[edit: this page seems to be a favorite for Russian spammers because it's linked to from a lot of blogs, and lj does a terrible job of blocking spam comments these days. I've turned commenting off.]
Avatar is worth seeing as a demo reel for state-of-the-art filmmaking technology that will completely revolutionize the industry and will eventually trickle down to independent filmmakers over the next 20 years or so. The movie was visually so stunning that I managed to more than tolerate a thoroughly mediocre plot. Lots of people have written about the special effects, the plot, and the issues of race the movie brings up. My questions is...
So, could a world like Avatar evolve?
The conclusion is very surprising.
In the film, the trees have nervous systems. One tree in particular has the memories of generations of sentient inhabitants and the ability to control all the animals of the world. The people of this world can also control the animals by doing a neural meld with them.
Basically, it's as if James Cameron decided to create a world where the Native American's spiritual beliefs were made literal through biology.
Why would a plant need a nervous system?
Nervous systems are only useful when an organism needs to take very rapid coordinated action in different places in its body. A few plants on Earth have rapid coordinated movements, but this is rare and the systems for movement tend to be horrible hacks with high energy cost. Plants could gain an increase in fitness if their movement helps them catch prey, defend against a predator, or get more energy via sunlight. The trouble is that nervous systems are expensive, energetically speaking. Plants get all their energy from the sun, and at least on Earth, solar energy conversion by plants is fairly low, I think around 1-2%. So basically plants have a very low energy budget, and that means that the luxury of rapid movement is probably out. (Hey, if photosynthesis provided a lot of energy, then we'd all have chlorophyll in our skin, and we'd make extra energy on the side just by walking around outdoors.) Carniferns (predatory plants) are very rare in our world - their evolutionary "strategy" just barely makes sense, and only under very specific circumstances. Trees couldn't do much to defend themselves from predators using rapid movement since they cannot uproot themselves and have made a choice for greater strength at extremely low ongoing energy cost (wood) instead of more limited strength and greater flexibility at very high energy cost (muscle) Plants do move in order to gather more sunlight. Phototropism is very common in the plant world, but the rate of movement of the sun is on the order of hours, so a nervous system is not necessary. Maybe if the planet rotated a full revolution once every half-hour then plants would have a reason to move rapidly. So overall there's not a strong reason for plants to develop rapid movement or a nervous system.
However, not only is there a plant with a nervous system, there's a sentient tree. In addition, the nerve architecture is the same across plants, animals, and the sentient humanoids. Thus it must have evolved before plants and animals split off from each other, which is kind of unlikely since large animals (with nervous systems) could only evolve once there were plants to eat, and plants with no predators or prey likely would not need nervous systems, as mentioned earlier.
Oh wait, Occam's razor. There has to be a much more simple explanation: Intelligent design.
Pandora is a post-Singularity world. Some time ago the Na'vi achieved sentience, built incredibly advanced computers, and created the technology necessary to upload their consciousnesses into these computers. Those who embraced technology saw huge advantages to living in the simulated world and departed their physical planet. However, some Na'vi chose to shun technology because they valued a deep connection with nature and the life of the noble savage. Some charitable world-builders decided to create the ideal world for these beings -- they genetically engineered a wide range of species to make literal the spiritual beliefs of their ancestors. They created a locally omnipotent computer, Eywa, to oversee the world, as well as an interface, the Hometree, that allows the luddite Na'vi to communicate with it in a way that's compatible with their spiritual beliefs and hides all the technology behind a veil of biology. When the world is sufficiently threatened, Eywa ensures that the threat is dealt with in a way that does not compromise the primacy of the Na'vi's worldview. Looking at the ecosystems of Pandora, the evidence for intelligent design is everywhere. Of course, this is James Cameron's intelligent design at work, but for the purpose of this little thought exercise, it indicates the clear presence of a post-Singularity civilization.