Life on Thalassa

How Earth might have look after 4 billion years of evolution, with Stromatolites (cynaobacteria and perhaps green algae) dominating the coastlines. Multi-cellular life was just starting to appear on the planet. By comparison, Thalassa is 1.8 billion years old in SOT.

First contact! What might life look like on other planets? We may not be far away from that experience. Although we have discovered thousands of exoplanets in the last decades we know of only one planet with life: Earth. So, as a marine biologist, life on Thalassa was created using the scientific foundations and evolutionary history of what we know about primitive life on Earth, with some added twists.

Since we don’t know what life might look like on another planet, it is possible some alien life will be similar to earth’s life in overall appearance and form due to the constraints of an organism’s functional anatomy, physiology, and its cellular and genetic foundations, which I assume is based on something like DNA and RNA. My bias and that of almost all science fiction writers, is that we tend to look at life on other worlds through a water-borne, carbon-based lens and life on other planets may be fundamentally different than Earth (see Irwin and Schulze-Makuch, 2011). At any rate, I surmised life on an ocean planet would be similar to our planet in basic life forms, but ecologically unique due to the differences of a younger, ecologically different setting.


Primitive Life : the “fronds”

The earth is very old, 4.6 billion years to be precise: an immense amount of time that is difficult to comprehend. The truth is that after the earth had a chance to settle down from its violent birth, which included coalescing from planetesimals, violent volcanic eruptions, a collision with a Mars-sized object that created the moon, and 300 hundred millions of years of bombardment by massive asteroids and comets, primitive life evolved relatively quickly and appeared after 1.1 billion years. However, it took another 3 billion years for multicellular life to begin. Thus, most of our planet’s history has been dominated by single-celled organisms such as archaea, bacteria, including cyanobacteria (blue-green algae), and single-celled creatures (eukaryotes). I imagine that Thalassa had a similar early history but an accelerated rate of evolution.

ediacaran_1

Depiction of Ediacaran Fauna, showing rangeomorphs and other multicellular species which was model for life on Thalassa.

Most scientists look to the “Cambrian Explosion” for the beginning of multi-cellular animal life, and rightfully so. It was at that time, about 540 million years ago (mya), that an amazing diversity of animals appeared to explode on the scene, or at least in the fossil record. First documented in the famous Burgess Shale of British Columbia and discussed in Stephen J. Gould’s incredible book Wonderful Life, complex animals appeared to burst onto the planet in an unparalleled record of animal diversity with most of the modern animal groups intact. But there is an older, and more recently discovered era, the Ediacaran, that actually represents the earliest appearance of complex life. And it’s a fascinating era with many puzzles. Chief among them are the Rangeomorphs, an enigmatic group of organisms that were ubiquitous in fossil assemblages over 580 mya, 40 million years before the “Cambrian explosion.” The rangeomorphs were the basis for the “fronds” on Thalassa.

Ediacaran fossil assemblages are common at only a few places:  Newfoundland, Arkhangelsk Russia, Namibia, Charnwood Forest in England, and at their namesake in the Ediacaran Hills in South Australia. Although there are differences, each site shows a common architectural organization of a group of primitive critters that lived for over 30 million years. To call them invertebrates is probably presumptive; they may not even be animals. Their unique frond-like fractal body plan consisting of petals branching off a central axis and occurs across dozens of taxa.

Although they were found in shallow water environments, their structure precludes filter feeding and they were widespread in deep water indicating they may have fed on dissolved organic matter which was common in the microbial-dominated Ediacaran seas and possibly in the early evolutionary history of any ocean planer. In some cases, they occurred at densities and distributions similar to modern-day invertebrate communities with major diversity patterns that exploited resources at different levels above the seafloor.  Early forms were simple and prostrate while more derived forms showed a complex, fractal-based structure elevated successively elevated above the substrate. These patterns indicate species may have been adapted to variation in water flow.

The leaf-like rangeomorphs, some of which grew to 2 meters in height, showing what Thalassaian seas might have looked like. Photo: Richard Bizley

In a recent review, (Liu et al., 2015) examined the ecology of the Ediacaran organisms, including the Rangeomorphs. Their conclusion: We don’t really know what they were or how they functioned. Here’s what they concluded:

  1. They probably aren’t related to any modern marine life: they are likely an early evolutionary experiment in multicellular life and a stem-group branching off just before or after the divergence of animals and fungi (Erwin and Valentine, 2013).
  2. Their frond-like morphology is unique: their fractal-based segmented morphology suggests their surface area: volume ratio was constant, unlike modern animals where it decreases with size. Thus, their bodies may have been filled with inert substance such as water or sediment to maintain thin tissue contact with the environment  (Laflamme et al., 2009).
  3.  They were common in both shallow- and deep-sea environments: therefore, they could not have used photosynthesis and had no obvious structures for filter-feeding so their feeding strategy was enigmatic.
  4. They may have fed passively: one hypothesis is they feed by osmotrophy (Laflamme et al., 2009) which involves simple absorption of dissolved organic carbon (DOC) from seawater. DOC may have been 2-3 orders of magnitude higher in the microbial-dominates seas of the Ediacaran and a common source of energy. Their constant surface area: volume area supports this possibility.
  5. They may not have been animals: one hypothesis suggests they were fungi (Erwin and Valentine, 2013), perhaps even lichens (Retallack, 2014), and these were early adaptations to primordial seas and shores.
  6. In a more recent study (Lui and Dunn, 2020) colonies of rangeomorphs have been found to be connected by small filaments, and hence were colonial.

Based on this science, I created the “fronds” on Thalassa which varied from small shallow-water balloon-like structures to deeper-water, more complex kelp forest-like structures. When touched, they quickly disintegrate into sand, which is one hypothesis for the anatomy of the rangeomorphs. But as with all life on Thalassa, I created them to be symbiotic with bacteria which helped them live off both sunlight (in shallow water) and DOC (in deeper water). Although not stated in the book, I believed the fronds shared the same cellular components as the lichens and hence were both fungi and algae, which is why they dominated the seas.


The Jellies, Sheets, and “Anemones”

Jellyfish, or the medusa-like life phase of cnidarians (like anemones and coral, are primitive and ancient animals so I included them in Thalassa’s seas. However, unlike Earth’s jellies, I created the “sheets” as large simple animals that used stinging cells, like the cnidocytes of jellyfish, as protection and a way to capture prey. Among the Cnidaria, cnidocytes can induce a range of responses from deadly stings, to stickiness, to slight numbing.

Jellyfish, the model for the “Sheets,” have two layers of tissue with a soft “jelly-like” middle layer.. Photo: Sharon Ang, Pixabay.

I decided to create a species that slowly becomes more deadly over time, allowing a prey item to not pull back at first encounter but eventually become immobilized and then consumed. The teachings of the Nesoi shows that they have had past encounters and remember their lessons. Sage tried to avoid the sheets based on her first encounter with a small one (which stung hard, like on Earth) but was helpless against the all encompassing larger ones. On Earth many species of jellyfish bloom during certain times to become very abundant and cover the surface of the ocean.

Moon Jelly fish blooms. Photo: Morgan Bubel.

I limited the distribution of the sheets to calm, inland waters as their simple structure would be easily torn apart in waves and during extreme tides, hence their absence after Thalassa’s tidefall. As like jellies on Earth, the sheets can harbor symbiotic bio-luminescence organisms which cause them to “glow” in the night.

Anemones are early life forms related to the sheets but I used their appearance to trick Sage into thinking that because they looked like anemones, they were the normally harmless anemone. Instead, they were a worm-like animal similar to the Bobbit Worm. These worms are notorious for their vicious and lighting-fast predatory attacks, as seen in the book.

A tube anemone I(left) and (right) a Bobbit worm in action.


Complex Life: Blobs, Marble Sponges, Pika, and Mantis Shrimp

During the Ediacaran, we also saw the appearance of species that indicate the ancestors of modern animals were clearly present, including the ancestors of amoeba, sponges and ctenophores. All of these are in SOT as the floating amoeba-like “blobs,” marble sponges, and the drifting white marbles. Our sponges are composed of millions of cells with tails (flagella) that drive water through their bodies for feeding and are related to free-living, single-celled choanoflagellates. One hypothesis is that choanoflagellates aggregated to form sponges. Based on that, I created the white marbles and marble sponges to be related, with marble sponges being an aggregation of white marbles, another example of symbiosis.

The floating amoeba-like red-orange “blobs” Georgia observes in the submersible were based on a symbiosis between shell-less snails, or sea slugs, and their internal chloroplasts they contain from eating plants. They are an animals that can live off sunlight. In their lab on the Duke, Sage and Georgia observe one transform from a single-celled amoeba to a slug-like animal with a head.

The green sea slug Elysia, model for the “blobs.” Symbiosis with chloroplasts it gets from eating plants allows it to live off of sunlight. Photo: Patrick Krug.

On Earth during the Ediacaran there were other invertebrates, such as Kimberella, a putative ancestral mollusk, indicating that bilaterally symmetric animals were likely on the scene, so they also appear in the book. These species eventually gave rise to our vertebrate ancestors in the Cambrian period 20 million years later. Early primitive ancestors of vertebrates (like fish) include Pikaia and our modern-day lancelets, like Amphioxus.

Pikaia is, of course, is the namesake for “Pika” in SOT, the principal dietary source for humans and the Nesoi. Lancelets feed using a ring of small projections (cirri) around the mouth which drive plankton-laden water over their gills slits and trap food for consumption, I created Pika to be similar but instead have a row of pores and cirri spirally arranged along their body that drive food-laden water through their pores and into their bodies. I created their (and other animals) spiral motion to be unique to Thalassa and a consequence of spirally-organized plankton in the water column. Ultimately the spiral motion in many animals is due to Thalassa’s small size, high rate of rotation (18 hr days) which results in a high Coriolis force.

Mantis squid were created as the principal indigenous marine predator. Their morphology was based on a combination of several creatures on Earth: 1) a crustacean-like segmented head with antennae, eyes, mouthparts, and lighting-fast appendages like a mantis shrimp; 2) a body and tentacles (including light organs) of a deep-water giant squid. Their behavior as sit-and-wait predators is consistent with many octopus, as is their use of blinking lights and camouflage to help them blend in with their surroundings.


Lichens

Lichens on Earth are a symbiosis between fungi, which live on organic matter, and algae which live off of sunlight through photosynthesis. Because they are among the oldest living symbiotic organisms, perhaps older than 400 million years and extending back into the Ediacaran, I used them as a model for the primary organisms dominating land on Thalassa. Another reason is that lichens are super-tough and live in extreme environments like the freezing tundra, hot deserts, and even in toxic waster dumps. They can even live inside solid rock, growing between the grains (Wikipedia). Amazingly, a European Space Agency experiment discovered that they can survive in the harsh conditions of space.

Thus, it is possible they would have evolved in a similar but somewhat different form on a planet like Thalassa and have colonized land. Given their extreme habitats on Earth, they could survive Procyon’s glare because it is a larger, brighter F-type star that emits higher levels of cell-damaging (UV) ultraviolet light. A study showed that DNA molecules under the glare of an F-type star such as Procyon would suffer 2 to 7 times more damage from UV light compared to that inflicted by our Sun, something a symbiotic lichen might survive.

Fruiting bodies of lichens, the base of the slug-bug-chimera food chain.

I based the growth forms of lichens on Thalassa on those found on Earth. Most were encrusting, and covering the rocks, but some were also crustose (which Sage tried to eat) or branching. Because they were photosynthetic I created them to be red, orange, or yellow because Procyon’s light output would be in higher wavelengths and their pigments may be different than our green plants (Kiang, 2008). The base of the slug-bug-chimera food chain were the fruiting bodies of lichens, which occur when they reproduce.

Slugs, and Bugs, and chimera, oh my!

One of the hardest parts of the books was coming up with an idea of what life would be like on land on Thalassa. As the ocean buffers extreme temperatures and UV light it was more likely to have rich life, as it does in SOT. But land is much harsher and the first colonizers of land on Earth failed several times before they were successful (McGee, 2013). But when the cycles came into the plot, along with clouds and rain, I developed an idea based on periodic cicada life cycles. Although they are different, period cicadas emerge from the ground every 13-17 years in a population explosion that swamp out predators so enough survive to make it to the next generation. I hypothesized that a similar adaption could occur during the two-year rain cycles resulting from Hina’s orbit.

Every food chain needs a base, a source of primary production. In SOT I used the fruiting bodies of lichens , which contain algae, as that source. From there other animals consume them, and then their predators eat them, and so on up the chimera. The slugs and bugs were based on primitive invertebrates from the Cambrian period. Here’s some of the primitive species (mostly Cambrian) that inspired the slugs and bugs in SOT.

Chimeras had another origin and were inspired by the “Grendel” in Niven, Pournelle, and Barnes Legacy of Heorot (1987) and the POV they used in the book. If you haven’t read the book you should as it is fascinated to look at how the first colonizers of a planet might deal with an alien life form. In SOT, the chimera, which by definition is a “mix” of several different features, was constructed using several Earth-based life forms including an arthropod and squid. It was designed to be a relative of the mantis squid that invaded land by colonizing rivers and burrowing the in the mud until the massive rain events. On Earth, most animals that colonized land first adapted to lakes, ponds, and rivers, then migrated onto land. Like the Grendel, chimeras can only survive out of water for a short period of time and must return to their river lair to survive. From there, they launch horrendous attacks on the early human explorers from the Proteus and the Duke, threatening their existence. Although, as you can see on the cover of the book, the Grendel looks quite different than a chimera as envisioned in SOT, the general idea of a river-based creature attacking the first explorers of Thalassa is a major feature of the plot.

The idea of burrowing in the mud for long periods of time then emerging with the rains follows the typical lifestyle of many desert animals, like the spade foot toad. They are adapted to dehydrating during long periods of intense heat and lack of moisture and rehydrating when the rains return, just like the chimera.

In summary, this is some of the science behind the basis for life on Thalassa. I document the cetacean-like animals in another section. Most all life is based on earth ecology and evolution with changes due to the unique environment and age of Thalassa. There is plenty more to learn, just check out the references here and explore the overall reference list.

References:

  • Erwin, D.H., and Valentine, J.W. 2013. The Cambrian Explosion: The Construction of Animal Biodiversity. Roberts and Co., Greenwood Village, CO. xii + 406 p.
  • Gould, S. J. 1990. Wonderful Life: The Burgess Shale and the Nature of History. W. W. Norton & Company, New York, NY. 352 pp,
  • Irwin, L. N. and Schulze-Makuch, D. 2011. Cosmic Biology: How Life Could Evolve On Other Worlds. Praxis Publications Inc , 337 pp.
  • Lumbsch, H. and J. Rikkinen. 2017. Evolution of Lichens. In Chapter 4, The Fungal Community: Its Organization and Role in the Ecosystem, Fourth Edition. Edited by John Dighton, James F. White. 597 pp.
  • Kiang, N. 2008. The color of plants on other worlds. Scientific American, April 2008, pp. 48-55.
  • Liu, A. G., C. G. Kenchington and E. G. Mitchell 2015. Remarkable insights into the paleoecology of the Avalonian Ediacaran macrobiota. Gondwana Res. 27: 1355-1380.
  • Liu, A.G. and F.S. Dunn. 2020. Filamentous Connections between Ediacaran Fronds. Current Biology 30, 1–7, https://doi.org/10.1016/j.cub.2020.01.052
  • Retallack, G. J. 2014. Precambrian life on land. The Paleobotanist  63(2014): 1-15.
  • Valentine, J.W. 2013. 2004. On the Origin of Phyla. University Of Chicago Press. 608 pp.
  • Xiao, M., S. and M. Kowalewski. 2009. Osmotrophy in modern Ediacaran organisms. Proc. Nat. Acad, Sci. 106(34); 14438-14443.