrum of information-gathering-and-analyzing systems that begin with the simple sensors of the earliest single-celled organisms.
Entropy, of course, keeps a beady eye on all of this. If more complexity means more information, then when you increase complexity and information, you are reducing entropy and its accompanying uncertainty or disorder. And entropy will notice. Entropy is rubbing its hands at the thought of the energy taxes and fees it can levy as complexity and information increase.7 Indeed, some have argued that entropy actually likes the idea of life (and may encourage it to appear in many parts of the universe), because life degrades free energy so much more efficiently than nonlife.
Explaining the origins of life on Earth and trying to figure out if something similar might have emerged elsewhere in our universe are among the most difficult problems facing modern science. At the moment, we know of only one planet with life. Astrobiologists are searching for life elsewhere through the Search for Extraterrestrial Intelligence (SETI) program, which began in 1960, but so far they have found none. For now, we are confined to studying the origins of life on Earth. Even that is extraordinarily difficult, as it means trying to determine what was happening on our planet almost four billion years ago, when Earth was very different.
Defining Life
Having only one sample makes it difficult even to know what life is. What distinguishes life from nonlife? Life is as hard to define as complexity or information, and there seems to be a murky border zone between life and nonlife.
Most modern definitions of life on Earth would include the following five features:
1. Living organisms consist of cells enclosed by semipermeable membranes.
2. They have a metabolism, mechanisms that tap and use flows of free energy from their surroundings so they can rearrange atoms and molecules into the complex and dynamic structures they need to survive.
3. They can adjust to changing environments by homeostasis, using information about their internal and external environments and mechanisms that allow them to react.
4. They can reproduce by using genetic information to make almost exact copies of themselves.
5. But the copies differ in minute ways from the parents, so, over many generations, the features of living organisms slowly change as they evolve and adapt to changing environments.
Let’s take each of these features in turn.
All living things on Earth consist of cells. Each cell contains millions of complex molecules that react with one another many times every second as they push their way through a watery, salty chemical sludge full of proteins in the gooey realm known as the cytoplasm. The cytoplasm is bounded by a sort of chemical fence, the cell membrane, that controls what comes in and goes out. Like the walls of a medieval city, the membrane has gates and guards that decide which molecular travelers can enter and when. Cells really are like cities. In a book on cells, Peter Hoffmann writes:
There is a library (the nucleus, which contains the genetic material), power plants (mitochondria), highways (microtubules and actin filaments), trucks (kinesin and dynein), garbage disposals (lysosomes), city walls (membranes), post offices (Golgi apparatus), and many other structures fulfilling vital functions. All of these functions are performed by molecular machines.8
All living organisms depend on carefully managed flows of free energy. Stop the flow, and they die, like a besieged city starved into submission. But if the flow is too violent, they will also die, like a city under aerial bombardment. So flows of energy need to be managed with great delicacy. Usually, cells take in and use energy in tiny doses, electron by electron or proton by proton. Though small enough not to be disruptive, these flows are large enough to provide the activation energies needed to drive lots of interesting chemistry. Etymologically, the word metabolism comes from the word meaning “change.” It’s a reminder that cells never stand still. Like birds in flight, they use flows of energy to keep adjusting to ever-changing environments.
Living organisms must constantly monitor and adjust to changes in their environments. This constant adjustment is known as preserving homeostasis. To maintain some sort of equilibrium in changing surroundings, cells must continually access, download, and decode information about their internal and external environments, decide on the best response, and then respond. The word homeostasis means “standing still,” which is the opposite of “change.” But it makes sense if you think of standing still in the never-ending molecular hurricane of the cell’s environment.
Impressive as these abilities are, they would be of little interest if living organisms appeared and vanished like spray on an ocean wave. And that may be what has happened on some planets around some stars, and perhaps even early in Earth’s history. But today on planet Earth, living organisms don’t just stand up in the hurricane of change and entropy. They also make copies of themselves, so that when particular cells fall down (and eventually they will all fall down), others can take their place. Reproduction is the ability to make viable copies of cells. Reproduction means that the template for making an organism (its genome, in modern terminology) can survive even after individuals have died. Like an instruction manual, the genome stores information about the proteins needed to build a copy of the parent as well as some basic assembly rules. Today, most of this information is stored in molecules of DNA. But early in the history of life on Earth, it was probably stored in RNA, a molecular cousin to DNA that still does a lot of heavy lifting inside cells.
Though the templates are more or less immortal, the copying process is not perfect. This is good news, because it means the templates can slowly change as a result of tiny copying errors, and that is the key to adaptation and evolution. Tiny genetic changes give life its remarkable resilience because they allow species to adapt to their environments by randomly creating slightly different templates. As environments change, so, too, do the rules that determine which templates will survive and which will perish.
This is the mechanism Charles Darwin described as natural selection. Natural selection is a fundamental idea in modern biology because it is an extraordinarily powerful driver of increasing complexity. Natural selection filters out some genetic possibilities, allowing only those compatible with local rules. So natural selection is a ratchet, like the fundamental laws of physics, because it locks nonrandom patterns in place. But in the biological realm, it is the local rules of particular environments, not the universal rules of physics, that determine what survives. And the biological rules are much more persnickety. Don’t expect a giraffe to survive underwater.
Like the mechanisms that generated the universe’s first structures, natural selection links necessity and chance. Variation provides multiple possibilities; natural selection uses local rules to pick out those that will work under local conditions. Here is how Darwin put it in The Origin of Species:
Can it… be thought improbable [that] variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations? If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection.9
Darwin’s idea, when linked to a modern understanding of genetics and heredity, explains life’s creativity, its ability over many generations to explore possibilities, tap new energy flows, and construct new types of structures. It explains how, in the biological realm, structures of staggering complexity can emerge through repetitive algorithmic processes as they are filtered out from myriad variations, step by step and generation by generation, over millions and billions of years.
The idea of natural selection shocked Darwin’s contemporaries, because it seemed to do away with the need for a creator god.10 And that idea was fundamental to the Christian origin story that most people accepted in Victorian England. Even Darwin was worried, and his wife, Emma, feared she and Charles would end up in different places in the afterlife. But the mechanism Darwin described really does seem to be fundamental to the history of life. Let finches breed on one of the Galápagos Islands that Darwin visited in his youth. If this island’s trees produce nuts with tough shells, over time those finches with beaks that can crack the shells most efficiently will survive better and produce more offspring than others. Wait a few generations, and you will find all the finches on this island have this type of beak. Over time, as some individuals are selected by “nature” (in fact, by the rules of the local environment), a new species will eventually emerge. Here, as Darwin showed, is the basic mechanism of biological evolution. This is Darwin’s complexity ratchet; this is how life builds more and more complex things, step by step.
The Goldilocks Conditions for Life
How did life first sputter into motion somewhere in the rich and varied Goldilocks environments of the young Earth?11
What Darwin did not know was that mechanisms similar to natural selection, in which random changes are filtered out by local rules, can also work in rough-and-ready ways in a world without life. Where there are complex mixtures of chemicals and plenty of free energy, molecules can arise that encourage the formation of other molecules and eventually create the molecules the reaction started out with. This is an autocatalytic cycle, a reaction whose components enable, or catalyze, the production of other components of the cycle, including its original ingredients, so the cycle can repeat itself. Fire up one of these cycles, and it will produce its components in larger and larger quantities as it extracts more and more food energy until it starts starving other, less successful reactions. The cycle may even modify itself slightly if new types of food appear. This is beginning to look like the survival of the most successful chemical reaction. So here we already have something a bit lifelike, something that can persist and reproduce by tapping energy from its surroundings. “Before we can have competent reproducers,” writes Daniel Dennett, “we have to have competent persisters, structures with enough stability to hang around long enough to pick up revisions.”12 This idea of chemical evolution will help us explain, at least in general terms, how the preconditions for life emerged on the young Earth.
