Alien Biospheres: Part 4 – The Invasion of Land

Last time, our two ancestral bodyplans diversified into different clades to fill the niches of the ancient oceans But now, 100 million years after the bodyplans first appeared, the stage is at last set for their descendants to emerge from the sea and take the first steps onto dry land So far, all life on our alien planet has been restricted to the water, and for good reason At this point in history, dry land represents an extreme habitat, and poses a huge challenge to potential colonists If an animal comes out onto land, there will obviously be no water to swim through, and so it will be unable to support itself, and so collapse and die Not only that, but outside of the ocean, any water in the animal’s body will quickly evaporate through its skin, and so the animal will dehydrate, desiccate and die Not to mention the fact that, as of yet, there’s no food outside the ocean, so any animal that comes onto the land will starve and die So, dry land seems like a pretty inhospitable place, and yet on earth, it’s been colonized innumerable times by a huge diversity of clades, the most obvious examples being the forerunners of tetrapods and terrestrial arthropods that came onto land hundreds of millions of years ago, but also including more recent examples like mudskippers, air-breathing catfish, and terrestrial sea slugs So, why have all of these different groups independently come onto land despite the numerous difficulties involved? The simple answer is a lack of competition The oceanic ecosystems are already thoroughly colonized, and so the marine species must compete furiously to obtain food and avoid predators But if a species comes onto land, especially at such an early stage in the history of multicellular life, they’ll be no other species to compete with and no predators to worry about, so selective pressures will pretty much inevitably drive organisms to come ashore However, going immediately from wholly aquatic to totally terrestrial would be a very drastic change in lifestyle, but fortunately, the transition doesn’t have to be so immediate The intertidal zone, sometimes called the littoral zone, forms a perfect medium between land and water Organisms living in this zone will become exposed as the tide ebbs, and so will benefit from adaptations that help them survive until the tide returns Over many generations, individuals that are able to survive out of the water for longer will enjoy the less competition and will thus be selected for, until they’re ultimately able to live entirely on land Luckily for our alien lifeforms, their planet’s large moon means the high tide will be almost three times higher than earth’s, creating a large intertidal zone that will allow for a very gradual transition to terrestrial life However, there’s still no reason to go further than the high-water mark if there’s no food outside the sea While this will halt the progress of the animals, it won’t be a problem for the autotrophs, who can produce their own food, so they’ll be the first organisms to make the jump to a permanently land-dwelling existence On earth, terrestrial autotrophs consist almost exclusively of phototrophs, most notably plants, but on our alien planet, we also have chemotrophs that gain energy from oxidizing hydrogen sulfide Up until now, we’ve assumed that these autotrophs have remained relatively simple and unspecialized, mostly existing as plankton or other basal organisms But over the past few hundred million years, as the animals have been diversifying, the autotrophs will have been evolving as well Perhaps one kingdom of chemotrophs evolves to cluster together to form colonies and ultimately become full-fledged multicellular lifeforms Since hydrogen sulfide dissolves in water, these organisms can only carry out chemosynthesis when in contact with the atmosphere, so they may form large rafts or mats that float on the ocean’s surface If some of these rafts specialize to live in the intertidal zone, they’ll be out of reach of many pelagic predators such as the large filter-feeding acanthopods At low tide, they’ll be exposed to the atmosphere, allowing them to take in hydrogen sulfide, and at high tide, they’ll take in water to stave off dehydration As they evolve to become increasingly terrestrial, their outer membranes will thicken to better retain water However, making the membrane waterproof will also make it harder for hydrogen sulfide to pass through it, so these organisms may evolve specialized pores, or stomata that can be opened to take in hydrogen sulfide, and closed to prevent water loss These stomata may connect to a layer of vascular tissue that transports the hydrogen sulfide to every cell in the organism Also, recall that these organisms produce elemental sulfur as a byproduct of their chemosynthesis, so these stomata may secrete excess sulfur to prevent it from building up in their tissues This means a coating of sulfur will gradually accumulate on their cuticles, which will get

washed away as the tide comes in When it comes to reproduction, these chemotrophs will need to tackle the usual problems faced by sessile organisms Their colonial ancestors may have reproduced asexually, perhaps by budding or fracturing If the terrestrial forms inherit this mode of reproduction, they’ll be able to colonize unsettled areas of the shorelines very quickly But at higher population densities, they may be incentivized to switch to sexual reproduction to increase genetic diversity and give them an edge over the competition In such a case, they may grow gametangia, specialized organs that produce gametes, which are then released into the air in huge quantities, very similar to the broadcast spawning used by sessile marine creatures like the anthostomes However, in the water, gametes can drift among the currents more or less indefinitely, and so will have a considerable span of time to encounter other gametes, but on land, without the density of the water to keep them buoyant, the gametes won’t remain airborne for long, and once they reach the ground they’ll die very quickly One way of alleviating this problem is to have fertilization take place within the gametangium itself; the male gametes are released immediately, while the female gametes are withheld until a male gamete lands on the gametangium The two gametes then merge to form a spore, which is then dispersed and can immediately begin growing into a new organism once it lands These will be some of the first terrestrial lifeforms, forming sulfur-coated carpets of lichen-like crust growing over the rocks and sand But as they begin to populate the coastlines, they’ll pave the way for other colonists Like the chemotrophs, the phototrophs will need access to the atmosphere to take in carbon dioxide, which dissolves in water, and so they’ll mostly live near the water’s surface These organisms, though, gain energy from chemical reactions that occur between sunlight and photosynthetic pigments within their cells Among phototrophs on earth, the dominant plant kingdom and the related clades of green algae use Chlorophyll for photosynthesis, giving them their characteristic green color, while other clades of algae make use of a multitude of other pigments, each one gaining energy from a different portion of the spectrum of light produced by the sun Our alien planet’s star is a class GV star just like earth’s sun, so it will produce a similar spectrum, meaning the various kingdoms of phototrophs will show a similar variety of pigments to those on earth Let’s say there’s a class of algae on this planet that uses a red pigment for photosynthesis, and that one clade of these algae specializes for life in the intertidal zone Once again, any individuals that are left exposed at low tide are at risk of desiccating, but instead of evolving their own mechanisms to resist dehydration, they may simply take advantage of the abundant colonies of chemotrophs that already cover the shore By incorporating themselves inside the membranes of the chemotrophs, the algae can rely on their hosts’ toughened cuticles for protection from desiccation This will also provide them with extra nutrients, as they can take up the sulfur and water given off as waste products by the chemotroph Since they now rely on the chemotroph for nutrients and shelter, it’s in the algae’s best interests to make sure their host stays alive and healthy, so they may evolve to provide the chemotroph with a portion of the glucose they produce from photosynthesis, which is more chemically efficient than the formaldehyde created from chemosynthesis This sort of symbiosis between phototrophs and other organisms happens often in evolution; corals rely on photosynthetic zooxanthellae to provide them with sugars in return for minerals and carbon dioxide, lichens consist of a photosynthetic alga that shares the sugar it produces with the fungus that shelters it, and about 80% of all plant species have fungal or bacterial symbiotes in their root systems to aid in nitrogen fixation and acquiring minerals from the soil However, this partnership poses a difficulty when it comes to reproduction; whenever the chemophyte reproduces, it will need to ensure some of its algal symbiotes are transferred to its offspring, so when it grows a female gametangium, some algae may migrate inside and attach themselves to the gametes When a male gamete lands within the gametangium and fertilizes the female gamete, the algae will be released with the spore as part of a pre-packaged dispersal unit, or diaspore, and once the diaspore lands and the new plant begins growing, the algae will multiply asexually to fill the new chemotroph’s tissues I’m going to dub these symbiotic organisms “chemophytes”, although for simplicity I’ll be referring to them informally as “plants” from now on, even though they’re technically not plants in the strictest sense The carbon dioxide these plants need for photosynthesis will be supplied by the respiration of animals and aerobic microorganisms, while the hydrogen sulfide for chemosynthesis is given off by organic decomposition and by sulfur-reducing microbes As these plants proliferate, they’ll cause the atmospheric proportions of these gases

to steadily decline and oxygen levels to rise Once they become fully terrestrial, they may increase their structural complexity To take up as much sunlight, carbon dioxide, and hydrogen sulfide as possible, they’ll benefit from a large surface area, which once again, in accordance with the square-cube law, will favor a thin, flat shape, which is part of the reason why the leaves of plants on earth are shaped the way they are However, large leaves will also have high air resistance, increasing their likelihood of being be damaged by the wind This can be mitigated by having spaces within or between leaves for the wind to pass through, which may take the form of a branching structure For maximum exposure to sunlight, they may evolve stems or stalks that grow upward towards the sun The tallest plants will have the best access to light, so an arms race will ensue, with plants growing taller and taller, eventually evolving into forms that might be called trees Since there are no terrestrial herbivores as of yet, these trees may spread uninhibited across the landscape, forming vast forests of uninterrupted greenery, or in this case… redery, I suppose? And as the plants establish themselves on land, they’ll present a new food source that may coax the animals into taking their first steps out of the water The sessile anthostomes rely on the ocean currents to bring food to them, meaning they can’t feed above the water line, so the land won’t have very much to offer them, but the motile tentaclostomes stand to gain much more from making landfall Perhaps some tentaclostome species come into the intertidal zone to spawn, as the shallows and tide pools form an ideal shelter for their developing larvae These coastal species may evolve to supplement their diet by feeding on the chemophytes along the shore and eventually be enticed out of the intertidal zone by the abundance of food offered by the land plants Initially, they’ll need to constantly return to the water to prevent desiccation In particular, their gills are especially vulnerable to drying out While in the water a large surface area allowed increased oxygen uptake, on land it means a greater area for water to evaporate through, so having a large breathing surface is now a detriment Recall, though that the tentaclostomes capable of retracting their limbs, so the gills may be withdrawn into the body when not in use, minimizing their exposure to the air and preventing water loss And as the tentaclostomes become increasingly terrestrial, perhaps their gills evolve to become permanently retracted into pockets within body, somewhat like a pair of lungs Air passes into these lungs through spiracles or pneumostomes on the creature’s sides, and the muscles at the base of the lungs may become a pair of hearts to pump the oxygenated blood around the body Since this passive form of breathing relies on a constant flow of oxygen-rich air into the lungs, the energy they get from respiration will be directly proportional to the levels of atmospheric oxygen The less oxygen available, the less energy they’ll have to maintain their growth, and so will have an upper limit on the size they can reach before their volume becomes too large to sustain This is one reason why terrestrial arthropods on earth don’t get larger than a few kilograms at the most, as they rely on a similar mechanism of passive respiration Luckily though, the proliferation of the land plants will steadily oxygenate the atmosphere, allowing these tentaclostomes to attain fairly large sizes However, they’re maximum size will still be limited by their lack of internal support structures To help them locomote on land, their fins may lose their hydrodynamic specializations and evolve into simple feet made of pure muscle This is very inefficient and energetically demanding, as the muscles will need to be tensed continuously to keep the body supported Beyond a certain size, this becomes impractical, posing another limit on their potential size, although recall this planet’s gravity is 20% lower than earth’s, so this constraint won’t be quite as severe as it would be for animals on earth The only structural support the tentaclostomes have is their shell The shells of marine anthostomes are made of calcium sulphate, which they form by taking in minerals dissolved in the seawater However, there’s obviously no dissolved minerals in air, so forming a calcium sulphate shell on land may be infeasible On earth, the various groups of arthropods have independently invaded land numerous times, and in almost every instance have lost the mineral component of their shells, forsaking the ancestral calcium carbonate shell for one primarily formed of chitin Our terrestrial tentaclostomes may do the same, turning their mineralized shell into a lighter, more flexible substance Since the shell is impermeable to water, it helps prevent desiccation, which may also be averted by evolving thick water-proof skin, allowing them to venture out of the water

indefinitely However they’ll still be tied to the water for reproduction Just as with the chemophytes, the tentaclostomes’ gametes and larvae are prone to desiccation on land, so the tentaclostomes will need to return to the water to spawn, and their larvae will need to remain in the water until they develop the water-proof skin and air-breathing capabilities of the adults To become totally independent of the water and colonize dryer habitats, the terrestrial tentaclostomes will need to evolve internal fertilization With this reproduction innovation, instead of the male and female expelling gametes into the water and having the larvae develop on their own, the male will transfer his gametes into the female’s body through what’s called a “cloacal kiss” The female will then contain the fertilized eggs within an oviduct, where they’ll develop a waterproof outer casing that allows them to be laid on land without desiccating This will also dramatically increase reproductive efficiency, as while within the oviduct, the eggs will be protected from predation and adverse conditions until they reach a suitable stage of development Note that this strategy closely mirrors the reproductive adaptations of the terrestrial chemophytes, which similarly carry out fertilization within the relative safety of the female gametangium No longer reliant on the water to reproduce, these tentaclostomes may now push in land, undergoing further specializations for their new terrestrial habitats along the way To cope with a varied diet, the setae along their feeding arms may evolve into rasping surfaces, like the radulae of gastropods, while their stomach may develop a branching structure to increase surface area, as well as a number of caeca, or pouches to store digested food, which when full will then evert back into the stomach so waste can be regurgitated These tentaclostomes will be the first true land animals and are set to spread across the continent to occupy the abundant niches of the new terrestrial ecosystems I’ll call these creatures lophostomes But though they may have won the race to land, they’ll be shortly followed by other colonists Among the polypods, the planktonic tachypods rely on the ocean currents for movement, so they may not fare well out of water, and the acanthopods are very specialized for aquatic life, but the sarcopods may be adaptable enough to make the transition to a terrestrial existence By this point, the chemophytes may grow into large coastal swamps around the mouths of rivers and estuaries, where the sarcopods might come to spawn or scavenge As they evolve to inhabit shallower waters, their eyes might move towards their dorsal surface to allow them to see above the waterline, a common condition found in many semi-aquatic animals Vertebrates on earth have only two eyes, which provide a fairly narrow range of vision regardless of how they’re positioned This may have contributed to the evolution of the neck, which allows the head and the eyes attached to it to be aimed in different directions to take in visual information over a wider range Having a neck also allows for the mouth to be brought closer to food without moving the entire body, which conserves energy and makes feeding easier The sarcopods, on the other hand, have six eyes that grant a much greater field of vision without needing to be repositioned, and they have anterior limbs that can be used to reach food and pass it into the mouth, so evolving a neck may not be necessary Instead of having a head distinct from the body, the sarcopods will have a somewhat head-like anterior tagma, or cephalothorax, and a posterior tagma, or abdomen In these tidal swamps, coming onto land will present more opportunities for scavenging and predation, so some species of sarcopods may adopt an amphibious lifestyle To stay out of water for longer, the sarcopods will need to solve the same problems as the lophostomes, and so may converge on similar solutions, developing a tough waterproof integument and adapting their respiratory system for air-breathing Just like with the lophostomes, the sarcopods’ large ventral gill surface will lose water very quickly when exposed to air, and so is likely to be internalized to prevent dehydration, forming a lung-like chamber However, unlike the lophostomes, the sarcopods may evolve a mechanism for actively pumping air into and out of this lung to increase respiratory efficiency The aquatic sarcopods use the constant beating of their 5th pair of limbs, or pleopods to continuously pass water over the gills for greater oxygen uptake, but in the terrestrial sarcopods, these limbs may also become internalized, where they may atrophy into a set of muscles that allow them to actively inhale and exhale, somewhat analogous to the diaphragm of earth’s vertebrates This mechanism is much more efficient than simply letting the oxygen diffuse into the lung as the lophostomes do, and so provides the body with extra energy, meaning the size constraint posed by the atmospheric oxygen levels won’t be anywhere near as limiting For full terrestrialization, they may also evolve internal fertilization, with the gonopods

assisting in the transfer of gametes from the male to the female But instead of laying batches of eggs that are each individually protected by an outer shell, perhaps the female sarcopod protects the fertilized eggs from desiccation by containing them within a leathery sac formed from the lining of her oviduct This egg sac, or ootheca, may contain dozens of developing young, and will serve as their first meal upon hatching With these developments, the sarcopods are now able to leave the water behind for good and specialize for life on land The sarcopods are, in a sense, already pre-adapted to terrestrial locomotion thanks to their hydrostatic skeleton, which lends their bodies a degree of support However, beyond a certain size, hydrostatic pressure alone won’t be enough to keep them from collapsing, and the legs will buckle under the increased weight One way of dealing with this is to stiffen the limb by evolving permanently inflexible components, so that the limb won’t deform as easily and therefore won’t need as much muscle power to maintain rigidity This may begin as filaments of stiffened muscle fibers within the walking legs, which will include the limbs evolved from the 4th, 6th, 7th, and 8th segments These fibers may become reinforced with layers of a hardened polymer, becoming a structure similar to the bones of earth’s vertebrates, although evolved along very different lines The legs will have several joints to allow them to articulate, while longitudinal muscles around the margins of the haemocoel may ossify to form a limb girdle, to which the leg muscles will be anchored Once the capability to produce this bone-like polymer has evolved, it may get co-opted for other purposes, namely protecting the internal organs Arguably the most important part of the body to protect is the brain and sensory organs, so it’s likely a brain case or skull plate may evolve, along with other deposits of bone around the cephalothorax, as well as along the lateral nerve cords that serve as the equivalent of the spinal cord This skeleton, as well as their efficient respiratory systems, will allow these sarcopods to grow to considerable sizes By the time the sarcopods come onto land, the lophostomes will have already occupied most of the available niches, but the sarcopods’ size advantage will give them access to megafaunal niches that the lophostomes are excluded from To further specialize for attaining large sizes, they’ll need to evolve a more efficient cardiovascular system, as extra weight puts more pressure on circulation The haemocoel used to function as both the hydrostatic skeleton and the circulatory system, but now that the role of structural support is taken up by a true endoskeleton, the haemocoel can evolve to specialize into a closed circulatory system, in which blood is moved continuously through a series of vessels to ensure a constant flow of oxygen to every cell in the body The ancestral polypod pumped blood through the haemocoel by contracting rings of muscle within each of its segments As these terrestrial sarcopods grow larger, the contractive muscles within the abdomen may fuse into a single dedicated heart This new circulatory system will also need to have a filtering mechanism to prevent the buildup of metabolic waste products and toxins in the blood Each of the segments of the ancestral polypod may have had a pair of nephridia, or glands involved in blood filtration, which may survive in these terrestrial forms as a series of kidney-like organs within the abdomen Their digestive system may undergo some changes as well The simple gastrointestinal tract of the ancestral polypod may develop a foregut, or gizzard, within the cephalothorax, lined with rasping teeth or plates to grind down swallowed food, and a second, larger stomach may occur within the abdomen, along with organs that supply enzymes to aid in digestion The anterior limbs may specialize to aid in feeding on their new, much more varied diet The first two pairs of limbs, once used for sifting through sand for detritus may now become mandibles to process food and pass it into the mouth, with the setae on these limbs becoming tooth-like cutting surfaces The third pair of limbs may become pedipalps, which may have a role in both handling food items as well as sensing the environment The tips may contain chemical receptors to smell and taste food before it’s ingested, and the setae on these limbs, used in marine sarcopods to sense vibrations, may specialize into a hearing mechanism Speaking of senses, since air is constantly being drawn into the lungs, a ring of olfactory receptors around the spiracles will pick up any scents carried on the breeze, thus giving their sense of smell a much greater range These sarcopods are now adapted to a fully terrestrial life and are poised to exploit the as-of-yet vacant megafaunal niches I’ll call this clade the osteopods Together with the chemophytes and lophostomes, they’ll serve as the blueprint for innumerable land-dwelling lineages to come It will have taken about 100 million years to gain a foothold on land, but now they’re

fully equipped to populate the nascent terrestrial ecosystems In the next episode, we’ll see how these pioneering land dwellers undergo an explosion of diversity into a myriad of new clades