Biology
“IN THIS BROAD EARTH OF OURS,
AMID THE MEASURELESS GROSSNESS AND THE SLAG,
ENCLOSED AND SAFE WITHIN ITS CENTRAL HEART,
NESTLES THE SEED PERFECTION.”
-WALT WHITMAN
Ɑ
—Biology is defined by self-replication.
—4.1 billion years ago (4.1 Gya): Self-replicating molecule emerges
—3.5 Gya: Photosynthesis, free oxygen (O₂) produced
—1.7 Gya: Multicellularity
—1.2 Gya: Sexual reproduction
—850 Million years ago (850 Mya): Land plants emerge
—600 Mya: Complex multicellular sessile blobs emerge (proto-animals)
—540 Mya: Cambrian Explosion, most major animal phyla appear
—200 Mya: Mammals emerge
—85 Mya: Primates emerge
—8 Mya: Gorillas diverge from chimps/humans
—5 Mya: Chimps diverge from humans
I. The Emergence of Self-Replication
Overall chaos in the universe increases over time; rivers of energy erode all things, draining into a dark ocean of dissolution. Things fall apart. Structures collapse and crumble into ash and dust.
But in that ocean, there are islands; little pockets of complexity, where order is increasing, things are being constructed, and entities are continually ascending into higher realms of existence.
All over the universe, particles gather into atoms, which gather into molecules and stars, then into black holes and galaxies. Lower-order entities mingle until, by some miracle, they assort themselves into the correct pieces of a puzzle and a new, higher-order entity is born.
Each entity is defined by its harmony with the environment, it's capacity to maintain its own existence. Nature declares than a hydrogen atom is a form of harmony—it’s existence is stable. Nature also declares that a gathering of enough hydrogen atoms will begin fusing helium and ignite into a star: a new, stable entity.
The star is an entity, made up of hydrogen entities, which are made up of proton and neutron entities, which are made up of quark entities—this is a hierarchy, a structure of nested entities, out of which the entire cosmos is built. The cosmos is a city, made up of city blocks, which are made up of buildings, which made up of bricks. The buildings are both wholes and parts.
A proton is similar: a proton is made up of quarks, but a proton is also a part of a hydrogen atom. If the quark entity collapses, so do protons, and then hydrogen atoms, and then stars and galaxies, and so on.
Each entity in the hierarchy is emergent, meaning its more than the sum of its parts. The star may be made up of a trillion hydrogen atoms, but only if those atoms are all together in the same gravitationally bound sphere. Separate each atom by a thousand miles, and, though you haven’t lost any atoms, you no longer have a star. Bring them together, and they begin to fuse, burn, and ignite into a light-producing, celestial object—something more than the sum of its parts; the whole has characteristics and processes not possessed by its constituent pieces.
In this vast dark ocean, on a very special island, a new entity emerged; something that was far, far more than the sum of its parts. This entity was vastly different than anything that had come before and its emergence led to an exponential acceleration in the magnitude of complexity and organization in the universe.
What set this entity apart was its ability to replicate itself. This strange new trait revolutionized the evolutionary process, resulting in a vast landscape of emergent entities that would give rise to many complex new properties.
It’s very unclear how, exactly, this self-replicating might have emerged. Based on our observations, a self-replicating entity being pieced together by purely physical, inorganic processes seems impossible. We might fathom how human engineers could build a device capable of building copies of itself—some kind of box with mechanical arms programmed to build another box with mechanical arms programmed to build another box with mechanical arms, and so on—but, even if that were feasible, it's unclear how that might come into existence without being deliberately built.
Hydrogen atoms gravitationally drifting into each other to form a star is one thing, but amino acids in water drifting together to form a molecule that constructs copies of itself is another thing entirely. It’s like completing a puzzle by shaking its box: it’s not just unlikely; it seems absolutely impossible.
But somehow it happened and I will attempt to summarize the events that led it.
II. The Perfect Earth
The emergence of a self-replicating entity requires a host of fortuitous cosmic circumstances:
First of all, we need a star system within the habitable zone of its galaxy. Our Sun is a nice distance away from the galactic center of the Milky Way, about 28,000 light years. Being this far away gives our Solar System an orbit that avoids huge asteroid fields, star clusters, and other debris that, if we were closer, would greatly increase the rate of collisions and likely wipe out any life that might occasionally crop up on Earth. The center of the Galaxy is also awash in harmful radiation. A star system nearer to the center would experience increased exposure to gamma rays, X-rays, and cosmic rays, which would destroy any life trying to evolve on a planet.
Then we need the right kind of star. For advanced life, our star must be highly stable, which is typical of middle-aged star, about 5 billion years old, that is of the proper size (about one million miles in diameter) and proper metallicity (contains the right amount of carbon, silicon, iron, etc). The star must have a system with small and rocky inner planets and outer gas giants, that aren’t close enough to disrupt the inner planets’ orbits. At the right distance, the gas giants also function as gravitational ‘vacuum cleaners,’ absorbing much of the debris that might otherwise collide with the life-bearing planet.
We need a planet that’s not too close or too far from the Sun so that the surface can support liquid water. It also needs to have a circular orbit around the Sun, as opposed to an elliptical orbit which is much more common for planets in the universe; an elliptical orbit means extreme temperature changes throughout the year, with ocean’s boiling as the planet approaches the sun and freezing as it drifts away.
A planet that is tilted on its axis gives rise to seasons, which is conducive to life if the tilt is moderate, but not if the tilt is extreme. The planet should rotate relatively quickly so that the day-night cycle is not overlong; if a day takes years, for instance, the temperature differential between the day and night sides will be extreme.
A large moon can play a crucial role in stabilizing the axial tilt, and will significantly contribute to ocean tides. These tidal forces not only help ensure that the oceans do not stagnate, but also play a critical role in maintaining a dynamic climate.
The planet needs to be big enough to hold its own atmosphere (through gravitation), but not so big that the atmosphere becomes dense like a gas giant. If it’s too small, the planet will quickly lose the heat of its core (leftover from the planet’s formation) and end up geologically dead, lacking the volcanoes, earthquakes and tectonic activity which supply the surface with life-sustaining material and the atmosphere with temperature moderators like carbon dioxide.
Plate tectonics appear particularly crucial, at least on Earth: not only does the process recycle important chemicals and minerals, it also fosters bio-diversity through the creation of continents, mountain ranges, and valleys.
The planet needs a large iron core. This allows for a magnetic field to protect the planet from stellar wind and cosmic radiation, which otherwise would tend to strip away planetary atmosphere and to bombard living things with ionized particles.
In summary, things need to be balanced (not too hot, not too cold) across many, many metrics or life cannot manifest; everything needs to have come together absolutely perfectly, just to get the right conditions for a self-replicating molecule to maybe have a change of randomly assembling itself.
All these conditions came together when the Earth took shape around 4.5 billion years ago. Everything was set up perfectly and the miracle of life began.
We had oceans of liquid water, washing up against mountainous land and forming warm, stagnant pools. We had radiation from the sun coming down to bake those pools. There was abundant carbon, nitrogen, and other elements in the atmosphere, born from dying stars. We had lightning storms occasionally electrifying water. And we had time—lots and lots of time.
Over the course of several hundred million years, there was an evolution of chemicals. Elements like carbon and nitrogen evolved into more complex compounds like methane and ammonia. A constant input of sunlight and electricity sloshed these compounds around and, eventually, they assembled into more complex molecules like amino acids, nucleic acids, and lipids.
This chemical soup evolved over time, with increasingly advanced molecules assembling, seemingly of their own accord, in ways we now find difficult to comprehend. Inorganic compounds transformed into organic compounds. The building blocks of a self-replicating molecule gradually came into being.
The next part isn’t so clear.
Somehow, the entity that would become life, succeeded in attaining 1) a cell membrane (which allows food to enter and waste products to leave), 2) metabolism (the ability to feed and repair itself), and, of course, 3) self-replication (the ability to produce copies of oneself).
These are the processes that define life. Biological evolution cannot be sustained without them. No self-replication, no offspring, no next generation. No cell wall or no metabolism and any entity that emerged would dissolve immediately—it could not maintain the organization and complexity required for repeated, sustained self-replication.
How exactly we would get from simple organic compounds to this finely-tuned reproduction machine is very unclear. Again, it doesn’t seem like a proposition that is remotely plausible.
Regardless, somehow, someway the self-replicating entity emerged, was able to maintain its own existence, and gave birth to whole new paradigm of behaviors and patterns that differed greatly from the cosmic dust and balls of fire that came before.
III. The Biological Timeline
Once self-replication gets established as a sustainable process, the phase of biological evolution begins.
Evolution has three universal components: 1) iteration; 2) variation; 3) selection. Any time you have these, you have evolution.
Physical evolution works like this: 1) many possible physical states; 2) variation in structure, configuration, or energy of those states; 3) nature selects certain states by dictating which states ‘work’ and which states don’t—the ones that ‘work’ are retained, the ones that don’t are discarded.
Quick illustration for review:
Life itself is a product of physical evolution. There are 1) many iterations of planets (a trillion trillion) in the observable universe, that 2) vary in their size, mass, proximity to a star, axial tilt, shape of orbit, degree of rockiness, presence of water, etc, and are 3) selected by nature, based on these characteristics, to harbor life. The planets that are ‘fit’ enough are chosen, and life evolves there. The planets that are not ‘fit’ are discarded, and life doesn’t evolve there. Earth is, within this context, the most highly evolved planet we know of.
Biological evolution works off the same universal mechanism of iteration, variation, and selection, but differs wildly in how those processes manifest.
While physical entities iterates through the independent emergence of multiple, similar entities (multiple planets taking shape independently of each other), biological entities iterate through the production of offspring, of multiple generations that inherit the traits of their parents. Physical entities vary as a product of their environment and history as they float through the cosmos, while biological entities vary through genetic mutation in their morphology, physiology, and behavior. While physicality is selected by nature for its adherence to the ‘rules’ or predetermined Forms, biology is selected also by predators for consumption, as hosts for parasitism, and by mates for reproduction.
Biology is emergent in this way: manifesting wildly different behaviors than that of its inorganic predecessors.
So 4.1 billion years ago—about 400 million years after Earth takes shape—this new phase of evolution begins. The self-replicating molecule starts replicating itself. Eventually, it’s offspring not only begin to mutate, but they pass those mutations down to the next generation. Each individual molecule varies in its morphology or behavior. The ones that vary in advantageous ways succeed in surviving and reproducing; the ones that vary in disadvantageous ways fail to survive and reproduce.
The advantageous genes get passed down, the cycle repeats again and again and again, and the molecule’s genetic package becomes increasingly adapted to its environment—it evolves.
This is slow going at first. It’s not sea monsters in an epic battle for survival, it’s just microscopic gunk floating of the surface of water, resembling something you might find if you don’t use chlorine in your hot tub. Complex processes are taking place within this gunk over millions of years, and gradually, large floating microbial ‘mats’ of single-celled organisms begin to take shape.
About 3.5 billion years ago, an important innovation is made: photosynthesis. Whereas before these organisms were utilizing geological chemicals as reducing agents in their metabolic processes, an adaptation that, instead, utilizes water and sunlight and produces oxygen would increase biological productivity in the microbial mats by a factor of 1,000. Organisms that developed photosynthesis, like cyanobacteria, would become radically more self-sufficient than their predecessors and be good candidates for striking out on their own as independent mats.
The advent of photosynthesis would radically alter Earth’s atmosphere, filling it with free oxygen (O₂), where previously there was none. The oxygenation of the atmosphere would serve as a prerequisite for the evolution of the most complex eukaryotic cells, from which all multicellular organisms would later be built.
About 1.5 billion years ago we see the evolution of multicellularity, yielding advantages like more efficient sharing of nutrients that are digested outside the cell, increased resistance to predators, many of which attacked by engulfing; the ability to resist currents by attaching to a firm surface; the ability to reach upwards to filter-feed or to obtain sunlight for photosynthesis; the ability to create an internal environment that gives protection against the external one; and even the opportunity for a group of cells to behave ‘intelligently’ by sharing information.
About 1.2 billion years ago, sexual reproduction evolved as a mechanism for genetic recombination that can yield advantages over asexual reproduction by creating a more diverse collection of traits, making a species less prone to extinction (due to one maladaptive trait) and encouraging mutations that can lead to revolutionary reproductive success.
Multicellular, photosynthesizing eukaryotes that became plants emerged around 850 million years ago, followed by the so-called Cambrian Explosion that occurred 600 million years ago with multicellular eukaryotes that would become animals, which evolved and diversified rapidly starting with strange, flat and tubular blobs, then jellyfish, sea anemones, and hydra.
An evolutionary arms race began, where various traits evolved and competed for survival, leading to the development of more and more advanced traits that actively influenced one another, resulting in the ‘circle of life’ we observe, where, for instance, predators have evolved certain traits to catch certain prey, who have, in turn, evolved traits to outmaneuver and hide from those predators.
Fast forward to 230 million years ago—greatly oversimplifying the complex evolutions and extinctions of many wondrous taxons of animals—and we get the more well-known dinosaurs; 200 million years ago and we get mammals; 85 million years ago, primates; 8 million years ago, we see gorillas diverge from chimps/humans; 5 million years ago, we see chimps diverge from humans.
Not long after that, the emergent phenomenon of culture would emerge, and a new evolutionary phase would begin.
IV. The Pervasiveness of Extinction
It’s Important to remember that evolution works by a survival of the fittest mechanism. It is accurately framed as this: virtually all entities get wiped out of existence, but a few don’t. We don’t like how that sounds; it’s too cold and brutal. We prefer to imagine it as if all entities survive and, as they do, they somehow morph into more advanced beings. That is a misconception aimed at avoiding the nasty truth of the overwhelming death and destruction intrinsic to the evolutionary process and intrinsic to the universe.
More than 99 percent of all species—amounting to over five billion species—that ever lived on Earth are estimated to be extinct. Extinction is the disappearance of an entire species. Extinction is not an unusual event, as species regularly appear through speciation and disappear through extinction. Nearly all animal and plant species that have lived on Earth are now extinct, and extinction appears to be the ultimate fate of all species.
These extinctions have happened continuously throughout the history of life, although the rate of extinction spikes in occasional mass extinction events.
The first of five great mass extinctions was the Ordovician-Silurian extinction, 450 million years ago. Its possible cause was the intense glaciation of Gondwana, which eventually led to a snowball Earth. 60% of marine invertebrates became extinct and 25% of all families.
The second mass extinction was the Late Devonian extinction, 375 million years ago, probably caused by the evolution of trees, which could have led to the depletion of greenhouse gases (like CO2) or the eutrophication of water. 70% of all species became extinct.
The third mass extinction was the Permian-Triassic event, 250 million years ago, possibly caused by some combination of the Siberian Traps volcanic event, an asteroid impact, methane hydrate gasification, sea level fluctuations, and a major anoxic event. This was by far the deadliest extinction ever, with about 57% of all families and 83% of all genera killed.
The fourth mass extinction was the Triassic-Jurassic extinction event, 200 million years ago, in which almost all synapsids and archosaurs became extinct, probably due to new competition from dinosaurs.
The fifth and most recent mass extinction was the K-T extinction, 66 million years ago, where a 10-kilometer asteroid struck Earth just off the Yucatán Peninsula—somewhere in the south western tip of then Laurasia—where the Chicxulub crater is today. This ejected vast quantities of particulate matter and vapor into the air that occluded sunlight, inhibiting photosynthesis. 75% of all life, including the non-avian dinosaurs, became extinct, marking the end of the Cretaceous period and Mesozoic era.
Ω
If you didn’t already get it, your existence is incredibly, incredibly unlikely. It’s astonishing to consider all the conditions that need to be perfect, all the cosmic and biological events that have needed to take place in order for you to be alive, right now, in a stable house, in a stable city, in a stable country, on a stable planet, in a stable galaxy.
There are so many things working against that. Everything in the universe—everything about the universe—is constantly trying to kill you. And, yet, you’re alive. You exist within some divine bubble of protection, a safe little cradle of harmony, the achievement of countless, simultaneous perfections.
Millions of generations of your ancestors lived and died to ensure that you came into existence. Millions of other sperm could have made it to the egg that night, but somehow the one that would become you did. You are life: the rarest, most miraculous thing in the universe.
Try to remember that.