PDF Summary:The Vital Question, by Nick Lane

Why does life work the way it does? Are life’s features the product of random genetic mutations, or did organisms have to evolve a certain way to overcome specific physical and chemical challenges? In The Vital Question, Nick Lane argues that genetics can’t give us the answer by itself. To explain the origin of life, we have to understand how cells make use of energy and what environmental conditions gave rise to the molecular power plants found in every living thing.

In this guide, we’ll explore the biochemistry of cells and how it may have developed on the very young Earth. We’ll explain why Lane believes all complex life emerged from a single instance of symbiosis between two very different single-celled creatures. We’ll expand on fundamentals of biology and genetics, discuss alternate views to some of Lane’s proposals, and point to the directions research is now taking to gain a fuller understanding of life’s origins.

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The second clue to where life may have formed is the electrochemical membrane gradient that’s used to power all living cells, as was discussed earlier in this guide. The original cell must have been born in a place where such gradients can occur naturally. Finally, there’s the requirement for the cell to metabolize food into the molecular building blocks it needs to grow and reproduce. In modern cells, the flow of food and waste into and out of the cell is guided by complex, specialized proteins that have evolved over eons. Lane argues that before life began, those specialized proteins couldn’t have evolved yet, so something else must have directed that flow—perhaps a stream of heated organic compounds funneled through a constricted passage.

(Shortform note: One component absent from the conditions Lane describes is the presence of light, which implies that life could rise in places where sunlight is hard to come by. This is why planetary scientists believe that one of the most likely candidates for non-terrestrial life in our solar system is the subsurface ocean of the icy moon Europa, whose rocky core is heated by tidal squeezing from nearby Jupiter. Since the atomic ingredients of life are common throughout the universe, the only questions are whether Europa has alkaline vents with organic molecules and whether a similar series of events took place there as did on the early Earth.)

Where Life Began

Because of these clues, Lane believes that a certain chain of events took place that created the housings for the first living cells. First, inside a porous geologic formation with microscopic chambers the size of tiny cells, natural electrical gradients combined carbon and other common elements into organic molecules. Those molecules clustered in the formation’s nooks and crannies, where a steady stream of heat prompted them to self-organize into even more complex patterns. Eventually, some of these molecules coalesced into chains that are capable of copying themselves, while others formed the basis of internal cellular structures. But where in the world could this sequence of events have taken place?

Lane writes that taking into account the first living cells’ chemical and geophysical requirements narrows the field of possible locations where life could have begun to a single, most likely candidate: alkaline hydrothermal vents at the bottom of Earth’s primordial ocean. Lane describes the nature of these hydrothermal vents, why they were ideal for the development of life, and a scenario by which life may have evolved the ability to break free from these vents and survive in the open ocean.

(Shortform note: While Lane cites the discovery of the Lost City Hydrothermal Field in the mid-Atlantic as evidence for the existence of alkaline hydrothermal vents, several others have been found as well. These include alkaline springs off the coast of New Caledonia in the South Pacific and the Strýtan Hydrothermal Field in the waters of the coast of Iceland. Unlike the Lost City, those other two systems are in much shallower waters than the vents Lane describes.)

Alkaline hydrothermal vents are different from the more commonly known “black smoker” underwater volcanic vents that exist along oceanic ridges where continental plates are pulling apart. Alkaline vents are farther from the rift, but where molten rock from the mantle is still relatively close to the Earth’s crust. Seawater filters down through the crust, where it reacts with the mantle and spews back upward, bringing a payload of hydrogen gas and minerals from beneath the Earth’s surface. These vents create porous towers of rock that are rushing with heated water rich in chemicals—conditions that are perfect for organic compounds.

(Shortform note: Though a handful of rocks have been discovered that date back to the era during which life began, much of Lane’s discussion about the primeval sea floor is by necessity conjecture since most of Earth’s crust has been recycled by tectonic activity in the last 4 billion years. However, clues to Earth’s early conditions might be found on other planets where the surface has been better preserved. On Mars, where there is much less tectonic activity, ancient seabeds still exist. A study of Mars’s Eridania basin shows chemical deposits consistent with the presence of underwater alkaline vents at the same time on Mars as when life began on Earth.)

Life in the Vents

According to Lane, the micropores in these vents were the ideal cell-like structures in which the chemical precursors of life could collect. The material found in the walls of these vents, such as iron sulfide and iron hydroxide, acted as inorganic catalysts for the basic metabolic reaction of life—the conversion of hydrogen and carbon dioxide into a variety of organic molecules until finally being expelled as methane. The flow of warm, chemically saturated water provided all the energy and fuel this burgeoning ecosystem needed, while also providing a motive force to flush away the buildup of waste from the first organic reactions.

(Shortform note: This process in which life uses inorganic chemistry for fuel is referred to as chemosynthesis—as opposed to photosynthesis, in which energy comes ultimately from sunlight. Though this process began at the bottom of the ocean, some bacteria retained this metabolic strategy as life migrated away from the vents. In I Contain Multitudes, Ed Yong explains that bacteria that adapted to live inside other organisms, such as symbiotic gut bacteria in many deep-sea creatures, still thrive via chemosynthesis and use it to generate chemicals needed by their host organisms. Chemosynthetic bacteria have even made it to the Earth’s surface, such as the bacterial symbiotes in flatworms on the coast of the island of Elba.)

Lane goes on to point out that oceanic chemistry was different 4 billion years ago, and this was significant to the formation of life. The seas of that time had no dissolved oxygen, but they were rife with carbon dioxide, meaning they were far more acidic than today. The water emerging from these vents, however, was strongly alkaline. The fatty acids created by the vents’ ongoing chemistry congealed into a membrane-like barrier between the vents’ porous interior and the seas outside. The iron that coated the walls of the vents conducted protons from the acidic (positively charged) ocean to the alkaline (negatively charged) water in the vents’ pores, creating the electrochemical gradient that still powers all living cells today.

Acids and Bases

Lane assumes a basic understanding of acidity and alkalinity and how they relate to chemical reactions. While it’s commonly taught that how acid or alkaline a substance is can be measured on the pH scale (with acids having a pH under 7 and bases having a pH over 7), what these values actually represent is the proportion of ionized molecules in a substance.

Neutral water has two atoms of hydrogen to one atom of oxygen. However, if a hydrogen nucleus is transferred from one water molecule to another, it produces a hydronium molecule (3 hydrogen, 1 oxygen) with a positive electrical charge and a hydroxyl molecule (1 hydrogen, 1 oxygen) with a negative charge. An acid is a substance with more hydronium molecules, and a base is a substance with more hydroxyls. Bases and acids cancel each other out by balancing their positive and negative ionic charges. An alkali is simply a base that’s soluble in water, such as the substances found in hydrothermal vents.

In other words, the physical properties of alkaline vents provided all the necessary pieces for living cells to develop within them. All that was needed now for life to start was for the proteins and organic molecules inside to organize into patterns that could store information and replicate themselves. Once that happened, life was ready to emerge, spreading throughout the alkaline pores in life’s original hydrothermal home.

Life Escapes the Vent

If alkaline vents were the perfect environment for the earliest form of life on Earth, what was it that pushed that life to leave its home and explore the rest of the planet? Lane says that the answer lies in how cells make their own energy. As discussed earlier in this guide, modern cells use an electrical voltage across a membrane to power the creation of the ATP molecule, a voltage that is created by tiny protein machines moving charged ions from one side of the membrane to another. However, inside an alkaline vent, there was no need for those proteins to evolve. The naturally conductive iron inside the vent provided all the electrical pathways that were needed—so long as the charge was from hydrogen ions.

(Shortform note: While common definitions of the term “membrane” assume it exists in a biological system, Lane uses the term in a broader sense to refer to a thin layer of material that separates chemical solutions, allowing some particles of matter through while denying passage to others. Early research on the properties of membranes was conducted by American founder and polymath Benjamin Franklin, who studied the behavior of oil floating on water and began the work that would eventually lead others to understand the role of membranes in the body.)

Lane and his colleagues have discovered certain enzymes that pump sodium ions across cell membranes in addition to hydrogen. These enzymes provide an evolutionary kick because sodium ions yield more power than hydrogen, and being able to pump them out of the cell reduces the buildup of sodium inside. However, this only works if the cell has a thicker, less permeable membrane to keep the excess sodium out. Lane proposes that the evolution of sodium-ion power prompted the evolution of stronger cell walls and freed life’s ancestors from the natural conductors found in alkaline vents. Once the cell wall and ATP-building enzymes were in place, our forebears floated free of their vents and into the ocean at large.

(Shortform note: While Lane suggests that sodium-ion power revolutionized cell development and enabled life to flourish, today some tout sodium-ion power as a possible alternative to the lithium-ion power used in electrical batteries. While sodium-ion batteries lack some of the advantages of lithium-ion power, they may turn out to be much cheaper, especially for industrial applications such as in electrical vehicles. Should sodium-ion power catch on, then our cars may soon be powered by a similar reaction to the one that drives our metabolic functions.)

Lane suggests the evolutionary leap to sodium-ion power actually happened two different times, among two separate populations of cells that sprang from our last universal common ancestor. This division gave birth to the two separate domains of single-celled life: bacteria and archaea. Scientists believe this because those two separate domains found different solutions to the same evolutionary problems. Their cellular membranes, enzyme pumping machines, and even the way they metabolize carbon all function differently enough to suggest that they took separate evolutionary paths to break free from the oceanic vents. The stage was set for life to flourish, but the next giant leap in its development wouldn’t occur for 2 billion years.

(Shortform note: All of the events described by Lane so far took place during the Archean Eon, from roughly 4 billion to 2.5 billion years ago. That period comes after the Hadean Eon—literally “Hell on Earth” as magma covered much of the still-forming crust and asteroids left over from the solar system’s birth rained down in the Late Heavy Bombardment. The fossil record of the Archean Eon shows that microbial mats along early coastlines created layered deposits known as stromatolites. These deposits are the only fossil record for over 80% of the history of our planet, but though they went into decline over 1 billion years ago, stromatolite-producing bacteria survive to this day in places such as Australia’s western coast.)

The First Complex Cells

The event that prompted the explosion of complex life from simpler, single-celled forms has long been shrouded in mystery. Lane proposes that eukaryotes—the domain of life that includes everything more advanced than bacteria and archaea—arose from a single, unlikely symbiosis between two very different single-celled creatures that unlocked the limits on how cells use energy and undammed the flow of evolutionary progress. Lane explains what distinguishes eukaryotic cells, the idea of evolution by symbiosis, and the energy problem that holds bacteria and archaea back while propelling eukaryotic life to ever greater organization and complexity, eventually evolving into the plant and animal kingdoms.

What defines eukaryotic life is that the cell’s DNA is contained within a protective nucleus, whereas in archaea and bacteria, the DNA is attached to the outer cell wall right next to the enzymes that produce ATP. Not every eukaryote is multicellular, but they all dwarf bacteria and archaea in terms of the size of their genome and the complexity of their molecular machines. Eukaryotes are full of mitochondria that generate the relatively enormous amount of power eukaryotic cells need to enact the properties of their multitude of genes.

(Shortform note: The single-celled eukaryotes Lane refers to include yeast, algae, and plankton. Yeast is a form of single-celled fungus, but unlike bacteria, yeast cells have nuclei, mitochondria, and other organelles that simpler life forms lack. The same is true for algae, which has both single-celled and multicellular varieties. Aquatic plankton, which form the base of the oceanic food chain, also fall into this category. Because of the structural complexity of their cells, yeast, algae, and plankton are more closely related to humans than any bacterial species.)

To be sure, bacteria and archaea are incredibly varied in terms of biochemistry, but they don’t come close to eukaryotes in terms of size and complexity. What’s more, bacteria and archaea haven’t evolved cellular complexity in 4 billion years. Lane suggests that there’s something physically different about eukaryotic cells that allows them the structural complexity that bacteria and archaea can’t achieve. What puzzles biologists is that they’ve found no “missing link” between simple, single-celled organisms and eukaryotes, as if the first eukaryotic cell sprang into existence fully formed.

(Shortform note: While Lane discounts proposed “missing link” candidates such as the anaerobic giardia microbes that exist in animals’ intestinal tracts, researchers have offered a new candidate since the time of Lane’s writing. The microbes in question are lokiarchaeota, a member of the Asgard superphylum of archaea that were discovered in vents on the ocean floor between Siberia and Greenland, and later near Japan. These archaea have several genetic and structural similarities to eukaryotes, and while their discovery does not invalidate Lane’s endosymbiosis hypothesis, it does suggest what type of archaea was the original host cell from which eukaryotes sprung.)

The Great Merger

An analysis of eukaryotic genes further confuses the issue of complex life’s evolutionary heritage. Eukaryotes share a third of their genome with single-celled life, but 75% of those genes come from bacteria while the rest come from archaea, as if eukaryotes somehow descended from both domains of simple organisms. Lane says that this is evidence that the first complex cell was a hybrid of a bacterium and an archaeon that somehow grafted themselves together. Because this shared genome is common to all eukaryotes, biologists are forced to conclude that this hybridization only happened once in 4 billion years, or at least only once that was successful and survived.

(Shortform note: The specific word Lane uses is “chimera” rather than hybrid. In biology, a hybrid is an offspring of parents from two different breeds, subspecies, or species, such as mules, which result from breeding horses and donkeys. A chimera, on the other hand, is a life form that has distinct cells with different sets of genes. Named for the three-headed creature of myth with a body composed of goat, lion, and snake, real-life chimerism has been observed in anglerfish, cats, and even humans. While some chimeras are developed in a lab, they occur in nature when one embryo absorbs another. In humans, chimerism is sometimes unnoticed, though it can also result in mottled skin, mismatched eye colors, and autoimmune disorders.)

The process by which such grafting can occur is called endosymbiosis, in which one cell is physically engulfed by another, but instead of being consumed, the two cells become a larger, combined organism. The evidence that this happened lies in our mitochondria, which have a completely separate genome from the nucleus of the cell. Mitochondria are, in fact, an evolutionary remnant of that first coupling.

Once, long ago, an archaeon absorbed a bacterium in such a way that allowed the bacterium and its descendants to flourish inside the archaeal host cell. Lane explains that the host stripped the bacteria’s DNA, adding it to its own repository, while leaving the bacteria just enough genes to produce ATP and fuel the host cell’s growth. As evidenced by the mitochondrial genome, our modern mitochondria are the descendants of those bacteria, reduced to the status of power batteries that are fed and protected by the larger eukaryotic cell.

Tracking Mitochondrial DNA Across the Ages

Lane explores more biological implications about the relationship between nuclear and mitochondrial DNA than can be covered in this guide, but he skips the most commonly known aspect of mitochondrial DNA—that it can be used to trace your maternal ancestry. While the genes in your cells’ nuclei are inherited from both of your parents, your mitochondrial DNA comes solely from your mother. And since historically the human race was far less mobile than it is at present, past mutations in mitochondrial DNA can be tied to diverging ethnic groups as they slowly migrated across the ancient world.

Today, there are several mitochondrial DNA testing services you can use for genealogical purposes. Mitochondrial testing is also used as a diagnostic tool for hereditary diseases and in forensic analysis of human remains in which nuclear DNA has otherwise decomposed. Mitochondrial DNA testing has uncovered surprises in people’s family trees, allowed descendants of enslaved people to reconnect with their cultural heritage, and even helped to identify the remains of King Richard III. All this is merely a small-scale representation of Lane’s deeper truth—that the mitochondria in all complex life share a single genetic ancestor.

The Energy Problem

The fact that the initial archaea-bacteria chimera thrived suggests that its odd symbiosis provided the first eukaryotes with an evolutionary advantage. As it was with the original genesis of life and the adaptation that let it leave its ocean-vent nest, the development of complex cellular structures revolved around a new way to harness energy. Lane describes the energy restrictions that have kept bacteria and archaea from achieving complexity for billions of years, while the novel approach that eukaryotes took to overcome that energy barrier not only allowed for greater structure and complexity but actually drove genetic development.

One fundamental difference between eukaryotes and simpler life forms is that because of their mitochondria, eukaryotes have vastly more energy available for use per gene. That energy goes into constructing proteins based on DNA’s genetic designs. Without mitochondria to fuel them, bacteria and archaea have to rely on the ATP their genes produce along their outer cell wall. Lane says this encourages them not to grow bigger because as a bacterial cell’s volume increases, its available energy only grows proportional to its increase in surface area. A large cell is therefore less energy efficient and would be outcompeted by its leaner, meaner cousins.

(Shortform note: While Lane describes energy efficiency as a main driving factor behind the evolution of bacteria, some biologists believe that energy efficiency is the primary evolutionary driver for all life on earth. In this view, efficiency is measured as the ratio between the energy a life form takes in versus the energy it had to expend to obtain it. For instance, to calculate a cheetah’s energy efficiency, you’d divide the amount of energy it obtains from eating a gazelle by how much energy the cheetah expends while hunting. It’s been suggested that humans evolved to walk upright because bipedal motion is more energy-efficient than walking on all fours like our chimpanzee cousins.)

Because of the efficiency problem, Lane argues that there’s no evolutionary reason for archaea and bacteria to grow in size and complexity. By adapting their inner metabolic processes, single-celled organisms have adapted themselves to flourish in every corner of the ecosphere, including the Earth’s most extreme environments. There’s no need for more complexity or larger genome size, so evolution has instead selected for bacteria and archaea that are slimmed down to the basic needs of survival.

(Shortform note: The advantages of efficiency that Lane says guide bacterial evolution have led to some very small creatures indeed. The smallest known bacteria—less than a micron (a millionth of a meter)—in diameter, is mycoplasma genitalium. M. genitalium also has the smallest genome of any living microbe capable of reproducing on its own. M. genitalium inhabits the human urinary system, can be sexually transmitted, and causes a host of medical problems in both men and women. M. genitalium is so lean and efficient, it doesn’t even have a cell wall, which makes it resistant to antibiotics that target the cellular structure of larger bacteria.)

During the endosymbiosis event that launched the beginning of eukaryotic life, the evolutionary pressure for lean, efficient cells got flipped on its head. The newly absorbed mitochondrial bacteria provided far more energy than the archaeal host cell needed. Lane suggests that rather than being drowned with ATP molecules, the host cell had to find something to do with all that excess energy. Its solution came from the extra DNA it absorbed and repurposed from its bacterial symbiotes. The flood of new energy demanded it be spent, and the only way for eukaryotes to survive was to open the possibilities of complex cell structures, charting new frontiers for evolution to explore.

(Shortform note: To understand Lane’s idea that a dramatic increase in cellular energy demanded an increase in size and complexity, consider the problem in terms of economics. The archaeal host cell is like a small town—lean, efficient, and relatively stable. However, the inrush of mitochondrial ATP is like a population explosion within the town’s borders. To survive, the town has to grow into a city by creating new infrastructure, housing, and development so that it can employ its booming population. If it doesn’t, then rampant unemployment leads to poverty, hunger, and disease—exactly what happens inside a cell that doesn’t fully employ its ATP.)

The Path of Evolution

Because of the lack of intermediary steps on the evolutionary path to eukaryotes, it’s not easy to reconstruct the steps from that first symbiosis to the modern complex cell. But just as Lane reverse-engineered the properties of the original cell, he is able to do the same for eukaryotes, tracing our ancestors’ rapid evolution from a chimeric hybrid to a new branch of life. The clues he draws upon are the introns—so-called “junk DNA”—that populate eukaryotic genomes but not that of bacteria and archaea. He shows that a sudden flood of such introns, stripped from bacteria in the eukaryotic host cell, populated the newly forming species and caused the evolution of the nucleus’s wall.

One thing that’s certain is that the nucleus evolved after endosymbiosis. Lane explains that we know this because a similar structure doesn’t exist in any species of archaea or bacteria. Also, because all eukaryotes have a nucleus, we know that it evolved relatively quickly, before eukaryotes had a chance to evolve into separate species. Evolution favors stability, and new adaptations only appear when absolutely needed for survival. Therefore, the merger of archaea and bacteria created an unstable internal situation that natural selection had to address quickly.

A Genetic Mosh Pit

Lane mostly eschews a focus on genetics in favor of discussing life’s energy demands, but exploring the leap from archaea to eukaryotes requires understanding genetic competition and cooperation. In The Selfish Gene, evolutionary biologist Richard Dawkins states that living systems are merely the mechanisms by which genetic information perpetuates itself. He affirms that living systems have a natural preference for long-term stability and that they’ll strive to restore that stability as quickly as they can if their equilibrium is disrupted.

Within a cell, few things could be more disruptive than a sudden influx of foreign DNA looking for a means to reproduce. However, genes don’t live in a dog-eat-dog world—for genes to survive, the host organism must survive. Therefore, while some genes compete directly with each other for survival, others have been seen to cooperate with each other for mutual benefit. None of this is conscious behavior—genes are just molecules, after all. Genetic cooperation is an artifact of evolution’s push toward self-replicating populations of genes with the best chance of survival. In the case of eukaryotes, that “best population” of genetic material takes shape in a cellular nucleus that deactivates counterproductive DNA.

The instability of the original endosymbiotic merger is reflected in the chaotic state of eukaryotic DNA. Between useful strands of genetic information, the eukaryotic genome is full of dead lines of code—known as introns—that don’t produce any proteins at all. Lane traces the lineage of these introns to genetic parasites that inhabit bacterial DNA. These parasites aren’t living things themselves but are self-replicating bits of DNA that hitchhike inside bacteria and spread through lateral gene transfer. Bacteria and archaea have internal defenses such as enzymes that act as genetic pest control to stop these parasites and keep their numbers down.

(Shortform note: The genetic parasites Lane refers to are commonly known as transposons, and are found in modern eukaryotes such as plants and animals. These transposons can spread from one part of your genome to another by using your cells’ RNA messengers to insert themselves into different parts of your DNA. Eukaryotes such as ourselves aren’t susceptible to lateral gene transfer from other eukaryotic species, some animal species have absorbed genetic traits from symbiotic bacteria, and our genes can also acquire and spread transposons that travel as passengers inside viral infections. By actively rearranging DNA, transposons may be responsible for many of the mutations that drive evolution.)

Lane hypothesizes that something different happened during endosymbiosis. As the absorbed bacteria lost DNA to the archaeal host cell, it also released a flood of genetic parasites, and because they were parasites from bacterial DNA, the archaeal host cell had no defenses. These parasites, the precursors of introns, flooded the burgeoning eukaryotic genome and began to make use of excess ATP to generate a torrent of unusable proteins. Since the cell had ample energy, this wasn’t a problem until those proteins began to clutter the insides of the cell. Only then did evolution come up with a way to turn the introns off.

(Shortform note: Introns are different from exons, which are the sequences of DNA that actively code for the production of proteins. Not everyone agrees with Lane’s interpretation that introns are merely leftover bits of code from bacterial genetic parasites—some biologists argue that introns serve their own genetic functions in addition to the more commonly understood features of exons. These functions include regulating genetic transcription within the cell and organizing nested genes within DNA strands. It’s unlikely that introns served these functions when the original eukaryotic genome formed, but they were later repurposed through millions of years of evolutionary tinkering.)

The Nucleus Is Born

According to Lane’s proposal, our first eukaryotic ancestors evolved a way to shut down our introns by walling them off behind a membrane, the one that defines the outline of the nucleus. This membrane interrupts the process of expressing intron genes as useless protein strands while allowing the genes that should be active to carry out the tasks they’re assigned for. The nuclear membrane is able to do this because it’s based on bacterial DNA and can therefore defend against parasitic bacterial genetic code. While our ancestral bacteria became mitochondria, our ancestral archaea appropriated bacterial traits, such as the kind of protective membrane that could deactivate the introns blocking our single-celled forebears’ evolution.

(Shortform note: For the very reasons Lane describes, the integrity of the nuclear membrane is of vital importance to the cell. New research shows that the boundary of the nucleus is susceptible to being ruptured by viruses and congenital defects, which leaves the DNA inside exposed to potential chromosomal damage. Cancerous cells are particularly vulnerable to this kind of rupture, leading to additional genetic cell damage that may cause the cancer to mutate and spread further. Scientists have recently discovered that the protein vimentin may prevent this kind of rupture by fortifying the nucleus’s wall.)

With the formation of the nucleus and the birth of mitochondria, the evolutionary floodgates of eukaryotes were opened. The powerhouse of the cell and the flexibility of a large genome enabled the many evolutionary wonders of the natural world. Gone are the days when bacteria and archaea were the preeminent forms of life on the planet. All this, Lane says, took place because of a single instance of symbiosis that’s only happened once in the history of our world.

The Resilience of Eukaryotes

The adaptive versatility of eukaryotes has been shown by how they repeatedly bounce back from mass extinction events as whole arrays of new species. While Lane argues that bacteria and archaea have never been threatened by the extinction-causing disasters that have periodically decimated more complex life, eukaryotes have had to claw their way back from the brink on multiple occasions.

In The Sixth Extinction, Elizabeth Kolbert lists five prior global waves of extinction that have happened since eukaryotes first emerged on the macroscopic scale. In each of these, more than three-quarters of all eukaryotic life was extinguished, and yet every time, new species emerged with different, complex evolutionary solutions to the problem of surviving in the world they were born into—from trilobites to ancient sharks, dinosaurs, and us. Though many people such as Kolbert believe that we’re currently in the middle of another mass extinction, eukaryotic life will most likely survive and adapt to new forms long after we’re gone.