Infinite Life E-Book

Jules Howards

• Herausgeber: Elliott & Thompson
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch
• Veröffentlichungsjahr: 2024

Seitenzahl: 318

Beschreibung

Every animal on the planet owes its existence to one crucial piece of evolutionary engineering: the egg. It's time to tell a new story of life on Earth. 'Jules Howard's egg's-eye view of evolution is dripping with fascinating insights' ALICE ROBERTS If you think of an egg, what do you see in your mind's eye? A chicken egg, hard-boiled? A slimy mass of frogspawn? Perhaps you see a human egg cell, prepared on a microscope slide in a laboratory? Or the majestic marble-blue eggs of the blackbird? Every egg there has ever been, is an emblem of survival. Yet the evolution of the animal egg is the dramatic subplot missing in many accounts of how life on Earth came to be. Quite simply, without this universal biological phenomenon, animals as we know them, including us, could not have evolved and flourished. In Infinite Life, zoology correspondent Jules Howard takes the reader on a mind-bending journey from the churning coastlines of the Cambrian Period and Carboniferous coal forests, where insects were stirring, to the end of the age of dinosaurs when live-birthing mammals began their modern rise to power. Eggs would evolve from out of the sea; be set by animals into soils, sands, canyons and mudflats; be dropped in nests wrapped in silk; hung in stick nests in trees, covered in crystallised shells or secured by placentas. Whether belonging to birds, insects, mammals or millipedes, animal eggs are objects that have been shaped by their ecology, forged by mass extinctions and honed by natural selection to near-perfection. Finally, the epic story of their role in the tapestry of life can be told.

Leseprobe

‘Like a shower of stars the worlds whirl, borne along by the winds of heaven, and are carried down through immensity; suns, earths, satellites, comets, shooting stars, humanities, cradles, graves, atoms of the infinite, seconds of eternity, perpetually transform beings and things.’

– Camille Flammarion (1842–1925)

CONTENTS

Prologue

1.   DUST FROM DUST

Hadean Eon, 4,540 million years ago, to the end of the Cryogenian Period, 635 million years ago

2.   THE GARDEN OF MORTALITY

Ediacaran Period, 635 million years ago to 538.8 million years ago

3.   THE EARLY WOMB

Cambrian Period, 538.8 million years ago to 485.4 million years ago

4.   STARBURSTS ON SHORES

Ordovician Period, 488.3 million years ago, to the end of the Silurian Period, 419.2 million years ago

Interlude: A post-Ordovician moment

5.   A TALE OF TWO FISHES

Devonian Period, 419.2 million years ago to 358.9 million years ago

6.   A MOST MARVELLOUS INVENTION

Carboniferous Period, 358.9 million years ago to 298.9 million years ago

7.   THE LARVAL STORM

Permian Period, 298.9 million years ago to 251.9 million years ago

Interlude: A post-Permian moment

8.   THE TRIASSIC TAKEOVER

Triassic Period, 252 million years ago to 201.4 million years ago

9.   NAVEL-GAZING

Jurassic Period, 201.4 million years ago to 145 million years ago

10.   THE ART IN THE ISTHMUS

Cretaceous Period, 145 million years ago to 66 million years ago

Interlude: A post-Cretaceous moment

11.   THE INVASIVE PLACENTA

Cenozoic Era, 66 million years ago to today

Epilogue: A future for eggs

Acknowledgements

Glossary

Chapter notes and further reading

Index

PROLOGUE

Under a winking strip light in the corner of my middle infant classroom, a fish tank had been placed upon the dingy, cork-covered worktop. I can see this new addition now, in my mind’s eye, from across the room. We lined up like normal that day, then planted our bottoms on the floor. As usual, the register was taken and the teacher told us about our day, yet at no point did my eyes leave the thing behind the glass: the great globule; this gelatinous dropping. This was my first encounter with frogspawn and I had never seen anything like it in my life. This mass of eggs was so strange that if you had told me an extraterrestrial visitor had broken into the class in the night and dropped this germ into our midst, I might have believed you. In the weeks that followed, I would take it all in: the dots; the twitching embryos; these assembling protoorganisms. The gathering of tiny tadpoles on the surface of the slime blob; their first lap of the tank; their second and third. The fog of my eager commentary condensed upon the glass each breaktime. Each day, my tacky fingerprints peppered the glass surface; each night, someone’s job must have been to wipe them off. My quaint, rundown primary school never repeated this activity again while I was there. Probably, the teachers cursed the cleaning that having a tank of frogspawn required, as well as the smell that belched from it. But I am extremely grateful to whoever it was who said back then, in the cigarette fog of the dingy staffroom, that it might be something worth trying. I still think about those eggs today.

When you think of an egg, what do you see in your mind’s eye? A chicken egg, hard-boiled? A mermaid’s purse, the egg of a shark or ray, entangled in seaweed thrown onto shore? The eggs of head-lice, perhaps, being scraped off a nit comb? Perhaps you see a human egg cell, prepared on a microscope slide and telegraphed onto a TV screen in a laboratory? Or the majestic marble-blue eggs of the blackbird? Each egg is unique, and that is one of the finest things about them. Each egg on Earth has its own charisma, allure and evolutionary backstory, easily (I have learned) as diverse and interesting as the animals that hatch out of them. Every egg there has ever been is an emblem of survival; a product whittled, chiselled and crafted by the unthinking forces of natural selection for the purpose of passing genetic lineages forwards in time – days, weeks, months, sometimes years. Eggs have an evolutionary depth to them that animal-lovers don’t consider enough. And so, Infinite Life is a biography, of sorts; a true history of the egg – the most unifying, resilient life structure that Earth has ever cooked up.

In the chapters of this biography, we journey through the Cambrian explosion, when animal life surged into the lineages we recognise today, when eggs were first nursed and cradled; we chart the egg’s magnificent assault on the land, first through the ancestors of spiders and scorpions, then insects, then fish that first walked the shores and, later, the forests. Our journey takes in Triassic ponds, brimming with mating amphibians; the rise of maggots and other insect larvae; the marsupials thriving in newly evolved pouches and the rise of the most diminutive egg of all – the mammal egg as you and I know it. A single cell, the width of a hair, from which every human alive has passed. There is, naturally, a special place in our story for the eggs that encapsulated themselves in a layer of crystalline calcium and became the shelled eggs of some dinosaurs and, later, birds – a model example of how natural selection can modify egg shape, structure, colour and form and how beauty can manifest itself upon nature without design or deity. But the evolution of the bird egg is just one story among hundreds of others that feature here.

To write this evolutionary biography has, at times, been challenging. It took time to shift my perspective and view animals as bit-part players in the story of life, for once. I have been writing books about animals for fifteen years, yet this is the first time I have found myself having to look past them, at this new, earlier, evolutionary frontier. In time, as I worked through life’s chapters, writing from this new perspective became clearer. In fact, by the end of Infinite Life, I began to see animals as little more than vehicles to make more eggs, which of course, in an evolutionary way, they are. However, there are times in this book when it may seem as if eggs have desires, wants or needs: that eggs wanted to move from the sea to the land; that eggs sought safety in the mammal uterus or hid themselves in crystalline bird eggshells. Nevertheless, eggs – devoid of a brain and incapable of an instructive thought – can clearly do very little else than simply be an egg. Eggs are not capable of knowing their journey. I give them agency at particular moments only to better tell an engaging story.

Another challenge, which likewise required careful navigation, was on which animal eggs to focus. After all, there are many millions of animal species alive today and millions more that lived in the past. I decided to choose as diverse a spread of eggs as possible – from insects to mammals, from sea urchins to sharks – ‘zooming’ in and out of their lives to best describe the ways that the egg was revolutionising itself. My goal was to explore how evolutionary changes in the egg affected, shaped even, animals and their ecosystems through time: from before the Cambrian Period, before animals as we know them today existed, through the Silurian Period and Devonian Period, when coastlines shifted and climates see-sawed; into the Carboniferous, where bony land animals made advancements across continents; then to the Age of Reptiles – the Triassic, Jurassic, Cretaceous – and into the modern day, where mammals are now having their turn at the top. Eggs were there, the whole way, evolving in a host of incredible directions.

Some evolutionary ‘flashpoints’ in egg history include the evolution of free-swimming sperm and egg in the midst of the pre-Ediacaran Period; the evolution of the ‘soma’ (‘body’), the seat of animal mortality; the evolution of internal fertilisation and the implications this had for the sex lives of land animals. And, more recently, the evolution of the placenta, whose scar, once a fracture point between mother and offspring, sits a few inches above your lap.

A further challenge in the writing was the fact that, at many points in Earth’s history, very little is known about what some eggs looked like. The reason for this is that the fossil record is, by and large, biased towards hard things – bones, teeth, claws, shells. The things that fossilise well. Eggshell is, in many cases, in most species, too soft to fossilise, and so most prehistoric eggs have been lost to the sands and muds (and death-consuming micro-organisms) of time. To fill in any gaps in our knowledge about eggs in evolutionary history, I have looked to closely related modern-day animal species for clues and hints or, where DNA analysis allows, I have investigated some of the secrets found deep in animal genomes. That makes Infinite Life, in general terms, more of a gentle journey (or a ‘thought experiment’ even) through evolutionary history than an exhaustive and exacting account of every egg that there ever was. Ultimately, the question I hope this book answers is: can we re-frame the story of animal evolution through the lens of the egg? It has been a great privilege to try.

As a child and as an adult now, the joy of eggs is that they sit between the boundary lands of life and death. They represent potential. That’s what I see in frogspawn each year to this day – an exciting, dizzying, extreme form of potential. And so, for me, the egg will always be what I saw in that school tank all those years ago. Something to smear a nose up against; to question; to express wonderment at; to find new perspectives of life in.

Eggs are, I now realise, the mechanism through which animal lineages are propelled forwards through time, like threads woven into a giant tapestry. Eggs are the stitches. Eggs hold together life generations and lineages; they bind the animal narratives known to us today, allowing us to marvel both at the big picture and the intricate needlework of life. Told from the perspective of the egg, there are surely things we can learn about what it is to be alive.

Jules Howard

1

DUST FROM DUST

Hadean Eon, 4,540 million yearsago, to the end of the CryogenianPeriod, 635 million years ago

‘But if (and oh what a big if) we could conceive in some warm little pond with all sort of ammonia and phosphoric salts, — light, heat, electricity present, that a protein compound was chemically formed, ready to undergo still more complex changes . . .’

– Charles Darwin, in a letter to Joseph Hooker (1871)

Among the billions of stars taking shape in our galaxy long ago, the yellow dwarf sun that would come to light our days would not have immediately stood out as anything unique. In the long view, it was just part of a constellation, nothing more. Another pinprick of light, born from a spinning disc of super-heated matter, like almost everything else. Out of the planets that formed around this disc, one single rocky sphere near the middle of the pack started to develop differently to any other we know about so far in the universe. On Earth, the sun energised oceans and continents. Our planet soaked up its radiation and things started to stir. In time, life would evolve here. And with it, in the life lineage that would become animals, came the egg.

How far back does the egg go in Earth’s history? Right to the start? Not quite. At first, 4.5 billion years ago, this place was barely a planet at all. It was more like a condensing cloud forming from debris – ice, rock, dust – that circled our fledgling star. It was an aggregation of matter, one of many aggregations orbiting our Sun. Earth would have looked like a misshapen blob in those earliest millennia, pulled out of shape by the gravity of the objects caught in its grip. But as this sphere grew, its gravitational pull increased and, slowly, it became rockier.

There was nothing gentle about those early Earth days. Not a single rotation passed without the bombardment of new rocks and ice and new showerings of space dust. Some chunks of rock were as big as countries are today, some as big as continents and some, rarely, were even bigger still. Although the explosions caused by their impact were violent and tumultuous, these intemperate collisions, without any thought or forward planning, delivered the precursors for life – the elements, especially metals, that would become the building blocks of proteins and the structural units of cells and their membranes. These gifts from the solar system are in you now. Many comets in this early period delivered ice, which vapourised upon impact. In our paper-thin atmosphere, water molecules grouped together again, coalesced and, for the first time, formed beads at the mercy of gravity. Clouds formed. Rains fell. There was something akin to weather.

Cauterised through millions of years of bombardment, the Earth’s surface became a hellish melting pot, but there was no stopping the maelstrom of chemical interactions now. Elements shuffled against one another to form minerals which would go on to become extra ingredients in life’s cauldron. Unstable isotopes of uranium sank deep into the Earth, stoking a fire that still rages beneath our feet today, and the potential of carbon, an atom which can unite with other elements in multiple arrangements, was being explored through a trillion or more chanceful, random interactions. Some carbon arrangements held for moments. Others, in the form of the oldest diamonds, were forged 100 kilometres below the Earth’s crust at temperatures of up to 1,200 Celsius and they are still locked in their arrangements as you read these words.

In that first few hundred million years, Earth’s atmosphere was gossamer thin. Yet, as its crust hardened, gases began to belch out from the chaos churning kilometres beneath. These gases burst upwards through turbulent volcanic eruptions, through cracks and fumaroles. In the sea, chemicals boiled from vast seams that must have looked like open wounds in the ocean floor. Some of these produced ocean vents, where water heated from deep underground thrust itself upwards into the cold ocean waters. Here, where hot met cold, minerals in the water deposited themselves into tubes that loomed upwards like monolithic organ pipes in a vast cathedral.

The Earth is one character in this, our setting of the stage for the egg. But there is another character too. It is Luna – our moon.

Luna also has history. It begins as a cosmic congealment of dust caused by an explosive collision unrivalled in our history, when Earth was struck by a planet-sized object named Theia. Some 6,500 kilometres wide, Theia is likely to have had its own molten core and, perhaps, water. After engaging itself with Earth’s orbit, Theia glanced across the face of the planet, at close to a 45-degree angle. This happened very early on in our history, perhaps only a few hundred million years after the Earth began to form. The collision could so easily have been a near miss. The cloud of debris that formed after the connection of these two celestial objects was so great that, once pulled back into shape by the workings of gravity, it became our Moon. The collision affected the Earth forever, knocking its rotation off its axis, causing it to spin at an angle. From this point onwards, Earth would spin 23.5 degrees off kilter. This is the first of a number of flashpoint moments for the egg, for this wonky spin meant that our planet ended up with an exaggerated seasonal change – spring, summer, autumn, winter. Seasons bring with them chaotic periods of ebb and flow; feast and famine; life and death. And in time, eggs would evolve in ways that would navigate animals, and their genetic material, through hard times like these. But we are getting ahead of ourselves. For billions of years back then, life was mostly about relentless reproduction – single-celled organisms dividing and dividing, throwing variations to the anarchy of the oceans; a sea of losers, speckled with accidental winners, dodging the certainty of uncertain environmental change.

Evidence for these early micro-organisms abounds in numerous forms, many going back 3 billion years or more. This evidence comes from biogenic graphite, loosely described as the fossilised remnants of early cells left as chemical imprints on graphite and carbonate, dug out of rock faces by geologists. Littered within these samples are carbon arrangements derived from organic reactions common to life today. Evidence also comes from garnet crystals, formed in the Earth’s earliest pressure-cooker environments, that contain carbon molecules preserved in a chemical death grip with other atoms seen commonly in biological systems today. These biological arrangements are composed of oxygen, nitrogen and even phosphorus, a common component of cell membranes and internal cellular components, including DNA. Indeed, rock impressions from the Pilbara region of Western Australia contain pyrite-bearing sandstones that have within them micro-fossils of tube-like cells once thought to oxidise sulphur using a primitive form of photosynthesis. Life was, to turn a phrase, finding a way 3.7 billion years ago.

But there were no eggs or anything much like them at that time.

There is evidence from 200 million years later than this – an eyeblink in evolutionary terms – of cement-like structures (made by micro-organisms) known as stromatolites. Stromatolite colonies were so large back then that they could have been stood upon like giant slimy toadstools, protruding from coastal pools and lagoons. Like weary soldiers, survivors of evolutionary wars untold, these ancient organisms remain in a handful of tidal lagoons today (most famously at Shark Bay in Australia) where salinity remains so high that grazing animals such as sea snails stand no chance of sustaining a slimy foothold. These glistening, bulbous stromatolites are formed from centuries of microbial growth, each generation building upon the last in an orgy of photosynthesis. But, again, eggs remain absent from this diorama of early life.

By 1.9 billion years ago, cyanobacteria were thriving. Gorging on sunlight and producing energy for growth in the presence of carbon dioxide, these micro-organisms flourished in surface waters. Great clouds of oxygen, their waste product, began to cause our atmosphere to change. Minerals in rocks exposed to the atmosphere began to oxidise, stripping oxygen atoms from the atmosphere to create flaky red deposits rich in iron ores that are still found today. The Earth’s surface, literally, started to rust. Oxygen levels rose. The colour of our planet changed. Within another evolutionary blink of the eye, these photosynthetic organisms had painted parts of the Earth’s surface in greens, reds and blues, giving life to water in slimy mats upon the shores. Through cyanobacteria, for the first time, the planet Earth had been endowed with signature colours visible from light years away.

These vast populations of cyanobacteria, many trillions strong, started to evolve. Through accumulations of mutational copying errors, individuals and their populations became slightly different from one another and it was upon this variation that the cogs of natural selection bit. As the sun’s radiation poured across the Earth’s surface, the rate of these copying errors waxed and waned. With each incoming deluge of photons, the broad-brush forces of natural selection set to work, removing from the gene-pool those most ill-equipped for survival. Seasons must have come and gone like impeccably timed plagues back then. And in the shadow of their retreat, the ones that were left – the toughest, the hardiest – continued their evolution. There was no higher aim or purpose to any of this. No ‘desire’ to make something new or better. It was just that those individual micro-organisms best at not dying began to fill up the seas. The world was, in other words, beginning to host a growing accumulation of successful mistakes.

In one group of single-celled organisms, the cell’s structure evolved and became more complicated. This sort of cell had within it a range of smaller sac-like structures, one of which, the nucleus, would provide the housing for DNA. These were the ‘eukaryotes’ – the ancestors of plants, fungi and animals, organisms that (sometimes but not always) freely swap DNA between them through a phenomenon we call sex.

This wasn’t sex as we know it today. Instead, back then, for eukaryotic cells, sex was a far more primitive activity, probably involving large free-living cells engulfing smaller free-living cells and assimilating some or all of their genes in the process. Once it took off, this simple form of sex happened probably quite predictably at certain times or in certain seasons. At other times of year, these individuals reproduced in an ‘asexual’ manner too, the classical style, with single cells splitting in two to produce identical, daughter cells.

And so, although there was a simple form of sex 1,000 million years ago, there was no true egg, defined as an organic vessel grown by an individual to carry offspring to term. The egg is a very ‘animal’ endeavour. It was far too soon for that.

That’s not to say that some of these cells did not become very egg-like, of course. Between 1 and 2 billion years ago, a very resilient form of vessel started to evolve. A resting stage (or ‘resting cyst’) evolved in cyanobacteria. This was an armoured sleepsuit within which organisms could see out hard times. In life, before becoming these resting stages, these single-celled organisms were likely to have lived like modern-day plankton, floating in surface waters, using energy from the sun’s rays to grow.

Their resting stages are visible in the fossil record, clear as day. The tiny fossils are identified and studied by the most diligent, yet underappreciated, of all fossil scientists – the palynologists.

Palynologists (from the Greek verb paluno, meaning ‘dust’) obtain these tiny egg-like fossils from fine-grained shales and silt-stones by bathing them in hydrochloric or hydrofluoric acids for days on end. The acids strip away the minerals from the rock face and create a gooey residue of what was once organic matter. Within this residue, sieved and sieved again with ever finer mesh sizes, are the hard remains of resting cysts, the walls of their cellular ramparts toughened with a layer of complex molecules that resemble those found in modern pollen grains. A single 25-gram sample of rock can contain hundreds of these peculiar microfossils. A single sample could occupy an entire PhD.

Fossilised cysts really do have a lasting charm. When first released from their rocky matrix and spilled onto microscope slides for closer examination, palynologists rarely know how many there will be or what types of cyst they might see. Magnified by forty or one hundred times, the microscope slide becomes like a two-dimensional continent. In fact, so huge is the slide in comparison with the fossilised cyst that, when a potential suspect is spotted, its location on the slab of glass is noted with a special grid reference. Each microscope slide is like a historical map, with each grain of dust its ornate treasure.

Under a light microscope, resting cysts resemble seeds. Their spherical surfaces have upon them patterns of ridges and furrows which look as if they have been carved in drying cement. Most contain within them a hollow internal chamber in which a single unicellular organism once lived. The coverings of some specimens appear almost mesh-like. Or corrugated, like roofing. There are pores visible in many. Some cysts come complete with rows of tiny portholes. It is through these pores that, many millions of years ago, the environment influenced the micro-organism, now lost.

Using a scanning electron microscope to provide far better magnification, the intricacy of the cyst surface becomes stunningly complex. There are ripples and folds of protein-laden sheets that overlap with one another like leaves on a cabbage. Some specimens are covered in distinct patterns of dimples or, as they evolved and diversified, lumps and spikes. Not all examples are spherical. Far from it. Some are cube-like; some are adorned with wisps and ribbon-like structures. In life, scientists argue, such arrangements may have helped these organisms float in the surface waters of the long-ago world. Now, like beads in a broken necklace, they litter the late-Proterozoic rock layers.

Many scientists subscribe to the view that the cysts are ancestors of today’s dinoflagellates – unicellular planktonic life forms that bear a resemblance to baroque shields, powered by a pair of asymmetrical whip-like ribbons that provide the engine for movement.

In the modern day, dinoflagellates are best known for causing ‘red tides’, where populations increase exponentially and, within the space of days and weeks, turn surface waters into a ghoulish red mist. At this point, when dinoflagellate populations peak and resources deplete, starvation ensues across the population and the inevitable die-off begins. Not all dinoflagellates perish in these moments, however; many become resting cysts. In this hibernation chamber, the dinoflagellate sinks to the sea floor, ready to re-activate when the good times come back; when Earth, at the behest of the seasons, shall provide. Many scientists suspect that it was the same for the ancient ‘cysts’ described from fossils.

For a billion years, up until about 650 million years ago, the humble cyst is an enduring theme of the fossil record. A hardened wall, mostly impermeable to environmental chaos, which maintains an internal environment that can be controlled. A private space, tightly maintained to ensure the survival of molecular threads, conglomerations of enzymes and oozing cytoplasmic subspaces.

The resting cyst was a device that propelled genetic material forwards in time. The product of a sun, a moon, a changing planet riddled with environmental uncertainty. In the presence of death, the egg – as a concept – was forged. Yet, only in the biological kingdom known as Animalia would it become so refined, so natural, so crucial a part of the life and life story of Earth organisms.

And so, in the millions of years that followed the evolution of the resting cyst, a new sphere-shaped structure began to fall upon sea floors across our world. This vessel contained cells which divided, over and over, which coalesced into the shape of an organism, a unique genetic individual that could, by activating muscles and a primitive brain, escape its casing and live, truly, a new life.

The egg, as you and I know it today, was coming.

2

THE GARDEN OF MORTALITY

Ediacaran Period, 635 million yearsago to 538.8 million years ago

‘Omnis cellula e cellula.’

(‘Each cell is of the cell.’)

– Rudolf Virchow (1821–1902)

Imagine an art gallery where great works are not categorised by century or by geography and not ordered by chronology. A gallery in which the art styles of thousands of years are caked, daubed and plastered upon one another; where famous works can be added to and tinkered with by others that follow. For more than 60,000 human generations, that art has resided upon the rocks of Namibia’s Aar plateau, one of the most important geological strata in the world for understanding pre-animal life. Traditionally this incredible landscape was home to the San people, one of the first indigenous hunter–gatherer cultures in southern Africa. The San came to these rocks because, when impacted with a pointed rock, a white dot is produced upon the dark grey limestone background. These dots, when collected into shapes and patterns – of snakes, zebras, antelope, elephants – are the expression of human minds past. They are still added to today, creating a miasmic mood board of the human experience over time. And interspersed between and under these artworks are fossils. It is upon these rock faces that the so-called Ediacaran Garden has been exposed, where details of the organisms that lived in the geological age before animals as we know them today existed. When palaeontologists study this layer of rocks, they see a jumbled mess of creation – a biological puzzle, still being solved, featuring strange and non-sensical biological beings whose fossilised lives are yet to be fully understood.

Some Ediacaran organisms, like Charnia, resembled living feathers, collecting nutrients (we assume) using globular lobe-like structures. Others, we think, were worm-like and dug branching burrows through the sand. Some were large, like Dickinsonia, whose fossil imprints show it to have been the size of a pillowcase, engorged and close to bursting, perhaps, with some unseen, viscous fluid. Did it have eyes? A stomach? Gills? No one really knows. Among these curious Ediacaran organisms are fleeting glimpses in the fossil record of one of the earliest true eggs. Seeing or studying these eggs requires a steady hand, endless patience and an extraordinary eye for detail.

For Ediacaran eggs to avail themselves and be studied properly, the rock samples first need to be partially ground up and sliced into thin sections, some one twentieth of a millimetre thick, then mounted on slides for examination under 200 per cent magnification. To see them at their best, polarised light is often used, which lights up the crystal elements of each egg, helping scientists to gauge their orientation. The polarised light gives a cosmic quality to the images that the scientists produce. Each tiny sac looks like a distant supernova or a far-off galaxy, bordered on all sides by a ring formed from some forgotten explosion. As with NASA images, there is an element of confusion and chaos here too, mostly because the Ediacaran eggs are far from intact. Each is battered and fragmented, so much so that some members of the palaeontological community still deny that they are eggs at all.

Most notably, in many of the 120 types described, there is the suggestion that, at the time of their death, when they were fossilised and frozen in time, the cells in these eggs were forming a recognisable embryo. In fact, some fossils seem to show cells at a stage in embryonic development known as gastrulation, when the embryo transforms from being a hollow ball of cells (known as the blastula) into a three-dimensional cup-shaped structure (the gastrula) from which development continues. Some of these specimens appear to show the tiny fossil eggs in familiar states of division: four-celled structures, for instance, and eight cells, sixteen cells and so on. These, surely, were true eggs – embryonic cells, contained within a protective envelope, starting to develop into new life.

But what produced these eggs? And are they comparable to those of any modern-day species?

Some fossil specimens, particularly those from China’s Doushantuo Formation, show Ediacaran eggs with a defined single cell layer (the ectoderm) and a clear inner (endodermal) layer, folded and divided into finger-like protrusions that resemble the early stages of the free-swimming larvae of jellyfish and other representatives of their taxonomic group, the Cnidaria. Many suspect this is where the eggs came from. Jellyfish and their kin, the cnidarians, are, quite probably, the first true, most profligate, egg-layers.

With no blood and a limited nervous system, jellyfish have an obvious ‘before-animal’ feel about them. They seem to fit right in among the Ediacaran organisms. They digest things with a simple pouch-like bag, with one single opening; their internal and external worlds are divided by a body wall, just a mere cell or two in thickness. Compared to animals as we recognise them today, jellyfish make no sense at all. But, in the context of the Ediacaran, with its miasma of blobs, slime and semi-symmetrical splurge, the cnidarians fit right in. Today, almost 15,000 cnidarian species are known. The group includes corals, sea anemones, sea pens, the box jellies, hydrozoans (which include all freshwater cnidarians as well as the free-floating Portuguese man o’ war) and the ‘true’ jellyfish known the world over. The latter include in their ranks some that measure barely a few millimetres in diameter and others with bell diameters of more than 2 metres, and tentacles that can extend up to 30 metres in length.

Cnidarian eggs are small, sometimes the size of a full stop, sometimes less than the width of a single human hair. Under a microscope, their eggs appear as tiny, gelatinous spheres, occasionally with a slightly granular, sometimes speckled, veneer. But eggs are a tiny part of the life cycle of this primitive animal group. For great chunks of time, sometimes for years, these organisms reproduce asexually, without the need for egg or sperm. This may well have been, for many Ediacaran organisms, for most of the time, the way that things were done back then.

Many cnidarians live in colonies in which every individual is a clone of their neighbour. These colonies are extended by individuals ‘budding’ themselves – releasing into the water small amounts of body tissues that fix themselves to nearby rocks and grow into completely new, genetically identical, individuals. Here they are known as ‘polyps’ – a cnidarian life stage where the organism consists of a body, attached to the rock face via a stalk, with a single upward-pointing opening surrounded by multiple tentacles. In colonies of cnidarians, hundreds of polyps can become gathered together, producing a tangled carpet of silky tentacles that appear to lick and flick at each passing plankton-filled current. As the colony grows and conditions in the water change, often seasonally because of food availability, the young cnidarians ready themselves for a new stage: they produce free-swimming forms (that look like true jellyfish) known as medusae. In many (but not all) species, the tiny medusae drift off to make a life in the open sea where they become, essentially, free-swimming sex machines – sporadic purveyors of sperm and eggs to ensure each lineage continues.

It is in sunlit waters that the medusae produce eggs and sperm and where sexual reproduction takes place, externally, in clouds of activity unseeable to the human eye. Once fertilisation occurs, the egg, its cells undergoing their earliest divisions, drifts downward onto the sea floor where it becomes a new polyp, which begins to develop and ‘bud’ into ‘daughters’ that go on to become a new population of asexually reproducing clones. Thus, these organisms go through waves of both sexual and asexual reproduction, as many Ediacaran animals may have done. They produced true eggs, in other words, fertilised by a new character: sperm.

The significance of this cannot be understated. Although the distant ancestors of animals (shared with plants and fungi long ago) were able to transfer their genetic material, now, here in the Ediacaran, was a gang of (in many cases) multicellular interlopers actively growing and expelling cells that could mix with others in the waters to grow entirely new genetic individuals. The egg had evolved, but it was not to be a solitary character. In this era, sperm evolved to become its accomplice.

It is hard to say what those early sperm may have resembled. Their small size is probably ancestral – maybe once, tail-less, they actively bumbled into eggs, ‘urging’ the larger egg cell to engulf them entirely so that DNA could be incorporated and a simple form of fertilisation could occur. Regardless, from the earliest days of the animal egg, sperm were there. Proof of this can be seen clearly in the genomes of all modern-day animals. The gene that humans rely on to activate sperm production (known as the Boule gene) is found in animals as distantly related as insects, crustaceans, starfish and snails. Jellyfish (and other cnidarians) also possess the Boule gene, suggesting that, together, we inherited this gene from a common ancestor from which we collectively evolved, more than 600 million years ago. Thus, the instruction manual for making sperm and eggs is, almost certainly, an early (there or thereabouts) Ediacaran innovation.

The universal propensity for animals to make sperm in the same manner, using the same genetic tools, is not readily disputed by scientists. What is disputed, on the other hand, is why sex became more and more important to so many animal lineages at this time. Because, when you think about it, sex makes no sense at all. That an animal would spend so much effort, engaging a mate, producing eggs, producing sperm, to attempt to ensure just 50 per cent of its genes make it into the next generation . . .? You would expect natural selection to correct that quickly. One would assume that in such a world of sex, where individuals are working tirelessly to pass on just half of their genes, that cheats would prosper – that those individual organisms that focus solely on producing eggs with 100 per cent of the individual’s genes, without any need for sperm, would flood gene pools and flourish. But this is almost universally not what we see in animal lineages. At some point around the Ediacaran, some of the earliest animal groups (among them, the distant relatives of insects, spiders, fish and many more) appeared to invest more heavily in sex as a means of reproduction, pulling away from the budding (asexual) strategy championed by jellyfish and others. So why? Why sex?

Science writer Carl Zimmer describes the trio of jostling theories for the origins of sex as the ‘good’, the ‘bad’ and the ‘ugly’. Like playing cards being shuffled, sex opens up the possibility of ‘good’ hands cropping up in populations, which natural selection favours. This, over millions of years, leads to the generation of successful, adaptable organisms, far superior to the asexual organisms that are left behind in their wake. So that’s the good, but what of the bad? In asexual organisms, a naturally occurring random mutation can be lethal, which is, in no uncertain terms, a very bad thing for genetic lineages. If an individual acquires a mutation before asexually reproducing – a corrupted cell molecule working comparatively inefficiently, say – then every daughter cell will have the same mutation. ‘Bad’ genes are inherited, passed down, ad infinitum. There is no way to flush these bad ‘cards’ from the population and so they persist. Worse than that, through additional mutations, more bad genes can join them. Compare that to sex, where individuals share cards, swapping the best ones around, while natural selection flushes the worst hands from the deck round after round.