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Orbital photo of Lyr, an experimental world-model, over twice Earth's diameter, six times Earth's air pressure, and 15 times Earth's water. 95% of the surface is sea, 5% land. Orbital photo of Lyr, an experimental world-model, over twice Earth's diameter, six times Earth's air pressure, and 15 times Earth's water. 95% of the surface is sea, 5% land. EVOLUTION ON LYR

by Chris Wayan, 2006

Lyr (home) - map - creatures - cultures - evolution - climate - geology - gazetteer - nomenclature - definitions - building Lyr - more worlds? Planetocopia!


Lyr's a world-model challenging exobiologists like Peter Ward Douglas ("Rare Earth"), who say complex life will only evolve on worlds almost exactly like Earth. Lyr is emphatically not Earth! Seven times as massive, in an eccentric orbit too far out from its dim little sun, with the wrong density, wrong tilt, wrong satellites, wrong geology, wrong water content... can you get wronger? Douglas says big wet worlds like Lyr will be (at best) world-seas, poor in minerals, with sparse unicellular life at most, and if it's multicellular than not intelligent, and if intelligent than not technological.

Tell that to the Lyrans.


Earth has remarkably thin air for a world its size. Smaller Venus and Titan have more. What would Earth be, if we had, say, double or ten times our current atmospheric pressure? Looking at Venus, you might assume the only possible answer is "unlivably hot due to runaway greenhouse effects." Are you sure? Earth has survived much warmer conditions in the past. Life does fine in hot water--it's a very limited niche on Earth, so such organisms are simple, but if our oceans were hot, why shouldn't multicellular life evolve? The biochemistry, enzymes in particular, would have to be heat-stabler, but similar shapes and behaviors should evolve--tails still propel, senses orient and warn, shells protect. Maybe hot-water, thick-aired worlds make BETTER biospheres. How can we know without examples?

Yes, Venus boiled off. But it has twice our sunlight and ONE MILLION times our CO2 level. Of course, life may CREATE thin air--mining the atmosphere for carbon and nitrogen. And liquid oceans do dissolve and precipitate things, including CO2. Primal Earth presumably had much more CO2 and a higher surface air pressure--but was it as dense as Venus? Anyway, today in our solar system, Earth is a thin-aired anomaly. Of the worlds with atmospheres thick enough to visibly affect their surfaces (in order of atmospheric density: Jupiter, Saturn, Neptune and Uranus, Venus, Titan, Earth, Mars, Triton) we're seventh of nine--and only one, Titan, has a pressure anything like ours.

Say I'm wrong, and Earth's an average biosphere. Let's do a thought-experiment. Can you imagine a planet with advanced life and healthy biosphere, with an atmosphere one-tenth of ours? Just barely. Thin air means harsh climate! Yet it's easy to picture worlds with atmospheres ten times thicker supporting life. A hundred? Why not? Such a world would have to orbit further out to balance its greenhouse effect, but so? Where air is concerned, we're nearer the bottom than the top of the range.

Such thick-air worlds increase both the number and richness of potential biospheres:

  1. With dense air to distribute heat, slow-spinning worlds and highly tilted worlds may be still viable even if large parts of such planets face long dark nights--or sunless winters. Thick air may even allow life on worlds with highly eccentric orbits, mitigating the harsh orbital seasons.
  2. If a world's habitable at all, most of it will be. Thin-aired worlds like ours can have wide sterile belts--desert zones and polar caps. Temperature gradients are much evener on thick-aired worlds. Venus's poles and equator may be hot, but they're equally hot, unlike the Terran/Martian patttern. Dense life competing in a stable environment's more likely to turn multicellular (and intelligent) than life struggling mainly against a marginal, Martian landscape.
  3. Thick-aired worlds, with their stronger greenhouse effect, can orbit further out, where the zone in which water is liquid (and life can evolve) is much wider. This doesn't automatically increase the number of such worlds--the match between atmosphere and orbit is still a matter of chance--but rigid formulas declaring outer solar systems totally sterile are just plain wrong. Small red stars, for example, have been written off, since their liquid-water zone was so close that tidal drag becomes a problem. But thick-aired worlds could orbit further out, where they run no risk of ending up with one face always to the sun. Even if most small stars are likely to have equally runty inner planets, a minority will have an Earth (or a small Neptune) in this outer life-zone. And such stars are so common that we're talking a LOT of worlds.
  4. Multiple worlds in one system can be viable, if a thick-aired world or two have outer orbits while thin-aired ones huddle close to the sun. If life evolves on any one world, meteor strikes will spread it to others where it might never have evolved but can survive, adapt, and flourish. It's quite possible that optimal birthplaces for life and ideal biospheres (big, diverse biomass, complex forms, intelligence) are quite different. That could even be true of us--it now looks possible life evolved on Mars and spread to Earth (admittedly, Earth's abyssal rift gardens are strong candidates for biogenesis).
Two thin-air scenarios: Mars Reborn, and Serrana. Thick-air scenarios? Venus Unveiled and Lyr itself.


Let's look closer at thick air's effects on a biosphere--beyond simply making life possible on many otherwise sterile worlds.

One obvious effect on flora is that winds will pack a heavier punch. They'll be slower, and it's tempting to conclude from the savage winds of Mars and the slow surface winds of Venus that thicker air just slows down, creating about the same wind-force on most rocky worlds. But on fast-spinning planets like Lyr this is unlikely; winds may not pack SIX TIMES the force of Earth's hurricanes (there's less sunlight to heat the air, and less of a temperature gradient from equator to pole) but I expect total wind forces to be substantially higher than Earth's.

Trees will respond with either great flexibility (the kelp solution), increased buttressing (the mangrove solution), or lower wind resistance (the, uh, fennel solution). The huge platelike leaves found in Terran jungle understories would soon be torn apart on Lyr; most leaves will be highly divided, even feathery.

A corollary: vines, which are largely parasitic on earth, could be symbiotic in Lyr's denser forests--they tie together the whole canopy, supporting weak links among the trees. It'd be a trade-off for a tree--reduced light for some extra buttressing.

The effect on fauna? Flight! Such a dense medium will allow quite modest wings to support larger animals than have ever flown on Earth--despite the somewhat higher gravity. The competitive advantage is so large and the flight-specializations so much smaller that I'd expect all small and most midsized creatures to fly. Creatures big enough to have...


Air pressure affects the evolution of intelligence. On thin-aired worlds like ours, the weight limit for flying creatures limits body size--and therefore brain size. The limit's not absolute--we can imagine, even perhaps bio-engineer intelligent fliers (scaled-up ravens or parrots, or pygmy angels, or giant bats, or smart pterodactyls, whatever) but such forms are unlikely to evolve in the wild, where no living fliers weigh over 15 kilos. Yet pound for pound, the smartest brains on Earth are avian--parrots and ravens rival chimps with many times their brainweight! The fact that Earth birds are as bright as they are, given their weight constraints, suggests that life on the wing stimulates the mind: 3D acrobatics, the sheer mental stimulation of travel between varied ecozones, the need to memorize landmarks and navigate intelligently, and especially the stimulation of social contact over long distances. I'd add one less obvious feature of life on the wing. Birdwatching is popular among mammals (not just humans--think of cats!) because birds are uniquely visible. They can afford to be--on the wing or high in trees, safe from most nonfliers, they needn't hide as mammals must. But the reverse is also true--birds are mammal-watchers. They constantly observe life below them--not just for food or safe perches, but for sheer entertainment. Birds are voyeurs. We're spread out to them; they witness the survival strategies of other species--strategies with useful lessons, if they're bright enough to learn them.

Flying, in short, is educational. A rich information environment favors clever, social, observant, opportunistic generalists, and that's just what parrots and corvids are--up to the limit Earth's current air-pressure imposes!

I conclude that thin-aired worlds like Earth may not develop intelligent fliers, but worlds like Lyr certainly will. The, uh, pressure is there.

High orbital photo of Lyr, a sculpted 30


Two evolutionary forces are at war on big, wet worlds like Lyr. One is continental isolation far more extreme than Earth's--our continents have all touched within the last 100 million years, swapping genes like bacteria mating--while most of Lyr's lands have never touched. The counter-force is winged migration, for Lyr's thick air means most species will fly. But will they migrate over such wide seas? Is it worth the risk? Earth's harshly seasonal poles host a lot of migratory species; but will Lyr's milder polar lands, dark but not deadly cold in winter, really motivate fliers to risk long migration?

After designing Lyr, I stumbled on an old science fiction novel with an ecological model like this: THE MAN WHO COUNTS, by Poul Anderson. Diomedes is twice Earth's diameter, with a sharp axial tilt, low density, Earthlike gravity, swift rotation, thick air, and a metal-poor surface. Practically Lyr! It's a masterful job of speculation, especially since it was written nearly 50 years ago. Time has exposed only a few flaws in Diomedes:

  1. An unconvincing mineralogy--not just metal-poor, but a near-total absence of heavy elements. Anderson was trying to keep the gravity down, of course (and make local food useless to his human castaways, for plot reasons). I'm leaving Lyr's exact composition a black box, but I do think middleweight worlds forming in a cooler zone than Earth will have some heavy elements. Even Jupiter has an iron core. And on a rocky world, convection and eruption bring some of those metals to the surface.
  2. Diomedes is sharply tilted like Uranus, so winters are long, dark, and harsh. Life hugs the equator, and ice covers even mid-latitude islands. Anderson naturally but incorrectly assumed dark means cold. Our better knowledge of greenhousing suggests that Lyr's (and Diomedes') dense damp air would even out worldwide temperatures and keep the poles thawed out, even in darkness. It happened on Earth: ancient Antarctica was polar yet wooded. Where two miles of ice lie today, dinosaurs roamed under the trees through the dark winters. That's what just a little extra CO2 can do--now picture the effect of SIX atmospheres loaded with water vapor (another strong greenhouse gas). Anderson, needing these harsh winters for his plot, asserts Diomedes has low CO2 levels, even though it's tectonically active enough to raise many lands above its world-sea--do Diomedean volcanoes have CO2 scrubbers on them? Worse yet, he claims there's not much water vapor (above a world-sea? No way!) And even if we conceded him such an atmosphere, I think it'd still be warm enough to keep the ice away... sheer quantity counts for something.
  3. Diomedes' air is odd in a second way--it's 79% neon! It's poor in nitrogen and oxygen, not just CO2 and water vapor. All that neon smells like desperation to me--Anderson needed SOMETHING his shipwrecked human characters could breathe at six atmospheres, and he was worried about nitrogen narcosis and oxygen-burned lungs. I'd rather let tourists suffer and make Lyr's air more plausible. Titan suggests that over a wide range of temperatures, nitrogen will predominate. I've allowed far more light, inert gasses (helium, neon, argon) on Lyr than Earth has--but nothing like Diomedes! And would six atmospheres be intolerable? Cousteau and Sylvia Earle, both pioneer divers, say oxygen is breathable short-term up to 8 times normal pressure (it's 3.6 on Lyr); nitrogen narcosis varies by person but often starts around 4 or 5 atmospheres, growing severe around 7 or 8; Lyr's partial pressure of nitrogen is 5.2, so for many of us the rapture would be mild. Could your body adapt over time? I've found no data--as far as I know, no one's lived in such an atmosphere long-term. Deep-sea missions understandably add helium to the mix, not wanting drunken divers! But consider how humans adapt to altitude--it takes time. We may not know our outer limit on the high end. Anderson's desperate neon strategy is unlikely--and possibly unneeded.
Diomedes, despite its flaws, is still far more nuanced than Peter Ward Douglas's simple dismissal of big wet worlds. Consider just one of its implications: given Diomedes's icy winters, Anderson logically postulated migratory, seasonally breeding, winged people. Would this hold for Lyr? Facing milder winters, Lyran fliers have less reason to migrate--and Lyr's deep seas and lonely continents make migration riskier. Prohibitively so? I'm not sure.

My best guess: some athletic species will migrate, following the light, but they'll pay a price. The deep-sea crossing will limit and shape their physical evolution and culture. They'll have to be light and travel light. Most species will stay put through the dark rainy winters... and that will shape them. Even Anderson's model had a few sedentary cultures breeding year-round--though the migrating majority called them perverts!

Is either path more conducive to intelligence? To civilization? Migrators get exposed to other lands, cultures, perhaps to other intelligent species... On the other hand, migrators' material culture must stay light. And fire would give the stay-at-homes a great competitive advantage--any group adopting it would flourish. Pressure toward civilization? But would fire be discovered that easily, on a damp, lush world like Lyr? Prehumans discovered fire on a dry savanna crackling with thunderstorms. Are we so sure ALL early people find fire? Even elephants, living on the same savanna, with bigger brains than our ancestors had, never picked up the torch... But then, elephants had no larger predators to fear; we did. Fire may have saved our species. The elephants' lesson is: four factors lead to fire: brains, hands, opportunity and need. Elephants didn't need fire. Lyrans hunkering down for winter surely will, but opportunities may be scarce. My guess is, fire will be late on Lyr, and metalwork even later.

Some species won't migrate--not all intelligent species will fly! Amphibious reef dwellers, arboreals in the warmer rainforests, hoofed or elephantine grazers (and their predators) on the prairies... Unless migrators build a worldwide culture based on their informational and trade advantages, these wingless peoples (far more varied and isolated than early hominids) may develop wildly diverse cultures.


Lyr is warm and wet, but sunlight's dim and reddish. The main effects on life:

Rainforests with slow turnover and relatively open understories. In the damp warmth, decomposition is nearly as fast as in the Terran tropics, but plants can't grow as fast, and ground-level herbiage is lightstarved. The result is a dense canopy of mature trees, but little brush and few seedlings--just low grasses and herbs on the ground. Even canopy trees will be economical, shedding as little as possible; leaves will have a long life and be highly efficient. Most of the mid-story will be vines.
The center of animal life will be in the canopy. Flyers and climbers will be lightly built and big-eyed, with slit pupils to maximize the range of apertures and a reflective layer like a cat's. Skulls will widen to make room; faces will be flat or wedge-shaped to accomodate the huge eyes. Even diurnal Lyrans will look nocturnal to us piggy-eyed Terrans! To protect such large eyes in flight, a transparent inner eyelid may evolve in many winged species. All will have long rain-shedding lashes. Basically, Lyrans look like overly cute cartoon animals--anime characters with too much makeup. And here you thought that was just my bad taste! Heavens no, I'm not a furvert. This is logical extrapolation. Ignore the snickering in the underbrush.
Map of Lyr, a world-building experiment. Click a feature to go there.
Gazetteer: index of places, with descriptions. Or...

TOUR LYR! Climb volcanoes, swim seas, meet weird creatures. First: survival tips! Then, pick a region:
Ythri -- Polesotechnic Chain -- Troisleons -- Roland -- Oronesia -- Gaiila -- Flandry -- Diomedes -- Ak'hai'i -- Averorn

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