The discovery of life on other planets may be just around the corner – but will we recognise it when we see it? Lewis Dartnell reports.
REAL ESTATE IS BOOMING in outer space: in the past decade, over 400 new planets have been discovered, and the number is set to grow even more in the years ahead. What’s more, experts predict that evidence for extraterrestrial life is probably just around the corner.
Several exciting telescope missions aiming to seek out this life are already underway. NASA’s Kepler telescope was launched last year, joining France’s COROT telescope in the search for habitable planets in our neck of the galaxy. Many other ground-based telescope programs are also hunting intently.
Plenty of these potential abodes are expected to be spotted in the next few years, and follow-up observations over the next two decades could confirm that one of these worlds is not only habitable, but inhabited (see “Sun lovers” box below).
But what might these extraterrestrial animals and plants actually look like? Intelligent aliens are unlikely to look anything like the characters in our sci-fi films and TV shows.
Chewbacca and Mr Spock are, let’s be honest, no more imaginative than a bloke in a furry suit or bits of plasticine glued to someone’s ears. These designs are thought up for the convenience of the make-up department, rather than being based on scientific rationale.
Although evolution is driven by random mutations, the truth is that
natural selection is far from haphazard.
SO WHAT CAN WE PREDICT about alien evolution before our interstellar explorers finally make landfall on a new world? Well, it turns out from studying evolutionary biology on our own planet, in particular the two principles of convergence and contingence, that the development of alien animals and plants might actually be fairly predictable.
Although evolution is driven by random mutations, the truth is that natural selection is far from haphazard; basically, the individuals best suited to their particular environment prevail. An alien animal or plant developing on a world on the far side of the galaxy would be subject to exactly the same laws of physics and principles of design as terrestrial biology. An alien fish species with the density and shape of a brick wouldn’t get very far in the survival game, and over time the species would develop a streamlined shape and fins or tail for propulsion – and end up looking uncannily like a salmon.
This is the principle of convergence in biology: there may only be very few good solutions to a particular survival problem, and so evolution hits upon the same design time and time again. For example, the camera lens eye, just like the one you’re using to read this article, has evolved independently several times on Earth, in animals as diverse as squid and birds.
“These design features that arise independently multiple times by convergence are said to be ‘universals’ of terrestrial evolution,” said Jack Cohen, a British reproductive biologist and honorary professor at the University of Warwick, “and so are likely also to be seen in aliens.” As Cohen delightfully puts it, “The four universal Fs of evolution are fur, photosynthesis, flight and…mating!”
Of course, random events can have enormous effects on the evolutionary trajectory of a planet. It is widely believed that it was a chance asteroid impact 65 million years ago that wiped the dinosaurs off the face of the Earth and so vacated evolutionary niches for us mammals to inhabit.
But evolution after such catastrophes is still broadly predictable. Mammalian whales and dolphins developed to be remarkably similar to the extinct marine species plotosaurus, and bats closely resemble the aerodynamic shape of pterosaurs. Convergent evolution is exceedingly common on Earth and so in many respects we’d expect alien evolution to produce surprisingly similar solutions to similar survival problems.
ON THE OTHER HAND, it appears as if some features are not optimal solutions to any particular survival problem. They are instead dependent upon previous chance outcomes; no more than historical accidents frozen in time. For example, there is no reason to suspect that our five fingers or toes offer any particular advantage: we could balance upright or grasp objects just as easily with four or six digits.
So why five? Because land vertebrates – including amphibians, reptiles, mammals, and birds – inherited five bones from our fishy ancestor that clambered onto dry land.
“Such chance outcomes aren’t likely to be repeated in alien evolution and so are parochial to us,” said Cohen. An intelligent alien produced by a similar evolutionary route could just as easily give her friends a ‘high-four’.
So while convergent evolution tells us that many universal features of an alien would be broadly predictable, some of their traits would be contingent upon quirks within its own evolutionary history.
And of course, if an alien is evolving in an environment very different to the Earth it could hit upon completely novel design solutions.
Perhaps instead of striving to reach skyward like a towering column,
alien plants would simply float upwards in the air.
THESE ENVIRONMENTS ARE OF interest to Karl Niklas of Cornell University in upstate New York, who uses computer models to study the evolution of plant design. “Any land plant has to simultaneously satisfy four basic requirements for survival: it must intercept as much light as possible for photosynthesis and disperse its pollen or seeds as far as possible, all the while ensuring it doesn’t topple over or lose too much water.” For example, a plant maximising light collection would evolve leaves in a very wide flat canopy, like a parasol, but this would make it vulnerable to uprooting in high winds.
The optimal plant design for different environments is always a compromise of these four requirements, so the shapes of trees growing in alien forests are likely to be similar to those found in a comparable habitat on Earth. A plant adapted to desert conditions on another world would need to be squat and unbranched like a cactus; a tree maximizing stability would be structured much like a fir; and a tree in moist soils would have a large photosynthetic surface and a broad crown like an oak.
But what about the colour of their leaves? On Earth, plants and trees look green because they contain a pigment called chlorophyll that absorbs the blue (which is most energetic) and red (which is most plentiful) much more strongly than green in the spectrum of sunlight.
In fact, if our eyes were sensitive to a slightly wider range of light wavelengths, plants would not seem green but a lush near-infrared colour. This is because land plants reflect infrared wavelengths very brightly, probably to avoid over-heating in the sunshine.
“The absorbance of plants is closely tuned to the spectrum of sunlight,” said Nancy Kiang, a biogeochemist studying how to detect photosynthesis on other planets at the NASA Goddard Institute for Space Studies in New York. “In general, plants on Earth use the broad range of visible light (red-orange-yellow-green-blue-indigo-violet), but there’s a slight bump in reflectance in the green. So forests growing by the light of a different sun – a star bluer or redder than our Sun – would need photosynthetic pigments tuned to this different spectrum.”
M-class dwarf stars – such as Gliese 581, the four-planet system that was the target of the Hello From Earth project in 2009 – glow much more dimly than our Sun, and with a redder light. Plants growing on a planet orbiting such a red dwarf star would need to absorb as much of this sunlight as possible to gather the energy to grow, and so would probably appear black to our eyes, she explains. Of course, the eyes of any animals evolving on the planet would also have been tuned to the cooler spectrum, and so they would see their plants as a lush infrared colour.
On the other hand, an F-class star burns hotter and brighter than our Sun. “Although the spectrum is shifted towards the blue, the sunlight has a lot of visible light like on Earth, and so the colour of plants growing on an F-star world could look pretty similar,” said Kiang. “However, the flood of energetic blue light might be so intense that plants need to protect themselves by evolving to shield themselves and reflect more blue light.”
IN ADDITION TO STRANGE LEAVES, perhaps plants evolving under very different environmental conditions would find a totally novel way to reach sunlight, reproduce and access water. On a super-Earth – a planet more massive than our homeworld – the increased pull of gravity would make developing stout trunks and branches a less successful strategy.
Perhaps instead of striving to reach skyward like a towering column, alien plants would simply float upwards in the air. Zeppelins are buoyant because they are pumped up with hydrogen gas and so are less dense than the atmosphere around them. Photosynthetic plants on Earth harvest the energy of sunlight to split apart molecules of water, using the hydrogen to produce food and releasing oxygen as a waste gas.
All an alien plant would need to do is release this hydrogen inside an inflatable sac and it could float up into the sky, anchoring itself to the ground with a vine-like tether. Indeed, kelp forests hold themselves up in Earth’s seas by pumping numerous small floatation bladders full of oxygen or carbon dioxide. For extra lift, an alien balloon plant might even develop dark pigmentation on the topside of the hydrogen-bag so that its sun’s heat expands the gas within, creating extra buoyancy.
An alien land animal, descended from its own finned swimming ancestor,
could just as easily sport three pairs of limbs.
A VERY EFFECTIVE REPRODUCTIVE STRATEGY is also open to balloon plants; they could detach from their tether and be carried on the wind to disperse seeds across the landscape below. Perhaps the only reason that flying balloon plants haven’t evolved on Earth is that there isn’t the necessity for it.
Animal evolution would also be expected to follow the principles of good design, even on another planet. On Earth there are features that are common across the animal kingdom, regardless of environment.
For example, extracting nutrients from food is achieved with an internal gut – a through-flow pipe with an entrance and a waste exit at the other end. The human body, or indeed that of any other vertebrate, is essentially an elaborate gut tube with a few internal upgrades, supported by a backbone and accessorised with sense organs and grasping limbs.
There is also a common need for a feature that allows creatures to access oxygen once it grows over a certain size, and so they must develop structures such as gills or lungs with a very high surface area. There’s good reason to expect alien animals would enjoy a lungful (or gillful) of oxygen as much as we do – burning organic fuel in oxygen is the most energy-rich form of respiration, able to satisfy the enormous power demands of active animal life.
Getting these nutrients and gases to all cells throughout a large body also presents a distribution problem, and so an internal circulation system of blood vessels and a pumping heart becomes greatly advantageous. Alien blood needn’t be red with iron-bearing haemoglobin like ours (horseshoe crabs bleed blue because of copper): it just must be able to transport oxygen.
In order to find tasty morsels, or avoid becoming a tasty morsel for a predator themselves, it becomes crucial for animals to be able to sense their surroundings. Animals on Earth have developed a great range of senses, and sight provides such a useful survey of the environment that it is present in practically all terrestrial higher animals. It seems the best strategy is to situate eyes at the front of the body.
All this incoming information must be processed and acted upon using a large cluster of nerve cells, close to the sense organs for quick reactions, and protected within a hard case: that is, a head. Larger animals colonising the land also require structural scaffolding to support their body against gravity, and a frame against which muscles can push: a skeleton that can be either an ‘innie’ (like ours) or an ‘outtie’ (like crustaceans).
All of these examples and speculations are based on the fact that every animal needs to gather energy, protect itself, and sense and respond to its environment. And we can be confident that alien animals would be startlingly familiar in these respects.
But there could also be strange features of alien animals that would have evolved as a result of quirks of their ancestors.
A MAJOR EXAMPLE OF CONTINGENCY in Earth animal evolution is the four-limb body plan of all land vertebrates, due to our fishy ancestor – by chance – having two pairs of lobed fins.
An alien land animal, descended from its own finned swimming ancestor, could just as easily sport three pairs of limbs. “On another world, you might find alien lizards and gazelles prancing around on six legs: hexapods like terrestrial insects,” said Gert van Dijk, who is active in the field of speculative biology and editor of Life on the Planet Furaha.
And you might find that large six-legged animals can do without the front pair of limbs for running and so evolution may give them new functions. Van Dijk has speculated on the possibility of alien horse-like predators that have readapted their front limbs into slicing or clubbing weapons. “I call this evolutionary process ‘centaurism’, after the mythical creatures half-human and half-horse, and it’s exactly what’s happened in terrestrial biology for the claws of the praying mantis or crab,” said van Dijk.
But maybe we’re not thinking alien enough. How different could alien animals realistically be? Most familiar animals on Earth – birds, reptiles, fish, insects, and so on – are based on a body plan with bilateral symmetry: the left and right halves are reflections of each other.
Even more amazing is the possibility of enormous beasts taking to the skies of a super-Earth. Contrary to what you might expect, it’s actually easier to fly on a planet with more powerful gravity.
PERHAPS ALIEN ANIMALS COULD be based on another, fundamentally different, body plan, for example, radial symmetry – where an organism has no left or right side, only a top and bottom, like jellyfish. The progression of radially-symmetric life forms on Earth stalled, as they were apparently unable to develop greater complexity of limbs, senses or nervous control system. But given a fresh chance on another world, they might flourish.
Evolution in Earth’s rivers and seas has produced a great number of successful swimmers. Salmon, whales, penguins and water boatmen all come from very different lineages, yet have independently converged on the same body plan: it turns out there aren’t many ways of remaining streamlined while pushing yourself through the water. So any alien fish would also be expected to boast a sleek bullet-like shape, and perhaps swim by beating alternate pairs of flippers like the extinct plesiosaurs.
But what about a more innovative style? The crucial feature here are the two pairs of long, wing-like flippers that beat out of synch with each other, but other structures could change. Van Dijk has suggested alien fish may exist that don’t have a jaw-like mouth, but a four-way orifice that gapes open to envelop prey.
Van Dijk has also speculated on the possibility of tube sharks speeding through alien oceans using sustained jet propulsion. Unlike terrestrial squids, which repeatedly contract an outer cavity to lurch backwards in pulses, these tube sharks would have a hollow cylinder running the length of their bodies, with a gaping intake at the front. Muscle contractions squeeze water through the inside of the tube (not unlike the peristaltic contractions that move food down your throat) and squirt it out the back, smoothly jetting the shark forwards.
Flight is an incredibly nifty trick for foraging or escaping predators, and has been used by insects, birds, mammals and extinct saurians. It’s no surprise that the need for an aerodynamic body and large wing surface ensures flyers have a convergent body plan, and the same is expected in alien ecosystems.
ALIEN BIRDS WOULD definitely not be confused with Earth species, however, if they evolved from hexapod ancestors. With two pairs of limbs available for adapting into wings, alien evolution could produce a biplane bird. Such a configuration would be extraordinarily manoeuvrable in the air for hunting, and holding one pair of wings rigid like an airplane could be energy efficient when soaring over long distances.
Even more amazing is the possibility of enormous beasts taking to the skies of a super-Earth. Contrary to what you might expect, it’s actually easier to fly on a planet with more powerful gravity. Although your weight increases, the air becomes denser, and so wings can generate greater lift.
Animals as massive as elephants would be able to soar among the clouds, while sky-whales would be carried on immense wingspans and rise on thermals. Such gigantic flyers would never be able to land, and so sky-whales would need to sleep and mate ‘on the wing’, as terrestrial swifts do, and perhaps filter-feed on dense blooms of aerial plankton held aloft in the soupy-thick atmosphere.
While it’s all good fun discussing what weird and wonderful forms of alien might be produced by evolution on another world, this goes far beyond empty speculation. Thinking about the necessary substances and processes that alien life might share with us, and also important ways in which it could differ, is crucial for detecting an extraterrestrial biosphere.
WITH LUCK, TELESCOPES such as Kepler and COROT will find some Earth-like planets close enough that we can scrutinise for signs of life. Astrobiologists are particularly keen to check nearby worlds for signs of photosynthesis (see “Sun lovers” box below).
But how might the signatures of alien plants be different from on Earth? We need to consider how alien evolution might have moulded extraterrestrial lifeforms to suit their own environment in order to know what to look for.
Nonetheless, astrobiologists are optimistic that within our lifetime we’ll have found convincing evidence of an alien biosphere. However, imagine what it would be like finding a ‘second Earth’ with clear signs of thriving life in our stellar neighbourhood, but not being able to voyage to this new world for hundreds of years to see what these aliens are actually like. It could turn out to be the most frustrating discovery in the history of science!
IT’S LIFE, JIM
As far as we know, complex animal and plant life has the best chance on an Earth-like planet: a world with continents, oceans and a thick atmosphere. But there are other potential habitats for life – even in our own Solar System. While Mars and Saturn’s largest moon, Titan, almost certainly could host nothing more complex than microbial life, Europa, the icy moon of Jupiter, could potentially harbour more advanced ecosystems in its subsurface ocean. Photosynthesis in Europa’s ocean, sealed away from the sunlight by a thick shell of ice, is impossible, but perhaps animal life like small jellyfish or tadpoles could be supported by oxygen transported into the dark ocean from the irradiated ice above.
Even more exotic life could potentially thrive on worlds very unlike Earth. Terrestrial life is assembled from carbon-containing building blocks – organic chemistry – and based on liquid water as a solvent.
One could consider the potential for life built not from carbon, but silicon, its close elemental relative. And alternatives to water for filling alien cells might include ammonia, methane or formamide – an organic compound derived from formic acid.
Although these are possibilities, it’s true to say that we can’t even begin to flesh out the details of how life might be built using these materials as yet.
However, many of the arguments in this article about the evolutionary design of life are based on principles of physics and engineering constraints – it doesn’t matter what biochemistry underlies it. A silicon-based snake would still need to extract nutrients from whatever it calls food with a through-flow gut. And a fish would still need to be streamlined, even if swimming through an ocean of liquid methane.
Terrestrial plants and photosynthetic microbes called cyanobacteria are able to capture the energy of sunlight with the help of a pigment called chlorophyll. This absorbed energy is used to split apart water, with the plant using the hydrogen to build up food molecules, such as sugars from carbon dioxide, and releasing the oxygen as a waste product. The enzyme that enables plants to ‘fix’ carbon dioxide, RuBisCO, is the most abundant protein on Earth, and the productivity of photosynthetic organisms supports practically all surface life on the planet.
There is good reason to think that chlorophyll might also be used by alien photosynthetic life. The molecular core of chlorophyll, as well as that of the iron-binding component of our blood, is a member of a class of organic molecules called porphyrins, which are thought to be available in the chemistry of any warm, wet planet.
Astrobiologists are already working on ways to detect evidence of photosynthesis on potentially habitable exoplanets discovered. The last two billion years of photosynthesis operating on Earth, and the harnessing of sunlight to split water molecules and release oxygen as a byproduct, has resulted in a high concentration of this gas in our atmosphere. Oxygen cannot persist in a planet’s atmosphere for any substantial length of time other than through the action of life; so finding that an exoplanet has oxygen-rich air is a promising ‘biosignature’.
Detecting both oxygen and methane, a very reactive mix, would really clinch the argument. Astronomers are also hopeful that they could pick-up the bright reflectance of near-infrared light from plants coating an exoplanet’s continents, the so-called ‘red-cliff of vegetation’.