Taxonomy on Mars

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Taken at face value, Martians are not all that different from Earthlings. They are carbon-based, need liquid water and also require classic elements like nitrogen, hydrogen, oxygen, sulphur and phosphorus to function. But it is the combinations of these elements that makes them different. For storing information and replicating, Martians use a polymer derived from glycol nucleic acid, although billions of years of evolution made it more complex and perfected it so much that it does not resemble anything we were yet able to synthesize in our laboratories. Functionally it works almost the same as DNA, though the majority of Martian lifeforms have five rather than four nucleotide bases like we do. They also have a simpler, even less well understood, polymer that most likely serves as an analogue to RNA. Genetic studies comparing the heredity of the extant lifeforms to each other have only started in the last few decades. Until then, the reconstruction of the Martian tree of life was largely reliant on anatomic and fossil analysis, which is not always reliable as we have seen in the case of Earth. Two lifeforms looking similar to each other does not automatically mean that they are closely related, due to convergent evolution. Studies comparing embryological development has also been difficult, as we have struggled with raising these extraterrestrial lifeforms in captivity.

Important Kingdoms and Phyla

For the majority of this work, we will be using cladistics, a system that classifies organisms based on their ancestry that uses almost infinitely stackable clades, ranked by increasing exclusiveness. But for a start we will still use the classic taxonomic ranks of Linnaeus, such as kingdoms and phyla, as they serve as an easy and illustrative introduction into the basic parts of the Martian biosphere (although not without problems, as we shall see). They are here listed by increasing complexity, but this does not represent a line of descent.

The Tree of Life on Mars (click to enlarge). It is always in flux as our scientific understanding about these organisms grows.

“Domain” Xenovira: If there was ever an example of a perfect waste-basket taxon, Xenovira would probably be it. A waste-basket taxon is usually an unnatural taxon into which various unrelated species are lumped in due to misleading similarities and lazy taxonomists. In this case, Xenovira has become a catch-all term for any extraterrestrial, biologic molecular structure that, as the name implies, even slightly resembles a virus. And by any I do mean any, as even similar structures found in Jupiter’s moon Europa and Venus’ atmosphere have been called Xenovira. They are all characterised by being too simple to be, by most definitions, considered life, but still showing enough reproductive and darwinistic capabilities to be in some kind of pre-biotic limbo. Another defining trait is that all of them are exclusively parasitic, preying on and abusing the metabolisms of actual lifeforms. But that is really where the similarities end. Some resemble classic viruses from Earth, by being genome-strands encased in membranes or protein-shells, while others are just free-floating strands of biopolymers or even just self-replicating proteins, similar to prions like mad cow disease. Some may be survivors of an ancient pre-biotic primordial soup that evolved to prey on later lifeforms, while others may be the descendants of actual life that lost complexity over time as an adaptation to parasitism. Due to this heterogeneity, many astrobiologists have tried breaking up Xenovira into multiple natural groupings. The “true viruses” of Mars, which possess genetic material similar to the other Martian lifeforms and membranes/shells, are called Euxenovira or Areovira. They are predictably the most common pathogen in the Martian biosphere, though they are thankfully not capable of infecting humans, as our DNA and cellular systems are incompatible with theirs. The same is true for any other Martian pathogens, as well as terrestrial ones in reverse, but handling foreign microbes or introducing our own is still considered dangerous for both parties, as, even if we cannot infect each other, some microorganisms might still release waste products that are unexpectedly toxic to other lifeforms.

Domain Nanobacilli: Informally also known as Nanobacteria. This taxon has no real equivalent to Earth-life. Originally grouped among the Xenovira under the name Nanoglobuli, they were long thought to be too simple and primitive to be considered life as we know it, but this quickly changed as we discovered that they met all criteria to be considered lifeforms, as they, unlike viruses, are capable of independent reproduction and metabolism, despite most of them being smaller than 200 nanometers. This is rather challenging, as the existence of “bacteria” in this size-range was previously deemed impossible, since the bodies of such organisms were considered too small to hold all the necessary molecular machinery for independent replication. This may have been true for terrestrial biochemistry, but apparently things on Mars work a little differently. By some poorly understood process the Nanobacilli seem to have adapted an internal machinery that consists of protein-based alternatives to the genome used by most other Martian life, allowing them a rudimentary metabolism and reproduction without having classic genetic material. Yet, they are all encased within membrane structures closely matching those of areonts, making it possible that they descend from more complex cells that have simplified to an extreme degree. In a sense they are a prion trying to be a bacterium or, rather, the other way around. Nanobacilli can live in a variety of lifestyles and habitats, though autotrophy is relatively rare. Commensalism with more complex organisms is instead the most common way of life for these organisms.

Domain Areonta: Areonta are the closest analogues on Mars to earthly prokaryotes (Bacteria and Archaea). Although still single-celled, they are usually far larger and more complex than Nanobacilli. They either have heterotrophic or autotrophic lifestyles, reproduce asexually, are one of the major backbones of every ecosystem, make up the majority of Martian biomass and have probably ruled this planet for over four billion years. Just your average “bacteria”. In an interesting contrast to terrestrial prokaryotes, areont cells have a significantly higher propensity to evolve intercellular membranes, meaning walls and tendrils that extend into the cytoplasm. These internal membrane-extensions function more similarly to the endoplasmatic reticuli of  Earth’s eukaryotes and often serve as “work-spaces” for enzymes and proteins. Why Martian prokaryotes have a higher tendency to evolve such intercellular membranes than Terran prokaryotes is not known. This might simply be a consequence of differing biochemistries. All higher lifeforms on Mars and likely even the Nanobacilli are descendants of the Areonta. The Kingdom could therefore be considered a paraphyletic and therefore unnatural clade, as it does not include all of its descendants. Important phyla include the Phytoareonta (photosynthetic cells similar to cyanobacteria), the Methanoareonta (methane-producers) the Perchloareonta (perchlorate consumers).

Kingdom Macroareonta: Macroareonta are what one could call a multicellular prokaryote, as they are not just simple cell-colonies, but complex organisms made up of differentiated cells, but their individual cells are still structurally areonts. A “multicellular bacterium” does indeed sound strange, but there are similar lifeforms even on Earth, such as the prokaryotic Myxobacteria and several species of Cyanobacteria that have multicellular life stages, complete with cell-differentiation and fruiting bodies. Macroareonta are similar to these, although far more widespread on the red planet and more complex, some resembling microscopic versions of plants, fungi and even animals. Despite this complexity, none of the known species grow larger than 15 centimeters. Some larger fossil relatives possibly show that this did not use to be the absolute size-limit. If this is true, modern members instead seem to be adapted for smaller niches due to competition with Rhodokarya. Rhodokarya have on average much larger cells than Macroareonta, meaning that a macroareont of the same size as a multicellular rhodokaryote is at a significant disadvantage, since its body is made up of many more cells, making it more prone to cancer and similar issues. Macroareonta seems to be polyphyletic, as apparently not all of its members have the same common ancestor, rather several lineages of Areonta evolved multicellularity independently from each other. The Macroareonta were long thought to be transitional forms between Areonta and Rhodokarya, but this seems increasingly unlikely, as Macroareonta lack cammaculae and the ancestral rhodokaryotes seem to have been unicellular.  Some extant photosynthetic species build shells made of silicon, similar to diatoms. Entire sediments made of these silicon shells have been found in marine fossil assemblages, showing that Macroareonta once constituted a major part in ancient planktonic communities, making them important for biostratigraphy. The clade of the filulithophores are today also among the most important nitrogen fixers.

Domain Rhodokarya: Rhodokaryotes are characterized by their cells having something akin to mitochondria (in their case called cammaculae) that engage in oxygenic respiration, as well as other organelles, which is why many declare them the Martian analogue to Earth’s eukaryotes. They encompass many taxonomic kingdoms that come in unicellular, multicellular and intermediate forms and constitute the majority of the macroscopic biosphere, though they have lost a lot of their former glory. It was originally thought that their organelles evolved out of Gigananobacilli, the largest known Nanobacteria, originating through an endosymbiotic event, where an areont cell was permanently inhabited by a nanobacillus, either through symbiosis or perhaps even through parasitism. The parallels of this idea to Earth’s eukaryogenesis, where an archaean cell was entered by a bacterium, is obvious (and might reflect some form of chauvinism by earlier researchers), but recent research has cast significant doubt on this hypothesis. The cammaculae bear no resemblance to Nanobacilli, do not float freely inside the cytoplasm, have no genetic material of their own and are not shared between cells during reproduction. Instead, they develop directly out of the inside of the cell membrane after mitosis. The more likely scenario is that, as Mars gradually built up more oxygen in its atmosphere, the ancestral hydrogenotrophic areont cells began to protect their anaerobic methanogen-metabolism by folding their intercellular membranes onto themselves to create a separate chamber inside the cell. The protein-studded walls of the resulting chamber could then effectively filter out free oxygen to create a controlled anoxic environment inside the chamber while the rest of the cell and the environment were oxygenated. Afterwards, aerobic-respiration may have originally evolved to more effectively eliminate free oxygen from the cell, before this gradually became the main metabolism. What corroborates this are recent genetic studies which show that rhodokaryotes which only engage in hydrogen respiration are actually more ancient than oxygen respiring ones (and not the other way around, as originally thought by analogy to Earth). The fact that life on Mars evolved the “complex cell” completely autonomously and not through an “accident” like Earth’s eukaryotes seems to have had a tremendous effect on the story of the biosphere. Biogeochemical signs of rhodokaryotes show up in remarkably ancient rocks 3.4 billion years ago (Sivgin 2345), far older than the first signs of eukaryotes on Earth, and as fossils show, this seems to have facilitated the evolution of complex multicellular life only about four hundred million years later (interestingly, the time of appearance between the first rhodokaryotes and their first multicellular members is similar to that between the first eukaryote cell and the Francevillian biota in Gabon).

Superkingdom Proteroareozoa: Try saying that three times fast. Proteroareozoa is technically another paraphyletic taxon that encompasses several kingdoms of unicellular rhodokaryotes, far too many to mention them all here. The many lineages of Proteroareozoa can be distinguished from each other by their modes of life, cell-structure and the way they move using their flagella. All rhodokaryotes ancestrally reproduce sexually, although asexual reproduction has independently evolved in multiple lineages. A prominent group of proteroareozoans are the flechtoids, a semi-multicellular group which resembles a mix between slime molds and lichen.

Kingdom Arephyta: Arephyta is the kingdom that includes large, multicellular, photosynthetic rhodokaryotes that share one common ancestor. They are in some ways the Martian analogue to plants and algae, by having cells with cell-walls and being immobile autotrophs. However, they are not actually the dominant constituents of the global flora. They originally seem to have been, during the Thermozoic Era (Sivgin 2345), but have gradually declined in favour of Spongisporia and Fractaria. A plausible explanation for this might be their metabolism. Arephyta do not engage in oxygenic photosynthesis like Earth-plants, but instead use sunlight to react hydrogen sulphide with carbon dioxide to generate sugar and, as a waste product, elemental sulphur. This is a form of photosynthesis only used on Earth by some of the most archaic organisms. Arephytes are of a principally two-layered build, with an external germ-layer consisting of oxygen-respiring cells, which protect an inner layer of anaerobic cells that are specialized for this mode of photosynthesis. Such a metabolism seems to have been viable even on land in the earlier periods of Mars, as constant volcanic outgassing continuously enriched the atmosphere with sulphuric gases. But as volcanic activity died down, the arephytes lost much of their livelihood and the increasing oxygenation likely made it even more difficult to maintain their metabolism. Today, Arephyta without special adaptations, such as the divisions (the botanic equivalent to a phylum) Porphyta, Cochleophyta and Arechlorotia, are solely found close to the hangs of active volcanoes or along geothermal hot springs. More derived groups have gained a more flexible range by living in endosymbiosis with sulphur-reducing areonts, which recycle the waste sulphur back into hydrogen sulfide. Part of this group are the Arthrophyta, which are notable for possessing a segmented body with both internal and external bilateral symmetry, something not seen in any living plant on Earth. Lastly there are also the Pennatophyta, possible offshoots of the Arthrophyta, which have lost their autotrophic lifestyle and have instead become heterotroph detrivores, fulfilling a similar ecological role on Mars as fungi do on Earth. Unlike Earth-plants, Arephyta have no alternating generations.

Kingdom Spongisporia: Like the Arephyta, spongisporians possess cell walls (except for the dedicated filtering cells), though composed of different polysaccharids than in arephytes. Spongisporia have no true differentiated tissues and they are immobile their whole life, but since they are heterotrophic, they are not reliant on sunlight or chemosynthesis. These traits combined with their rhizome networks makes it easy to draw parallels between these lifeforms and Earth’s fungi, but the similarities are superficial. Fungi can exist both in unicellular (think yeast) and multicellular states and are mostly made up of loosely connected hyphal networks without a central body (mushrooms are just spore-bearing fruiting-bodies that grow out of the mycelium for mating). Spongisporia on the other hand are exclusively multicellular and have a more defined morphology. They have a central, often tube-like trunk and a root-like network of hyphae underneath. The earliest members of this group seem to have lived on the seafloor (Sivgin 2345), had porous walls with a hollow central-cavity, a sclerite-skeleton made of silicon dioxide and ate by filtering the seawater with cilia. This made the earliest spongisporians, as the name implies, more similar to Earth’s sponges and indeed many extant members still have the characteristic porous body-cavities (although they now serve a different function) and sclerite-skeletons.  Consequently, the majority of spongisporians live as filter-feeders, not just underwater in cave systems and glacial lakes, but also on land, where the low gravity allows for the existence of an aeroplanktonic community. Some forms also live in symbiosis with photosynthetic areonts, essentially being giant sponge-lichen. Spongisporia reproduce through spores and have haploid and diploid generations, which often differ greatly from each other.

Kingdom Fractaria: These are a lot more complex than Spongisporia and Arephyta, with clearly visible symmetrical builds and differentiated tissues, but they are still too “primitive” to be comparable to animals. The majority lack a gut, mouths or any digestive organs, as well as the cell walls and hyphal roots of the spongisporians. While they ancestrally were filter-feeders like the Spongisporia, many have also become autotrophs by living in symbiosis with colonies of Areonta that live in their tissues. These forms live off osmotrophy, chemotrophy and even photosynthesis. Because they are relatively easy to keep in captivity, their biology has been well researched. A Fractarian starts out life as an immobile propagule (either by budding off the stolon of a parent, as a free-floating spore or in more derived species from a capsule sometimes called an egg or a seed) which grows into a base not unlike that of a spongisporian embryo. Out of this, a fern-like frond develops with a simple internal system of interconnected filtering-chambers. This frond gives rise to another frond, which also gives rise to another one, until the whole organism ends up with the characteristic glide-symmetry that the group derives its name from. Such basic fronds seem to be the simple building block of this kingdom, with most members having found some way of iterating in remarkable ways on this template, for example by evolving multiple vanes or increasingly more fractally growing fronds. The majority of the “higher” Fractarians however reach their complexity instead through colonial living. Like corals, seapens or sessile versions of the Portuguese Man O’War, many clones growing from a single ancestor grow into larger, plant-like superorganisms. An important group among these colonial forms are the Hylozoa, which have evolved outer walls made of cellulose, similar to wood, which allowed them to colonize terrestrial habitats and through symbiosis with Phytoareonta have become a major part of Martian flora, in the past even growing into forests (Sivgin 2345). Fractaria are generally not classified as Martian animals, since at no point in their lives do they have a mobile stage. The sole exception to this, as has only recently turned out, is the superphylum Pseudarticulata. Originally classified as primitive Arezoans (Martian animals proper), we now know that these are actually fractarians, which evolved from solitary fronds that found a way to use the filtering cilia on their skin to lie flat on the ancient seafloor and crawl about the microbial mats like giant amoeba. Out of these primitive forms eventually derived descendants, which, independently from Martian animals proper, evolved their own versions of nervous systems, muscles and sense organs. Pseudarticulata, with their fake-segmented body, today largely resemble flattened worms and maybe arthropods. Their most successful and famous clade are the Chirorbita, sometimes nicknamed “Martian trilobites”, which have evolved a tunicine exoskeleton, camera-eyes, and move about with hydraulic tubefeet similar to starfish.

Kingdom Arezoa: In older literature also called Zoomimia/Zoomimida, the “animal mimics” of Mars (technically an alien creature cannot be called an animal, since an animal can only be a member of Animalia, a taxonomic kingdom from Earth. However, for convenience, Arezoa will still be called animals here). These naturally make up the bulk of what can be called the planet’s fauna. They have differentiated tissues, lack cell walls, are heterotrophic and are all capable of free movement at least at some point in their life cycle. Interestingly though, the majority of them possess metabolic pathways that are capable of recycling much nitrogen from their waste, which makes them more similar to Earth-plants than to Earth-animals. The exact number of arezoan-phyla that exist or have existed on Mars is a debated question. Therefore, we will only list the most important phyla here:

• Mollizoa
• Brachiostoma
• Aspiderma
• Spiriferia
• Trichordata
• Onychognatha
• Antitremata

Instead of discussing each in detail here, you will get to know them by example in the upcoming pages. The exact origin of the Arezoa is a controversial issue as we lack a lot of fossil or embryological data. Anatomical comparisons predict that the Fractaria are their closest relatives, but researchers have also noted similarities to the Spongisporia. It is interesting to note that, unlike most animals on Earth, the arezoans are predominantly hermaphroditic, with only a few known species having something akin to differentiated sexes. During mating either both partners get pregnant or, what more often happens in the more derived groups, the partners first fight each other to determine who gets to impregnate the other. Many of the simpler forms also practice self-fertilization, similar to Earth-plants. Why this difference exists is unknown. There are hypotheses whereby the difficult inheritance of mitochondria between cells is what drove sex-differentiation in Earth’s eukaryotes. Since rhodokaryotes do not have true mitochondria, the true need for sexual differentiation may have never arisen on Mars.

References:

• Sivgin, T.K. : Life on a Dead Planet. The first 3 billion years of Evolution on Mars, Zürich 2345.