The Science of Avatar

²¹ A CLIMAX ECOSYSTEM

Among Avatar’s most wondrous sequences are those showing Jake Sully’s first encounter, in his avatar body, with the rich environment of the Pandoran forest. OK, it ended up with a lot of running away from a thanator. But who could forget Jake’s discovery of those big spiral trumpet-like plants (the helioradians) that, at a touch, shrank down into the ground?

How did Avatar’s designers dream up such a marvellous and convincing world?

First impressions: the ecosphere we see on Pandora is evidently a kind of rain forest, dominated by the tremendous trees that are so important to the Na’vi. Various other flora include what look like Earth’s ferns, palms, bamboos and grasses. Pandora is evidently an environment as rich in resources and energy flows as tropical Earth, and natural selection has produced an ecology as diverse and complex as anything on Earth.

However Pandora’s conditions differ from Earth. The lower gravity, thicker air, strong magnetic fields and different day–night cycles have all shaped the evolution of life, as we will see. One obvious example is gigantism; thanks to the lower gravity, many of the plants we see are like terrestrial forms grown huge. As for magnetism, the anemonid is a carnivorous plant that absorbs metals from the soil, giving it the ability to use Pandora’s magnetic field for movement, a feature RDA’s biologists refer to as “magnetonasty.” And a plant called sol’s delight, or Calamariphyllum elegans—“elegant squid-like plant”—is “magnetotropic,” that grows in the direction of magnetic fields. The “delight” name comes from the fact that the plant helps RDA’s miners detect unobtanium deposits by conveniently straining towards them.

But in devising this ecology, as with other aspects of the movie, the designers have always kept in mind the audience’s needs. They have given us a world that is strange, but with elements of the familiar from Earth, twisted and distorted to give an impression of the alien. That’s why Pandora is green! Plants on Earth are green because of the chlorophyll in their cells, the chemical compound that supports photosynthesis, processing the energy of sunlight for growth. Maybe, as sunlight is such an easily accessible energy source, on worlds with transparent atmospheres like Pandora and Earth some kind of photosynthesis is always likely to evolve. But there are different chemical ways to achieve photosynthesis; leaves don’t have to be green. The greenness of Pandora is a design choice.

The trees are the single most important element of the Pandoran forest, as in all forests on Earth. The main canopy tree is called the beanstalk palm, growing as much as a hundred and fifty metres tall. To the Na’vi it is tautral, the “sky tree.”

Earth’s greatest trees, the sequoias, don’t grow as high as this, but they are remarkable organisms in themselves. Today, sequoias are confined to a strip of the Pacific coast of North America. They flourish in the mountains, which trap moisture coming off the ocean; the tallest specimens grow in valleys and gullies where streams flow year-round and there is regular fog drip, which helps keep the trees’ upper leaves supplied with moisture. The sequoias are part of a habitat which supports many species of plants and animals. In the 1990s, tree-climbing biologists discovered a treetop ecology based on soil that had formed high above the ground from leaf mulch and other decayed vegetable matter.

Sequoias can be as tall as a Saturn V rocket, and older than Christianity. They are remarkable inhabitants of planet Earth.

Meanwhile in the undergrowth, deep, rich, dense and luminous, visually the Pandoran forest has something of the feel of the underwater world—you might be reminded of a coral reef, perhaps. In fact on some coral reefs there is a shrinking-trumpet plant like the helicoradians Jake encountered, the “Christmas tree worm,” Spirobranchus giganteus, that does indeed withdraw into a tube when disturbed. The woodsprites, “seeds of the sacred tree,” look a lot like jellyfish. Much larger jellyfish-like beasts float by like natural airships. The Mother Tree in the Tree of Souls has tendrils that resemble the tentacles of sea creatures. This oceanic influence is no surprise. After James Cameron completed the very aquatic movies The Abyss and Titanic, he made six deep ocean expeditions, filming in 3D. At the time of writing he is planning an expedition to the Pacific’s Mariana trench, the deepest point on Earth, a point nobody has visited since 1960.

And Cameron did base his vision of the forests of Pandora (partly) on the coral reefs he encountered in the ocean’s depths. This is appropriate because a coral reef, like a rain forest, is an example of a “climax ecosystem,” a complex and rich environment in which large numbers of animals and plants have coevolved.

It was Charles Darwin himself who first figured out how coral reefs work. Corals themselves are tiny anemone-like organisms that leave behind tough little skeletons. (In the past, reefs have also been built by other organisms such as algae, sponges, molluscs and tube worms.) With time these skeletons can heap up into huge reefs; Australia’s Great Barrier Reef stretches for two thousand kilometres around the north-east coast of Australia.

The secret of a reef as a habitat for life is that it “fills in” what would otherwise be an empty column of water above a flat ocean floor. A reef is a highly complicated three-dimensional structure, full of crevices and folds and cracks, ripe for colonisation by other life forms. On land, forests do the same thing, the tall trees rising up from the ground to vastly increase the effective surface area available for life. And so coral reefs are thick with fish, molluscs, sponges, echinoderms (starfish and sea urchins) and other forms of life, all shaped by evolution into complex chains of symbiosis, competition and cooperation—just as we glimpse in the Pandora forests. (Ironically, on Earth the coral reefs that are such an inspiration for Avatar are dying back. This is an ecological disaster but also a human one, as there will be economic losses for fisheries and tourist resorts, and coastlines will be left less protected from the ocean.)

But on Pandora, within that intricately interconnected biosphere, there are a rather large number of living things that bite.

The first truly spectacular animal that avatar-Jake confronts is a hammerhead titanothere. This is a massive six-legged quasi-rhino, heavily armoured, with a “hammerhead” muzzle reminiscent of another aquatic creature, a hammerhead shark. And the beast has a spectacular threat display, designed to scare off any ambitious predators, and indeed would-be rivals from within the titanothere’s own species; these are very territorial animals.

But the hammerhead, a herbivore, is somewhere near the bottom of Pandora’s land-based food chain. The hammerhead eats the Pandoran equivalent of grass, shrubs, leaves, and in turn is eaten by predators like the viperwolves. These scary beasts are six-legged pack hunters that run like dogs, but are also nifty climbers thanks to their ape-like paw-hands. And they are highly intelligent, as you can tell from onscreen evidence of communication as they hunt Jake. There are evidently creatures that prey on the viperwolves in turn, such as the thanator, a beast like a lion or a panther, a relative of the viperwolf.

Similarly there is a food chain of the air. The mountain banshees, graceful pterosaur-like flyers, are also pack hunters, aerial equivalents of the viperwolves—and again an even scarier hunter preys on them, the mighty leonopteryx.

We always see the thanator alone, like the leonopteryx, and this makes sense from what we know of food chains on Earth. On our planet each step of consumption up the chain is only about ten per cent efficient, in terms of nutrient value. A thousand tonnes of grass can support a hundred tonnes of hammerhead meat, which can only support ten tonnes of viperwolf meat, which can only support one tonne of thanator meat… So if you are an “apex predator,” as a thanator or a leonopteryx is believed to be on Pandora—or a lion, or a tyrannosaurus rex on Earth—the land can only support a small number of your kind. It’s thought that the range of a single tyrannosaur might have been hundreds of kilometres; a thanator’s range is three hundred square kilometres. On Pandora or Earth, for six limbs or four, the rules of the natural economy are set in stone.

This dog-eat-dog (or viperwolf-eat-viperwolf) aspect of life on Pandora is reflected in something else we see onscreen: arms races between predator and prey.

To escape a big fast scary predator, you either evolve to run fast, like the slender deer-like hexapedes, or you evolve heavy armour, like the hammerheads—or you do both, like the direhorses. Another possibility is to use threat displays like the hammerheads, effectively startling away the hunter, if you’re lucky. Meanwhile your hunter in response is evolving to run ever faster, brandishing ever sharper teeth… The end result of an evolutionary arms race is a killing monster like a thanator or a tyrannosaur hunting down a tank-like prey animal like a hammerhead or a styracosaurus—which was a rhino-like dinosaur with a horn on its nose, bony bosses over its eyes and cheeks, and a bony frill over its neck with even more long and pointy horns.

Although Pandora is presented to us as a world of natural harmony, nature is evidently red in tooth and claw here: a world so tough that even apex predators like the thanators need to be armoured. And while the forest has a dreamy oceanic visual feel, the ferocious predators and their heavily armoured prey drew inspiration from the mighty creatures of Earth’s dinosaur age.

But there are gentler elements too. Many of the animals are social, the direhorses, the buffalo-like sturmbeests in their herds, the banshees in their flocks. And we glimpse family groups, the sturmbeest on the move protecting their calves, the direwolf cubs playing.

You’ve no doubt observed that many of Pandora’s animals share common features: six legs, two neural whips (called “queues” in the Na’vi, as they are wrapped in braids of hair), supplementary breathing holes, and four eyes. This applies to flying creatures like the banshee and leonopteryx, as well as to the ground animals from the hammerheads to the thanators. (The exceptions to the general plan are the Na’vi and their apparent relatives the prolemuris, as we’ll see in the next section.) This convincing consistency is a testament to the disciplined imagination of the movie’s designers, and to their inventiveness, such as in the plausible-looking gait of the many six-limbed animals, and the sensible-looking flapping of four wings.

The antenna-like neural whips are used to link the nervous systems of animals, and to link Na’vi to animals, and indeed to link Na’vi to Na’vi. While a Na’vi has just one queue, many animals have two whips. The equine direhorses connect with each other through their whips, bonding emotionally but also passing on information about food sources and threats. We’ll look more closely at neural queues when we come to consider the Na’vi themselves, as well as the Eywa neural network.

What of the multiple breathing holes shown on many of the animals? On Earth some insects have “spiracles,” additional body vents to take in air. On Pandora the vents are for supercharging—taking in more oxygen quickly, a feature that is particularly useful for flying creatures, and we do see prominent vents on the banshees, which, like birds, burn up a lot of energy and need efficient heat-loss systems. But the vents are also a relic of an early stage of the movie’s design process; Cameron wanted some of the animals he envisaged to have the feel of automobiles, and the air vents are a trace of that source of inspiration!

Those multiple eyes are another striking feature. Why would you need two sets of eyes? On Earth, though insects may have many sets of eyes, one pair seems standard issue across the animal kingdom—though a bivalve mollusc known as the “thorny oyster” (Spondylus) has multiple eyes scattered around the edge of its shell. There is a South American fish called the anableps that rises to the water’s surface to seek prey in the air, but while it hunts it is in constant danger of threats from below. So each of its eyes works as two separate optical systems, an upper one for aerial vision and a lower one for aquatic vision; the creature can watch for danger from below while it stares up into the air for its food. These systems have separate retinas, but there is only one optic nerve per eye—two eyes acting as four.

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 their 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.

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.

And, remarkably, it appears that some microbes could survive the multi-million-year journey from Mars to Earth, even without the benefit of cryosleep. On Earth, microbiologists have found fossilised microbes in salt strata two hundred and fifty million years old, some of which, when treated with tender loving care, revived.

All this makes panspermia a terrifically exciting idea once again. And since it seems likely that Mars was “warm and wet” and a suitable haven for life long before Earth ever was, though it has “aged” much faster, and since it’s easier to get from Mars to Earth than the other way around, perhaps Earth life actually originated on Mars. Perhaps we are all Martians!

But what of Pandora? Could the Pandorans be descended from Earth life? Or even, could we all be Pandorans? The latter possibility actually seems the more likely of the two, given that Alpha Centauri is older than the sun by a quarter of a billion years or so (as astrophysicists can tell from the composition of the stars).

But the transfer of material across interstellar distances seems much less likely than between the planets. Mathematical simulations by Jay Melosh showed that over the life of the solar system probably only a handful of rocks from Earth have ever made it to Alpha, and vice versa, and even then the chance of any of them landing on a planet is slight. But it’s not impossible.

And there’s one other extreme possibility, which is referred to by the dry title of “directed panspermia.”

Never mind hitchhiking on rocks. In Avatar the starships of RDA have transported Earth life to another star—and have brought Pandoran life back to Earth too. If we have moved life between the stars, maybe other intelligent species have done it before. Maybe in some sense this is the purpose of intelligence, to be carriers of life between the worlds, whatever else we think we are doing.

I leave it to you to tell Colonel Miles Quaritch that he is actually an interstellar spermatozoon.

Philosophically, panspermia (directed or not) is something of a cop-out. It just puts off the deeper question of life’s ultimate origin. But what a marvellous idea it is: how emotionally satisfying. And if some day we do reach Alpha Centauri, and if we do find life on an Earth-like world, I don’t know if it will seem more wonderful to find our cousins, or a different sort of life entirely.

But even if we knew how life started on Pandora there would be more questions to answer. Whether or not Pandoran life is related to our own, we have been separated by light years, and presumably by billions of years of divergent evolution. How likely is it that Pandoran life will have any similarities to our own?

²³ FOUR LEGS GOOD, SIX LEGS BETTER

Some elements of what we see of the living things on Pandora are familiar. There are complex multicelled life forms all over the place. Among the fauna many are vertebrates, with interior skeletons, just like us. Among the flora there are trees and flowers. There are herbivores and carnivores, and food-chain distributions of predators and prey.

Then there are differences. The land animals generally have six limbs where we have four. If they have fingers, unlike us “pentadactyls,” they have three plus a thumb—like the Simpsons of the TV show.

How likely is it that if we were to travel to Pandora, even assuming life got started there, living things would have anything even remotely in common with us? Or, you could ask alternatively, why should we expect the Pandoran biosphere to have any significant differences from our own? Why should the Na’vi look even slightly different from humans?

Exploring such questions is illuminating the history of life on Earth, as well as giving us the means to guess at what we might find on alien planets.

It used to be a given, I think, that other worlds would be inhabited, and probably dominated by humanoids more or less like us. That’s what John Carter found on Barsoom, in Burroughs’ Princess of Mars. Dejah Thoris tells Carter, “Nearly every planet and star having atmospheric conditions at all approaching those of Barsoom, shows forms of animal life almost identical with you and me.” This idea has become known as “convergent evolution,” the notion that similar conditions must mandate similar evolutionary outcomes.

Confidence in this idea was dented during the twentieth century as a result of a growing understanding of the complexity of life from the intricacies of DNA on upwards, and the discovery of the apparent chance events that have shaped life’s evolution, such as the wipe-the-slate-clean asteroid impact that rid the world of the dinosaurs.

By 1985, biologists like Stuart Kauffman were asking what would happen if the story of life were to be rerun from the days of the earliest Precambrian era, when the first life formed. If you could act out the drama again, how much of the result would be familiar, and how much not? Or to put it another way, what properties are “easy” for evolution to produce, and what difficult? What properties of life are “necessary,” and what are “contingent”—just one-off accidents? The debate has intensified since, with the late American biologist Stephen Jay Gould in one corner, who claimed that practically nothing would be repeated, to the British biologist Simon Conway Morris in the other, who has argued for inevitability both at the morphological level—the alien must look more or less human—and the metabolic—it must use something resembling our DNA wet chemistry.

Remarkably enough, the history of life on Earth has provided us with a series of natural experiments to test these ideas.

Thanks to continental drift many landmasses have spent tens or hundreds of millions of years more or less in isolation, including Australia, New Zealand, Madagascar and South America. There is isolation in time too: the long dinosaur evolutionary experiment was cut short by the asteroid, to be replaced by a mammalian equivalent later. It is as if the world has been filled with a series of its own Pandoras, isolated by sea rather than space, years rather than light years—and each has been a laboratory of evolution.

And what we observe in this natural laboratory is that life on Earth does seem to keep rediscovering familiar patterns.

The tree, so important on Pandora, is a classic example of convergent evolution. A “tree” is actually defined, for a biologist, by its form: a woody plant, with secondary branches supported clear of the ground on a single main stem, or “trunk.” And tree forms have emerged in many divergent classes of plants. Most trees today are fruit-bearing (angiosperms) or coniferous, but the earliest trees on Earth were tree ferns, horsetails and club mosses, which grew in the forests of the Carboniferous era some three hundred million years ago. These could be every bit as tall as modern trees. There are still tree ferns around, but the descendants of the horsetails and club mosses no longer have tree-like forms. The tree body-plan is obviously a universal response to similar environmental challenges: trees arise wherever a plant has to grow tall to compete for the light, while staying rooted in the ground for nutrients. So it’s no great surprise to see trees on Pandora.

Among the animals, too, we see divergent creatures evolving similar forms to fill particular roles. Whatever animal kingdom is dominant there are always herbivores and carnivores, grazers and browsers, runners, flyers and swimmers; there are always food chains and predator-prey hierarchies, just as we observed in Pandora. This applied among the dinosaurs as it does among the mammals; it applies in the oceans as well as on land. Thus the mammals, starting from the runty, squirrelly stock that survived the dinosaur era, quickly evolved ferocious predators and fleet prey to fill the stage vacated by the dinosaurs.

One of the most fascinating examples, to my mind, is New Zealand, where there were virtually no native mammals at all aside from bats, and all the usual roles were filled by descendants of the birds and insects and bats that flew there, or were blown over from the mainland. Thus the huge moas were flightless browsers, preyed on by giant eagles. This unique ecology was broken up when humans arrived around thirteen centuries ago.

It is as if there are a series of “niches” out there in evolutionary space, idealised roles which if left empty will be filled by some creature or another, given time for natural selection to work. Evolutionary biologist and science-fiction writer Jack Cohen says there are evolutionary “universals”: features that will usually, perhaps always, crop up in an ecology, and which we could then expect to find in an alien ecosphere.

This applies to features of the body too. Eyes are a famous example. Life on this planet, from insects to crustaceans to humans, seems unreasonably eager to evolve eyes. Nine different physical principles have been used to evolve eyes, each of them occurring many times in nature. Perhaps on a planet with a transparent atmosphere and abundant light, like Earth, like Pandora, eyes are such an obvious advantage they should be considered a universal. As we saw in Chapter 21 another common example of multiple evolution is flapping flight (bats, pterosaurs, birds, insects). We see plenty of flying creatures on Pandora.

On the other hand, there are body features, behaviours and life strategies that have emerged only once on Earth, as far as we know. An example is the diving bell spider. It’s not the only air breather that has taken to the water, but unlike such creatures as dolphins that need to return to the surface to breathe, the spider uniquely (apart from humans) takes down its own air supply with it.

Meanwhile, Jack Cohen says, alongside evolutionary “universals” there are “parochials”: unique solutions, or one-off details the choice of which doesn’t make much difference in the grander scheme of things. Earth’s backboned animals all share a four-legged body plan because we happened to inherit it from the first fishy mud-skipper-like beasts that crawled out onto the shore. If those early ancestors had happened to have six limbs or eight, then, I suppose, so would we—and we can guess that the equivalent pioneer mud-skipper on Pandora must have had six limbs, given the prevalence of that body plan among the fauna there.

But we may be thinking on too small a scale.

I suspect a convergent-evolution sceptic would protest that I’ve been much too narrow in picking examples of convergence mostly from multicellular vertebrate creatures. Well, we are multicellular vertebrate creatures, and it’s natural for us to think that evolution had to produce something like us, and so we look for similarities with creatures like us. But multicellular vertebrates are a small subset of the panorama of possibilities for life—and possibly not an inevitable one.

Consider this. Life on Earth is some four billion years old, and got going here as soon as it could. But multicellular creatures only arose some six hundred million years ago, in the last one-seventh part of life’s long history. Maybe that’s telling us that life is common, for it arose quickly on Earth, but multicellular life isn’t, for it arose so late. Maybe when we get to Pandora we are most likely to find life, but not multicelled complex life—nothing but slime and algae and huge dreaming mounds of bacteria, with not a snail to feed on them.

And even if you have multicelled life, it doesn’t have to have a skeleton. The Na’vi and plenty of other fauna of Pandora evidently do have internal skeletons, as we see from the leonopteryx skull hanging up in Hometree, and the frame of bones, vertebrae and ribs, to which Grace and Jake are strapped during the assault on Hometree. But again, on Earth, vertebrates evolved relatively recently, something like five hundred million years ago. The first vertebrates were the fish, and today the class includes the mammals, the birds, the reptiles and the amphibians. But they, we, represent only five per cent of the planet’s animal species. The majority of the planet’s multicellular organisms, such as sponges, flatworms and mollusc, get along fine without internal skeletons. Maybe if you re-ran evolution it wouldn’t be necessary for vertebrates to evolve on Earth at all—or, indeed, Pandora.

As noted before, in the context of Avatar we always have to remember “creative licence.” The viewing audience needs to be able to recognise what it sees onscreen, and yet have a feeling of alien-ness. There is a tension between the familiar and the strange. Thus the direhorse is recognisably a “horse,” even though once you recognise that you immediately start to spot differences from terrestrial horses. Images of a world without vertebrates would, I suspect, have simply been so strange visually as to baffle us.

As with the issue of the origin of life itself, the jury is out on convergent evolution. There’s only so far you can go with speculations based on the single example we have, Earth. We’re just going to have to go out to Mars and Titan and Pandora to find out.

But certainly we can say that the flora and fauna of Pandora as depicted onscreen, with its mixture of universal features—predators and prey, eyes, wings—and parochials—six limbs and four fingers—do present in many ways a pretty plausible picture of how life on other worlds might appear. And it’s a tribute to the imaginative discipline of the movie’s creators that nothing on the screen is there just because it looks pretty; everything has a role in the greater ecology—everything we see is there because it needs to be there.

On Pandora, however, human explorers found, not just life, but intelligent life.

They found the Na’vi.