The Science of Avatar

by Stephen Baxter

  On Pandora the multiple eyes have primarily evolved because of the varying light conditions. Maybe there is no single eye design that can handle the brilliance of a double-sun open sky, the bioluminescent shade of the forest, and the occasional deep dark night. For example a banshee’s primary eyes see in full colour, with vision roughly equivalent to a human’s. Its secondary eyes see in the near infrared, for night hunting: they are like military night-vision technology, capable of detecting prey through its body heat.

  Perhaps the most visually impressive of all Pandora’s creatures are the flyers.

  The banshees are reminiscent of pterosaurs, the flying reptiles of the past, or of bats, rather than birds. But they are also a little like stingrays or manta rays, another oceanic visual reference, and have jaws rather like fishes’, indicating a possible line of evolutionary descent. Flying is aided on Pandora by the lower gravity and the thicker air, which gives the flyer’s body more impetus with each stroke. But a downside is that the thicker air is harder to move through, and good streamlining is needed to achieve high speeds.

  On Earth, flight seems to have evolved independently among three groups of vertebrates (backboned creatures), the birds, the dinosaur-age pterosaurs and the bats (the insects also evolved flight, again independently). All these three groups descended ultimately from the same four-legged bony fish that crawled out of the ocean some four hundred million years ago, to become the progenitor of all vertebrate life on land and in the air. Each of the three groups used adapted forelimbs as wings—but in each group a different evolutionary strategy was used, as if the primordial skeleton was pulled this way and that into new forms. In the birds, the whole forearm flaps; a reduced hand with lost or fused fingers is an anchor for feathers. In the pterosaurs, the wings were sheets of membrane that stretched from a grossly extended fourth finger and were attached to the rear legs. And the bats don’t flap their forearms at all; their wings are membrane sheets attached to a frame made of hugely extended fingers. A bat’s wing is essentially its hand.

  The wings of a banshee consist of membranes stretched over a framework of bones, a little like a tent over a frame; they look something like the wings of a bat or a pterosaur. Each main fore-wing has three sail-like structures on the end, stretched over struts of bone. These vanes are used to generate extra lift and give fine control in flight. The wing also has an impressive claw.

  But there is the complication that the banshees also have hind wings. There are no vertebrate four-winged animals on Earth, though some insects have four wings—some of the Lepidoptera, for instance, the big group that includes moths and butterflies. These insects have various kinds of coupling mechanisms to ensure the wings work together. Those extra rear wings, plus the wing-tip panels, give the banshee additional control over its flight, as well as providing additional power when required.

  Flying animals have differing wing shapes, described by a number called the “aspect ratio”—the ratio of wing length to wing breadth. A long, narrow wing is aerodynamically efficient, but is energy-consuming to flap. So long wings are best suited to creatures that can fly in open airspaces, especially where you can just jump off a ledge to get your lift: these include the albatrosses, and the big pterosaurs of the dinosaur age, and the mountain banshees of Pandora. If you live in wooded country, the ability to take off from the ground, powered lift and manoeuvrability are paramount, so shorter wings are favoured. Thus the forest banshee has a much shorter wingspan than its mountain cousin.

  When Jake, undergoing the Iknimaya initiation trial, is taken to the banshee rookery to choose his mount, we see the banshees on the ground, where they look big, clumsy, ill-adapted; with their hind limbs having been adapted to wings they have no “legs” and must stump about on folded leathery wings. The great pterosaurs were similarly poorly adapted to the ground. The banshees have given up everything else for the sake of efficiency in flight, even their manoeuvrability on the ground. But then nothing will prey on them on the ground.

  Nothing save the leonopteryx.

  The “Last Shadow,” as the Na’vi call it, has a superficial similarity to the banshees, but a quite remote evolutionary relationship. The banshees evolved from four-limbed creatures, but the leonopteryx’s ancestors were six-limbed; it has two sets of wings like a banshee, but also a set of true legs, which the banshee does not. And its wings are composed of individual panes that can separate like a Venetian blind, or close over to form a solid surface; the vanes are a little like the big flight feathers of a bird on Earth.

  On a world where even great creatures like the banshees have something to fear, at least on Pandora you rarely need be afraid of the dark.

  The first time we really become aware of the ubiquitous bioluminescence of the Pandoran forest, the glowing of the living things, is during Neytiri’s first encounter with Jake as she saves him from the viperwolves. When she douses his torch it turns out he doesn’t need it to see, for almost everything around him shines of its own accord.

  The Greek roots of the word bioluminescence are “living” and “light.” Living creatures can emit light by releasing stored energy through chemical reactions, though the details differ from species to species. On Earth, bioluminescence is common in the deep sea, below around a thousand metres. Down there in the eternal dark, too deep for sunlight to penetrate, it’s thought that some eighty per cent of creatures exploit bioluminescence. On land, by comparison, it is used by very few—fireflies, glow-worms, a few fungi.

  In our oceans, bioluminescence is used for a variety of purposes. Some creatures use the living light to attract mates. But mostly bioluminescence is used in the endless game of predator versus prey. Many prey animals use the dark to hide in; they will descend into the deep dark during the day, and ascend to the food-laden surface waters only at night. So if you are a hunter, having a built-in headlight, as do many predators among the shrimps, fish and squids, can be very useful in tracking your elusive prey.

  Meanwhile some prey creatures like the benttooth bristle-mouth use bioluminescence as a kind of camouflage, to muddle their own silhouettes if they are shadowed against light from above. Another tactic is to raise a “burglar alarm,” to lure an even bigger predator to chase off the guy attacking you. And still another tactic, used by some shrimps and squids, is to startle a would-be predator by releasing bioluminescent material into its face.

  On the other hand, some predators use bioluminescence to attract prey. In the ocean, some of the decaying matter drifting down from above can be riddled with glowing bacteria; if you can mimic that glow, your prey animal can swim right up to you expecting to find lunch, only to become your lunch.

  In the Pandoran forest, bioluminescence is common among plants, and animals, such as the direhorses, exploit it too. Even the Na’vi have glowing skin-spots, yellow on blue, and they light up their Hometree with sacs of bioluminescent life forms.

  Why it is that so many land-based creatures on Pandora have chosen to exploit bioluminescence, compared to so few on the Earth? The answer is that so few nights on Pandora can rarely be truly dark in the first place, thanks to the spectacular light show put on by the two suns of Alpha Centauri, Polyphemus and the other moons. While the banshees for example have developed good night vision with their secondary eyes—and perhaps other animals have developed echolocation, a sound-based detection system like that of bats—many creatures have joined in a kind of cooperative light-based “arms race.” If everybody is kept flooded with light all the time you don’t need to evolve night vision or echolocation.

  Visually, the living lights of Pandora are one of the most charming aspects of the movie, even if bioluminescence isn’t used quite the way it is on Earth.

  There’s a great deal more detail on Pandora’s flora and fauna available in sources like the online encyclopaedia Pandorapedia. If you check it out you’ll find that Pandora’s invented biosphere has both intellectual and emotional depth.

  Intellectually, the designers have given all the
ir creations formal species names: thus the hometree species is Megalopedians giesei, Latin meaning the Great Tree. (And of course the tree has a Na’vi name, Kelutral.) This mirrors the biologists’ classification of life forms on Earth, which sorts out living things into hierarchies: you belong to a species, which belongs to a genus, which belongs to a family, which belongs to an order, which belongs to a class, which belongs to a phylum, which belongs to a kingdom. The five kingdoms, including animals, plants, fungi and bacteria, are at present the highest level of division; all living things on Earth are supposed to belong to one of them. Biologist Peter Ward has suggested that if we do ever discover life on another world we may need to extend the hierarchy upwards to include super-kingdoms, each covering all life on Earth, Mars, Titan, Pandora, the details depending on whether or not life on the different worlds is in any way related.

  And emotionally, the designers have tried to give us a visual sense of the interconnectedness of the Pandoran biosphere. Think of the ubiquity of the touch response we see in many of Pandora’s creatures, such as the helicoradian, and the way the mosses on the tree branches and light up in response to Jake’s footsteps, like a Michael Jackson video. Everything reacts to everything else, everything is connected.

  This quick tour of Pandora’s flora and fauna has shown us that some aspects of Pandoran life have parallels with Earth life—there are predators and prey, carnivores and herbivores—and some don’t have such parallels, such as the ubiquity of bioluminescence. But we’re in another star system here, on an entirely alien world. Why should life on Pandora have any similarities with life on Earth at all?

  And why, indeed, is there life here in the first place? Pandora is evidently habitable. Was it necessary that it should be inhabited?

  22

  WARM LITTLE PONDS

  We would have a much better idea of how likely it is that a world like Pandora will be found to host life if we had a clear idea about how life started on Earth itself. We have lots of plausible theories about that, but there’s no consensus.

  The question puzzled Darwin himself. His theory of evolution gives a convincing account of the story of life once it got started, but he says nothing on how that start came about in the first place. An old idea had been that life could simply burst into existence through “spontaneous generation.” For instance it had been believed that rotting meat spontaneously generated maggots. By Darwin’s time such ideas were already under attack from scientists like Pasteur.

  Darwin himself believed that for a whole life form to be generated from scratch was too much to swallow. Instead he mused about some kind of chemical evolution which might have led to the building blocks of life: “If we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, &c, present, that a protein compound was chemically formed ready to undergo still more changes…”

  A century and a half later this is still the essential thrust of thinking about life’s origin. If life emerged spontaneously on the Earth (and later I’ll consider the alternative, that it came from somewhere else), then it must, by definition, have emerged from some prebiotic (non-living) medium. And since Darwin’s time we have made some progress in figuring out how this happened.

  When did life form?

  Traces of life have been found in very ancient rocks, for example in the old and stable heart of Australia. Life seems to have got going on Earth almost as soon as it could—as the planet cooled from its formation, and as it recovered from the tremendous bombardment it suffered in the late stages of the solar system’s genesis. This leads to optimism about finding life elsewhere; if it started up on this world as soon as it was physically possible, maybe it will start up everywhere.

  As to where it first formed, Darwin’s suggestion of a warm little pond has been supplemented by ideas like the “deep hot biosphere,” prompted by the extraordinary discovery in the 1970s of life forms on the deep seabed, living in perpetual darkness, feeding not on sunlight but on heat and mineral seeps from volcanic vents. Some bacteria live even deeper, in the warm womb of the subsurface rocks. Some biologists suggest that even today most of Earth’s biomass may be down there in the rocks (and safe from the depredations of mankind, as I suggested in Chapter 2).

  How did life form? With Darwin, we don’t imagine that complete organisms emerged fully formed from some warm little pond, but more basic components of life may have: cells, perhaps, or self-replicating material. Some scientists argue for cells first, some kind of containment, perhaps based on mineral structures, that gave pre-life an isolated environment in which to develop. Others, like Richard Dawkins, believe replication must have come first. After all, replication, the transmission of information from one generation to the next, along with the ability to construct that generation, is the very essence of life.

  Baby steps towards working this process out were made through such experiments as that of Stanley Miller and Harold Urey in Chicago in 1952. They took a flask full of what was believed to have composed Earth’s early atmosphere—methane, water, ammonia, hydrogen—simulated lightning by passing electrical sparks through it, and were pleasantly surprised to find that a black sludge that collected in the bottom of the flask contained amino acids, constituents of proteins, which in turn are the building blocks of organic life like ours. This experiment itself turned out to be something of a dead end. An amino acid is a long way away from a protein in terms of complexity, and such acids are actually common in the universe, in interstellar molecular clouds. But still, this was conceptually at least a demonstration of how Darwin’s “warm little pond” might have worked to produce the materials of life from something non-living.

  Where did life’s complexity come from, though? Recent years have seen the rise of new ideas of “self-organising systems,” in which the repeated application of a few simple rules can lead to great complication. Examples in mathematics include the famous “Mandelbrot set” of fractal theory, an object of literally infinite complexity generated by applying a simple mapping rule over and over. American biologist Stuart Kauffman has developed ideas on how life might have arisen, and biological complexity developed, from the self-organisation of “auto-catalytic sets,” networks of chemical reactions with self-sustaining feedback loops. A catalyst is a substance that helps a chemical reaction take place. An autocatalytic reaction doesn’t need an external catalyst to work but generates its own, so once it gets started it just keeps going, rather like a spreading fire. Kauffman argues that the propensity of the universe to support self-organisation and the resulting emergence of complexity is the fundamental cosmic property that underpins the origin of life.

  Maybe these different threads of research will lead us eventually to a specific picture of how life like ours got started. Richard Dawkins has suggested that when we do figure out the answer, then rather like Darwin’s theory of evolution, it will turn out to be such a simple and compelling idea that in retrospect we will wonder how we missed it for so long.

  But until we have that answer opinion will remain divided as to whether life is likely or unlikely, and whether it is rare in the universe or commonplace.

  You can see that how likely you think it is that life emerged on a world like Pandora depends on whether you think the origin of life is likely or not. Francis Crick, the co-discoverer of DNA’s spiral structure, once wrote, “The origin of life appears at the moment to be almost a miracle, so many are the conditions which would have to have been satisfied to get it going.” But on the other hand the biologist Christian de Duve believes that life may be a “cosmic imperative,” its formation hard-wired into the laws of the universe, as much as are the formations of atoms and stars.

  At least we can cling for comfort to the basic fact that life clearly was created at least once. Otherwise, we wouldn’t be here debating the subject. That proves that the formation is life is possible. Given that undeniable truth, there’s at least a basis for hope that it could happen elsewhere.

&n
bsp; And one candidate answer to the question of how life began on Earth is: it didn’t begin here at all. It started up somewhere else, and travelled here…

  The idea of “panspermia”—life propagating between the worlds, perhaps even between the stars—goes back to the Greek philosopher Anaxagoras who as long ago as the fifth century B.C. imagined “seeds of life” spreading through the universe. A modern panspermia hypothesis was developed in the 1970s by astronomers Fred Hoyle and Chandra Wikramasinghe, who thought the process might be so commonplace that new viruses might be delivered to the Earth by comets almost daily.

  In the 1990s the study of the famous “Mars meteorite,” found in the Antarctic and presented by NASA as containing possible traces of Martian life, gave the idea renewed credibility. This rock had been blasted off the surface of Mars when an asteroid or comet struck, then drifted in space for perhaps millions of years, before happening to fall towards Earth. It endured a severely hot entry into Earth’s atmosphere before landing on the polar ice. Could this horrendously violent process transport, not just fossils as may have been present in the NASA meteorite, but living things between the worlds?

  Possibly. Jay Melosh, a specialist in impacts, has shown that a large enough impact can throw rocks off a planet without necessarily overheating them; a giant impact causes the surface rock layers to flex, and boulders are hurled away like dried peas off a trampoline. Melosh showed too that because Earth’s gravity well is a pretty large “target” for a drifting Mars rock, there has probably been quite a hefty transfer of material from the red planet to the blue over the aeons—though not so much the other way.