Matchmaking life

Oxygen’s got a brand new pair of roller skates, hydrogen’s got a brand new key.

You can find romance if you want in the periodic table, with elements on the right side of the table, just an electron or two short of a full shell, cruising for partners over on the left side, with an electron or two to share. The folk-singing sisters Anne and Kate Macgarrigle made a song out of this, NaCl, about a love affair between a chlorine atom and a sodium atom. “Think of all the love you eat when you salt your meat.”

And living things are matchmakers, middlemen making a living by arranging liaisons between the two sides of the periodic table, harvesting the energy released by moving electrons around. The basic mechanism is shown below.

Take a reducing compound willing to give up an electron and an oxidant looking for an electron. Electrons go where they ain’t: starting on the left, electrons (e-) from the first compound pass through a series of protein complexes (grayish circles) embedded in a membrane (horizontal lines). The passage of electrons from one complex to the next in the membrane powers the movement of protons (H⁺) from one side of the membrane to the other. For each pair of electrons passing down the chain, ten protons are moved across the membrane. Finally the electrons are united with the oxidant (shown as O₂ generating H₂O). Electrons keep being pulled through the chain as long as reducing and oxidizing agents are available.

This is the first part the job. The result is a lot of extra protons on one side of the membrane and a large difference in electrical potential between the two sides. For the second part, another protein complex (ATP synthase) uses this potential difference to turn ADP (adenosine diphosphate) into ATP (adenosine triphosphate). ATP is the fundamental energy carrier of life, powering the cell’s chemical reactions when it turns back into ADP. (Creatine supplements work for athletes and bodybuilders because creatine helps with ATP synthesis.)

This is the basic mechanism of respiration (and most of photosynthesis), just as DNA replication is the basic mechanism of heredity. Understanding the origin of this mechanism is a major challenge in understanding the origin of life. Nick Lane walks through some of the theories.

Up this point in Earth’s history, living things are tiny and not very interesting structurally. But they are hugely diverse biochemically, making use of a great array of different reducing and oxidizing compounds. For example, methanogens use hydrogen seeping from undersea vents to reduce carbon dioxide, producing methane. This may have been of some importance to the planet as a whole. Methane is a greenhouse gas, trapping infrared radiation even more effectively than carbon dioxide. The Sun three billion years ago was fainter than it is today; it may be methanogens that kept a young Earth from freezing over.

Better living through chemistry

2.43 – 2.31 billion years ago

There are some interesting parallels between chemistry on the one hand, and linguistics on the other. Remarkably, a recent article makes a strong case that there is an actual historical connection between the science of linguistics and the science of chemistry. Specifically, Mendeleev’s construction of the Periodic Table of Elements was probably influenced by Pāṇini’s classic generative grammar of Sanskrit, the Aṣṭādhyāyī. This was written somewhere around 500-350 BCE. It has been said to be as central to India’s intellectual tradition as Euclid’s Elements is to the West’s. It probably reached the attention of Mendeleev thanks to the work of his friend and colleague, the Indologist and philologist Otto von Böhtlingk, who translated it.

[F]oundational to the Aṣṭādhyāyī was a two-dimensional, periodic alphabet, which may have intrigued Mendeleev as he struggled to create his own periodic array.

The physicist Eugene Wigner wrote about “the unreasonable effectiveness of mathematics in the natural sciences.” (Here is a cute recent example where repeated rebounding collisions produce successively close approximations of pi.) Perhaps Mendeleev’s debt to Pāṇini via von Böhtlingk is an example of the unreasonable effectiveness of linguistics.

Here’s more on the parallels:

Chemistry plays a big role once Earth forms. Different mineral species appear, with different chemical compositions. Magnesium-heavy olivine sinks to the lower mantle of the Earth. Aluminum-rich feldspars float to the top.

Chemistry is an example of what William Abler calls “the particulate principle of self-diversifying systems,” what you get when a collection of discrete units (atoms) can combine according to definite rules to create larger units (molecules) whose properties aren’t just intermediate between the constituents. Paint is not an example. Red paint plus white paint is just pink paint. Mix in a little more red or white to make it redder or whiter. But atoms and molecules are: two moles of hydrogen gas plus one mole of oxygen gas, compounded, make something very different, one mole of liquid water. Add in a little more hydrogen or oxygen and you just get leftover hydrogen or oxygen.

A lot of important chemical principles are summed up in the periodic table.

On the far right are atoms that have their electron shells filled, and don’t feel like combining with anyone. Most, but not all the way, to the right are atoms with almost all their shells filled, just looking for an extra electron or two. (Think oxygen, O, with slots for two extra electrons). On the left are atoms with a few extra electrons they can share. (Think hydrogen, H, each atom with an extra electron it’s willing to share with, say, oxygen.) In the middle are atoms that could go either way: polymorphously perverse carbon, C, star of organic chemistry, with four slots to fill and four electrons to share, and metals, that like to pool their electrons in a big cloud, and conduct electricity and heat easily. (Think of Earth’s core of molten iron, Fe, a big electric dynamo.)

Another example of “the particulate principle of self-diversifying systems” is human language. Consider speech sounds, for example. You’ve got small discrete units (phonemes, the sounds we write b, p, s, k, ch, sh, and so on) that can combine according to rules to give syllables. Some syllables are possible, according to the rules of English, others not. Star and spiky, thole and plast, are possible English words, tsar and psyche are not (at least if you pronounce all the consonants, the way Russians or Greeks do), nor tlaps nor bratz (if you actually try to pronounce the z). Thirty years ago app, blog, and twerk were not words in the English language, but they were possible words, according to English sound laws.

You can make a periodic table of consonants.

Across the top are the different places in the vocal tract where you block the flow of air. Along the left side are different ways of blocking the flow (stopping it completely –t-, letting it leak out –s-, etc.) The table can explain why, for example, we use in for intangible and indelicate, but switch to im for impossible and imbalance. (The table contains sounds we don’t use in English, and uses a special set of signs, the International Phonetic Alphabet, which assigns one letter per phoneme.) This is why a book title like The Atoms of Language makes sense (a good book by the way).

So sometimes the universe gets more complex because already existing stuff organizes itself into complex new patterns  – clumps and swirls and stripes. But sometimes the universe gets more complex because brand new kinds of stuff appear, because a new particulate system comes online: elementary particles combine to make atoms, atoms combine to make molecules, or one set of systems (nucleotides to make genes, amino acids to make proteins) combines to make life, or another set of systems (phonemes to make words, words to make phrases and sentences) combines to make language.

The curve of binding energy

9.86 – 9.33 billion years ago

More on stardust and us.

Looking at the abundance of different elements in the universe, we get the following:

Note that the vertical scale is exponential. Each tick marks a hundred-fold increase in abundance over the tick below, so there is vastly more hydrogen and helium in the universe than any other element. As noted in the last post, all the elements except hydrogen and helium were formed after the Big Bang, spewed out by supernovas and the collisions of neutron stars. In general, heavy elements are less abundant because it takes more steps to produce heavy elements than light ones. But the curve is not smooth. The lightest elements after hydrogen and helium (lithium, beryllium, boron) are relatively rare, because they get used up in the nucleosynthesis of heavier elements. And there is a saw tooth pattern in the chart, because nucleosynthesis favors atoms with even numbers of protons. So we get lots of oxygen, magnesium, silicon, and iron, the main constituents of our planet. Lots of carbon too. Finally, iron (Fe) is more than 1000 times more abundant than might be expected based on a smooth curve. Iron nuclei are especially stable because binding energy, the energy that would be required to take the nucleus apart into its constituent protons and neutrons, reaches a maximum with iron. Here’s the famous curve of curve of binding energy (nucleons are protons and neutrons):

An implication of this curve is that if you can split a really heavy nucleus, of Uranium-235 say, into smaller nuclei (but still heavier than iron), you will release energy equal to the vertical difference between U-235 and its lighter fission products (not shown) on the vertical scale. This is lots of energy, way more than you get from breaking or forming molecular bonds in ordinary chemical reactions. And if you can fuse two light nuclei, of hydrogen say, into a larger nucleus, you can get even more energy. When we split uranium, we are recovering some of the energy that colliding neutron stars put into synthesizing the heaviest elements. When we fuse hydrogen, we are extracting energy left over from the Big Bang that no star got around to releasing. (This doesn’t violate the Law of Conservation of Energy, because the negative gravitational potential energy of the universe cancels the positive energy represented by the matter. So the total energy of the universe is zero.)

Starting to figure this all out was part of a scientific revolution that made physics in 1950 look very different from physics in 1900. The new physics resolved a paradox in the study of prehistory. Geologists were pretty confident, based on rates of sedimentation, that the Earth had supported complex life for hundreds of millions of years. But physicists couldn’t see how the sun could have kept shining for so long. The geologists were right about deep time; it took new physics to understand that the sun got its energy from fusing hydrogen to helium (via some intermediate steps).

As the scientific revolution in atomic physics was picking up steam, it was natural to assume that it would be followed by a revolution in technology. After all, earlier scientific revolutions in the understanding of masses and gases, atoms and molecules, and electrons and electromagnetism, had been followed by momentous innovations in technology: the steam engine, artificial fertilizers, electrification, radio, to name just a few. But in some ways, the Atomic Age hasn’t lived up to early expectations. The atom bomb brought an earlier end to the Second World War, but didn’t change winners and losers. The bomb was never used again in war, and it’s a matter for debate how much the atom bomb and the hydrogen bomb changed the course of the Cold War. Nuclear energy now generates a modest 11% of the world’s electricity (although this number had better go way up in the future if we’re serious about curbing carbon dioxide emissions). And a lot of ambitious early proposals for harnessing the atom never got anywhere. Project Plowshare envisioned using nuclear explosions for enormous civil engineering projects, digging new caves, canals, and harbors. Even more audacious was Project Orion, which developed plans for a rocket propelled by nuclear explosions. Some versions of Orion could have carried scores of people and enormous payloads throughout the solar system. Freeman Dyson, a physicist who worked on the project, said “Our motto was ‘Mars by 1965, Saturn by 1970.’”

On the purely technical side these plans were feasible. There were concerns about fallout, but the problems were not insurmountable. Nevertheless both Plowshare and Orion were cancelled. Regarding Orion, Dyson said “… this is the first time in modern history that a major expansion of human technology has been suppressed for political reasons.” The history of the Atomic Age  and its missed opportunities is one more refutation of pure technological determinism. How or even whether a new technology is exploited depends on social institutions, politics, and cultural values.

Speaking of which, Where Is My Flying Car?

We are stardust

10.4 – 9.9 billion years ago

The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.

Carl Sagan (h/t to commenter remanandhra)

There’s a long gap between the origin of the universe, the first stars, and early galaxies, and the origin of our Solar System and our planet Earth. If we were using a linear scale for our calendar, the Solar System would get started in September. Even on our logarithmic scale, Sun and Earth wait until late January. A spiral galaxy like the Milky Way is an efficient machine for turning dust into stars over many billions of years. But the earliest stars it produces are poor in “metals” (to an astronomer, anything heavier than helium is a metal). It takes generations of exploding stars producing heavier elements and ejecting them into space before a star like the Sun — 2% metal – can form.

And just a few years back, a spectacular discovery provided support for another mechanism of heavy element formation. Astronomers for the first time detected gravitational waves from the collision of two neutron stars, 300 million light-years away. Such collisions may be responsible for the formation of some of the heaviest atoms around, gold and silver in particular. So your gold ring may be not just garden-variety supernova stardust, but the relic of colliding neutron stars. Here’s a chart showing where the elements in our solar system come from:

Alchemists thought they could change one element into another – lead into gold, say. But it takes more extreme conditions than in any chemistry lab to transmute elements. The heart of a star makes heavy elements out of hydrogen and helium; it takes a supernova to make elements heavier than iron, and something even more spectacular, the collision of neutron stars, to make the heaviest elements. So it’s literally true, not just hippy poetry, that “we are stardust” (at least the part of us that isn’t hydrogen).

Matchmaking life

You can find romance if you want in the periodic table, with elements on the right side of the table, just an electron or two short of a full shell, cruising for partners over on the left side, with an electron or two to share. The folk-singing sisters Anne and Kate Macgarrigle made a song out of this, NaCl, about a love affair between a chlorine atom and a sodium atom. “Think of all the love you eat when you salt your meat.”

And living things are matchmakers, middlemen making a living by arranging liaisons between the two sides of the periodic table, harvesting the energy released by moving electrons around. The basic mechanism is shown below.

Take a reducing compound willing to give up an electron and an oxidant looking for an electron. Electrons go where they ain’t: starting on the left, electrons (e-) from the first compound pass through a series of protein complexes (grayish circles) embedded in a membrane (horizontal lines). The passage of electrons from one complex to the next in the membrane powers the movement of protons (H⁺) from one side of the membrane to the other. For each pair of electrons passing down the chain, ten protons are moved across the membrane. Finally the electrons are united with the oxidant (shown as O₂ generating H₂O). Electrons keep being pulled through the chain as long as reducing and oxidizing agents are available.

This is the first part the job. The result is a lot of extra protons on one side of the membrane and a large difference in electrical potential between the two sides. For the second part, another protein complex (ATP synthase) uses this potential difference to turn ADP (adenosine diphosphate) into ATP (adenosine triphosphate). ATP is the fundamental energy carrier of life, powering the cell’s chemical reactions when it turns back into ADP. (Creatine supplements work for athletes and bodybuilders because creatine helps with ATP synthesis.)

This is the basic mechanism of respiration (and most of photosynthesis), just as DNA replication is the basic mechanism of heredity. Understanding the origin of this mechanism is a major challenge in understanding the origin of life. Nick Lane walks through some of the theories.

Up this point in Earth’s history, living things are tiny and not very interesting structurally. But they are hugely diverse biochemically, making use of a great array of different reducing and oxidizing compounds. For example, methanogens use hydrogen seeping from undersea vents to reduce carbon dioxide, producing methane. This may have been of some importance to the planet as a whole. Methane is a greenhouse gas, trapping infrared radiation even more effectively than carbon dioxide. The Sun three billion years ago was fainter than it is today; it may be methanogens that kept a young Earth from freezing over.

Better living through chemistry

2.42 – 2.30 billion years ago

There are some interesting parallels between chemistry on the one hand, and linguistics on the other. Remarkably, a recent article makes a strong case that there is an actual historical connection between the science of linguistics and the science of chemistry. Specifically, Mendeleev’s construction of the Periodic Table of Elements was probably influenced by Pāṇini’s classic generative grammar of Sanskrit, the Aṣṭādhyāyī. This was written somewhere around 500-350 BCE. It has been said to be as central to India’s intellectual tradition as Euclid’s Elements is to the West’s. It probably reached the attention of Mendeleev thanks to the work of his friend and colleague, the Indologist and philologist Otto von Böhtlingk, who translated it.

[F]oundational to the Aṣṭādhyāyī was a two-dimensional, periodic alphabet, which may have intrigued Mendeleev as he struggled to create his own periodic array.

The physicist Eugene Wigner wrote about “the unreasonable effectiveness of mathematics in the natural sciences.” (Here is a cute recent example where repeated rebounding collisions produce successively close approximations of pi.) Perhaps Mendeleev’s debt to Pāṇini via von Böhtlingk is an example of the unreasonable effectiveness of linguistics.

Here’s more on the parallels:

Chemistry plays a big role once Earth forms. Different mineral species appear, with different chemical compositions. Magnesium-heavy olivine sinks to the lower mantle of the Earth. Aluminum-rich feldspars float to the top.

Chemistry is an example of what William Abler calls “the particulate principle of self-diversifying systems,” what you get when a collection of discrete units (atoms) can combine according to definite rules to create larger units (molecules) whose properties aren’t just intermediate between the constituents. Paint is not an example. Red paint plus white paint is just pink paint. But atoms and molecules are: two moles of hydrogen gas plus one mole of oxygen gas, compounded, make something very different, one mole of liquid water.

A lot of important chemical principles are summed up in the periodic table.

On the far right are atoms that have their electron shells filled, and don’t feel like combining with anyone. Most, but not all the way, to the right are atoms with almost all their shells filled, just looking for an extra electron or two. (Think oxygen, O, with slots for two extra electrons). On the left are atoms with a few extra electrons they can share. (Think hydrogen, H, each atom with an extra electron it’s willing to share with, say, oxygen.) In the middle are atoms that could go either way: polymorphously perverse carbon, C, with four slots to fill and four electrons to share, and metals, that like to pool their electrons in a big cloud, and conduct electricity and heat easily. (Think of Earth’s core of molten iron, Fe, a big electric dynamo.)

Another example of “the particulate principle of self-diversifying systems” is human language. Consider speech sounds, for example. You’ve got small discrete units (phonemes, the sounds we write b, p, s, k, ch, sh, and so on) that can combine according to rules to give syllables. Some syllables are possible, according to the rules of English, others not. Star and spiky, thole and plast, are possible English words, tsar and psyche are not (at least if you pronounce all the consonants, the way Russians or Greeks do), nor tlaps nor bratz (if you actually try to pronounce the z). Thirty years ago app, blog, and twerk were not words in the English language, but they were possible words, according to English sound laws.

You can make a periodic table of consonants.

Across the top are the different places in the vocal tract where you block the flow of air. Along the left side are different ways of blocking the flow (stopping it completely –t-, letting it leak out –s-, etc.) The table can explain why, for example, we use in for intangible and indelicate, but switch to im for impossible and imbalance. (The table contains sounds we don’t use in English, and uses a special set of signs, the International Phonetic Alphabet, which assigns one letter per phoneme.) This is why a book title like The Atoms of Language makes sense (a good book by the way).

So sometimes the universe gets more complex because already existing stuff organizes itself into complex new patterns  – clumps and swirls and stripes. But sometimes the universe gets more complex because brand new kinds of stuff appear, because a new particulate system comes online: elementary particles combine to make atoms, atoms combine to make molecules, or one set of systems (nucleotides to make genes, amino acids to make proteins) combines to make life, or another set of systems (phonemes to make words, words to make phrases and sentences) combines to make language.

The curve of binding energy

9.86 – 9.33 billion years ago

More on stardust and us.

Looking at the abundance of different elements in the universe, we get the following:

Note that the vertical scale is exponential, so there is vastly more hydrogen and helium in the universe than any other element. As noted in the last post, all the elements except hydrogen and helium were formed after the Big Bang, spewed out by supernovas and the collisions of neutron stars. In general, heavy elements are less abundant because it takes more steps to produce heavy elements than light ones. But the curve is not smooth. The lightest elements after hydrogen and helium (lithium, beryllium, boron) are relatively rare, because they get used up in the nucleosynthesis of heavier elements. And there is a saw tooth pattern in the chart, because nucleosynthesis favors atoms with even numbers of protons. So we get lots of oxygen, magnesium, silicon, and iron, the main constituents of our planet. Lots of carbon too. Finally, iron (Fe) is more than 1000 times more abundant than might be expected based on a smooth curve. Iron nuclei are especially stable because binding energy, the energy that would be required to take the nucleus apart into its constituent protons and neutrons, reaches a maximum with iron. Here’s the famous curve of curve of binding energy (nucleons are protons and neutrons):

An implication of this curve is that if you can split a really heavy nucleus, of Uranium-235 say, into smaller nuclei (but still heavier than iron), you will release energy equal to the vertical difference between U-235 and its lighter fission products (not shown) on the vertical scale. This is lots of energy, way more than you get from breaking or forming molecular bonds in ordinary chemical reactions. And if you can fuse two light nuclei, of hydrogen say, into a larger nucleus, you can get even more energy. When we split uranium, we are recovering some of the energy that colliding neutron stars put into synthesizing the heaviest elements. When we fuse hydrogen, we are extracting energy left over from the Big Bang that no star got around to releasing. (This doesn’t violate the Law of Conservation of Energy, because the negative gravitational potential energy of the universe cancels the positive energy represented by the matter. So the total energy of the universe is zero.)

Starting to figure this all out was part of a scientific revolution that made physics in 1950 look very different from physics in 1900. The new physics resolved a paradox in the study of prehistory. Geologists were pretty confident, based on rates of sedimentation, that the Earth had supported complex life for hundreds of millions of years. But physicists couldn’t see how the sun could have kept shining for so long. The geologists were right about deep time; it took new physics to understand that the sun got its energy from fusing hydrogen to helium (via some intermediate steps).

As the scientific revolution in atomic physics was picking up steam, it was natural to assume that it would be followed by a revolution in technology. After all, earlier scientific revolutions in the understanding of masses and gases, atoms and molecules, and electrons and electromagnetism, had been followed by momentous innovations in technology: the steam engine, artificial fertilizers, electrification, radio, to name just a few. But in some ways, the Atomic Age hasn’t lived up to early expectations. The atom bomb brought an earlier end to the Second World War, but didn’t change winners and losers. The bomb was never used again in war, and it’s a matter for debate how much the atom bomb and the hydrogen bomb changed the course of the Cold War. Nuclear energy now generates a modest 11% of the world’s electricity (although this number had better go way up in the future if we’re serious about curbing carbon dioxide emissions). And a lot of ambitious early proposals for harnessing the atom never got anywhere. Project Plowshare envisioned using nuclear explosions for enormous civil engineering projects, digging new caves, canals, and harbors. Even more audacious was Project Orion, which developed plans for a rocket propelled by nuclear explosions. Some versions of Orion could have carried scores of people and enormous payloads throughout the solar system. Freeman Dyson, a physicist who worked on the project, said “Our motto was ‘Mars by 1965, Saturn by 1970.’”

On the purely technical side these plans were feasible. There were concerns about fallout, but the problems were not insurmountable. Nevertheless both Plowshare and Orion were cancelled. Regarding Orion, Dyson said “… this is the first time in modern history that a major expansion of human technology has been suppressed for political reasons.” The history of the Atomic Age  and its missed opportunities is one more refutation of pure technological determinism. How or even whether a new technology is exploited depends on social institutions, politics, and cultural values.

Speaking of which, Where Is My Flying Car?

We are stardust

10.4 – 9.9 billion years ago

The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.

Carl Sagan (h/t to commenter remanandhra)

There’s a long gap between the origin of the universe, the first stars, and early galaxies, and the origin of our Solar System and our planet Earth. If we were using a linear scale for our calendar, the Solar System would get started in September. Even on our logarithmic scale, Sun and Earth wait until late January. A spiral galaxy like the Milky Way is an efficient machine for turning dust into stars over many billions of years. But the earliest stars it produces are poor in “metals” (to an astronomer, anything heavier than helium is a metal). It takes generations of exploding stars producing heavier elements and ejecting them into space before a star like the Sun — 2% metal – can form.

And just a few years back, a spectacular discovery provided support for another mechanism of heavy element formation. Astronomers for the first time detected gravitational waves from the collision of two neutron stars, 300 million light-years away. Such collisions may be responsible for the formation of some of the heaviest atoms around, gold and silver in particular. So your gold ring may be not just garden-variety supernova stardust, but the relic of colliding neutron stars. Here’s a chart showing where the elements in our solar system come from:

Alchemists thought they could change one element into another – lead into gold, say. But it takes more extreme conditions than in any chemistry lab to transmute elements. The heart of a star makes heavy elements out of hydrogen and helium; it takes a supernova to make elements heavier than iron, and something even more spectacular, the collision of neutron stars, to make the heaviest elements. So it’s literally true, not just hippy poetry, that “we are stardust” (at least the part of us that isn’t hydrogen).

In the beginning

13.8 – 13.1 billion years ago

Logarithmic History is now rolling into its ninth year. We’ll continue with a mixture of blog posts and tweets, some recycled, some new, with favorite Logarithmic History holidays, celebrating the origins of seafood, first flowers, beer, bread, and more. Welcome! 

Knowing what happened at the very beginning of the Universe is speculative. It depends on what the theory of quantum gravity looks like, which is up in the air. The theory of inflation (insanely fast growth before 10^-32 seconds , after which the universe settled down to merely explosive growth with the Big Bang) may explain why the universe is flat, uniform, and not very lumpy. In 2014, it looked like we had direct evidence for gravity waves generated by inflation, going back just 10 sec from the beginning of the universe. But it looks like this doesn’t hold up.

Later developments are more generally agreed on, although some of the exact times may need revision in the future. The late (1933-1921) Steven Weinberg’s  The First Three Minutes is a classic summary. Strikingly, a lot of familiar astronomical objects, including stars and galaxies, are already around within 100’s of million of years. However early stars are short on metals (to astronomers, anything heavier than helium counts as a metal), and the early Milky Way is dispersed and fuzzy, not the barred spiral galaxy we know today.

Matchmaking life

You can find romance if you want in the periodic table, with elements on the right side of the table, just an electron or two short of a full shell, cruising for partners over on the left side, with an electron or two to share. The folk-singing sisters Anne and Kate Macgarrigle made a song out of this, NaCl, about a love affair between a chlorine atom and a sodium atom. “Think of all the love you eat when you salt your meat.”

And living things are matchmakers, middlemen making a living by arranging liaisons between the two sides of the periodic table, harvesting the energy released by moving electrons around. The basic mechanism is shown below.

Take a reducing compound willing to give up an electron and an oxidant looking for an electron. Electrons go where they ain’t: starting on the left, electrons (e-) from the first compound pass through a series of protein complexes (grayish circles) embedded in a membrane (horizontal lines). The passage of electrons from one complex to the next in the membrane powers the movement of protons (H⁺) from one side of the membrane to the other. For each pair of electrons passing down the chain, ten protons are moved across the membrane. Finally the electrons are united with the oxidant (shown as O₂ generating H₂O). Electrons keep being pulled through the chain as long as reducing and oxidizing agents are available.

This is the first part the job. The result is a lot of extra protons on one side of the membrane and a large difference in electrical potential between the two sides. For the second part, another protein complex (ATP synthase) uses this potential difference to turn ADP (adenosine diphosphate) into ATP (adenosine triphosphate). ATP is the fundamental energy carrier of life, powering the cell’s chemical reactions when it turns back into ADP. (Creatine supplements work for athletes and bodybuilders because creatine helps with ATP synthesis.)

This is the basic mechanism of respiration (and most of photosynthesis), just as DNA replication is the basic mechanism of heredity. Understanding the origin of this mechanism is a major challenge in understanding the origin of life. Nick Lane walks through some of the theories.

Up this point in Earth’s history, living things are tiny and not very interesting structurally. But they are hugely diverse biochemically, making use of a great array of different reducing and oxidizing compounds. For example, methanogens use hydrogen seeping from undersea vents to reduce carbon dioxide, producing methane. This may have been of some importance to the planet as a whole. Methane is a greenhouse gas, trapping infrared radiation even more effectively than carbon dioxide. The Sun three billion years ago was fainter than it is today; it may be methanogens that kept a young Earth from freezing over.