PDF Summary:A Short History of Nearly Everything, by Bill Bryson

Have you always wanted to understand the natural world but found science classes tedious and science textbooks difficult to understand? Author Bill Bryson can relate—that was his motivation for researching and writing A Short History of Nearly Everything. The book is an accessible overview of the natural sciences that describes not only the important discoveries but also the unknowns and controversies that still exist in the sciences, like the mystery of missing mass in the universe and the puzzling details of human evolution.

In this guide, we’ve organized key topics from Bryson’s book into a concise chronology covering the origins of the universe, the geologic history of planet Earth, and the history of life on Earth. We’ll also analyze the scientific issues that Bryson brings up in light of recent scientific papers and compare his perspective to that of scientists like Stephen Hawking.

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(Shortform note: The model of Earth’s interior as four uniform concentric layers is arguably obsolete. As scientists have gathered more and more seismograph data over the years and used increasingly sophisticated computer models to piece it together, they’re developing a more detailed—and more complex—picture of the Earth’s interior. They’ve found thin layers between some of the traditional layers and an “inner inner” core inside the inner core. They’ve also found large lobes of dense material at the base of the mantle on opposite sides of the Earth. They’ve even mapped out inverted mountain ranges on the bottom of the continental crust that appear to be made partially of solid diamond.)

Plate Tectonics

Scientists can also observe continental drift: the motion of different land masses and portions of the sea floor relative to each other. These observations imply that Earth’s land masses are riding on “plates” of solid rock that slide around on top of Earth’s molten interior. Convection currents in the molten rock are thought to be the driving force behind the movement of these continental plates.

But, Bryson continues, even at Earth’s surface, plate tectonics is not an exact science either. He describes how scientists can infer the past locations of continents by matching fossils or other rocks that were unique to a certain area, but were split up when the continents drifted apart. However, as they try to piece together a map of Earth’s continents over geologic history, the map looks different depending on which fossils they use.

Multiple Pangeas

The traditional theory of plate tectonics holds that all of Earth’s continents were once united in a single supercontinent that eventually broke apart. As Bryson points out, there are numerous anomalies that make it difficult to figure out exactly how our modern continents fit together in the original supercontinent.

But new studies suggest that there wasn’t just one supercontinent (the so-called Pangea), but rather several successive supercontinents over the course of Earth’s geologic history. The periodic fragmentation of supercontinents into continents and reassembly of the continents into a new supercontinent could explain some of the anomalies in how the continents appear to fit together: The continents may have fit together in different ways at different times.

Ice Ages

According to Bryson’s exposition, while Earth’s molten interior has certainly played a role in shaping the planet’s surface, ice has arguably played an even greater role. All over the world, we find valleys that were carved out by glaciers, as well as moraines—deposits of rock and sediment that were carried along by glaciers and left in piles when the glaciers melted. From this, scientists infer that almost all of the Earth’s surface has been covered by ice at some point.

Bryson remarks that, while scientists agree that Earth has had numerous ice ages, there is no consensus about exactly what causes ice ages to begin or end. Slight variations in Earth’s orbit or the tilt of its axis may be a factor, as they would change the intensity of sunlight striking the Earth. Volcanic eruptions could also play a role, either by creating plumes of ash that block sunlight and bring down global temperatures, or by releasing large amounts of greenhouse gasses, raising global temperatures.

Furthermore, Bryson explains, there is evidence that global temperatures sometimes rise or fall enough in just a few decades to bring on an ice age or signify the end of one.

He also points out that technically we’re in a mild ice age right now, because Earth has polar ice caps and large temperate climate zones that are snow-covered in the winter. Fossil evidence indicates that tropical climate zones extended from the equator all the way to the poles for much of Earth’s geologic history.

(Shortform note: It’s debatable whether or not we’re currently in an ice age because the definition of an ice age is somewhat subjective. Most sources define an ice age as a period of time during which a substantial portion of Earth’s surface is covered by ice. But just how much is “substantial?” Since ice caps currently cover Antarctica and much of Greenland, some sources agree with Bryson that we’re in an ice age. But other sources say the last ice age ended about 13,000 years ago. And still others, while acknowledging a similar time frame for the last major ice age, make mention of a minor ice age that started about 500 years ago and ended about 200 years ago.)

Modeling the Ice Age Cycle

As computer models of global climate have become more sophisticated, scientists think they’re getting closer to understanding why ice ages come and go—and why they appear to have come and gone at regular intervals over Earth’s geologic history. The answer, according to the latest simulations, has to do with the interrelationship between Earth’s topography and the currents in the atmosphere and oceans that drive much of the planet’s weather.

Both global warming and global cooling tend to be self-reinforcing: Ice sheets absorb less heat from sunlight than land does, so the more ice accumulates on Earth, the less heat Earth receives from the sun, and the lower temperatures fall, causing still more ice to accumulate. Conversely, if ice sheets are receding, the more they recede, the more heat Earth receives, causing them to melt further. But what triggers the transition between these two processes?

Scientists now think it has to do with ocean currents and atmospheric air currents, which play a central role in shaping Earth’s weather. As ice sheets accumulate, ocean levels drop, and thick ice sheets on land fill in and smooth out the topography. When the changes to Earth’s water levels and topography reach a certain threshold, ocean currents and atmospheric air currents shift, resulting in different weather patterns. Some scientists now think this is the dominant mechanism in triggering the onset or end of an ice age.

The History of Life on Earth

Earth’s geologic history is intertwined with its biologic history since, as we’ve discussed, geologists often make use of fossils to infer information about Earth’s past climate and continental structure. Now we’ll turn our attention to what biologists can infer about the history of life on Earth, from the appearance of the first life to the appearance of modern man.

The Origin of Life

According to Bryson, the origin of life is still one of the great mysteries of science. Scientists thought they were close to solving it in the 1950s when Stanley Miller succeeded in synthesizing amino acids by passing an electrical current through a mixture of gaseous chemicals. Amino acids combine to make proteins, which in turn make up the tissues of living organisms. However, more recent discoveries indicate that the chemicals Miller used were probably not present in the atmosphere of the early Earth, so it’s hard to say how the first amino acids on Earth were produced.

And after the amino acids were produced, it’s a bit of a mystery how they were first assembled into proteins. Molecules like amino acids do naturally polymerize (bond together into long chains or other structures) to form proteins under certain conditions, but water inhibits the polymerization reaction, and Earth has always had an abundance of water, particularly in places like under-sea volcanic vents that are thought to be ideal sites for the first life to form.

Furthermore, Bryson continues, all modern life depends on certain proteins that are assembled from exactly the right sequence of different amino acids and then folded into just the right shape. These protein molecules are so complex and specific that they simply can’t form in sufficient quantities by random chance.

Thus, scientists infer that the first lifeforms or pre-lifeforms must have been made up of simpler proteins and become more complex over time. But so far they haven’t figured out what these early proteins were, let alone how to recreate the process in the laboratory.

Origin-of-Life Research

Although Miller’s experiments in the 1950s initially seemed to explain the origins of life, the problems that Bryson mentions have made it an active field of research ever since.

One relatively new approach that scientists are hoping will shed new light on the subject is computer simulation. Simulations are particularly useful for analyzing how theoretical proteins might interact because, as Bryson indicates, scientists hypothesize that the first life formed from simpler proteins than the ones that are found in living cells today. The ability to model how early proteins might have formed might also help scientists to unravel the mystery of how proteins could form in watery conditions.

Chemical reactions like protein synthesis are so complex that modeling them mathematically at the molecular level has only recently become possible. But now, chemical-reaction modeling software is starting to take off. For instance, an AI software developer recently produced an algorithm for predicting shapes of protein molecules based on the sequence of amino acids that they contain. These algorithms may improve our understanding of just how much random chance actually plays a role in protein folding.

At the same time, scientists are also making progress on artificial synthesis of organic molecules like DNA and protein. The more we learn about how these molecules are synthesized, the better we’ll be able to assess how they might first have been assembled into living organisms. A process for synthesizing custom proteins out of a desired sequence of amino acids was developed about 20 years ago, and has since been streamlined considerably.

Similarly, scientists have developed techniques for constructing DNA molecules in any desired sequence. They’ve even used this technique to create viable synthetic viruses. They do this by constructing a strand of DNA containing the entire genetic code of a virus and injecting it into a living host cell. The infected cell then copies the strand and makes more viruses, just as it would in the case of a natural virus infection.

The Theory of Evolution

Bryson asserts that one thing scientists do know is that all modern lifeforms share a common ancestor. This is because they all use the same genetic “language” and contain the same highly-specific proteins.

He asserts that while Charles Darwin is often credited with originating the idea that species evolve from one another through random variation and natural selection, the idea was already commonplace in Darwin’s day. Fourteen years before Darwin published his theory of evolution in On the Origin of Species, Robert Chambers anonymously published a book arguing that humans shared a common ancestor with other primates.

What Darwin did was present a more academically rigorous case for evolution than some of his predecessors and accept credit for it. Others were hesitant to take credit for the idea because it was so controversial.

(Shortform note: About 40 years before Chambers’ book, Jean Lamarck proposed a theory of biological evolution. Lamarck’s theory received so much criticism that it ended his career in science. This illustrates why others like Chambers were hesitant to ascribe their names to evolutionary theories, and also how much culture changed in the first half of the 19th century: By Darwin’s time, even though evolution was still highly controversial, enough people were open to his ideas that he could receive credit for them instead of being ostracized by the scientific community.)

Bryson explains that some of the original objections against Darwin’s theory of evolution have since been put to rest by new discoveries. For example, some people challenged the idea that animals could pass on beneficial traits to their offspring, but the development of genetic science showed how this was possible.

Other controversies continue to this day, such as William Paley’s “watchmaker” argument. Paley contended that the intricate mechanisms of living organisms imply that they were created by an intelligent being, just as the existence of a mechanical pocket watch implied the existence of a watchmaker.

Intelligent Design Theory vs. Evolution

As Bryson points out, the debate between evolutionists and creationists was ongoing even before Darwin published On the Origin of Species, and it continues to the present day. While the theory of evolution remains the more widely accepted position among scientists, some scientific discoveries have arguably strengthened Paley’s theory of “intelligent design.”

In Paley’s time, some argued that his watchmaker analogy was invalid because living organisms and their organs were quite different from watches or other man-made machines. But now scientists have identified nano-scale motors, valves, and other mechanisms within cells that are both functionally and mechanically similar to manmade devices. So Paley’s analogy turned out to be accurate after all, regardless of whether or not you accept his conclusion.

Moreover, many of these cellular mechanisms exhibit “irreducible complexity,” meaning that they are made up of many components, each of which is crucial for the mechanism to work. Proponents of intelligent design argue that these irreducibly complex mechanisms disprove the theory of evolution because they couldn’t have evolved from simpler mechanisms: Take away any part of the mechanism and it doesn’t work at all, so natural selection wouldn’t have selected for it.

However, there is debate over whether or not certain cellular structures truly exhibit irreducible complexity. Critics argue that simpler structures might have served other useful functions and thus evolved into their current form through cellular repurposing of increasingly complex molecular machinery.

Proponents of intelligent design also argue that it provides a better explanation for the origin of life. As we discussed before, scientists have yet to identify a natural process that assembled the first life forms. But if they were assembled by a superintelligent creator, then a natural assembly process isn’t required. Proponents of intelligent design believe that the common genetic language shared by all modern life forms indicates that they all share the same designer, rather than sharing a common ancestor.

The Progression of Life

As Bryson explains, scientists have been able to piece together a history of the successive types of life forms that have prevailed on earth from the earliest bacteria to modern man. We’ll consider the highlights of this chronology, as well as the evidence on which it is based and some of the lingering controversies.

Precambrian Life

Bryson explains that scientists can infer the history of early life forms from geologic information about the early Earth: Anaerobic bacteria must have been the first prevalent lifeform on Earth because Earth’s early atmosphere didn’t contain much oxygen. Then, at some point, cyanobacteria developed photosynthesis and started producing oxygen as a byproduct. Oxygen is actually toxic to most anaerobic organisms, but strains of anaerobic bacteria have survived to this day in swamps or other places where they are shielded from oxygen.

Bryson recounts how, once oxygen became abundant in the atmosphere, mitochondria started producing energy for cells by oxidizing nutrients. This facilitated the growth of more complex cells called eukaryotes, which eventually formed multicellular organisms. Scientists speculate that mitochondria may have begun as a separate organism that invaded bacterial cells, but then formed a symbiotic relationship with them. This is because mitochondria have their own DNA, which they keep separate from the rest of the cell’s DNA.

The Photosynthesis Chronology Controversy

As Bryson has pointed out elsewhere, there are often controversies and uncertainties in science, and the evolution of precambrian life is no exception. Some scientists challenge the once-accepted chronology that Bryson presents here.

As Bryson notes, it was widely believed that cyanobacteria developed photosynthesis, and all other photosynthesizing organisms evolved from cyanobacteria. But studies that analyzed the diversity of photosynthesis mechanisms across the spectrum of organisms concluded that modern cyanobacteria and modern plants diverged from a common ancestor as much as a billion years older than cyanobacteria.

If they are correct, that means photosynthesis was producing oxygen for a billion years before the oxygen concentration in the atmosphere increased appreciably (based on the oxygen content of rocks with various ages). Why the delay? Some scientists believe this was because the early photosynthesizers didn’t have enough of certain nutrients—particularly phosphorus—to grow large populations. Fossil evidence shows a marked increase in phosphorus right around the time oxygen levels rose.

So far, they haven’t specified the identity of this common ancestor from which cyanobacteria and other photosynthesizers inherited their capabilities, but other scientists theorize that photosynthesis first became possible when two different species of early bacteria combined in a symbiotic cellular relationship to form “double-walled” bacteria. This is similar to how mitochondria are thought to have started out as a separate organism that became integrated into other living cells.

The Cambrian Explosion

In the fossil record, a large number and diversity of multicellular organisms appear relatively abruptly about 540 million years ago. These organisms were all aquatic and included both plants and animals. As Bryson discusses, scientists refer to this sudden appearance of new species as the Cambrian explosion.

But, he notes, their sudden appearance in the fossil record doesn’t necessarily mean that they appeared suddenly on Earth. Scientists conjecture that many of the Cambrian creatures existed long before the presumed Cambrian explosion and became larger over time, making their fossilized remains progressively more discernible. Furthermore, the fossil record provides only sporadic glimpses of what life looked like in the past because fossils only form under certain conditions, which only occur occasionally. Scientists estimate that only about one out of every 10,000 species that have ever lived on Earth is preserved in the fossil record.

The Cambrian Chronology Controversy

As Bryson mentions, there are two competing schools of thought regarding the Cambrian explosion. One holds that the Cambrian explosion does indeed represent a relatively sudden appearance of many new species. The other holds that these species appeared gradually, beginning long before the Cambrian explosion, and either didn’t leave earlier fossils or left fossils that scientists haven’t found yet.

Genetic studies tend to support the gradual development hypothesis. Based on the rate of mutation and the number of mutations that differentiate modern species from one another, scientists estimate that the common ancestor of multicellular animals lived around 1.6 billion years ago—long before the Cambrian explosion.

They hypothesize that the Cambrian explosion may have been a case of parallel evolution, where many soft-bodied creatures on different evolutionary tracks began to grow exoskeletons. Since exoskeletons are much more likely to be preserved as fossils than soft tissues, this would explain the relatively abrupt appearance of these creatures in the fossil record. They also point to fossilized tracks in precambrian rock that may have been made by segmented worms or similar soft-bodied animals that lived before the Cambrian explosion.

However, other researchers believe that these tracks were actually made by unusually large single-celled organisms (of which they’ve found living specimens), and that multicellular animals really did appear abruptly at the time of the Cambrian explosion. They cite evidence that mutation rates were up to five times higher during the Cambrian period, explaining the rapid divergence of species.

Terrestrial Life

According to Bryson, the first lifeforms to emerge on land probably did so because of pressure from fierce competition for resources in the shallow water of the continental shelf. Scientists believe that at that time, all the continents were clustered together into a single land mass with much less coastline than Earth has today.

In any case, he says the first life forms to appear on land were plants such as tree-ferns and giant club moss. Later, animals such as millipedes and crustaceans emerged from the ocean to dwell on land. Eventually, the first vertebrates also migrated out of the shallows to become terrestrial amphibians and reptiles. Later, birds and mammals appeared as well.

Bryson notes that although mammals coexisted with dinosaurs, these early mammals tended to be small, burrowing animals similar to mice or gophers. Large mammals didn’t appear until after the dinosaurs died out.

The Significance of Terrestrial Vertebrates

Bryson emphasizes the importance of plants and crustaceans emerging as the first terrestrial life forms, but others, such as paleontologist Neil Shubin, emphasize the emergence of terrestrial vertebrates, seeing it as a more important milestone in the history of life than the terrestrial invertebrates that preceded them.

In Your Inner Fish, Shubin argues that understanding the first fish that crawled out of the water on primitive limbs and began living on land gives you a better understanding of all the animals that descended from it: reptiles, amphibians, birds, mammals, and ultimately humans. This is because the commonalities between all these creatures become more apparent when you can see how their differences are just variations of the original design. He also relates how his team discovered fossils of fish with primitive legs, unlocking some of these insights.

Hominids

According to Bryson, evolutionary scientists generally agree that humans and apes descended from a common ancestor that lived about seven million years ago. Since then, humans and apes have followed different evolutionary paths, although our ancestors remained very ape-like for several million years.

(Shortform note: Scientists have yet to find fossils of this common ancestor. The closest relative they have found is “ardipithecus,” a primate that lived about four million years ago. Studies of ardipithecus suggest that our ancestors may not have been as ape-like as formerly thought. It appears that ardipithecus had characteristics of both humans and apes, climbing through the trees like monkeys but walking upright on the ground like humans. If the common ancestor of humans and apes also had both these traits, perhaps each of the two branches perfected one and lost the other.)

Bryson says scientists think the transition from ape-like to human-like characteristics started with Homo Erectus about two million years ago. In addition to walking upright, Homo Erectus is thought to have built fires and cared for weak or injured members within a family or tribe, even though their intellectual abilities would only have been on par with a human baby. However, most of this is inferred from finds at a single dig site in Kenya, leading some scientists to question its validity.

(Shortform note: Scientists infer the intelligence level of Homo Erectus from the stone tools that they made: Their tools were simple enough that you could learn to make them just by imitating someone who was making them (much like babies learn by imitation) without any verbal instruction. These tools have now been found at multiple sites, as have remnants of their campfires. So only the inference that they cared for the injured is still based on a single fossil.)

Bryson notes that around the same time (two million years ago) there was another branch of the evolutionary tree called the Australopithecines, which are thought to have walked upright but otherwise be mostly ape-like. He says this branch was once thought to be part of humans’ ancestral tree but is now regarded as an evolutionary dead-end.

(Shortform note: Some scientists believe that Homo Erectus evolved from Australopithecines, which would put them back in the lineage of humans. The fossil record is compatible both with the hypothesis that the Australopithecines were the ancestors of Homo Erectus and that they were a separate branch that died off. Fossil evidence indicates that Homo Erectus coexisted alongside Australopithecines for a time, but the oldest Australopithecus fossil that has been found is considerably older than the oldest Homo Erectus fossil.)

As Bryson explains, the first Homo Sapiens, or modern humans, are thought to have lived around 100,000 years ago, but there are differences of opinion about their lineage. Based on fossil evidence, scientists generally agree that Homo Erectus first appeared in Africa and spread all over the world. Some scientists think that Homo Sapiens also appeared first in Africa and spread out from there, displacing Homo Erectus. But other scientists think that Homo Erectus evolved into Homo Sapiens synchronously all over the world.

Neither hypothesis fits all the available data. Genetic studies (which compare DNA from different individuals and use the differences to determine how long ago they shared a common ancestor) tend to support the idea that all modern humans are descended from a small population that originated in northern Africa, perhaps as little as 25,000 years ago. But archeological studies tend to support the parallel evolution hypothesis because tools that the earliest Homo Sapiens developed in Africa don’t show up in places like East Asia—if humans had spread out from Africa, they would presumably have brought their stone-age technology with them.

The Interbreeding Controversy

The question of how much the different pre-human species (and animal species in general) interbred with each other over the course of their evolutionary history adds another dimension to the controversy over whether modern humans originated in Africa or evolved from Homo Erectus in multiple parts of the world.

Traditionally, scientists thought hybridization between species played a negligible role in evolution because hybrid organisms are often unable to reproduce. But studies have also shown that when two different species do produce viable hybrid offspring, it sometimes produces a whole new species in just a few generations. This is because combining two genetic codes in a hybrid animal can produce a new and different code faster than the accumulation of mutations would.

If new species evolve by hybridization as well as mutation, then their ancestral trees get more complicated. In the case of human origins, one hypothesis that hybridizes (no pun intended) the two competing theories is that after Homo Erectus spread over the world, a species called Homo Heidelbergensis also originated in Africa and spread over the globe, displacing Homo Erectus. Homo Heidelbergensis evolved into several different species, including Neanderthals and Denisovans. Finally, hybridization among these species gave rise to modern humans, or at least contributed to their emergence in various parts of the world.

But, as with other aspects of human origins, there is debate on the subject of hybridization. Some studies claim to have found strong evidence for hybridization between Neanderthals, Denisovans, and other ancient species in modern humans’ DNA. But other studies contend that this DNA evidence merely indicates a common ancestor, not interbreeding after the species diverged.

Extinctions

Bryson observes that there were several times in Earth’s history when the majority of species on the planet died out. The most recent major one was the Cretaceous extinction, which wiped out 70% of all species 65 million years ago. And the most devastating was the Permian extinction, which obliterated 95% of all species 245 million years ago.

Bryson says scientists aren’t sure what caused these extinction events (or other, smaller ones), but they speculate that volcanic eruptions, meteor impacts, disease, solar flares, and other factors that could trigger global climate change could all be factors. There is geologic evidence that a meteor impact at least contributed to the Cretaceous extinction.

(Shortform note: In his book Brief Answers to the Big Questions, physicist Stephen Hawking argues that to escape extinction, humans need to colonize outer space. Like Bryson, he points to the evidence of past extinction events and the numerous factors that could cause them, but he takes the discussion a step further to point out that sooner or later, another mass extinction on Earth is inevitable. In his view, the only way to ensure that humans aren’t wiped out in the next extinction is to establish human civilizations on multiple planets.)

Human-Caused Extinctions

Bryson also points out that humans seem to have a talent for making other species go extinct. In recorded history, humans have hunted many creatures to extinction, such as the dodos, passenger pigeons, and carolina parakeets. In most cases, he says there was no apparent reason for humans to kill them—we just killed them because we could. Even in prehistoric times, it appears that the first arrival of humans in an area often coincided with the extinction of many species in that area.

Why Do Humans Cause Extinctions?

While Bryson expresses concern over the tendency of humans to hunt animals to extinction, he doesn’t delve into possible explanations for this tendency. There are at least two possible explanations.

On the one hand, some religions (such as Christianity and Judaism) teach that humans are a “fallen race,” meaning that we all have a natural inclination toward evil. While these religions teach that individuals can overcome their evil nature by various means, it also explains why humans have committed so much violence and waste throughout history.

On the other hand, evolution provides an atheistic explanation: Early humans developed sophisticated hunting instincts to help them survive and passed them on to their descendants. Many predatory animals instinctively kill prey when the opportunity arises, and humans, at one point, were no different. Today, our killer instincts may be counterproductive to our survival, but they linger in our DNA as an artifact of our evolutionary history.