In the last episode we covered the first 4 billion years of our alien planet’s history,
including the evolution of complex multicellular life and the resultant explosion of biodiversity
from which emerged two body plans.
As of yet, these bodyplans only serve as theoretical blueprints, but with the passing of millions
of years, the various descendants of these ancestral forms may undergo heavy modification
as they diversify into separate lineages or “clades”.
A clade is a group of species that share a common ancestor, represented by a single branch
of the tree of life along with any and all branches that stem from it.
For example, snakes form a clade of reptiles, but also form part a larger clade along with
lizards, a group known as the squamates, and in turn, squamates can be grouped together
with tuataras under the even larger clade lepidosauria.
However, lepidosaurs and turtles do not form a single clade because not all the descendants
of the last common ancestor are included.
This visual representation of the relatedness of different clades is called a cladogram
or phylogenetic tree.
The next step in creating our alien biosphere is to construct some cladograms for our two
bodyplans, and to do that, we need to have them undergo diversification into different
clades.
This diversification is likely to be driven by adaptation to different modes of life,
or more specifically, different niches.
An ecological niche is essentially the role a given species plays in its ecosystem.
The precise definition is multifaceted and may take into account the species’ habitat,
feeding, reproduction, and its relationship to other species.
A clade will often specialize for a group of related niches, such as how the big cats
have occupied the niches of large predators.
However, a single niche cannot be occupied by more than one species within any given
ecosystem, or else the two species will be in direct competition, which would ultimately
lead to one of them going extinct.
This will be important to bear in mind later on, but for now, we first need to identify
the niches present in our ancient sea and see how our bodyplans can evolve to exploit
them.
To start with, the vast majority of energy input in almost all ecosystems will come from
sunlight, which in our ancient oceans will power the metabolisms of billions of photosynthetic
microbes.
Specifically on this alien planet, though, there is another source of energy in the form
of atmospheric hydrogen sulfide, which, although it’s been gradually depleted over the past
few billion years, still provides energy for rafts of chemosynthetic microbes that float
on the water’s surface.
Together, these autotrophs will form huge clouds of plankton in the surface waters,
serving as a virtually inexhaustible source of food, and thus the basis of the foodchain
for the majority of oceanic ecosystems.
The term ‘plankton’ refers to any organism incapable of swimming against the current,
including these single-celled autotrophs, or ‘phytoplankton’, as well as a host
of tiny animals, or zooplankton, that feed on them.
One group of animals that’s very likely to form a large component of the zooplankton
are the anthostome larvae, whom we’ve already said live near the surface before descending
to the sea bed to complete their metamorphosis.
These larvae will likely feed in a similar way to the adults, using their feeding arms
to catch food particles.
While they are capable of controlling their position in the water to a small degree by
waving their feeding arms, the larvae aren’t able to move independently of the currents,
and so still get classed as plankton.
The polypods may have some representatives among the zooplankton as well.
One major advantage the polypods have when it comes to adapting their body structure
is their segmentation, as individual segments can be specialized to perform specific functions,
allowing for increased body complexity.
This can be seen in many clades of arthropods, annelids, and even vertebrates, which, although
missing any external signs of segmentation, show a pattern during embryonic development
of tissues arranged in repeating units which then differentiate to form various aspects
of the body structure in much the same way as other segmented bodyplans.
One way this specialization of segments may manifest is through tagmosis, the fusion of
multiple segments into a single morphological unit or tagma.
On earth, many groups of arthropod have independently undergone tagmosis to organize their body
into different regions for a specialized function.
One very likely development is for some of the anterior segments to unite into a tagma
dedicated to feeding.
If we say our polypods undergo tagmosis of the first two segments, then the limbs on
these segments could specialize into appendages for handling food.
Further tagmosis may fuse the third, fourth, and fifth segments together into a tagma dedicated
to propulsion.
The limbs may broaden and develop elongated bristles, or setae, to enlarge their surface
area.
Spasmodically beating these limbs provides short bursts of jerky movement, reminiscent
of the antenna-driven swimming of daphnia or cyclops.
The segments posterior to the locomotive tagma, no longer being needed for propulsion, may
evolve to form an enlarged gill surface and house the internal organs.
In particular, their reproductive systems may adapt to increase fecundity, the number
of offspring an individual is capable of producing.
Plankton tend to have high reproductive rates, as their only real defense against predators
is sheer numbers.
These tiny polypods may live much like copepods on earth, comprising thousands of species
and forming a large proportion of the biomass in the ancient seas.
I’ll call this clade of polypods tachypods.
The tachypods and the anthostome larvae may serve as prey for larger zooplankton.
While most anthostomes are filterfeeders, there may be room to expand into other methods
of feeding.
Perhaps rather than passively straining the water for plankton, they can adapt their feeding
arms into tentacles for catching larger prey.
Then, to increase their feeding surface, they may evolve a body cavity, like a stomach,
which will make their feeding much more efficient.
I’ll call this innovative new clade of anthostomes Gastrozoans.
The larvae of these gastrozoans will develop a stomach as they mature, allowing them to
feed on zooplankton such as the tachypods.
While still classified as plankton due to their inability to swim against the current,
the gastrozoan larvae may grow quite large before their metamorphosis, possibly living
in a similar way to jellyfish, drifting through the water and ensnaring passing prey with
their tentacles.
All of these various planktonic clades form an abundant food source for larger, nektonic
clades.
Unlike plankton at the mercy of the ocean currents, nekton are species capable of independent
movement thanks to adaptations that enable efficient swimming.
The ancestral polypod accomplished this with back-and-forth strokes of the many limbs that
lined its body, but this is by no means the only option for effective aquatic locomotion.
By undulating from side to side, the whole length of the body can be used to exert force
on the surrounding water.
This is called anguilliform swimming, and is used by many clades of fish, as well as
aquatic reptiles and polychaete worms.
If a clade of polypod evolved this form of swimming, it would grant them much greater
speed than the limb-driven swimming of their ancestors, allowing them to diversify into
the niches of large active predators.
This clade may be descended from the same stock as the tachypods, and so inherit the
same feeding tagma consisting of the first two segments.
In these creatures, the feeding arms may become grasping tentacles lined with setae, which
may be adapted into sharp spines for catching and impaling large prey, or into long baleen-like
bristles to filter plankton.
Since thrust is provided by the lateral motion of the body, the limbs are no longer needed
for propulsion, and so the anterior limbs may now serve as stabilizing fins to support
the body and maintain depth while the posterior limbs may become completely vestigial and
thus shrink to reduce drag.
And as this clade further specializes for fast swimming, the final segment may evolve
a fin to increase the volume of water displaced by the tail’s movement and therefore provide
greater leverage against the water, allowing for the evolution of more specialized forms
of swimming, such as carangiform or thunniform swimming, in which the central mass of the
body is kept mostly stationary while the propulsive force is provided solely by movement of the
finned tail.
With this design, this clade is well equipped for predatory niches, including shark-like
pursuit hunters and whale-like filter feeders.
Let’s call these creatures acanthopods.
Meanwhile, Anthostomes aren’t anywhere near as mobile as polypods, even in their larval
form, so they may be excluded from such predatory niches in open-ocean or pelagic environments,
but they may have more luck in benthic environments on the seabed.
Here, nutrients filter down from the surface waters in what’s called “marine snow”,
which constitutes the main food source in the lower layers of water column.
The majority of marine snow will be taken up by benthic filter-feeders like the adult
anthostomes, while the remainder collects on the ocean floor, forming a layer of detritus
that serves as food for other benthic creatures, a niche that may be occupied by another clade
of polypods.
Lets say this clade splits off from the main line before the tagmosis of the first two
segments, and thus represents a sister clade to all other polypods we’ve discussed so
far.
Tagmosis will be a useful innovation in this clade as well, perhaps this time involving
the fusion of the first four segments into a feeding tagma.
The first, second, and third pairs of limbs may form a frond of tentacles for filtering
and handling food, while the fourth pair may be designed for burrowing or digging through
the sediment for organic detritus.
These limbs may bear setae like the other polypodian clades, which may help them sift
through the sand, but may also be used as a means of sensing vibrations in the water,
giving them a way of detecting incoming predators.
Since they’re specialized for benthic life, they won’t be able to swim away very effectively,
so they’ll need some other form of defense.
Some species may simply rely on burrowing to keep them safe, while others might adapt
some of their setae into an array of spines to repel attackers.
The remaining five segments may form a second tagma dedicated to locomotion.
The gills on all these segments may fuse into a single surface on the creature’s underside,
and the ffifth pair of limbs may evolve into specialized fins that beat back and forth
to pass water over the gill surface to increase oxygen intake, much like the pleopods of some
crustaceans on earth.
The sixth, seventh, and eighth pair of limbs comprise the primary locomotor limbs.
What were once fins used for swimming may now be adapted into rudimentary legs for crawling
over or burrowing through the substrate.
While the ancestral polypod had limbs made of simple muscle, these benthic polypods may
require something more robust to support their bodies and give them leverage to dig through
the sediment.
The limbs of the ancestral polypod likely evolved from extensions of the gill surface,
and so the base of the limb may occur close to the haemocoel, since oxygen needs to be
able to diffuse through the gills directly into the blood.
Perhaps in this clade, an offshoot of the haemocoel could extend into the limbs, which
would provide them with more oxygen and also help maintain their structure through hydrostatic
pressure.
A muscular valve around the base of the limb may allow this outgrowth of the haemocoel
to be successively inflated and deflated with blood, which, in conjunction with movements
of the leg muscles will allow the limb to function as a sort of hydraulic piston.
This mechanism is also used by starfish on earth, which have a complex water vascular
system that pumps fluid into and out of the tube feet to move them along the seabed.
The final pair of legs, however, may lose their locomotive function and instead develop
into gonopods, limbs specialized to aid in reproduction, in this case allowing these
polypods to lock their bodies together during spawning and to facilitate the transfer of
gametes.
This clade will live in much like crabs and lobsters, sifting through the detritus for
organic matter.
I’ll call these creatures sarcopods.
In the deeper layers of the sea, they’ll be safe from most pelagic predators such as
the acanthopods, but the benthic environment will come with its own predators.
We’ve said that the gastrozoans have evolved a gut and the ability to ingest prey, and
this capability extends to their developing larvae.
Perhaps this reaches its pinnacle with one final innovation.
Neoteny occurs when traits associated with the larval or juvenile stage of a species
are maintained into adulthood.
Sometimes this may manifest as a species losing the ability to metamorphose into adults, completing
their entire life cycle in the larval form.
If this happens in one clade of gastrozoans, then instead of attaching to the substrate
and becoming sessile upon reaching maturity, they may evolve to remain motile throughout
their whole lives.
Freed from the constraints of a sessile existence, this clade of gastrozoans may now specialize
for active predation, which may entail some severe changes to their body structure.
As we touched on last time, among animals on earth there is a strong correlation between
motility and bilateral symmetry.
It could be that bilaterally symmetrical animals are predisposed to motile niches and thus
occupied them before radial creatures had the chance to adapt to them, but there are
some examples of motile animals with bilateral symmetry that evolved from radially-symmetrical
ancestors, as can be seen in one of the most criminally underrated animals on earth, the
sea pig.
These are a clade of sea cucumbers that have evolved a motile lifestyle to help them forage
across wide areas of the ocean floor.
Sea pigs belong to the phylum Echinodermata, the same phylum as star-fish, which means
the sea-pigs’ ancestors exhibited radial symmetry, but as they evolved to become more
motile, they secondarily evolved bilateral symmetry.
This provides some evidence that bilateral symmetry will be favored in motile animals
even if they descend from a radial ancestor, so the same thing may occur in our neotenous
gastrozoans.
When they first evolve life-long motility, they may propel themselves with wafts of their
tentacles, or by dragging their bodies along the sea floor like octopuses or brittle-stars.
Eventually cephalization may kick in and a single direction of movement may develop,
around which a plane of symmetry may form.
As discussed last time, it’s likely the mouth will form the front end of the animal
for efficient feeding, and consequently the arms surrounding the mouth will move forward
as well.
In the ancestral anthostomes, these arms serve many purposes, including respiration, feeding,
and sensing the environment.
Upon evolving bilateral symmetry, these limbs may become individually specialized for specific
functions.
Some of the limbs may become sensory stalks that let the animal detect and zero-in on
prey, equipped with eyes and a series of vibration-sensitive setae.
These organs are especially delicate and would pose a huge threat to the animal’s survival
if lost or damaged, so the ancestral condition of being able to retract these limbs may come
in handy, in much the same way that snails are capable of retracting their eyestalks.
Other limbs may become specialized gills with an enlarged surface area to increase oxygen
uptake.
Like in the polypods, these gills may contain an extension of a haemocoel, and muscular
contractions within the base of the limbs may pump blood between the gills and the rest
of the body.
Meanwhile still other limbs may become feeding arms to force prey into the muscular mouth.
These limbs may evolve sharp hooks or teeth to help grip prey and masticate it before
ingestion.
Note that because their digestive system evolved from a single cavity formed from the feeding
surface, these creatures don’t have a through-gut like the polypods, where food is ingested
at one end of the animal and waste is expelled at the other.
Instead, food is digested in the stomach and then the waste is regurgitated out of the
mouth, similar to some flatworms and jellyfish on earth.
The remaining limbs may be used for locomotion, beginning as simple tentacles that drag the
creature along the sea bed, but then eventually evolving into more efficient feet or flippers.
While they never become fully sessile, they may still develop the same mineralized armor
that the adults do, still serving as defense against larger predators.
However, this armor will still at least partially restrict their movement, so they’ll likely
be outcompeted for the niches of pelagic predators by the acanthopods, but they’ll thrive in
deeper waters, feeding on the sarcopods or other benthic animals living among the detritus.
But the evolution of motility provides these anthostomes with another advantage.
Recall the sessile anthostomes are committed to broadcast spawning, meaning they produce
vast quantities of gametes, only a very small percentage of which are fertilized and survive
to adulthood.
By developing motility, the neotenous anthostomes are now able to actively search for a mate,
minimizing the distance the gametes need to travel and thus substantially increasing the
chances of a successful fertilization.
The ancestral anthostome would have produced gametes from internal gonads and then expelled
them into the water through gonopores at the base of their feeding arms.
However, in the gastrozoans, as the stomach evolved, these gonopores may have migrated
inside the body cavity, meaning that the gastrozoans, including these new neotenous forms, will
produce gametes from a gland on the inside of their mouth, which means they eat, expel
waste, and secrete sex cells all out of the same hole.
Yeah, nature can be gross sometimes.
And one final note on reproduction, most anthostomes only develop a reproductive system during
their metamorphosis into adults.
This means that before they mature, the larvae are neither male nor female, completely lacking
any sexual characteristics.
Then, once they metamorphose, they become either male or female, perhaps based on certain
environmental cues such as temperature or the availability of certain nutrients, rather
like the developing embryos of crocodilians.
But since these neotenous anthostomes never undergo metamorphosis, they will instead gradually
develop a reproductive system once they reach maturity, with the same delayed-onset sex-determination
as the other anthostomes.
With this suite of bizarre traits, I’m going to dub these bilateral gastrozoans tentaclostomes.
And with that, we’ve got a pretty solid outline for the ecosystem of our ancient oceans
and a fairly extensive cladogram for both of our body plans.
Keep in mind, of course, that evolution on this scale takes millions of generations,
and for every clade we’ve developed, there are likely to be innumerable intermediary
forms, as well as many other clades that have either gone extinct or simply aren’t included
for clarity.
This will represent our ancient ocean’s ecosystem for the first 100 million years
or so following the emergence of the two bodyplans.
However, while our oceans are brimming with life, there’s still one habitat yet to be
colonized.
In the next episode, we’ll see how the descendants of these marine creatures haul themselves
out of the sea and take up residence on dry land.