Life is hard to define, so how will we know when we have found life on other planets?

Fire grows, produces “offspring,” and consumes food, but it is generally not considered alive. Crystals also grow and produce offspring. Viruses are nothing but large molecules that interact with cellular machinery on contact to reprogram it to make more viruses. They can be thought of as alive, but they can equally be thought of as signaling molecules, such as pheromones or hormones. Within the nuclei of all eukaryotic cells are stretches of DNA that sometimes get themselves copied and inserted elsewhere in the genome. They are like internal viruses. Are they alive?

Some definitions of life include homeostasis, the ability of an organism to maintain itself in different environments. Fire is temporary, existing only when conditions are right and until the fuel is used up. In contrast, wolves sense when they are hungry and seek out prey. Of course, wolves are temporary too, and cannot exist in the vacuum of space or on the surface of pulsars. Seen over long enough timescales, even the entire biosphere on Earth is temporary, expected to die a couple billion years from now.

Some definitions include having complex biochemical cycles, with compounds being replenished from their own products. Of course, there are also simple cycles, such as what occurs in the “traffic light solution.” Pigment A converts to pigment B, which converts to pigment C, which converts back to pigment A, in a cycle that continues as long as the base reactants (which could be thought of as food) last. Is the solution alive?

Some definitions include reproduction, but this excludes mules and sterile worker ants. If on another planet there was an ecosystem so closely symbiotic that it resembled a single organism that never reproduced, would we recognize it as alive? What if it talked to us?

Some definitions include having a replicator capable of following the Darwinian algorithm. This means it must duplicate information with a high enough degree of fidelity so as not to quickly degenerate into non-life, but not so high that mutations do not occur, giving natural selection something to act on. In other words, life must evolve. This definition excludes self-replicating machines designed so precisely that they never make mistakes. It would also come as a surprise to the ancients, who thought they knew what life was, but had never considered evolution.

More recently, some attempts have been made to describe life in mathematical terms, giving astrobiologists five specific things to look for:

If an object maintains a gradient of compounds out of equilibrium with its environment, it might be alive.

If the sizes of the objects under study follow an inverse power law, with smaller ones more common than bigger ones, they might be alive.

If one molecule is activated only by one other molecule, as if they were lock and key, they might have been produced by life.

If the molecules present require many unique steps to manufacture, rather than simply being repeating polymers, they might be produced by life.

If the molecules present are asymmetrical and not accompanied by their mirror-image partners, they just might be produced by life.

Life is hard to define, but the more qualifications a phenomenon holds, the more willing scientists are to call it life. So far, they have only found life on Earth. We must keep looking.

Further Reading:
https://www.scientificamerican.com/article/the-search-for-extraterrestrial-life-as-we-dont-know-it/

Possibly the biggest exobiology project in history, Sagan 4 is the joint creation of many users. Anyone can add another species by picking a previously existing species (alive in the current generation) and “evolving” it into something new. This has led to an explosion of biodiversity on the planet. As of 2021, there were 41 kingdoms. Learn more at Sagan4.org and forum.Sagan4.org

A selection of drawings from Sagan 4 can also be seen here: disgustedorite

Or, as informed by the commenter below, here: hydromancerx

Reccembra is the creation of DeviantArt user Earlegs. It is a roughly Earth-like planet with the equivalent of prokaryotes and eukaryotes, algae, chemotroph fungi, and simple animals with some very strange embryology.

Aderrica is a world of sea-dwellers with six fins and land-dwellers with six legs. There are flyers, climbers, burrowers, grazers, and predators. There are those that lick up detritus and those that snap their oversized jaws around prey.

There are many seed world concepts in speculative biology projects. Serina is a world populated by guppies and canaries introduced from Earth. Kaimere is populated by many Earth lineages introduced over long periods of time. Much more exotic, the Mammalian Ectoparasite Seed World (MESW) by Twisting Depths is a world of ticks, mites, fleas, flies, and lice. Some have grown to monster sizes and include cuddly blade-faced predators, grass-eaters, and shark-like organisms. Check them out.

Katie is the creator of four worlds: Xenosulia, Amthalassa, Nemoros, and Eurus. Each has its own blog out in the internet wilderness (Amthalassa shares its blog with stories of a British garden). The posts are in the order of most recent articles on the top, so you’ll want to go to the bottom to read about the climate, biochemistry, and anatomy of the major clades. Later posts cover specific biomes and genera.

Amthalassa is a world of ammonia, methane, and hydrogen. In deserts, moss fixes its own nitrogen to create ammonia to keep from drying out. Soft-stemmed kelp-like “trees” fill their bladders with hydrogen to support their weight in the thick atmosphere. Smaller plants crawl very slowly from place to place. Animals are radially symmetrical, with one eye per leg, a ventral mouth, and a dorsal tail containing an anus and genitals. Many are saucer-shaped. Others are snake-like with their legs clustered around the mouth. Still others have become bilaterally symmetrical, but retaining one eye per leg. There are herbivores and carnivores. There are shells, spines, and feathers. There are floating grazers that hold symbiotic, hydrogen-containing plants. There are eyes with inner and outer lenses able to move relative to each other. It’s a beautiful place.

Xenosulia is a tidally-locked world of acid rain and air algae. Trees are bilaterally symmetrical to better face the unmoving sun. Land animals sift algae from the air with the equivalent of baleen. Others are predators or grazers. The dominant animals are descended from radially symmetrical ancestors, with mouth and genitals on top surrounded by a compound eye ring and the anus on the bottom at the end of a tail. In most lineages, there are two front limbs and the anal tail has become a third limb for hopping. Skeletons are silica, nerves operate on triggered bioluminescence, muscle cells shrink by bonding gas from bubbles into specialized proteins, and many animals carry their haploid offspring as symbiotes. Wings are supported by blood vessels, spines provide thermoregulation (and more), and jaws evolved several times in different ways. The biodiversity is filled-out, with vaguely sloth-like, turtle-like, snake-like, mole-like, scorpion-like, and dromaeosaur-like forms. Another interesting lineage with potential is the “hingeflies.”

Nemoros is a moon of planet Aulea. It is a world of ammonia volcanos and very cold temperatures. Methane lakes dot the icy soil. Grass-like and tree-like producers cover the landscape. The nitrogen atmosphere is thick, allowing animals to tumble along with little effort. Those with hydrogen bladders can easily swim/fly. The atmosphere is also very hazy. Chemosynthesis is more common than photosynthesis. Sonar is more common than sight. Plants take in methane and store their food as acetylene. Animals eat the plants, inhale hydrogen, and exhale methane. The dominant clade is one of tetrahedral symmetry, though most lineages have evolved various forms of radial or bilateral symmetry. The animals flower, producing larvae that fuse in pairs to produce chimerical adults. Limbs move by the opposition of contracting muscles and expanding muscles.

Eurus is a low-oxygen world with an axial tilt of 87 degrees. The winds are severe. The plants resemble bamboo. The land animals are descended from molluscoids and have their anuses near their mouths. Their muscles exist inside hollow bones. Making use of bioluminescence, their nerves operate like those on Xenosulia. There are turtle-like and ferret-like forms. Smaller relatives with more legs also have wings derived from fins.

DeviantArt user Concavenator writes about the planet Ea. Unlike most projects that focus on one ecosystem, this project gets into the interactions of organisms with partly incompatible biochemistries. After several decades of environmental and political turmoil on Earth, humans have moved to Ea to repeat their mistakes – and they’ve brought their cats with them.

Few projects go into as much depth as this one, covering plant reproduction, cellular structure, biochemistry, and so much more. As of this writing, it still only has a few lineages though, including descendants of starfish-like animals, some who have converted one arm into a head, leaving four limbs left over, and some who have merged two arms into a head, leaving three limbs left over. It’s great!

YouTube user Artifexian, who creates videos covering a variety of subjects, including languages, has created a playlist of helpful tips to make scientifically credible worlds for your creatures to live in. Subjects include climates maps, geology, orbital mechanics and how they relate to any calendars your beings might create, plant color, and much more. Watch it here.

Isla is the creation of DeviantArt user bionautic (also on Instragram and YouTube). It is a tidally-locked world where life lives only near the “twilight zone” between the cold night side and the hot day side. Plants are often flat to deal with the constant wind in this area, though some rise above the surface somewhat to better capture the very horizontal sunlight.

Though lower in biodiversity than Earth, there are several interesting bidirectional animals described, as well as some of the variety in spore dispersal used by the plants.

Isla is also a world I can feel at home in while it captures the imagination. It has many cozy nooks and crannies created by the plants, whether it is the gaps between the generational reefs of flat desert plants, the root mesh underneath large plant colonies, or the world under the solid canopies of the “spore forests.”

Although I have seen many equally-good artists, this is the best art I have seen that was coupled with clear biological prose into a real project.

One of the most imaginative projects I’ve seen in a long time, planet Polinices created by DeviantArt user doublejota is dedicated to the question of how far we can take the concept of a colonial organism. There are worms that bud smaller versions of themselves from within their stomachs, who in turn bud more. Some colonies remain wormlike while others branch and house symbiotic algae. Some colonies are circular and roll like tank treads while others are donutlike and lined with needlelike teeth on the inner surface.

Some colonies have three members that resemble a fish and two large pectoral fins. They can move by fins or by siphons. Some colonies have two main members that face opposite directions and resemble Earth fish except for the four eyes in the middle of the body (derived from the gastric sacs of smaller members). Some colonies grow a long tubular shell that curves back and forth and some of these resemble fish in overall appearance with the propulsion tail member surrounded in two dimensions by serpentine shell.

In some colonies, the members detach and cooperate in ways reminiscent of a beehive. Forager drones gather food and bring it to the queen for digestion. Some swim. Some crawl. They communicate by radio and require repeater drones due to the limited range of radio waves in water. There are many variations on this theme, including the drones lining up as in a buffet, drones stringing together in chains while they filter-feed and are passed through the queen, and queens without drones that mimic radio calls to lure in the drones of others as prey.

There are also members that fully detach and roam in packs. Only some can digest, some can capture prey, and some can reproduce. It’s an extreme form of caste system similar to something I thought up once.

Furthermore, I also enjoy the variety within the variety. Some colonies have eyes on each end, but in some of those colonies the lenses of the eyes have overgrown into giant shells that the colony can pull together to cover itself. There are many kinds of shells and in some colonies the two shells can have different shapes and sizes. A lot of thought has gone into this project. I have barely scratched the surface in covering it and have left quite a lot unsaid.

The art is great and includes a mixture of diagrams and landscapes. Many of the animals are multicolored with sharp contrasts like tropical reef fish on Earth. It sets up a feeling that’s enough to just immerse yourself in after a long day. Fortunately, there is talk of a future book.

​Legacy by Greg Bear is a book whose story takes place on the planet Lamarckia, named after scientist Jean-Baptiste Lamarck, who is famous for his pre-Darwinian hypothesis of evolution through acquired traits that has since fallen into disfavor. There are only a few dozen genetically distinct organisms on Lamarckia, taking the form of vast forests or other biomes called ecos. Animal-like and plant-like objects are merely parts of the whole that perform their functions. These ecos compete with each other and steal genetic innovations from each other, giving them acquired traits, hence why the planet was named after Lamarck. While the core of the story is the human drama, there is much description of the biology that makes up the setting.

The free online magazine Astrovitae features artists, projects, and individual organisms from all across the speculative biology genre, stretching it to include even drawings of reimagined mythical creatures. They currently publish twice per year. The layout and graphics are professional, but most of the content is contributed directly from the artists without editing. They write about themselves, their goals, their attitudes towards art, but mostly about the life forms, and that’s what it’s all about.

​Curious Archive is a YouTube channel covering an eclectic bunch of subjects including history, cryptids, myths, and speculative biology in all its flavors. It is doing what I had hoped for this blog to do, shining a light on spec projects and encouraging art. If you need inspiration to start your own project, watch the videos sometime. Serina is as good a place to start as any.

If the life of planet Kaimere looks familiar, that’s because it originally came from Earth, teleported by mysterious means over the eons. Many clades extinct on Earth continue to live on Kaimere. Felids, creodonts, and theropods share the same hunting grounds. Bats, birds, pterosaurs, and strange flying therapsids share the skies.
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Keenan Taylor has written an anthology of short stories of the people who live on this world, available at Barnes and Noble. Concept art can be found on his DeviantArt page. Updates can be seen on his YouTube channel.

​Kalagari is the creation of YouTube user Ben Pebbles. It is a planet of purple plants with swimming fruits. There are also animals and green plants. I enjoy watching the animations of the seal-like and buffalo-like creatures. The narration style is humorous and tends to jump from one organism to the other by their ecological relationships. Watch the videos here.

Project Rose includes not just one, but three planets, Nusku, Mazu, and Ullr, all within the habitable zone of a red dwarf star. Nusku is tidally locked, giving it a permanent typhoon where it faces the sun, surrounded by desert, surrounded by temperate forest, surrounded by ice near the windy twilight zone. The “night” side of the planet is dead. Mazu is a water world with little or no dry land. Ullr is like a colder, larger version of Earth.

​As of this writing, only Nusku life has been described and plants are just beginning to move onto land. They resemble purple mushrooms, ferns, or palms. There are two basic lineages of animals: Filter-feeding, radially-symmetrical animals evolved into bilaterally-symmetrical fish-like creatures, some with armor and some with jaws. Flat animals with four mouths and one anus evolved into burrowing, swimming, and barnacle-like forms, some with parapodia, shells, hair, backbones, beaks, hooked tentacles, “swords,” or “saws.” Some filter the water with all four mouths. Others bring water in two of the mouths, filter it for plankton, and expel it out the other two.

Watch the ongoing saga on YouTube.

The planet Almaishah is the creation of YouTube user Dapper Dino and its inhabitants are the collaborative effort of many speccers and artists. Every few months, the project enters a new phase, setting the clock forward ten million years and causing extinctions. New plants and animals are added, but must derive only from those species already in existence. Because it is a collaborative effort, there is some variety in the art style and the length and detail of the descriptions read aloud by Dapper Dino. The large-scale groupings of organisms are more diverse when everyone is trying to take things in different directions. Some focus on behavior, others internal anatomy, others external anatomy.

As of this writing, phase three has just ended. Life still lives primarily in the oceans, but a few animals have adapted to taking short strolls above the tide line. The biodiversity is exciting. The art is professional.

There are boneless tetrapods, some resembling tiny horses or crocodiles, jellyfish-equivalents taking the roles of limpets, leaches, and corals, echinoderm-equivalents sometimes taking the roles of jellyfish, and segmented animals already diversified into forms vaguely reminiscent of fish, turtles, earthworms, shrimp, spiders, beetles, lobsters, swimming centipedes, and giant, blind, heavily-armored worms that slither along the bottom and eat whatever is stupid enough to stumble into their oral tentacles. There are purple plants filling the roles of seaweed, true plants, and corals. There are animal-like swimming green plants, some of which have lost their chlorophyll and become red plankton, and some of which have lost photosynthesis entirely and become vermiform detritivores. There are also animal-like radiotrophs that feed on the energy from radioisotopes, some of which have also developed photosynthesis and chemosynthesis. I don’t understand their anatomy at all.
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Watch the ongoing saga on YouTube.

C. M. Koseman, creator of Snaiad, also wrote and illustrated the book All Tomorrows, a future history of the human species. Over millions of years, genetic engineering by both humans and hostile aliens leads to the creation of some truly bizarre monstrosities among our descendants. There are human races that resemble moles, dolphins, flounders, and even colonial siphonophores. A good overview of the book, including some of the art, can be seen at this link.

YouTube user Phrenotopia, who creates videos covering a variety of subjects, including alternate history, science in fiction and popular culture, reviews, and biology, has created a playlist detailing how animal life is thought to have arisen on Earth. It is a good beginner’s guide to different bodyplans and embryology, as well as providing speculation on what patterns of biological evolution are likely to be seen on other planets – and which are not. It is a good resource if you are just getting started trying to develop plausible alien animals. Watch it here.

YouTube user Biblaridion, who normally covers language creation in world-building, has also created a playlist of videos featuring the step-by-step evolution of life on an unnamed, Earth-like world rich in hydrogen sulfide. The graphics are great. The concepts are plausible. Watch it here.

Preradkor is the creator of not one, but two worlds, planet Coatlique and planet Ullr. Coatlique is home to large animals reminding me simultaneously of mammals and arachnids, although some resemble sharks with whip-like jaws. Coatlique is hot, stormy, and irradiated, leading to a high level of mutation such that all animals and plants are chimeras. Ullr is much colder. It is home to the askewbugs, asymmetrical insectoid organisms. Check out his DeviantArt gallery.

Your mission in this game is to explore planet ARY-26 to make sure it is suitable for human colonization. Catalogue animals, plants, and...er...plantimals. The creators can explain it better than I can. Visit SavagePlanetGame.com

The Conceptual Evolution Forum is where you can connect with other speccers, posting about your own projects and commenting on others. Get feedback on aesthetics and scientific plausibility from thousands of people all over the world. There are different sections to discuss future evolution on Earth, what might have happened on Earth had evolution run differently, evolution on other planets, and other universes where the laws of physics are different. Popular projects include Nemo and Sagan4. Register and say hello.

Illuria is the name of a planet with its own wiki that anyone can edit. According to the introduction, it was discovered in 3018, has a 48-hour day, and a year that is 730 Earth-days long.

In addition to mountains, deserts, scrublands, savannahs, temperate forests, rainforests, marshes, and prairies, it has acid swamps and the “mineral sea,” a body of water rich in iron, gold, and silver. Its sub-biomes include rust reefs, crystal caves, and wiregrass meadows.

Among the animals of Illuria are crystal borers, shrimp-like, colonial animals that make nests out of crystals.

Among the plants of Illuria are crawling boom creepers, plants that grow rapidly at night on stored fuel, seeking out sleeping animals to infect with its spores, which it often delivers so explosively that the animal is killed.

As of this writing, the ecosystem is still incomplete. The life on the continents is undescribed. The gold shiners eat shellfish that have no page of their own. The hovizards lay their eggs on stilt plants with no page of their own. Visit and add your own ideas.

Evolution is a board game of strategy. Each of 2-6 players maintains one or more species that feed from a shared watering hole. To remain alive through the player’s turn each species must take in enough food for their body sizes and population sizes. Those that do not go extinct. Phenotype cards can be combined to alter each species to make it better at getting food. Sometimes, this means becoming a carnivore that can only attack other species for food. Some traits include defenses against predation. When the card stack is empty, the game is over and the player with the most food wins. I want to play this sometime.

Species is less of a video game than an evolution simulation program. One can simply sit back and watch creatures adapt and speciate or one can introduce selection pressures to see what happens. Challenge yourself to evolve something resilient. Among other things, players can change the oxygen levels, temperature, sea levels, and radiation. Changes in terrain can force creatures to expend more calories in order to move. Specific attributes can be added to the gene pools of any population, including not only shape and color, but sex drive, the maximum genetic distance allowable for successful mating, and metabolism – affecting secondary traits such as how often they sleep and how much food they need to keep from going extinct. The software tracks phylogenetic history to produce cladograms and comes with a DNA toggle board. It looks awesome.