Eukaryotes are the complex cells that make up all large life forms, from plants and fungi to animals. While their lifestyles are vastly different, the internal machinery of their cells is almost identical, suggesting they all arose from a single event about two billion years ago. This event gave rise to all complex life on Earth.

Unlike eukaryotes, bacteria and archaea have a much larger collective genetic repertoire but never evolved complexity. This suggests that the key to complexity isn't just about genetic information. Instead, it seems to be the acquisition of mitochondria, the power packs within our cells. These mitochondria, derived from bacteria, generate energy through respiration by creating an electrical charge across a membrane.

The charge is about 150 to 200 millivolts, but the membrane is 5 nanometers in thickness. If you shrank yourself down to the size of a molecule and stood next to that membrane, you would experience 30 million volts per meter, which is equivalent to a bolt of lightning.

This powerful and universally conserved energy-generating system likely dates back to the earliest life forms. A compelling theory for its origin, proposed by Bill Martin and Mike Russell, points to deep-sea hydrothermal vents. These vents are not like black smokers but are more like mineralized sponges with many cell-sized pores. The early ocean was acidic, while the fluids from the vents were alkaline, creating a natural proton gradient across the mineral barrier of the pores. This geological setup mimics the energy-generating membrane of a cell.

These vents also contained the necessary ingredients for life. The minerals, like iron and nickel sulfides, are the same metals used by modern bacteria as catalysts. The natural proton gradient could have powered the reaction between hydrogen and CO2 to create the first organic building blocks. This theory suggests a continuity between geology and biology, explaining why cells have a charged membrane—it was a feature of their environment from the very beginning. The internalization of this power system in the form of mitochondria later freed eukaryotes from these environmental constraints, allowing them to become larger and more complex.

Life may have originated from simple chemical reactions in hydrothermal vents. These reactions use hydrogen gas and carbon dioxide to create organic molecules. This process happens along a membrane with catalysts, which are like early enzymes, and it creates a proton gradient, much like a living cell does.

The first organic molecules produced are Krebs cycle intermediates. These small carboxylic acids serve as basic building blocks. Adding ammonia to them creates amino acids. Adding more hydrogen forms sugars. Reacting amino acids with sugars can then produce nucleotides. Longer-chain fatty acids can also form, and they spontaneously assemble into bilayer membranes. Lab experiments show these membranes can form robustly under various conditions, including high temperatures and different pH levels. These early cell-like vesicles are dynamic, constantly fusing and breaking apart.

This theory presents the origin of life not as a sudden "Frankenstein" moment, but as a continuous process. Life is seen as an extension of spontaneous chemical reactions.

A powerful analogy frames this concept: the cell is a miniature version of the Earth. A cell is reduced and alkaline on the inside, while being oxidized and acidic on the outside. This mirrors the Earth, which has a reduced and alkaline core and mantle, while its surface, with oceans full of carbon dioxide, is relatively oxidized. The Earth's crust acts like a membrane, and hydrothermal systems are the pores where exchange happens.

The idea that the Earth is a giant battery that produces little living cell mini batteries, it's a rather beautiful idea. I mean, you can't allow yourself to get too hung up on a metaphor, but it's a beautiful image.

In this view, the Earth acts as a giant cell. From its hydrothermal vents, smaller cells bubble off like mini-copies of the planet.

The fundamental chemistry of life, built on carbon, is likely to be a universal feature, not a contingent accident of Earth. Carbon is exceptionally good at forming the strong bonds needed for complex molecules. Nick Lane describes CO2 as a kind of "Lego brick that you pluck out of the air" to build structures like DNA and RNA. This type of spontaneous chemistry is not possible with alternatives like silicon without an intelligent designer.

The necessary conditions for this chemistry are probably widespread throughout the galaxy. The Milky Way may contain 20 to 40 billion wet, rocky planets. Many of these planets would likely have hydrothermal vents, which are not unique to Earth. They form when a very common mineral, olivine, reacts with water, producing hydrogen gas and alkaline fluids. Evidence suggests these vents existed on early Mars and are active now on moons like Enceladus and Europa.

This suggests that life's basic building blocks could be abundant. Nick speculates that a substantial fraction, perhaps even 50%, of wet, rocky planets could have nucleotides. This is because the core metabolism, reacting hydrogen with CO2, is a thermodynamically favored process. Even meteorites, formed under very different chemical conditions, tend to produce similar molecules like amino acids and nuclear bases. However, these ingredients would not be diluted in an open ocean but concentrated within the small pores of a vent system.

While the initial steps from geochemistry to the building blocks of life might be easy, there are still major bottlenecks. The leap from simple nucleotides to complex structures like RNA, DNA, and ribosomes is a significant challenge. Despite this, Nick is optimistic that a substantial proportion of these planets could develop life, and that this life would share fundamental similarities with our own, including its genetic code and its use of membrane potentials to drive work.

There are likely hundreds of millions of planets in the Milky Way that have life with building blocks similar to our own, such as ribosomes and DNA. This is because serious planetary driving forces create a fairly deterministic chemistry. These forces produce the same kinds of intermediate compounds and feedback loops, pushing things in similar directions. However, the degree of similarity likely decreases as you move from basic chemistry, like CO2 fixation, toward more complex genetics.

The idea that the laws of the universe strongly favor the chemistry that leads to life can seem like a vindication of intelligent design. Nick Lane, who is not religious, finds this idea almost disturbing but understands the connection. He notes this concept is consistent with a deist God, one who simply set the universal laws in motion and then let them play out. This aligns with a concept sometimes called "Einstein's God."

This is a very cold kind of goddess. Thermodynamics sets the laws of the universe in motion, reproducibly gives rise to the same kinds of things. Yes, you could interpret it in a kind of theistic, natural, theistic way, but I don't think many people would get that much comfort or meaning from that way of seeing the world.

If simple life is almost inevitable on rocky planets, the major bottleneck preventing the universe from being filled with aliens is likely the emergence of eukaryotes, which are necessary for complex life. Nick Lane identifies this as "the big one." He doesn't argue that eukaryotes could only arise on Earth, but he pushes back against the view, associated with Carl Sagan, that once life begins, it inevitably progresses towards complexity and intelligence.

Earth's own history contradicts this idea of inevitable progression. There was a 2-billion-year period of stasis with only simple life. This was followed by the apparent singular event of eukaryotes arising, another long gap before animals appeared, and the very recent emergence of humans. As Nick puts it, "we're just icing."

A successful endosymbiotic event, where one cell lives inside another, is incredibly difficult and rare. While prokaryotes like archaea and bacteria are small, making it hard to host another cell, the main challenge is maintaining the relationship. Often, the partnership fails, and the endosymbiont is lost, perhaps after transferring some of its genes to the host. Modeling suggests that under most conditions, both the host and the potential endosymbiont are better off living independently. Given the trillions of prokaryotes that have existed, the fact that this succeeded only once to create eukaryotes is remarkable.

Even the most eukaryotic-like prokaryotes, such as the Asgard archaea, are not truly complex. They have some eukaryotic-like genes and proteins, but they lack the complex internal structure and large genomes of true eukaryotes. In contrast, all eukaryotic cells, from a single-celled algae to a human kidney cell, share the same complex internal kit. This uniformity suggests it was not an adaptation to an external environment, but rather to an internal pressure: the complex relationship between the host cell and its new inhabitant. Nick Lane explains this internal struggle likely drove the evolution of features like the nucleus. He suggests it may have formed to protect the host's genome from genetic parasites emerging from the mitochondria.

So construct a lot of this history of eukaryogenesis... you start with simple cells with a cell inside, and you end up with the same cell structure everywhere.

This single, rare event appears to be the major bottleneck for the evolution of intelligent life. Eukaryotic cells solved a fundamental problem: how to support a large genome. A large genome is essential for complex multicellular organisms, where different cell types like liver or brain cells express different genes from the same master blueprint. Starting from a single cell with one genome prevents the genetic infighting seen in simpler multicellular forms like slime molds.

The question arises whether endosymbiosis is the only solution to this problem. Nick notes that giant bacteria exist, but they solve the size problem differently. They have 'extreme polyploidy,' meaning they carry tens of thousands, or even hundreds of thousands, of copies of their entire genome. The energy cost for this is colossal. Endosymbiosis offers a more efficient solution. It is still a form of polyploidy, but the mitochondrial genome is stripped down to only what is necessary for its function, creating a complementary relationship that allows the host cell to become much larger and more complex without the massive energy overhead.

When considering the possibility of life on other planets, one might argue that evolution is clever and would find other ways to create complexity. Nick Lane counters that this is a form of hand waving. A true scientific approach requires proposing a testable alternative. Instead, he argues that the basic constraints of biochemistry are likely universal. Wet, rocky planets are common, and they will likely have similar conditions, including CO2 and water. This leads to the emergence of bacterial cells with a charge on their membranes, a feature that constrains them from easily becoming larger and more complex.

On Earth, every known instance of prokaryotes attempting to grow larger resulted in limitations, such as extreme polyploidy, without developing sophisticated transport networks. The only time this barrier was truly broken was with the emergence of eukaryotes. This suggests it is a profoundly difficult and rare event. Nick admits this is not a viewpoint he enjoys holding, as it conflicts with the imaginative possibilities of science fiction.

I grew up watching Star Wars and Star Trek and reading Hitchhiker's Guide to the Galaxy. I love the idea that the universe is full of all kinds of stuff as much as anybody. So I don't like my position of saying, actually it's quite limited and you're going to see the same kind of things elsewhere. It's just a position that I've been forced into by everything that I've learned about life on Earth.

This is a probabilistic argument. Out of a thousand planets with life, perhaps 999 will follow this constrained path because the ingredients—carbon, water, and specific chemical reactions—are so common. This theory is becoming testable. For example, Enceladus, a moon of Saturn, has plumes containing water, organics, and hydrogen, with an alkaline pH. This suggests conditions similar to those that may have given rise to life on Earth exist under its icy surface, making it a prime target for exploration.

Protocells likely formed in hydrothermal vents, where organics would self-organize into structures with fatty acid bilayer membranes. Driven by deterministic chemistry and the pressure of hydrogen, these protocells would simply grow by making more molecules. Once they grew large enough, they would divide, similar to how a bubble splits. This creates a basic form of heredity, as the new protocells inherit the same molecules. This process happens relatively early.

However, this deterministic growth is a dead end. The cells are entirely dependent on their environment and lack the ability to evolve greater complexity. The introduction of random bits of RNA changes everything, creating what Nick Lane calls "evolvability." With genes, life can begin to resist its environment and evolve. But naked RNA is also a dead end; it's typically just selected for replication speed.

If you've got naked bits of RNA, what tends to happen is they're selected for their replication speed. They just go on making copies of themselves. They don't become more complex... they just go on copying themselves and it's a dead end. If you're trapping them inside growing protocells, then effectively they're sharing the same fate. And if some of them are capable of making that protocell grow faster, then they will get more copies of themselves because they're inside this protocell.

The conversation then shifts to the role of mitochondria in the evolution of two sexes. In biology, the female sex is generally defined as the one that passes on mitochondria, while the male does not. Sexual reproduction increases the variance in the nuclear genome, allowing selection to favor the best combinations. Mitochondria, however, are passed on asexually. They have a small genome with multiple copies. This presents a problem: mutations can accumulate over time without being weeded out, a process called Muller's ratchet.

If you've got 100 copies of mitochondrial DNA and two of them acquire mutations, but you've still got 98, which are doing their job fine. What's the penalty for those two mutations? It's not very much. You'll hardly notice them. So now you acquire another couple of mutations and you can degenerate over time.

To prevent this degradation, the variance of mitochondrial genes must be increased to make them more visible to selection. Having two sexes, where only one passes on the mitochondria, acts as a sampling mechanism. This process increases variance and allows for better quality control of the mitochondrial genes passed down through generations.

Uniparental inheritance of mitochondria helps increase variance between cells through a process of random sampling. Instead of creating identical clones, each new cell receives a small, random sample of the parent's mitochondria. By chance, one cell might get all the good copies, while another gets all the bad ones. This creates greater variation for natural selection to act upon. The cells with good copies thrive, while those with bad copies are eliminated. Uniparental inheritance is a form of this sampling, taking a subset of mitochondria from only one parent.

This creates an evolutionary niche for two distinct roles: one parent that passes on mitochondria and one that does not. This is the foundation for two sexes. Nick Lane explains that having only two sexes can seem like "the worst of all possible worlds," because you can only mate with 50% of the population. In contrast, hermaphrodites could mate with everyone, and some fungi have thousands of mating types. However, even in those complex systems, a pecking order determines which type passes on its mitochondria. A simple two-sex system might have evolved to minimize errors.

This fundamental difference also explains the distinct development of eggs and sperm. Since females pass on their mitochondria, their reproductive system is designed to protect them. The oocytes are essentially "put on ice" and shielded from mutations. Males, however, do not pass on their mitochondria. This frees them up to mass-produce sperm, which is often full of mutations. Nick highlights this with a quote from geneticist James Crow.

There's no greater genetic health hazard in the population than fertile old men.

Ultimately, the different constraints on each reproductive system, rooted in the role of mitochondrial inheritance, drive the biological differences between the sexes.

The Y chromosome is degenerate, meaning it has shrunk and lost most of its genes. This is because, like mitochondrial DNA, it does not undergo recombination. Its primary role is not as complex as one might think. The Y chromosome essentially encodes a growth factor via the SRY gene, which tells the developing embryo to grow fast. In fact, the earliest discernible difference between sexes during embryonic development is not the activation of the SRY gene itself, but the resulting growth rate.

This difference in growth strategy stems from evolutionary pressures. Males do not pass on their mitochondria, so they face no constraints on 'trashing' them to fuel rapid growth, which can be an advantage in securing resources. Females, on the other hand, must preserve their germ line to ensure the quality of their oocytes for the next generation. This requires them to protect their mitochondria, leading to a delay in rapid growth. This fundamental difference in energy management might also explain why females often live longer than males, a pattern observed across various species, not just humans.

The degradation of non-recombining DNA is explained by a concept called Muller's ratchet. The rate of this degradation is influenced by both population size and genome size. Large genomes, like those of bacteria, cannot be maintained without recombination; they accumulate mutations and shrink. This is precisely what happened to both the Y chromosome and mitochondrial DNA. Nick Lane explains this shrinkage in mitochondria:

Starting from the original bacteria that was involved, it has gone down from say 3 or 4,000 genes to in our own case, 37 genes.

The Y chromosome can afford to be so degenerate because it only needs a few functional genes, most notably the SRY gene that signals growth. Natural selection weeds out men with a non-functional SRY gene, ensuring this crucial component is preserved even as the rest of the chromosome decays.

Bacteria use a process called lateral gene transfer, which involves picking up random bits of DNA from their environment. This often happens when a bacterium is stressed and needs to adapt to changing conditions. By incorporating a new gene, it hopes for a beneficial outcome that will allow it to thrive and take over.

This method works because bacteria maintain very small genomes to replicate faster. While an individual bacterium's genome is small, it has access to a vast "pan-genome"—all the genes available across different strains of its species. For example, a single E. coli cell might have 4,000 genes but can access a pool of 40,000 genes from other E. coli living in different environments. They remain competitive by keeping their own genome streamlined and borrowing genes as needed.

Eukaryotes took a different path, developing much larger and more unwieldy genomes. According to Nick Lane, this was possible because the acquisition of mitochondria provided the vast energy surplus needed to maintain such a large genome. However, lateral gene transfer becomes highly inefficient for a large genome; the probability of picking up the right piece of DNA to replace a specific gene becomes incredibly low.

To solve this, eukaryotes developed a more systematic approach: sexual reproduction. This process involves pooling an entire genome from a partner, lining up the genes, and creating reciprocal crossovers. It is a structured way to maintain the quality of genes within a much larger genome, something bacteria never needed. An analogy can be drawn from software development.

Sexual recombination is like creating a new branch in a GitHub repository. You make organized changes to a specific function, and the maintainer can see the difference between the original and modified code before merging it. It's systematic. Asexual reproduction is like forking a repository and making a random change to a single bit of code. It will almost always be harmful, and even when an improvement occurs, there's no systematic way to merge beneficial changes from different versions back together.

Lateral gene transfer can be compared to software development. Imagine taking a random 500-line sequence from web page editing software and inserting it into airline management software. However, it's not entirely random. With lateral gene transfer, the ends of the genetic material must match something already present in the host's DNA. It's like plugging a module into a specific part of the code where a similar component could fit.

Despite this, the process has limitations, especially when compared to recombination. The primary issue is scale. If a genome is ten times larger, it needs to pick up ten times as much DNA from the environment to achieve a similar effect, which may not be feasible. There is also a significant penalty involved. Much like a mutation, the content of the transferred genetic 'cassette' is unknown. While the placement is correct, the function of the inserted genes is a mystery. Therefore, the more an organism engages in lateral gene transfer, the higher the risk of introducing harmful code and degenerating itself. This creates a cost-benefit trade-off for the organism.

When considering which experiments could provide the most information about the origin of life, observation is key. For example, if a giant bacterium were found that didn't have multiple copies of its genome, it would challenge existing ideas. While it might seem useful to visit a deep-sea hydrothermal system like Lost City, the ocean's chemistry is completely different now than it was four billion years ago. Today's oceans are full of oxygen, lacking the iron and nickel that were present in early oceans. The vent walls are now made of different minerals, so they can't perform the same catalytic chemistry.

Therefore, the most informative work is happening in labs. In anaerobic glove boxes, researchers are reacting hydrogen and CO2 to see how many molecules of biochemistry can be produced. This is slow and laborious work, with challenges like contamination and small yields. It will likely take decades to drive flux through all of metabolism under a specific set of conditions. Certain steps, like making purine nucleotides, are particularly difficult because the intermediate molecules are unstable and break down easily in water.

This work requires a specific mindset. One must believe the hypothesis could be wrong and be prepared for setbacks. Nick Lane emphasizes this approach to scientific inquiry.

There's so many beautiful ideas killed by ugly facts. So there's no good believing that you're right. You've got to believe you're probably wrong and keep going anyway.

A fascinating area of research connects anesthetics and mitochondria. Nick Lane explains that anesthetics affect mitochondria, a fact that holds true even for organisms without nervous systems, like amoebas. This raises a profound question: if an amoeba can be made unconscious, does that imply it was conscious in some form before? This moves beyond the typical understanding of consciousness as a product of complex neural nets.

This connects to what philosopher David Chalmers called the "hard problem of consciousness," which questions the physical basis of feelings like pain or love. From an evolutionary biologist's perspective, if feelings are real and have evolved, they must be a physical trait that natural selection can act upon. This implies feelings must be measurable and offer a survival advantage, yet we don't know what to measure.

To explore this, consider a bacterial cell. It performs about a billion chemical reactions per second, all of which must be controlled for the cell to behave coherently. Nick suggests that the cell senses its overall metabolic state not just through molecules, but through the electrostatic and electromagnetic fields generated by its membrane potential. These fields provide an integrated, real-time handle on its internal state in relation to the external environment.

This leads to a hypothesis: a feeling is effectively the electromagnetic field generated by a cell's membrane potential, signaling its physical state. When applied to anesthetics and mitochondria, this opens up two possibilities. One is that anesthetics simply cause an energy deficit that shuts down the brain. The more exciting possibility is that they interfere with informational fields generated by mitochondria.

Do mitochondria generate the kind of fields that I was talking about in bacteria that are giving some kind of indication of your status in certain mitochondria, certain neurons, and the anesthetics interfere with that? That would be magical if that were true, that would be a whole new direction of research, which would be fantastic.

While difficult to measure, early research points to specific parts of the mitochondria, like Complex One, as potentially being involved in generating these fields. For Nick, pursuing these kinds of questions is what makes science fun, a crucial element for any researcher.

Nick Lane's book, The Vital Question, is highlighted for its unique approach to science writing. It fills a niche between dense academic textbooks and anecdotal popular science books, which often focus on scientists' lives rather than their work. The book successfully explores the actual science for a lay audience.

Nick explains that physicists are very good at writing about the universe's big questions, captivating readers with complex topics like the Big Bang or black holes. He aims to do the same for biology, tackling universe-sized questions about the origin and trajectory of life. He believes in taking readers to the edge of current knowledge and honestly presenting his personal perspective on these questions.

These are big questions, universe sized questions, and there's not many people writing about them and trying to take you to the edge of what we know in the way that the physicists very often do and just say, well, here's how I see it.

Modern tools like LLMs can make such dense books more accessible. For instance, a book club of non-biologists used LLMs to clarify fundamental chemistry and biology concepts while reading. This helped them grasp complex ideas, such as the chemical reactions in early life environments.

Nick acknowledges the challenge of explaining biology's inherent complexity. He constantly wrestles with finding the right balance, trying to identify simple common denominators without oversimplifying or failing to make concepts clear enough. He values feedback from readers to improve his writing in the future.