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.
YO IT'S FINALLY OUT! Thank you so much for posting this!
This is a neat documentary series, great balance of Earth convergence and alien divergence.
Me no...
Yes it goes but be small and I cool
Yanggang!