On super-Earths, animals as large as elephants would be able to soar among the clouds, while some giant flyers may never develop the capacity to land.
Credit: Alex Ries
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).
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.
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.
