23 December 2009

A Brunch Film Review: Avatar Part II: A biologist examines Pandora

If Avatar is a film that works for the viewer, a large part of that will be because of the convincing ecology found on Pandora (easily the most stunning creation in science fiction cinema to date). Unlike many sci-fi world creators, it is hard not to think of George Lucas here, Cameron, and his partners at Weta Workshops, ILM and Stan Winston studios (to name just the three large effects houses involved) have put in the time to properly attempt the creation of an entire ecosystem, as well as a convincing anthropology of its native intelligent lifeform. There is certainly a great deal of biological mystery on Pandora (I don't understand its ecosystem fully for instance) however Pandoran ecology all seems so plausible -no surfeit of ecologically inexplicable giants here thank you very much- that even a well trained biologist shouldn't be required to suspend her disbelief in the service of a rewarding two and half hours. And a film that leaves a biologist (like myself) wondering about the evolutionary history of a make believe world without needing to make allowances, and giving passes, is a film going reward in itself.

Pandora is an Earthlike world, though not so Earthlike that humans can breath without aid. (For a detailed account of why this is click here) It is a warm world, or at least the part dominated by the story is and it orbits a gas giant very much like our own Jupiter. The gas giant is probably too much like our own Jupiter, but I will leave all non-biological observations for the astronomically inclined. Pandora is not quite as large as Earth, but large enough to hold an atmosphere capable of trapping heat.

Avatar takes place in what appears to be forests of high photosynthetic productivity. Pandora may have arctic and temperate biomes, but our story takes place in something very like a rainforest. There is a vertical structure to the system, massive canopy trees, and then below a massive complexity to the understory producers. No doubt the books that discuss the planet for other fans will dwell on the film's spectacular fauna, but the structure of that forest is the first step of anchoring the film in reality. Because the makers gave the forest a convincing and diverse understory, as well as the majestic canaopy for its wide shots the world created has a believeable biological depth. It is the brief, or undetailed glimpses of the epiphytes, and ephemerals, of the small game and the creatures of the undergrowth and detritus that will be the unsung heroes of Avatar.

Another compelling detail is the day, night switch in forest ecology that appears to occur on Pandora. Pandora isn't earth, but like Earth, its ecology is not dominated by the same players at all hours of the day and night. But that is probably where the similarities between the two worlds end. The change in players and signals among the players seems more like a pelagic switch on Pandora. Bioluminescence dominates the Pandoran biota. Does this make much sense biologically? At first, I thought it might be a bit of Weta, or ILM or some effects house demonstrating their coolness (and as such it works magnificently) but upon reflection I've come to a different conclusion. Bioluminesence makes some sense considering Pandora does not orbit its solar system's star like Earth. It orbits a gas giant on an outer orbital track. This would mean somewhat reduced exposure to sunlight, and maybe selective pressures pushed toward more intense visual signalling apparatus. Stephen Jay Gould would probably say I am crafting 'just so stories' but regrettably I'm unable to go to Pandora and prove my hypothesis, so just you quiet down ghost of Stephen Jay Gould. The point is that this weird morphological trait, shared by much of the biota (notably absent in at least two of its alpha predators) makes plausible evolutionary sense. We could probably infer from the prevelance among the biota, that bioluminescence is evolutionarily ancient. I suspect that if Grace Augustine's (Sigourney Weaver) research lab had its own Genome Project it would reveal that bioluminescence was actually lost in the alpha predators but that the ghosts of those genes still exist hidden in their genomes. Here the creators have crafted a scientifically believable ecological backdrop. The time and effort that this must have taken is extreme, and it is richly rewarded on screen.

Taxonomy is important and Cameron and his team have given this some thought as well. The plants are not dwelled on enough to examine their taxonomy, but among the land based vertebrate type animals we see at least two major evolutionary families, tetrapods and hexapods, that is four and six limbed types. Hexapods enjoy greater representation in the Pandoran biota (at least in the region where the film takes place). Is this a split similar to the marsupial/placental split in mammals? It isn't discussed. From an evolutionary perspective this would be a fascinating development (oh the research we could do!). On Earth only one major body plan emerged among big land animals, it has been greatly elaborated on, but all vertebrates are basicaly tetrapods. Not so Pandora. The Na'vi, the indigenous people of Pandora, are notably tetrapods. While potentially very interesting, Pandoran morphology is not discussed in any detail in the film, but remains an interesting point of mystery for the serious science fiction fan. There are substantial differences in morphology among the tetrapods and hexapods though both share at least one organ, even if they differ in limb number. I don't want to go to deeply into detail about what is shared, and what is not, because to do so would be to violate spoiler protocols. I will say that there are no evolutionary barriers to plausiblity to this state of affairs. What is clear is that Pandora is a planet that has produced a fascinating biota, that has had a different evolutionary history than our own.

It probably goes without saying that the ethology of the organisms is also meticulously and plausibly created. I will dwell on only one example, the megafaunal predator called a thanator. This intimidating, hexapod, alpha predator, resembles in many ways the big cats of our own planet, except of course for the extra limbs, and the ten senosry quills, attached to flat chitinous plates. There is something insect-like in the quality of its body. It's teeth and claws resemble material found in the chelicerae of spiders, or the stinger of a scorpian. The color of a thanator, an inky blue-black, also hints at some insect like evolutioanry roots. When we see it for the first time it attempts to attack a young Hammerhead Titanothere. Through this vinette we see more of its behaviors (it is a surprise predator, and has an impressive threat display of rattling plates and quills, pulled back lip flaps that reveal a strange set of gums, and a toothy maw). Also terribly crucial to establishing biological reality, we don't see many of these alpha predators in the landscape (this was probably the crucial ecological "flaw" of Peter Jackson's excellent King Kong). Big predators sit atop an energetic knife edge. That is to say consumers (animals that eat stuff and do not produce their own food like plants) are not terribly efficient at aquiring nutrients from their food stuffs. From the base of the food chain, the plants and other producers, to the top of the food chain, the alpha predators, lots of energy is lost by animals as they consume food. If plant consumer X has only managed to acquire 10% of the energy available it becomes immediately apparent that any animal that eats consumer X will need to be eating a lot of X to stay alive. An ecosystem can only support a few such predators (especially if they are "warm-blooded") in a given area.

All of this behavior has the feel of things I've seen in the field. If you have ever watched a bird in your back yard, a coyote in the wild, or had to have a face to face with a big black bear, found fresh mountain lion scat on the trail you use, or nearly been run down by elk, watching the thanotar, or indeed any of the nature in Avatar will seem like authentic nature watching. From climate, to taxonomy, to evolutioanry history Avatar works as a very convincing bit of science fiction.

There are some animals in Avatar that will seem too much like analogues of Earth creatures, the Direhorse will seem too much like our own horses, the Viperwolves to obviously like wild dogs found on Earth and so on through much of Pandora's fauna and flora. I suspect that would be one of the major objections for the avid science fiction fan. It is an echo of the old complaint, "Why so many humanoids?" that science fiction fans have been offering for decades. It is an important objection but one that I think evolutionary theory allows us to dispense with, at least in the broad strokes, as we watch Avatar.

Stephen Jay Gould rather famously put the problem as follows,

"But if I could rerun the tape of life from the origin of unicellular organisms, what odds would you give me on the reevolution of this complex and contingent insect-flower system? Would we see anything like either insects or flowers in the rerun? Would terrestrial life originate at all? Would we get mobile creatures that we could call animals? Fine-scale predictability only arises when you are already 99 percent of the way toward a particular result -- and the establishment of this 99 percent lies firmly in the domain of unrepeatable contingency."

The notion of historical contingency set ideas about progress in evolution on their ears. There was a tendency among both lay people and many working biologists to think of evolutionary processes as an inexorable march toward humanity. It was a kind of recapitulation, in natural terms, of the idea of the Scala Naturae but without all the angels, or a god. Gould thought his ideas about contingency were fairly radical, but his ideas were often not quite as revolutionary as he made them out to be in his less reserved moments. That isn't to say that contingency is unimportant, clearly accidents of history play huge roles in evolutionary history. Allow me to point out an obvious accident that was a massive biotic regime changer. If, Sixty-five million years ago, a large astroid had managed to slide by Earth, you and I are not here to have this little electronic exchange of ideas. No earth shaking astroid impact in the Yucatan Peninsula, means no age of mammals, and no age of mammals means no humans. Likely it would mean 65 million more years of dinosaurs engaging in the business of being dinosaurs. Contingency is important.

But so is convergence, which Gould simply neglected in his analysis (as does Michael Shermer, Gould's most staunch defender on the point of contingency). The philosopher of evolutionary biology/psychology Dan Dennett, and the evolutionary biologist Richard Dawkins have been most articulate in their (mild) critique of Gould's Radical Contingency. But what is convergence in evolutionary biology? Or to put the question as it may be framed in the reader's mind, "what is convergent evolution and how, Mr. Blogger Fanboy, can it help us see the creatures of Pandora as plausible products of evolutionary processes?"

Convergent evolution is a process whereby organisms only distantly related, in evolutionary terms, exhibit similar morphology, or behaviors because their lineages have been exposed to similar selective pressures. That is to say organisms take on similar traits and come to look like one another because they evolved in either similar environments or engaged in a similar ecology. Below is perhaps my favorite example of evolutionary convergence. This looks like a wild dog. It is not.

This is a marsupial, now very likely extinct, that not only looks like a dog but acts like a dog. The similarities between old world canids and the thylacine exist only because they are the products of similar selective pressures. Evolutionary convergence is not at all uncommon. Another text book example is the form of fish, ichtyosaurs and dolphins. All are only distantly related, but all have converged on a similar form because they evolved in a similar environment. Convergent features are something Dan Dennett has referred to as a "good move" or "forced move" in animal evolution. That is to say certain strategies, forms and structures evolve again and again (not precisely copied mind you and bearing the stamp of contingency) because selective processes favor their discovery. Even the rudimentary stages, it appears, of these evolutionary "good moves" offer such advantages, that selection favors elaboration of the trait, strategy, or behavior.

Eyes have evolved independently on Earth at least forty times. They are not identical, but certainly they are identifiably "eyes." Fin like structures have also evolved dozens of times independently. Insulation, endothermy, parental care, pack hunting, complex problem solving intelligence, chemical warfare have all evolved independently several times on Earth. It is the phenomena of convergence in evolutionary processes that makes the flora and fauna of Pandora more than mildly plausible. The animals have the stamp of their evolutionary history all over them. They are not identical to their earthly analogs, but represent another iteration of a "good evolutionary move." Both contingency and convergence are satisfied.

To return to Gould's famous question, would we be here if we could rewind the tape of life to its unicellular beginnings? If by we Gould means big brained mammalian primates, with opposable thumbs, limited body hair, and a penchant for sweets, then the answer is an obvious no. But if by we he means an intelligent, technology using species then the answer is-given enough time- probably yes. Intelligence, like fins, insulation, and wings, is a good evolutionary move that has been hit upon again, and again by evolutionary processes. So if the presence of horse like things bothers you as you watch Avatar think about contingency, but also convergence and maybe that will resolve your dilema.

The AVATAR program hinges on the genetic hybrid avatar bodies, which are clones of their human "drivers," mixed with DNA of the Na'vi. There is an implication here, wholly unexplored by the film, of quite a bit of extreme genetic similarity between Earth and Pandoran life forms. This is harder to explain. Two hypotheses come to mind, and I will leave it you to essay others.

The first is that our genetic code isn't particularly difficult to make. That is to say, given the right chemical environment, early self-replicating molecules will hit upon nucleic acids regularly. DNA/RNA are simply the most stable of the alternatives. If this is so, we can expect this mechanism of heredity (DNA, RNA)commonly wherever we find life- given a certain set of evironmental conditions.

The second hypothesis belongs to Sir Fred Hoyle and Chandra Wickramasinghe, and is referred to as panspermia. This hypothesis predicts that the origin of life is exceedingly rare, but several forms of that life (unicellular) will be extremely hardy and able to survive interstellar travel (provided it is suitably sheltered) in astroids or comets. These bacteria like organisms find themselves blasted out into space during extreme volcanic eruptions, or impact events and then go on to seed other worlds with life. If this is the case the DNA/RNA mechanism might also be somewhat common in our galaxy as these unicellular space travelers would make planet fall and seed evolutionary processes on different worlds. If you think this is far fetched, there are chunks of Mars that have landed on Earth after being ejected by some phenomena. There is even some evidence that at least one of those Martian bits experienced ancient Martian life. So the mechanism isn't implausible. However there is little evidence for it.

As for the great biological mystery (I will not spoil) at the heart of Pandora, I have no explanation. Is it plausible? I don't know. Possible, but contingency would predict the Pandoran system to be rare I think. But I am not bothered. I will remember Orgel's Second Rule (remember to place tongue in cheek) when thinking about it...."Evolution is cleverer than you are."

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At 5:58 AM , Blogger Titania said...

Wonderful review. Now I have to see the film again and concentrate on the biology instead of the action. The horse and dragon creatures did bother me a bit, but I shall keep contingency and convergence in mind.

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