Origin Story: A Big History of Everything

by David Christian

percent of their lifetimes at some point on the main sequence. Where they sit depends on their mass, but all stars on the main sequence generate the energy they need by fusing protons into helium nuclei. And that’s what our sun is doing right now, too. It is middle-aged and still on the main sequence. In the top right of the diagram you find red giants, like Betelgeuse, which is at one corner of the constellation Orion. These are aging stars that have used up most of the protons in their cores and are fueling their furnaces by burning other, larger nuclei. They have cooler surfaces because they have expanded to perhaps two hundred times the radius of our sun. But the total amount of light they emit is huge because they are very large, which is why they are near the top of the diagram. The third

the red giant has enough mass, gravity will compress it so tightly that its core gets hotter than ever before, hot enough to start fusing helium nuclei into heavier nuclei, such as carbon (with six protons) and oxygen (with eight protons). The star has revived, but fusing helium nuclei is a more complicated process than fusing protons and generates less energy, so stars at this stage have a much shorter life expectancy. Very large stars will go through several stages of increasingly frenetic expansion and contraction. Carbon and oxygen will fuse to form elements from magnesium to silicon and eventually iron. As the stars heat up, another mechanism

The core will gradually become a huge ball of iron surrounded by layers of other elements. And that’s the end of the road, because you cannot generate energy by fusing iron nuclei. Eventually, most stars will blast away their outer layers and end up as white dwarfs, which are down in the bottom left corner of the Hertzsprung-Russell diagram. White dwarfs are stellar zombies, with no

Most supergiants, having blasted away their outer layers in supernovas, will contract so violently that protons and electrons are squashed together to form neutrons. Now the entire massive blob is crushed into a neutron star, an object made

neutrons packed together as closely as the particles in an atomic nucleus. This is a very unusual and extremely dense form of matter, as most atoms consist mainly of empty space. A neutron star just twenty kilometers across would weigh twice as much as our sun, and a teaspoon of neutron-star stuff would weigh a billion tons.5 There is some evidence that many of the heavier elements in the periodic table may have been formed, not in standard supernovas,

Astronomers can tell a star’s surface temperature from the color (or frequency) of the light it emits, so we know that surface temperatures can be as low as 2,500 K and as high as 30,000 K. And, as we have already seen, they can calculate the total amount of light a star emits (its luminosity) by measuring its apparent brightness and then calculating how much brighter it would

Electrons are the key players. Like human lovers, electrons are unpredictable, fickle, and always open to better offers. They buzz around protons in distinct orbits, each associated with a different energy level. Wherever possible, electrons head for the orbits closest to an atom’s nucleus, which require the least energy. But the number of spaces in each orbit is limited, and if no places are left in the inner orbits, they have to settle for places in an outer orbit. If that orbit has just the right number of electrons, everyone is happy.

This is the situation of the so-called

called noble gases, such as helium or argon, which you find over on the right-hand side of the periodic table. They don’t combine with other atoms because they a...

But if the outer orbits of an atom ar...

Some electrons jump ship and head for neighboring atoms. If they do that, the atom they left will have lost a negative charge, so it may pair up with an atom that has an extra electron to form an ionic bond. This is how salt is formed from atoms of sodium, whose

Our solar system lies in the galaxy we call the Milky Way in a stellar suburb on one of the Milky Way’s spiral arms, the Orion spur. The Milky Way is one member of a group of about fifty galaxies,

unromantically, as the Local Group. The Local Group lies in the outer regions of the Virgo Cluster, which has about a thousand galaxies. This is part of the Local Supercluster, which includes hundreds of groups of galaxies. It would take you one hundred million years traveling at the speed of light to cross it. In 2014, it was found that the Local Supercluster is part of a vast cosmic empire with perhaps a hundred thousand galaxies, and to cross that would take you four hundred million years traveling at the speed of light. This empire is the Laniakea (Hawaiian for “immeasurable heaven”) Supercluster. At present, this is the largest structured entity we know of in the universe. We assume that Laniakea is built around a scaffolding of dark matter whose gravitational pull holds all these galaxies together as

English surveyor, William Smith, showed that identical suites of fossils appeared in rock formations in different places. On the reasonable assumption that similar fossils must have come from about the same time, you could identify strata around the world that had been laid down at the same time. Taken together, these principles allowed nineteenth-century geologists to create a relative timeline for Earth’s history.

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,

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.

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,

Rich chemistry is possible within only a narrow range of fairly low temperatures, and you find these in habitable zones that are close to stars but not too close. Our Earth’s orbit is in about the middle of our sun’s habitable zone. Venus and Mars orbit at the inner and outer edges, respectively, of our system’s habitable zone. But we are learning that some moons farther away from the sun, such as Saturn’s moon Enceladus, may also have internal furnaces and chemistries that make them life-friendly.

The presence of liquids depends on chemistry, temperature, and pressure. There is a narrow range of temperatures in which water exists in liquid form (most water in the universe is in the form of ice). But at these same temperatures, you can also find gases and solids, which makes for very interesting chemical possibilities. So, we should expect the most interesting chemistry to be on

whose average surface temperatures lie roughly between zero and one hundred degrees Celsius, the freezing and boiling points, respectively, for water. That’s rare, but our Earth happens to be at just the right distance from the sun to have liquid water. A fourth

chemical diversity. It’s no good having the right temperature if you’ve got only hydrogen and helium to work with. And today, even in the chemically rich regions within galaxies, hydrogen and helium still make up 98 percent of all atomic matter. What chemistry needs is those rare environments in which the other elements of the periodic table are more common. In our solar...

boiled away much of the hydrogen and helium from the solar system’s inner orbits, leaving a conce...

elements in the perio...

In both cells and stars, concentrated flows of energy are needed to pay entropy’s taxes and overcome the universal tendency of all things to degrade. In living organisms, energy has a new function that we don’t find in stars:

understand how plate tectonics works in detail, it helps to focus on the borders between tectonic plates. At divergent margins, like those described by Harry Hess, material rises from the mantle and pushes plates apart. Elsewhere, though, at convergent margins, plates are pushed together. If the two plates have about the same density—if, say, they both consist of granitic continental plates—then, like two bull walruses competing for mates, they will rear up. This is how the Himalayas formed; within the past fifty million years, the fast-moving Indian plate traveled north from Antarctica and smashed into the Asian plate. But if two converging plates have different densities—if, say, one consists of heavy, basaltic oceanic crusts

heavier oceanic plate will dive under the lighter plate at a subduction zone. It will travel downward, like a runaway elevator crashing through a concrete floor, carrying crustal material back into the mantle, where it dissolves. As the descending plate drills into the mantle, it will generate so much friction and heat that it can melt the crust above it, splitting it and punching up new volcanic mountain chains. This is how the Andes formed, as the Pacific plate burrowed beneath the plate carrying the west coast of South America. Finally, there are transform margins. Here, plates grind their way past each other like two pieces of sandpaper jammed together but pushed in opposite directions. Friction will stop the plates sliding until so much pressure builds up that there is a sudden, violent lurch. This is the source of the pressure building up along the

The astronomer Carl Sagan (one of the great pioneers of a modern origin story) pointed out that answering this question is vital because it may solve another puzzle. Stars like our sun emit more and more energy as they age, so the amount of heat arriving on Earth has slowly increased. When Earth was young, the sun was emitting 30 percent less energy than today. So why was the early Earth not a ball

The answer, it turned out, was the amount of greenhouse gases in the early atmosphere. Their levels were high enough to warm the young Earth so that life could evolve. There was hardly any free oxygen in Earth’s first atmosphere, but there were lots of greenhouse gases, particularly water vapor, methane, and carbon dioxide, belched up from the mantle through volcanoes or ferried in by asteroids. A greenhouse atmosphere was one more important Goldilocks

to put it more generally, what ensured that as the sun began to emit more energy, Earth’s surface would remain within the magical temperature range of zero to one hundred degrees Celsius? In

One of the most important of these thermostats is purely geological, so it would have begun to work even before there was life on Earth. It links tectonics and another driver of planetary change: erosion. While tectonics builds mountains up, erosion

Wind and water and chemical flows of various kinds break down the rocks of mountains and move them down a gravitational gradient into the oceans. Erosion explains why mountains aren’t much higher than they are; tectonics explains why they haven’t all vanished into a single, vast global plain. Erosion is itself a by-product of tectonics, of course, because both the wind and rain were burped up from Earth’s innards. And mountain

Here’s how the geological thermostat works. Carbon dioxide, one of the most powerful of the greenhouse gases, dissolves in rainfall and reaches the Earth in the form of carbonic acid. It dissolves material in rocks, and the by-products of these reactions, which contain lots of carbon, are swept

ocean. Here, some of the carbon gets locked up in carbonate rocks. Where tectonic plates dive back into the mantle at subduction zones, some of this carbon (much of it in the form of limestone) can get buried in the mantle for millions, even billions of years. In this way, the tectonic conveyor belt removes carbon from the atmosphere, and that should eventually reduce carbon dioxide levels and generate colder climates. Today we know that much more carbon is buried within the mantle than is present on Earth’s surface or in its atmosphere. Of course,

much carbon dioxide was buried in this way, Earth would freeze. That was prevented (most of the time) by the second feature of the geological thermostat. Driven by plate tectonics (a mechanism that is prob...

There is a balance between the two halves of this mechanism because higher temperatures generate more rainfall, which accelerates erosion, moving more carbon back into the mantle. But if the Earth cools too much, rainfall will dwindle, less carbon dioxide will be buried, carbon dioxide levels will build up as it is pumped up through volcanoes, and that will warm things up again. The geological thermostat has been adjusting to the increasing warmth of our sun over four billion years.7 We know of nothing

But eventually, other, backup thermostats evolved. These were created by the activities of living organisms. So now we must return to the role of life in the biosphere as living organisms stepped onto Earth’s geological stage and began to explore and eventually transform its many different ecological nooks and crannies. The Unity of Life

which different types of genomes diversify. So we can say with some confidence that humans and chimps shared a common ancestor about seven or eight million years ago, while humans and bananas have followed different genetic paths for about eight hundred million

The taxonomic levels become increasingly capacious from there; in ascending order, they are family, order, class, phylum, kingdom, and domain. So we can say that humans belong to the species sapiens, the genus Homo, the family Hominidae, the order Primates, the class Mammalia, the phylum Chordata (vertebrates), the kingdom Animalia, and the domain Eukarya. The first living organisms surely diversified fast, as they entered

That is the point at which natural selection, in Darwin’s sense, really took off. Once life got going, there were no guarantees it would survive. Mars and Venus may once have hosted simple life-forms. But if they did, life was soon extinguished on both planets. Even on Earth, the survival of a thin scum of life for almost four billion years depended on lots of things going right.

Both domains consist entirely of prokaryotes, minute single-celled organisms that have neither a distinct nucleus nor other specialized cellular organelles. Prokaryotes would dominate the biosphere for more than seven-eighths of its history, until about six hundred million years ago. If we meet living organisms elsewhere in our galaxy, we probably won’t be shaking hands with them but peering at them through a microscope. So small are prokaryotes that one hundred thousand of them could have a party inside the dot at the end of this sentence. Prokaryote

Today, prokaryotes still dominate the biosphere. On and within your body, there are probably more prokaryotic cells than cells with your own DNA. But we ignore them (until they give us a stomachache or cold)

world that we share with prokaryotes is known as the microbiome. Until recently, it was tempting to think that the history of single-celled organisms was boring, so we could happily skip the first three billion years of the biosphere’s history. Today we are learning that we can’t make sense of the biosphere’s recent history without understanding the much longer era of little life. As they evolved, prokaryotes

in the membrane and the flagella means that, in effect, E. coli has a short-term memory. It may last for just a few seconds but is powerful enough to say either “No problem, nothing to do!” or “This is not good, flagella, start flailing!” The short-term memory is based on tiny changes in the sensors and the chemicals they emit. This is simple information-processing equipment, but already we have the three key components of all biological information processing: inputs, processing, and outputs.

Quite early on, some prokaryotes learned how to eat other prokaryotes. These were the biosphere’s first heterotrophs, the prokaryotic equivalent of carnivores such as T. rex. You and I are also heterotrophs; we get our food energy by consuming other organisms, not by eating chemicals. But even eating other organisms has its limits if

about 3.5 billion years ago, a new evolutionary innovation, photosynthesis, was letting some organisms tap into flows of energy from the sun. This was life’s first energy bonanza, and its impact was the prokaryotic equivalent of a gold strike. Photons of light from the sun have thousands

How did living organisms use sunlight? There are several types of photosynthetic reactions that convert sunlight to biological energy with varying degrees of efficiency and release different by-products. All of them use energetic photons newly arrived from the sun to goose electrons inside light-sensitive molecules such as chlorophyll. This gives the electrons such a shock that they jump out of their home atoms and then get kidnapped, wriggling all the time, by proteins. The proteins pass the high-energy

This creates an electrical gradient across the membrane that can be used to charge up energy-carrying molecules such as ATP. This is chemiosmosis again, but this time, the energy that charges up molecules of ATP comes not from food molecules but from that vast generator in the sky, the sun. That’s

Prokaryotes that made their living from photosynthesis had to be near the surface of the oceans or on seashores. Many formed coral-like structures known as stromatolites, which grew into reefs at the edges of continents as billons of organisms accumulated over thickening layers

They are rare today, but from the time when they first appeared, more than 3.5 billion years ago, until about 500 million years ago—significantly more than half the history of our planet—they were probably the most visible form of life on Earth. If aliens had come looking for life on this planet, they would have found stromatolites. And perhaps that’s what we’ll find when we first detect life on rocky planets in other star systems.

general formula for this oxygen-generating form of photosynthesis is H2O + CO2 + energy from sunlight → CH2O (carbohydrates that act as stores of energy) + O2 (molecules of oxygen that are released into the atmosphere). Oxygen photosynthesis was much more efficient than earlier forms but still could extract only about 5 percent of the energy in sunlight, which is less than the most