Just outside the small town of Elkins, West Virginia, lies a stretch of US Route 33 that takes you into the heart of the Allegheny Mountains. If you know just where to look, it can also take you back more than 360 million years.
In the late 1970s, a geologist named Bill Gillespie got a nice tip-off about just where to look. Gillespie knew this part of West Virginia well, as he’d been born and raised just one county over and had worked for many years in the state departments for agriculture and forestry. He was also an avid fossil hunter and had spent decades collecting plant fossils from all over the area, amassing an impressive knowledge of West Virginia’s distant botanical past. So when a colleague from the West Virginia Geological and Economic Survey (WVGES) told him some plant fossils had been found near a thin coal bed near Elkins, he was eager to investigate. That’s how Gillespie found himself on a hillside next to US Route 33, not far from the Bowden turn-off, making one of the most intriguing discoveries in palaeobotany.
The thing about highways in uneven landscapes is that large sections of hills are often removed to allow the road to go through. These ‘roadcuts’ aren’t just useful to civil engineers. Geologists quite like them, too, because they serve as massive cross-sections where layers of earth, sediment, rock and other structures can tell a geological story. They’re also pretty good places for finding fossils. According to an article by Fred Schroyer in the WVGES publication Mountain State Geology, Gillespie found plenty outside Elkins, many in good condition, and estimated they were about 345 million years old.
Soon after, in the winter of 1980, Gillespie returned to the site while consulting for the United States Geological Survey. He excavated deeper this time, to see if even older fossils could be found. As Schroyer wrote in 1981: ‘He found masses of fossils, including bark, twigs, the leaves of several plants, and seed cupules.’ Those cupules were particularly intriguing. Cupules refer to small, cup-like structures that are believed to have been an important step in the evolution of seeds.
Around the time Gillespie was excavating, two experts in palaeobotany, Gar Rothwell and Stephen Scheckler, happened to be looking for some good fossil sites in West Virginia, and as Gillespie was the state’s foremost expert on such things, they got in touch. He told them he knew just the place and led them to the hillside south of US Route 33. Gillespie later told Schroyer that it was ‘the most fantastic two hours of digging I ever put in’.
They discovered hundreds of early seed plant fossils, so well preserved that on some, the internal anatomy of the reproductive structures was intact. Not only could they see cupules, but also ovules nestled within them. They found thousands of dispersed seeds as well. This plant species, which they named Elkinsia polymorpha, was a long-sought-after missing link between seed plants and all that came before.
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The story of seeds is very much a tale about life finding a new way to extend itself across space and time. We could begin that tale in any number of interesting places, like at the first twitch of life back when Earth was little more than a roiling hot mess and something mustered enough basic chemistry to reproduce and then reproduce some more. Or there’s that time more than three billion years ago when a single-celled organism evolved photosynthesis, allowing it to marshal the dual resources of sunlight and water to convert carbon dioxide into a sugary food source, all while flatulating pure oxygen – a neat evolutionary trick that forever changed the planet’s atmosphere. But let us fast-forward a little and begin with plants, specifically the moment when they showed up and made everything really weird.
Elkins lies almost precisely due west of Washington, DC, just on the other side of the Appalachian mountain range to which the Alleghenies belong. On a good day, the drive takes almost four hours, much of that on meandering mountain roads. I’m told it’s quite a pleasant trip, especially in autumn. But 500 million years ago, there were no Appalachians. In fact, Elkins and DC – more specifically, the patches of land they now occupy – were not even on the same continent. Back then, the entirety of the future West Virginia was underwater somewhere in the South Pacific. Meanwhile, the future home of Washington, DC, sat roughly at the South Pole.
Half-a-billion years ago is such a staggeringly distant point in the past, it is hard to imagine that anything more complicated than pond scum had yet summoned the audacity to evolve. But the oceans were already filled with complex life thanks to an event called the Cambrian Explosion, in which a sudden, rapid diversification of animal life took place. This event, which began around 540 million years ago, is writ large on the fossil record. It lasted around twenty million years and ushered in most of the phyla in the animal kingdom. By around 500 million years ago, creatures with entire nervous systems were swimming around, including our own direct ancestors, the first vertebrates. And yet, plants still did not exist. Not anywhere. Thanks to the Cambrian, animals had such a head start over plants that sharks evolved about 100 million years before the first trees.
The issue was this: although complex life was abundant, it was inextricably water-bound. It existed only in oceans, seas and waterways, as well as in the shallows lapping at the edges of vast, dry continents. This stands to reason: life evolved in water precisely due to water’s unique properties and its abundance. The structure of every strand of DNA, every functioning protein, the membrane of every living cell on the planet, is dictated largely by interactions with, or the avoidance of, water molecules. Water also provides pressure and buoyancy. It ebbs and flows from the minute scale of capillaries to the grand scale of oceanic tides. Water absorbs, reflects, diffracts and diffuses light, and has the capacity to absorb and transfer heat energy. Water, in other words, provides myriad opportunities for physiological systems to evolve and function. It also sets strict boundaries. Life evolved within the constraints of the chemistry and physics of water. Put another way, the very shape and behaviour of everything alive involves a concession to what water will allow.
Land, meanwhile, was desolate and wholly alien. It was a harsher, far more capricious environment than the ocean or even a good-sized pond. For one thing, it was far too bright and there was little protection from damaging UV radiation. For another, it was really dry. Earth’s ancient landmasses up to and including the Cambrian were arid, sun-baked wastelands that would have made California’s Death Valley look downright hospitable. There was rain, of course, and so there were rivers that flowed towards the seas where the tilt of the landscape allowed it. Rainwater also gathered into vast lakes or saturated expanses of porous rock and collected in underground streams. But in between and beyond these phenomena there was a lot of dry land.
This tableau held little to entice our vertebrate ancestors or their Cambrian cousins far inland. That said, there are fascinating examples of explorations at the water’s edge in the form of fossilised tracks dating back as far as the Cambrian. It’s believed these tracks were likely made by amphibious arthropods – segmented, many-legged creatures that once thrived in the shallow waters at the time. It seems that some of them made forays across ancient tidal flats or along riverbanks. The tracks are small, often only a few centimetres wide, and there are only fragments of the paths that were taken, but they provide wonderful snapshots of how all those tiny feet pushed into the dry land, the animals plodding forwards with intent. Their meandering patterns hint at body structure and maybe primitive curiosity in service of the next meal. Life, it seems, will always test its boundaries.
Back then, the land boundary was fierce. Even for amphibious creatures with respiratory systems that allowed such excursions, remaining on dry land was a death sentence. Dehydration was always imminent and starvation guaranteed. It was, in a word, unwelcoming. So for millions of years, complex animal life teemed along the shores of enormous continents, sometimes slipping and scrambling from one shallow tidal pool to another and sometimes day-tripping across tidal flats, but that’s as far as they got. Until, one day, some algae got adventurous.
Land plants evolved from algae, but not just any algae. They arose from a particular lineage of photosynthetic green algae called charophytes. These organisms lived in fresh water, and their genetic toolkits happened to contain a number of things that would prove useful in adapting to terrestrial life. Charophytes still exist today, and one group, the zygnematophyceae, are believed to be the closest living algal relatives to land plants. This means the zygnematophyceae and land plants share a common ancestor that lived at least 500 million years ago. Studying these algae provides hints as to why that ancestor was uniquely able to kickstart the transition to land. For example, zygnematophyceae have the ability to make a sugary mucus that facilitates water retention during dry conditions. They can also make their own sunscreen, tiny pigments that protect against harmful UV radiation. It’s now known that zygnematophyceae algae have an impressive genome size, comprising up to 50,000 genes – far more than the estimated 20,000 or so genes in the human genome. It appears that land plants’ algal ancestor provided quite a lot to work with.
For an evolutionary moment, there would have lived an organism at the edge of a body of fresh water, a green smudge of something that was no longer quite algae but not yet a plant. Its existence was precarious, ephemeral and nothing short of extraordinary. In October 2016, biologist David Domozych and his colleagues succinctly explained in the journal Frontiers in Plant Science that these
organisms adapted to terrestrial conditions, became capable of surviving and reproducing when fully exposed to the atmosphere, and some members ultimately evolved into land plants … This ‘terrestrialization’ of green plants represented a keystone biological event that forever changed the biogeochemistry and natural history of the planet.
Not bad for a green smudge, really.
The first plants may have had a foothold on dry land, but they remained effectively aquatic, breaking the surface of very shallow water or lingering just at the water’s edge in order to use the surrounding liquid for hydration and acquiring nutrients. Without the buoyancy afforded by water, early land plants lacked the structural integrity needed to support their own weight beyond a certain size. Some of their structures were only one cell thick and highly permeable. They also lacked the ability to transport the nutrients and water they absorbed from one part of the plant to another, beyond what was afforded by simple diffusion. These physical constraints placed stark limitations on size. As a result, the first land plants were tiny, many growing only a few millimetres tall at most. These first plants were the bryophytes, and three main groups exist today: liverworts, hornworts and mosses.
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It is the trailing end of a La Niña summer in Australia, and here in our pocket of south-eastern Queensland it has been raining for days. It’s a light, almost genial drizzle, at odds with the searing heat and bushfires that swept up and down the east coast a little over a year ago. This placid weather is a welcome reprieve, a moment somewhere between fire and flood, but we keep an eye on the weather radar, watching for gathering storms and silently willing the rain – any rain – to reach the drought-stricken lands on the western side of the dividing ranges. But here, and for now, the rain falls and, in response, everything is greening.
We live on a ridge not far from Brisbane that rises towards a small mountain. Its low, wide bulk is covered in protected forest, and today shrouds of mist drift over the eucalypts. Sometimes, flocks of cockatoos noisily burst from the trees and fly for a few moments, bright white against dark green, before finding shelter again. The forest extends much like this for many, many kilometres to the northwest, full of eucalyptus woods and subtropical rainforest, shifting sometimes from one to the other in the space of a few metres where the elevation is just right.
In this context, the sweetgum tree at the bottom of my backyard seems out of place. It’s a northern hemisphere species, but someone planted this one here years ago, and here it will stay. It’s big, not sequoia big but maybe 15 metres tall, and when I visit friends on the next hill over, I can see its rounded shape towering above the other trees, marking home. It seems protective somehow, and welcoming. Kookaburras and king parrots perch on its branches, sometimes even those cockatoos. Sometimes, on a warm spring day, the entire tree seems to hum as native bees spend hours gathering pollen. At the tree’s base, enormous roots splay out into the yard, covered in tangles of vines and eruptions of tall grass and weeds. There are patches of moss all along these roots and growing in streaks up along the trunk where the shade is good. This tree is central to, and shelters, its own little ecosystem, and it’s hard to imagine something so big came from something as small as a single seed. It’s harder still to comprehend that both the tree and the seed, in turn, arose from something very much like those patches of moss.
I don my rainboots and splash across the yard to take a closer look. The moss is vibrant against the dark, wet bark, and rivulets of water spill over and through it. It looks only tiny and green and pleasantly fluffy. It does not look old, yet it hasn’t changed much in hundreds of millions of years. It does not look revolutionary, but little bryophytes just like these most certainly were.
I gently tug at a small patch of wet moss, but it holds fast. And after days and days of rain it has stayed put quite nicely, never once washing away. I doubt even a fierce storm could separate it from the tree. Also of note, the tree itself has stayed put. I don’t mean that it simply hasn’t fallen, rather that it has occupied precisely the same spot the entire time I’ve lived here. I have never once woken up to discover it wandering around the backyard, or communing with its neighbours in the forest like one of Tolkien’s Ents. Likewise, I have never seen any of my house plants scrambling from their pots in search of better lighting or a decent drink. Indeed, they are entirely dependent on me for water – a predicament that, historically, has not always worked out well for them. The point is this: from the tiniest mosses to towering trees, plants are immobile.
Sure, they might bend to pressure or sway in the breeze. They might extend long branches or sinuous vines, feeling ever outwards and upwards. There are invasive brambles with stems that, when caught by a time-lapse camera, seem to crawl along a forest floor. There are Venus flytraps with gaping maws that snap shut in a fraction of a second. There are all those flowers that open and close in response to light, and many plants that follow the path of the sun across the sky. But though plants exhibit motion, they don’t, as a whole, pick up and move. The reason why my house plants are victims of my absent-mindedness, and why the huge sweetgum has not snuck up on me while I’m making coffee, and why that little patch of moss does not scuttle away at my approach, reveals something truly fundamental about the evolution of plants.
A major evolutionary step into a new and hostile environment like land was precarious. Should an emerging life form have found a hospitable microenvironment at the edge of some well-protected pond, it was worth sticking around for. In the slow progression from shallow water to dry land, anchoring became very useful. The first plants did not have true roots; nor do modern bryophytes, for that matter. Instead, they developed primitive root-like structures called rhizoids. Rhizoids function as a kind of anchor and would have afforded early plants an opportunity to establish themselves in whatever niches were available, preventing them from being washed away. Now it’s important to note that plants didn’t do this alone. Where they grew, they stayed put – no more wandering about – and with this came a big change in their sex lives.
In 2010, researchers in Argentina announced the discovery of fossilised cryptospores in a rocky outcrop in the Andes – not all that far, as it happens, from Santa Rosa de Tastil. Cryptospores were primitive spores used by the earliest plants, and these ones were dated to just over 470 million years old, making them the oldest fossil evidence of a land plant yet discovered. It was an exciting find because, for one thing, the earliest plants were small, soft and squishy and didn’t fossilise very well, so their appearance in the fossil record is rare. The discovery confirmed that land plants were alive and reproducing successfully by the middle of the Ordovician period. It’s important to note that plants didn’t achieve the successful transition to land on their own. Crucially, the earliest plants entered into a symbiotic relationship with mycorrhizal fungi. These ancient soil fungi interacted with plants’ early root-like structures, facilitating access to water and nutrients. In return, plants provided carbohydrates. It was a mutually beneficial alliance that enabled plants to truly colonise land. Those first fossilised spores also hinted at another important feature of early plants, perhaps the most important feature.
When it was time to reproduce, a primitive bryophyte would send out single-celled spores, each containing just one set of chromosomes. These would be carried by water, and sometimes wind, to a new location. If a spore landed somewhere nice and wet, it would germinate. It didn’t produce a whole new plant, however, but rather a kind of intermediate generation called a gametophyte, and it’s the gametophyte that developed the sexual organs capable of making either sperm cells or egg cells. Just like algae, the male gametophytes released sperm cells into water. The critical difference was that egg cells were not similarly released by the female gametophyte but instead remained anchored to the plant. Fertilisation occurred only if sperm cells swam to that anchored egg. The resulting zygote developed into an embryo that was, for a brief time, still safely ensconced at ‘home’.
Like algae, bryophytes are entirely reliant on water for reproduction to occur, but the anchored egg cell changed everything. And it led to a defining feature of all land plants: a protected embryo. It is for this reason that, in scientific parlance, all land plants are referred to as embryophytes.
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The arrival of a protected embryo was a stunningly successful evolutionary step. With the help of their fungal partners, bryophytes proliferated along the edges of streams, rivers and freshwater lakes, turning the edges soft, green and kind of fuzzy. In this way, they followed paths of water across vast landmasses. Many early bryophytes thrived in their new-found niche and changed very little over the ensuing epochs. But the story, of course, does not end there. In a 2020 paper published in the Progress in Botany book series, botanist Ulrich Lüttge argues that life will always spread and change in order to fill all available space. That’s just how entropy works: coded into the mathematics of this universe is a certain tilt towards disorder, and nothing escapes it. Even in the most hospitable of niches, resources will eventually dwindle, whether via environmental changes or competition, and not everything that would like to stay put is able to. In the case of plants crowding at the water’s edge, says Lüttge, some of them had to change in order to survive.
By 430 million years ago, plants were becoming increasingly complex, evolving vasculature and stronger structures. Vasculature provided the plumbing necessary to carry water, nutrients and minerals throughout the plant. This was a big deal because plants could now finally grow more than a few millimetres without sacrificing access to water or food. This was made possible because plant cells evolved the ability to make complex molecules, like lignin, that provided structural rigidity and quite literally bolstered the rise of bigger plants. Meanwhile, within the vasculature, branching turned out to be an elegant way to maximise surface area, so as plants got bigger, branching became more pronounced on larger scales, too, leading to the first plants with actual branches. All of these developments required more resources. Plants, as we know, did not ditch their anchors in pursuit of better mobility. Instead, they went all in and evolved true roots, which enabled them to effectively mine downwards into the ground to access minerals and groundwater. Roots also provided stability. In addition, plants developed highly specialised, solar-powered food factories, also known as leaves.
For the animal kingdom, this was all like an evolutionary dinner bell. In 2020, scientists from the University of Texas announced the discovery of the oldest fossil of a land animal. It was found by an ancient lakebed on one of Scotland’s Inner Hebrides islands, just a short ferry ride over cold water from the town of Oban, of whisky fame. The fossil was dated to 425 million years ago, a time when Scotland sat roughly at the equator, was volcanically active, unrecognisably balmy, and not yet in possession of any good distilleries. For better or worse, the landmass on which it sat was also in the process of slowly colliding with another vast landmass – this, incidentally, is how Scotland and England ended up stuck together. Whether this collision hastened things along is hard to say, but what we do know is that the fossil reveals a long-extinct millipede-like creature, perhaps a descendant of one of those curious amphibious arthropods. Intriguingly, some of the oldest fossils of vascular plants – belonging to a group of simple, branched plants called Cooksonia – were found in the same area and also date to 425 million years ago. Indeed, fossil evidence from this site and others strongly suggests that vascular plants and arthropods evolved together, forming symbiotic ‘pioneer’ communities that enabled each to thrive. They were so good at this that after about six million years, vascular plant communities were spreading at the edge of whatever waterways they could find and photosynthesising like crazy and spewing out oxygen. Eventually, something had to give. And so it was that by 419 million years ago, the very first wildfires began.
With this fiery entrance, the Devonian period arrived. It is said that, in terms of exploding diversity, the Devonian was to plants what the Cambrian was to animals. By 400 million years ago, this vast botanical experiment saw the evolution of the first ferns. They didn’t quite resemble the modern ferns we know and love. In fact, of approximately 10,600 species of ferns alive today, most appeared only in the past seventy million years. Their ancestors are long gone, yet their success as a family is irrefutable, and the secret lies in their adaptability. That said, the first ferns were still very much bound to the water’s edge because they followed a similar reproduction pattern to that of bryophytes, which relied on free water for fertilisation.
We also see lycopsids appearing on the fossil record during the Devonian – tall, vascular, woody trees that spent much of their life as a single massive stalk, some reaching circumferences of 1.8 metres and heights of 36 metres, giving them an appearance that has been likened to giant stalks of asparagus. They would, in time, produce branches at the very top and then reproduce with the aid of spore-bearing cones. Again, the same pattern: spore production, a gametophyte stage and then water-dependent fertilisation. And so all these strange and remarkable plants remained near water by necessity. Life had taken a spectacular step onto dry land but for millions of years had not been able to venture much further than that.
The timing of all this is still being refined. Molecular clock studies gauge the time required for genetic differences to gradually arise between different organisms. Recently, molecular clock analysis that involved scouring gene sequence data from more than 4000 living fern species suggested that the very first ferns – the first genetic glimmer of them, anyway – may have evolved as far back as 430 million years ago. The main point is that the Devonian was something of an extended heyday for ferns. They unfurled into a warm and humid world, where oxygen levels were fairly similar to what they are today, but the atmospheric CO2 was well over 3200 parts per million, more than seven times our current levels. Consequently, the average global temperature was about 4°C higher. This suited ferns just fine, and they spent millions of years proliferating and diversifying.
Around 385 million years ago – give or take a few million years – the first true trees appeared. Except they were not quite like the trees we have today. These were the Archaeopteris and there was something a bit chimeric about them, a beguiling in-betweenness. They had robust, woody trunks and branches covered in green foliage. From a distance they would have resembled conifers, with large branches lower down and tapering to a finer point at the top in that distinctive Christmas-tree shape. However, these branches were covered not with pine needles but loads of smaller branches, each resembling a fern frond. That wasn’t even the weirdest part. Archaeopteris belonged to a new group of plants called progymnosperms and, like ferns, they reproduced with the help of spores, but these spores were different in a very interesting way.
Up to this point, spore-producing plants were homosporous, meaning the spores produced by any one plant were all the same. Those spores would be released, land somewhere, then germinate to produce a gametophyte. It was the gametophyte’s job to produce egg cells, sperm cells or both. But progymnosperms dispensed with the need for the intermediate generation to germinate, grow into a plant and make sex cells. Instead, they produced two kinds of spores: a large one called a megaspore and a small one called a microspore. The cells in the megaspore matured to produce a female sex cell, also known as a megagametophyte, while the cells in the microspore matured to produce sperm cells. It’s not that the intermediate generation suddenly vanished – it still exists, but it all takes place much more efficiently and securely inside those two different spores. This was a critical shift that set plants on the path to seeds.
Of course, to make a whole new plant, those two types of spores still had to proverbially hook up. For a while, it seems, progymnosperms simply left this to chance, releasing plenty of megaspores and microspores in the hope some might land next to one another. Then came another fundamental change. Plants began to hold on to the female megaspore, keeping it in a cupule as it matured and became ready for fertilisation. The cupule afforded protection and also facilitated the capture of male microspores carried by the wind. Well played, really.
In 1968, palaeobotanists announced the discovery of a fossil plant, Runcaria heinzelinii, which they’d found in some 385-million-yearold sediment in Belgium. At the tip of a tiny stem was a strange little structure, a cupule with a petite stalk protruding from its centre, which was surrounded by an astonishingly beautiful swirl of fine tendril-like forms that partially enclosed it like a twisted birdcage. A new analysis of the fossil in 2004 revealed that this elaborate structure would have been capable of catching wind-blown spores, bringing them into contact with the female reproductive cell inside. Runcaria, it was determined, was the precursor to Elkinsia and a world full of seed ferns – and what a different world that was. Seed ferns did not need free water for fertilisation, not a stream nor even a puddle. They could spread across dry land, the seeds landing, germinating, and roots taking hold away from the water’s edge. They spent millions of years doing so, greening huge swathes of the planet that had once been barren. These seed ferns were a critical step towards the evolution of the two groups of seed plants that exist today: the gymnosperms and the angiosperms.
When gymnosperms came along they took things to a new level, encasing the female megaspore inside a structure called an ovule, which is surrounded by a covering layer called an integument. You see, that fine birdcage-like structure that Runcaria possessed never really went away. Instead, over millions and millions of years, it became more robust, a full cover – it became the integument. When fertilised, the ovule becomes a seed and, in the process, the integument becomes the seed coat.
It’s not entirely clear when the first gymnosperms evolved, but a variety called Cordaites began to appear in the fossil record around 318 million years ago. They were tall, gangly, narrow-trunked things with seeds protected in cone-like structures. It wasn’t too long before other gymnosperms evolved: conifers, ginkos, cycads and gnetophytes. For the better part of 200 million years, gymnosperms were the most prolific flora on land, and their presence invited more animals onto land. The gymnosperms also spent this time absorbing massive amounts of CO2 and converting it into carbohydrate building blocks, while releasing so much oxygen that atmospheric levels reached as much as 35 per cent by the early Permian, which is 14 per cent higher than current levels. It is hotly debated whether or not this oxygen spike fundamentally influenced the physiology of land animals to the extent that they became absolutely massive, the reasoning being that more oxygen allowed for larger bodies. This might have been the case, or perhaps it was a coincidence, but the fact of the matter is that there were a lot of very big animals wandering about during the reign of gymnosperms. Even the insects were enormous, including dragonflies with 70-centimetre wingspans, and 2-metre-long millipedes. It was a strange and wondrous world, full of lush greenery and terrifying fauna, but there wasn’t a single flower to be seen.
For a long time, it was thought that flowering plants, or angiosperms, first evolved about 125 million years ago during the Cretaceous. Reasonably enough, this was based on the complete absence of flowers in the fossil record up until that time. The problem was that the fossil record went something like this: no flowers, no flowers, no flowers, then – boom! – heaps of flowers. It seemed like the Cretaceous had been a huge evolutionary flower parade that no one had known was coming. Where did they all come from and how did it happen so quickly? It certainly bothered Charles Darwin no end. When he looked at fossil after fossil, he was sure he was seeing evidence of a rapid diversification of flowering plants, but he couldn’t for the life of him figure out how it had started. He famously called the sudden rise of angiosperms in the Cretaceous ‘an abominable mystery’ and wondered if an earlier origin was more likely. Today, many questions remain but, little by little, new evidence is suggesting that flowering plants may indeed be a lot older than the Cretaceous.
In 2018, the discovery of what appeared to be a tiny fossil flower in China indicated that angiosperms were at least fifty million years older than previously had been thought. Then, in 2021, another study combined fossil analyses with molecular clock estimates, and its findings pushed back the start date for angiosperms by yet another fifty million years. As computational palaeobiologist Daniele Silvestro and his colleagues explained in their paper in Nature Ecology and Evolution, it appears that multiple families of angiosperms existed during the Jurassic, so that by the time the Cretaceous rolled around they were poised to figuratively explode. The findings, the researchers said, showed that Darwin may have been right about an earlier origin of flowering plants, and that this would have provided them with the head start they needed to undergo rapid diversification during the Cretaceous.
Yet, abominable mysteries continue to abound. Gymnosperms and angiosperms are both seed plants, and it’s clear they’re both related – that they didn’t each spontaneously develop seeds independently. There are far too many commonalities, says Carol Baskin. There are parallels in minute structures and even metabolic behaviours within the seed cells. ‘I think the evidence is pretty clear they would have had a common ancestor,’ she tells me, but concedes no one knows precisely what it was or when it existed. Was it a seed fern or one of the first gymnosperms? In 2021, an international team of scientists from China and the United States published new fossil evidence that compellingly suggests it was an early but long-since-extinct gymnosperm.
There are some significant differences between gymnosperms and angiosperms, one of which relates to the covering layer around the ovule. It doesn’t sound like such a big deal, but it is. Gymnosperm ovules only have one integument. This leaves the ovules kind of vulnerable, and even when the seed forms and this becomes the seed coat, it’s not tough, just a thin layer of living cells. To work around this vulnerability, gymnosperms encased their ovules in protective structures – you might better know them as cones. Conifers, for example, have small, pollen-producing ‘male’ cones as well as larger ‘female’ cones, which contain many of these naked ovules. The female cones tend to grow on the higher branches where they are better poised to catch pollen on the wind.
But none of this happens in angiosperms because angiosperm ovules aren’t naked. They have a second integument wrapped around the first one, and all of that resides within a bigger structure – an ovary. Moreover, when angiosperm ovules are fertilised and develop into seeds, the second integument becomes a second, often tougher seed coat. And the cells inside angiosperm seeds can do something else that gymnosperms cannot do: they’re able to make endosperm, aka the lunch that’s packed inside the box with the baby plant. This nutritive tissue sustains the plant embryo through its early days of germination. As angiosperms evolved, they also developed additional elaborate structures. In time, some of those structures diversified to attract pollinators with scents, pheromones, colours and a variety of elaborate visual enticements. We know them as flowers.
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Today, there are at least 325,000 living species of angiosperms, which account for more than 90 per cent of all plants. They can be found in most climates and ecosystems on land. Meanwhile, only around 1000 species of gymnosperms still exist, their range mainly limited to cooler climates and higher elevations. They are still vitally important, though. In the climates to which they are now well suited, gymnosperms are the dominant forest tree species, especially in the boreal forests found in the frigid high latitudes of the northern hemisphere. Those forests cover approximately 11 per cent of Earth’s surface and are home to around 24 per cent of the world’s trees, most of them gymnosperms. Nevertheless, gymnosperms’ heyday has been over for quite some time, and the rise of angiosperms had a big hand in that.
I ask Carol Baskin why angiosperms have been so astonishingly successful. ‘We don’t really know,’ she tells me. ‘But my guess is …’ She stops herself and laughs, then searches for a slightly more formal term. ‘My opinion’, she says with her gentle Southern accent and a smile, ‘is that the seed production in angiosperms is much faster.’ Just look at the fertilisation process, she says, explaining: ‘It’s the difference between a mule walking along and a supersonic jet blasting through the air. I mean, it may take a whole year for a gymnosperm seed to be formed. But in angiosperms, it can be weeks or just a few months.’ Indeed, in cycads, which are gymnosperms, it can take six months just from the moment of pollination to the moment of fertilisation.
Gymnosperms just couldn’t keep pace with angiosperms, which outcompeted them by adapting much more quickly to new habitats and changing climates. For example, fossil evidence shows a direct correlation between a rise in angiosperms and increased extinction rates of conifers during the Cretaceous. Moreover, this pattern was sustained throughout the subsequent Cenozoic era. Yes, there was a massive asteroid strike at the end of the Cretaceous which generally mucked up life for everything, from Tyrannosaurus rexes to conifers, but close inspection reveals that long-term conifer extinctions can be blamed far more on flowering plants than on wayward asteroids.
It’s important to realise the first flowers were really small, says Andrew Rozefelds, who is Head of Geosciences and Principal Curator of Palaeobotany at the Queensland Museum. Some of the earliest flowers were tiny. Flowers reaching 1 millimetre were ‘huge’, he says. The seeds, as you might imagine, were smaller still.
We are sitting in Rozefelds’s office, surrounded by plant fossils, each a moment in time captured millions of years ago. It’s a strange juxtaposition, really, because we aren’t anywhere near the main museum in the centre of Brisbane, but in a room attached to a warehouse on a back road in the city’s north – kind of a netherworld between industrial storage depots, former workers’ cottages that now sell at eye-watering prices, and other quotidian trappings from this end of the Holocene. Set among all this, he Queensland Museum Collection and Research Centre is so unassuming from the outside that, even with satnav, I nearly drove past it twice. No passer-by would ever peg it as a mind-blowing gateway to the distant past. But it is.
Rozefelds hands me a tiny fossil and says, ‘Let your imagination run wild here.’ It’s small enough to fit easily in the palm of my hand and, at first, it almost looks like a tiny brain sculpted from rock. But I look closer and I can see individual sections, all pressed together in an irregular way. It’s an angiosperm fruit, he tells me, and it’s more than thirty million years old. With the exception of a few fleshy fruited conifers, the overwhelming majority of fruiting plants are angiosperms, where the fruit is simply a mature angiosperm ovary. As the fertilised ovule matures into a seed, the surrounding ovary also changes, often becoming fleshy and sweet. Many flowering plants produce fruits, though we don’t necessarily recognise them because the ovary doesn’t always become plump but might remain as a thin casing around the seed. Such is the case with rice and other grains, which are technically fruits.
The fossilised fruit I’m holding is actually many fruits. Each of those sections is a seed encased in a fleshy ovary and they are all fused together into one unit. It’s an ancestor of modern pandanus species, says Rozefelds. One of the challenges in palaeobotany is that the fossil record for fruits skews towards indehiscent species, Rozefelds tells me. These are the plants that make tough fruits that don’t split open when they ripen. They fossilised a lot better than anything with exposed, delicate tissues did. And even then you need to get the right conditions for fossilisation, which tends to happen when an organism is buried rapidly and cut off from oxygen.
That said, sometimes you get lucky, and maybe the seed grew in a place where there was a lot of volcanic rock around. Such rocks release silica as they begin to weather, and if that silica gets into a seed, it begins to infill. When this happens, says Rozefelds, ‘you can get an almost atom-by-atom replacement of the structure.’ He tells me that in some places where silicas are abundant, like near hot springs, this process can take place in a matter of hours.
Rozefelds picks up another fossil. This is much bigger than the first one and, to be honest, it really does look like a hardened slice of mouldy lasagne. It is nothing of the sort, of course. It’s a cross-cut of fine sedimentary layers, and right there, trapped within a few of them, is a tiny fruit with five bulbous points – it looks like a small star made of quartz. He shows me another fossil, this one cut free of whatever sediments it had been encased in and then sliced in half. It was once a woody fruit, says Rozefelds. Like the other, it is also a five-pointed star, but these points are sharper and I can see the dark outlines of where three seeds had been – in their place are curved indentations that sparkle with silica crystals.
There is another fossil, an early member of the spurge family. This one is not silicified but has a woody casing now tough as stone. It is half split open, revealing the empty space where its seed would have nestled millions and millions of years ago. With a quick glance in another context, it would easily pass for a nut that had been cracked open just yesterday. Yet, it reveals a profoundly old story. It tells us that, eons ago, this seed was able to transport genes forwards in time.