As seen in movies Aliens need not look like green blobs or scary exoskeleton creatures. They can range in size and shape.
Life there could microbial or enormous. But as of now all we can do is spectate about it. But there are experts who have different thoughts on how the life might be. Here are few:
Simon Conway Morris, an evolutionary biologist at Cambridge, thinks there’s a good chance intelligent extraterrestrial life will look a lot like us. Different species independently evolve in similar patterns, Morris argued in The Runes of Evolution, and would likely do the same on other planets. "The things which we regard as most important," he said in an interview, "cognitive sophistication, large brains, intelligence, tool making, are also convergent." If there are other planets that look a lot like planet Earth — and the Kepler spacecraft is discovering that there are — then the likelihood of human-like extraterrestrial intelligence on those planets isn’t a huge stretch. "If the outcomes of evolution are at least broadly predictable," Morris said, "then what applies on Earth will apply across the Milky Way, and beyond."
Bernard Bates a physics professor from University of Puget Sound.
"An octopus is a good example of an advanced-alien analogue on Earth. Octopuses are quite (probably human-level) intelligent and live in a totally alien environment (compared with ours). Evolution has had to find novel solutions to the pressures they're under—pressures completely different than those that shaped mammals on land.
"Dolphins and chimpanzees are extremely close to us—we're all mammals. The last common ancestor for humans and dolphins was around 100 million years ago, and for humans and chimpanzees was about 10 million years ago. Most of the evolutionary choices leading to intelligence were probably made before the splits occurred. The last common ancestor between mammals and octopuses is much, much further back in time, probably 800 million years ago.
"Aliens with advanced technology would have to be on land (technology needs fire to kick-start it). What we would expect in order to develop a technology comparable to ours? Hands with fingers (for delicate, precise manipulation) are important. At least two legs are needed for locomotion. If it has four legs, think centaurs—you need those hands to build things.
"You need binocular vision to judge distance (to prey). Elevated head to see predators. Eyes near the brain to reduce the time delay (or degradation) of the visual signal. Sound and smell sensors (ears and nose). Your survival chances improve if you can use all of the ways you can to detect food, mates, and predators. Living in an atmosphere means sounds and smells will arrive before the stinky, noisy predator."
Harry E. Keller, A chemistry Professor from Northeastern University:
"Somewhere in the universe, there must be, have been, or will be other advanced civilizations. The chances of anyone alive today seeing or even communicating with them are very slim. However, such problems should not stop us from imagining what they might be like.
"First, a real alien is bilaterally symmetrical, endothermic with excellent manipulative abilities, and has a hard container for a brain.
"Eyes? Of course. You have to see to build a civilization. The squid has better eyes than we do, and the eye seems to have evolved separately a few times at least. Our alien will have eyes that may only resemble ours superficially. A lens and iris are almost an absolute requirement. The whites do not have to be white. Neither does the iris have to be colored in anything like the way ours are. How many eyes? One just will not do. Inability to perceive distance will lead to rapid extinction. Unless you can find a good reason for it, extra eyes create evolutionary baggage and will not persist, spiders not withstanding. Eyes being important, they will be recessed and capable of being covered, as necessary. Our aliens are not likely to be bug-eyed, although we cannot totally rule that out.
"Nose? An alien must have the means to breathe. It must be symmetrical. The nose does not have to be in the middle of a face, although that is convenient for some purposes such as sharing the air pipe with a mouth for times of exertion. You can imagine other arrangements.
"Mouth? Must have means for ingestion of food. All large animals have some means to capture and swallow food. Some masticate it; others do not. Teeth are rather common on Earth but other systems of eating are possible. The problem is that plants and animals have been in an evolutionary war for millennia. We inherit the outcome of that war. Teeth were necessary to eat plants that became more fibrous, an evolutionary reaction to being eaten. There are other ways to sheer off plant food, such as what birds do. They 'chew' in their gizzards.
"Legs? Most certainly. Other means of locomotion will be relatively slow and be relegated to armored animals and of those living in holes. Neither of these lifestyles will lead naturally to greater intelligence. How many legs? In our case, we adapted the forelegs for manipulation from animals with four legs. No land animals with endoskeletons have more legs. The implication is that aliens with two legs are more likely than those with four.
"Fur? Hair? Feathers? Scales? Something else truly alien? The skin covering could be anything that makes sense. Real feathers are not so likely. Because feathers are used for flight, brains will be small. Scales are unlikely because they are particularly well-adapted for ectotherms, although a scaly sort of skin might be possible. Fur versus hair is hard to sort out because of my inherent bias. Fur has good reasons for being that are not related to intelligence. If furry, our aliens probably have short fur.
"Head? If you define head as a bony enclosure for the brain as well as the location for eyes, nose, and mouth, then it's likely. The nose could be located above or below the mouth. Above works better for swimming, but that's not a strict evolutionary requirement.
"Neck? Not at all necessary. Useful to swivel the head, but you can imagine other ways to look around. It's an annoying place for big cats to grab and strangle you with their powerful mouths.
"Arms? You must have something with which to manipulate things. Tentacles are unlikely because of their inability to perform fine manipulation. They usually have suckers on them and a very different purpose than toolmaking. The number of joints could be greater, and the direction of bend could be different.
"Fingers? You must have a way to grab tightly and a way to pick up small objects and twist and turn them. Two fingers are probably too few. Five is overkill. Three or four (as cartoon characters have) is about right. Our three joints in our fingers is about right. One would not do. Two, as with our thumbs, is unlikely."
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