1Sex in the Shallows
1.2 billion years ago
Arctic Canada
Little red threads waft back and forth to the slow rhythm of the waves. Each is so incredibly small that every wispy strand is anchored between individual pieces of crushed stone and quartz, relying upon what can only be a tenuous hold on the ever-shifting sea bottom. They can do little else but dance to the time called by the sea.
To pick out any single filament from the mass would be extremely difficult. These living things, stacks of cells that would seem to barely reach above the sediment, only span a couple of microns each, gathered around each other in organic strings growing from the tiny spaces between sand grains that were once stone. They make their own food by using energy from sunlight to rearrange organic molecules into forms able to nourish them during their sedentary existence. Despite their humble appearance, however, the threads are something novel on Earth. In a world of singular, self-contained bodies, the algae’s cooperative cells can carry out different functions—whether that’s holding tight to the substrate or sending a new generation out into the world. Their messy clumps are a foreshadowing, in miniature, of forests that will stretch both into the air and deep below the ground.
The strands are not Earth’s first photosynthesizers, nor are they technically plants. The multicellular strings are something between, a new expression of forms that have long existed but arranged in such a way that each part of the organism has its own role to play. The filaments have been shaped by the shifting demands of natural selection as well as a surprise cohabitation, a history in which the barriers between one living thing and another have become so permeable as to almost be intimate. And for the moment, the meek little algae are among the most complex life-forms among what is still—and will always be—a world densely populated by microbes.
So far as sheer longevity is concerned, organisms like these red algae will persist across incredible spans of time. Bangiomorpha, as they will eventually be named, wouldn’t look out of place in a twenty-first-century tide pool. But all those other organisms that we might associate with a trip to the beach are still a long, long way off. During this Mesoproterozoic time, there are no shells in the sand. Nothing large enough to be visible to the human eye swims through the water. No fish, no cephalopods, no bivalves, nothing at all that we would recognize as animal lives in these waters—or anywhere else on Earth. The earliest recognizable animal life, sponges that will form their own anchors to the ocean bottom, are still over 400 million years off in the future from this point in prehistoric time. And above the waterline, sand and stone gives way to a landscape of bare rock beyond. Snow-capped mountains in the distance are practically bald; no trees or other vegetation cover their slopes. Multicellular life hasn’t gotten there yet. In all, a visitor to this time might consider the planet lifeless despite all the tiny organisms constantly reproducing themselves, eating each other, and taking in the sun. Earth is an ocean planet, cradling most every living thing within the salty womb of the sea.
By this time, with Bangiomorpha growing in the shallows, it’s been more than two billion years since the earliest cells coalesced. During this incredible span of time the descendants of that first life have been busy reproducing, evolving, and filling the world’s waters with each successive generation. All the changes among these living things—from how they gather food to what conditions are most suitable for them to thrive—are setting up the shape of life in the hundreds of millions of years to come afterward. The possible array of forms life on Earth might take is already being influenced by the details of these microscopic lives. Consider it this way: The time between the first life on Earth and something as basic as Bangiomorpha is more than 2 billion years, while the amount of time between Bangiomorpha and the earliest humans is about 1,194,000,000 years. Most of Earth’s history is told through the lives of individual cells. It’s only in this moment that what were once single cells are beginning to combine and coalesce into new and unexpected arrangements, the essential foundations for everything that will follow.
On an overcast day like this one, skies weighted with a puffy gray blanket of clouds in every direction, the small strings simply shimmy in place and wait for the life-giving sun to warm the shallows again. They can’t make food for themselves without fresh photons to jump-start the essential process. But a human perspective is inadequate to understand life during this incredibly ancient slice of Earth’s history. Life is ubiquitous during this time, what will be called the Stenian Period by geologists, but it thrives at a scale that would require microscopes to see, much less understand. For more than two billion years, since something alive originated on the planet, biodiversity has existed almost entirely as arrays of single cells replicating themselves ad infinitum. Mats of bacteria and aggregations of cyanobacteria striving for sunlight are easily visible and dot these same shallows here and there, but we are so big in comparison that ever-busy aggregations of cells just look like puddles of muck to us. The smallest object that the unaided human eye can see would be about the size of an amoeba, just a tenth of a millimeter across. So much of what swarms and grows and responds on Earth in this primordial moment is many times smaller than that, only potentially visible when all those cells group together into something greater.
To understand this time, we need to think small. The shift only requires a few moments and a little concentration. As you stand in the shallows and feel the water slop and lap against your ankles, pay attention to your breathing and imagine shrinking with every exhale, becoming tinier and tinier in stature until you’re smaller than your childhood self, smaller than a housecat, smaller than a mouse, smaller than a lightning bug, the world as you know it shifting out into an impossibly broad horizon in which even a speck of stone seems like a house-size boulder. It’s so small that you could easily find the spaces between those broken-down pieces of rock and slide between them, an inch of seafloor now a vast and craggy field that feels impossibly deep despite the fact that you are still in sunlit waters. The air above the surface of the water may as well be outer space. Now you have a more fitting sense of scale, able to detect the small changes that—in time—will bring around sweeping consequences for the planet.
Shrunk down, the threads of red algae now stand taller than you are. Their full name is Bangiomorpha pubescens—the pubescent red algae form. It’s a strange name for this organism, not least because it’s difficult to imagine the strings of cells engaging in anything resembling teenage rebellion, but it’s a signifier of this essential turning point in life’s history. Not only can Bangiomorpha make its own food through photosynthesis, but it’s also one of the first living things on Earth to have sex.
It certainly took life on Earth long enough to accidentally happen upon this new way of reproducing. And the shift will open up new potentials for the green life that will follow in the hundreds of millions of years to come, responsible for pollen, flowers, and life’s irrepressible variety. Sex, as far as the potential for reproduction goes, is a gamble whose risks are offset by the new combinations of traits that evolution either discards or develops in the fullness of time. The path to this pivotal moment has been a winding one, in which accidents and novelty opened up new possibilities. The origin of organisms like the red algae wasn’t simply a matter of slowly refining cellular processes through time, but involved global changes, mass extinction, and unintended cooperation between cells that would generate an entirely new branch of life.
Photosynthesis is not synonymous with plant life, neither in our own time or in these chilly Stenian days. In fact, many of the first photosynthesizers might not be able to survive in the world Bangiomorpha now inhabits, and organisms like the little red algae acquired the ability to make their own food not through step-by-step evolution but a happy accident that changed the history of Earth forever after. As we continue to watch the Bangiomorpha twitch to the rhythm of the sea, let’s consider how such a living thing came to be.
The first cells capable of converting sunlight to energy evolved relatively soon after the origin of life. Instead of consuming other cells or organic molecules, the earliest photosynthesizers were able to use energy from sunlight to shuffle electrons from carbon molecules and other components of seawater to make sugars that nourished the cells’ other processes. Those early photosynthesizers tumbling and bobbing through the water didn’t give off oxygen as a byproduct, however. The cells were anaerobic photosynthesizers, meaning that oxygen played almost no role in the biological process. By accident, however, some of these ancient self-starters evolved a new way of feeding themselves that would begin to alter the Earth’s composition and fundamentally reshape the planet.
As individual cells nourished themselves and split, the genetic copying and pasting carried errors with it—mutations. It’s like running a passage from a book through a photocopier, scanning that copy once more, and again and again until smudges on the page appear to create a new word that changes the meaning of the page. One of these unforeseen changes altered the way some cells carried out photosynthesis, incorporating the oxygen from water molecules in the oceans—the O in H2O—in a way that gave off O2 molecules as a byproduct, nothing more than a buoyant gas released back into the water.
The O2 molecules produced by a single cell would have been negligible, even over the course of the microbe’s life. As the number of these oxygen-producing photosynthesizers grew, however, the chemistry of Earth began to change. Oxygen levels in the water climbed, saturating the oceans. Once the seas had become oxygenated, the gas began to escape into the atmosphere and altered the composition of the air—making oxygen molecules more prevalent than the previously abundant methane. What had once been a relatively rare molecule on the planet was produced in such vast quantities by early life that it forever altered the nature of the air and water. By a billion years before the time of Bangiomorpha, enough oxygen had seeped out of the ancient seas to make up about 3 percent of Earth’s atmosphere. Photosynthesis had changed the planet, and not all life would thrive under these new conditions. The rise of oxygen caused a mass extinction.
On our sun-soaked planet, early photosynthesizers thrived for billions of years. The main check on the proliferation of photosynthetic cells was their exposure to the sun. Green-tinged cells clogged the shallows and turned the waters emerald where the cells thrived. The sudden surplus of oxygen, though, was toxic to many forms of the more ancient, anaerobic photosynthesizers. The photosynthesizing cells that generated oxygen molecules were killing off their neighbors, and the dead cells fell to the sea bottom day after day, year after year after year. Over time, the carbon of their bodies became incorporated into the sediment. The sediment then turned to stone, all the while the oxygen-producing photosynthesizers continued to alter the planet. Over the course of millions of years, as the rocks enriched with carbon from long-dead cells were pushed up above the surface by the movement of Earth’s ever-shifting plates, something strange happened. Oxygen molecules in the air began to react with the organic carbon in the rocks, creating compounds like carbon dioxide. Oxygen levels in the air dipped, even if only temporarily. Life had caused Earth’s processes to find a new balance, and the success of oxygen-producing photosynthesizers ensured that it would be a lasting one. Life did not just live on the planet. Life changed the planet. Photosynthesis changed the world.
Such sweeping alterations were spurred by the everyday activity of tiny cells. The waters of the Earth had been greened. Plants were not an inevitable consequence of the change, however. Life could have very well stayed in a single-cell state for billions of years more. What happened next was not a matter of gradual refinement that we usually associate with evolutionary change. Something much more bizarre transpired, a cohabitation that would form the basis for organisms like Bangiomorpha and every plant that will come to stretch toward the sun.
Among the most prolific of the oxygen-producing photosynthesizers in Earth’s early days were cyanobacteria. Up close, they look like strands of green, pill-shaped cells. Their verdant tint comes from chlorophyll, which appears green to us because that’s the part of the light spectrum the pigment doesn’t use. Chlorophyll absorbs blue and red parts of light, leaving the green part of the spectrum to be reflected back out rather than taken into the plant’s tissues. The nature of light itself set up the spread of greenery, a near-useless shade for the photosynthesizers.
The oldest cyanobacteria lived in a world of cells that could easily absorb them. Eating a cell that creates its own food is nothing but a free, extra-nutritious meal. And so cyanobacteria lived side by side with other cells that surrounded them, absorbed them, and broke them down into nourishing components that allowed other forms of life to thrive. Not every attempted meal was wholly successful. On one unknown day, a cell searching through the water column for food encountered a cyanobacteria and wrapped its microscopic body around the floating cell. Every time before, the captured cyanobacteria was broken apart into its nutritious components. Not this time. Somehow, the cyanobacteria survived inside the cell and was capable of still feeding itself. The engulfing cell couldn’t bust the cell membranes to dissolve the cyanobacteria. Perhaps a prisoner, perhaps a roommate, the photosynthesizing cell was stuck inside of its host.
You can’t live in such close conditions without sharing a few things. The cyanobacteria cell could still split. The host cell could divide itself, as well, and when it did so, the engulfed cyanobacteria produced a new version of itself to go along with it. A cell meant to be a meal turned into a new cellular feature, the cyanobacteria tucked inside its host cell while also providing it with food that crossed through the photosynthesizer’s membranes. The cyanobacteria and host cell both had their own DNA, not packaged neatly in a nucleus but distributed like cooked noodles inside of themselves. In such close proximity, perhaps in those moments of mutual splitting, cyanobacteria genes were taken up into the core genetic storehouse of the host cell. The cyanobacteria was no longer just along for the ride. The host lineage began to reproduce the photosynthesizing cell inside of itself with every generation, two species now forever entwined as one. The new photosynthesizers were hybrid creatures, split evolutionary lineages that had come together like two streams becoming one. In time, the cyanobacteria ceased to resemble its original self and was transformed into a new feature within the parent cell. The modified version, called a chloroplast, had become a coordinated, food-generating component of its host.
The new cellular form began to thrive alongside its predecessors. The history of life isn’t a story of progress, after all, but is more like a play in which new characters arrive, many depart, and some seemingly minor characters stick around through all the dramatic changes that unfold. Descendant lineages often live alongside species that very much resemble their ancestors, and so, now that we can return to the Stenian seas Bangiomorpha call home, we can perceive both the red algae’s novelty and the persistence of the cyanobacteria.
Not far from the tiny grove of Bangiomorpha there are what would seem to be immense, rounded towers of compressed sediment. The structures aren’t purely geological phenomena. They were created by life. Atop each one is a mat of cyanobacteria, not much different from those that now act as the food generators inside the red algae, and the forms they have inadvertently created are called stromatolites.
Stromatolites huddle shoulder to shoulder in these waters, low and flossy mounds that would seem like a distortion in the sand if there were not so many of them. Domes like these have been around since the days of the oxygen crisis, not a single organism but rather a colony that remakes the very ground beneath it. The living part of the stromatolite is a woolly green shag on top made out of cyanobacteria, an early adopter of photosynthesis. As sand particles settle among the strands or little grains become stuck in the mat, the cyanobacteria readjust to get on top of the accreting mass, pushing the particles beneath and inadvertently fixing them in place. The process is too slow to see, but as this back-and-forth takes place day in and day out for year after year, generation after generation, the cyanobacteria collect small pillows of sediment and raise themselves just that much higher toward the shine of our distant sun.
Tiny Bangiomorpha resembled modern red algae, protoplants with differentiated cells.
Both the stromatolites and the Bangiomorpha growing alongside them rely on cyanobacteria, just in different ways. While Bangiomorpha is not technically a plant, the threads are close to that origin point. Within the walls of each algal cell is a chloroplast, not all that different from the cyanobacteria the component evolved from. The interior of the pocket is a mess of tiny discs, little green structures called thylakoids that absorb the sunlight needed for the Bangiomorpha to make its own food. The plant sits near the very roots of the plant evolutionary tree, an organism that could only exist thanks to billions of years of evolution, extinction, and chance events that led some cyanobacteria to take up residence inside other cells. And perhaps Bangiomorpha and its relatives could have become another evolutionary blip, one form of life that came into being but did not last. Nothing is guaranteed against the unfurling of time. With our gift of hindsight, though, we can perceive Bangiomorpha along a fizzing evolutionary fuse that is going to set off a burst of new living things. The protoplant is not just a collection of clones, but is made up of differentiated cells in which different parts of Bangiomorpha carry out specific tasks.
Bangiomorpha don’t just adhere themselves to the sediment like the cyanobacteria atop the stromatolites do. Superficial as their hold is, each stalk has a specialized cell that sends smaller threads into the spaces between the sand grains on the sea bottom. The fact that these holdfasts do something specific means that Bangiomorpha is a multicellular organism, a single living thing made up of multiple cells with different shapes and functions. The rest of the red algae exposed to the sunlight creates enough nourishment to feed the holdfast, which in turn allows Bangiomorpha to live in one suitably sunny place instead of being subjected to the uncertainty of floating through the water column. Perhaps it doesn’t seem like it in such a minimalistic state, but this differentiation will eventually be modified and tweaked such that specialized cells grow together to make tissues that combine to create organs, and which build upon each other into systems. The world will always belong to the small single cells that can adapt to just about any habitat and can simply split to reproduce, but the origin of multicellular photosynthesizers will allow everything from ferns to orange trees to evolve.
Living in just one place comes with its own risks, of course. The holdfasts mean that Bangiomorpha can’t move from their tiny patch of sediment. The benefit is that being anchored relatively firmly to the bottom means that Bangiomorpha can reach taller than many of the other photosynthesizers spreading out as mats and slimes across the sand. While something as simple as a wave carrying too much sand might mean disaster for cells growing close to the surface of the sediment, Bangiomorpha reach higher and have a better chance of still being exposed to the life-giving sunlight. Now that the red algae is able to grow upward, and not merely outward, Bangiomorpha can coexist with other species in a world crowded with constantly busy cells.
As the cloud cover begins to scatter and light shimmers here and there among the shallows, Bangiomorpha begin to photosynthesize once more. The threads are doing more than just making food. Differences between some of the red algae wisps indicate that there’s another biological process playing out. Some Bangiomorpha strands seem almost twice as thick as the smallest ones. The grove is a mix of different-diameter threads, each with a different role to play in the plant’s reproductive life.
For the vast majority of life’s history, organisms have reproduced themselves asexually. Split with yourself and suddenly you have a slightly different copy, not a clone and not a sibling, but something more like a mild variant of the original. And it’s worked out pretty well for microscopic life. Organisms can reproduce quickly and, if their constellation of traits works well in a given environment, they can rapidly spread. Such species can also respond quickly to changes like variations in temperature or oxygen levels, as a greater number of variations increases the chance that some of the population will be capable of withstanding the latest perturbation and passing the useful traits to their offspring. It essentially comes down to numbers—if you quickly produce a vast array of slight variations at any given time, there’s a better chance some will be able to survive the worst. Then again, a split can also perpetuate a vulnerability or limitation that can be difficult to evolve out of. On top of that, the amount a descendant can vary from its parent is relatively minor. Even though anaerobic photosynthesizers were numerous and reproduced rapidly, for example, many species didn’t vary enough to include those that could withstand Earth’s increased oxygen.
The red algae have spread by keeping tradition while adding a new twist to it. The various Bangiomorpha threads don’t begin their lives as slightly varied copies of the original. The way they’ll reproduce is more fortuitous. The stacks of cells start off as asexual, not differentiated into any reproductive role early in their lives. In time, some of the cells make asexual spores that can drift away and start the growth of a new plant much like its parent. But some of the threads change. Some cells split, and split, and split again, a thread producing two thousand or more sperm-like spores. Other spores transform in a different way, creating a kind of biological hook to gather those spores and fertilize a nucleus held within. The red algae is creating sex cells that recombine into new genetic arrangements—variations tested against the ever-shifting conditions of the planet. It’s a way to introduce more variety, faster, opening new possibilities for future change. More than that, harmful mutations have less of a chance to ride along with each generation. Every time the gametes meet each other and create a new mix, there is a possibility that liabilities to survival hidden in the DNA won’t be copied over. It’s a different response to an ever-changing, chaotic planet. Rather than throwing out a greater number of slightly different organisms of which a relatively few might survive and keep up with ecology’s pace, Bangiomorpha can vary more widely and have a greater likelihood that some chance change will open possibilities never seen before. In fact, it was sex that opened the possibility that Bangiomorpha could evolve its anchor and rise vertically from the seabed. In a rapidly changing world, sex is one way to stay ahead of tomorrow’s changes.
Here, in this ancient sea in which multicellularity is still a novelty, sex has opened possibilities that will be realized through the course of time. Cells can group together as part of one living thing and take on different roles. It’s no longer essential for the whole organism to copy itself. Cells can differ with each iteration, allowing the emergence of actual species with arrays of variations instead of more homogeneous groupings of near-clones that survive through sheer numbers. With each reproductive cycle, cells can potentially take on a new shape or role in the body of the entire organism, assisting in the survival of the whole. Sex requires luck, of course, and fortunes aren’t always good. Some mixes of genes come to nothing. Some differentiated cells aren’t particularly useful, or might even leave a living thing vulnerable. Such risks are unavoidable, yet the incredible flowering of life can only come about through such trade-offs. Sex drives such a novel spread that life could take on forms never before possible. What is simply another part of Bangiomorpha life, dancing to what the sea calls, will hasten how many new forms will originate and become extinct in the hundreds of millions of years to follow, a great flowering that will see life in the shallows wholly change the nature of the terrestrial realm.
Copyright © 2025 by Riley Black
