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 anot
her, 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.
PART SEVEN
NA’VI
“I see you.”
—Jake Sully, Neytiri and others
24
HUNTERS OF THE FOREST
In the movie Avatar the Na’vi we learn most about are Neytiri’s Omaticaya clan. There are however many other clans on Pandora, which we glimpse when Jake as Toruk Macto calls on the Na’vi to unite against RDA. Na’vi languages differ to a minor extent, as do their physiologies—details of their height, their skin tone—but they all seem to be of one species, as the many races of mankind are a single species.
The Na’vi live essentially by hunting and gathering, from the natural fruits of the world around them. Hunter-gatherers typically live off a variety of food sources. They really do hunt and gather. Among humans, hunting may be a prestigious male activity, though the women’s gathering of food from sources like roots, fruit, nuts and small animals may actually bring in more nutrition to the group. But it’s wrong to over-generalise and to draw gender stereotypes; every culture is different. Certainly the variety of food we see in Avatar is credible, from avatar-Jake’s first mouthful of fruit when he escapes from the lab, to the teylu larvae he eats during his first encounter with the Na’vi, to the hexapede he hunts down with Neytiri. The Na’vi do however have a kind of incipient agriculture, which they call ska’waylu which means encouragement, a kind of elementary husbandry of favoured plants. This behaviour is thought to have led to the development of full-blown farming on Earth.
Thus the Na’vi. Once, all humans lived as the Na’vi do.
In the long ages before the coming of farming—in Eurasia the period is known as the “Mesolithic,” the Middle Stone Age—everybody on Earth lived as a hunter-gatherer. The makers of Avatar based aspects of their depiction of the Na’vi lifestyle on the lives of hunter-gatherer forest dwellers in South America and Africa.
But surviving hunter-gatherer communities have been pushed to the margins by the spread of farming in prehistoric times, and by empires and colonies later. Today they are only allowed to subsist on land the farmers, loggers and miners can’t use, or haven’t got to yet. Modern examples probably don’t give us an accurate picture of how hunter-gatherers might have lived in the past, on the richest grounds, and before their ecologies and lifestyles were reshaped by contact with the farmers.
In particular, the Omaticaya clan in their Hometree are sedentary; they are based in one place all year round. We think of hunter-gatherers as mobile—nomadic, moving with the herds and the seasons. But there is plenty of evidence from the past that where resources are rich enough hunters would choose to be sedentary, like the Omaticaya. The North American Indians of the Pacific Northwest are an example. Modern hunter-gatherers generally don’t have concepts of “ownership” of the land. Either they are on the move in search of resources, or they live in a land so rich there’s no need for conflict over ownership; there’s enough for everybody. But perhaps the sedentary hunter-gatherers of the past were territorial, as the Omaticaya ar
e, and would have rallied to Jake’s battle-cry of defiance against RDA: “This is our land!”
Similarly, modern hunter-gatherer groups typically don’t have hereditary leaderships or rigid social hierarchies, and here again the Omaticaya, led by a named individual in clan leader Eytukan with a named heir in Tsu’tey, seem to be atypical. More usually “leaders” would be selected on the basis of skill or prestige for specific purposes, such as leading a hunt—rather as when Jake as Toruk Macto, “rider of the last shadow,” assumes the mantle of leader specifically for the fight against the SecOps forces. But again, we do know that stratified social hierarchies could arise among the sedentary hunter-gatherers of the past, and maybe in the Na’vi in that respect too are true to the past on Earth.
What about war? There’s clearly evidence of warfare among the Na’vi. The young male Tsu’tey defines himself as a warrior, and the clans come together willingly to face the external threat of SecOps. Large-scale warfare is thought to have been rare among hunter-gatherers because population densities were too low to support large armies; conflict was smaller-scale, ritualistic.
Among the Na’vi, the Earth explorers learn that warfare is rare unless brought about by external stresses—population displacements because of some natural disaster like a volcano, perhaps. Fighting tends to be fierce but brief, followed by intense efforts to resolve the conflict. Na’vi wars don’t lead to the elimination of whole peoples, as ours do. However, the Na’vi’s past must contain many stories we haven’t yet been told.
As with the Na’vi, human hunter-gatherers generally don’t see themselves as separate from the natural world which sustains them. The Na’vi even sleep cradled by nature. In the womb of their Hometree, the Omaticaya sleep in “hammocks” that are actually living plants. And the Na’vi know their world intimately, as we see onscreen when Neytiri hunts with Jake. In pursuit of their prey Neytiri is able to detect the subtlest clues: trails, tracks near the waterholes, the smallest scents and sounds.
The Science of Avatar Page 15