The Telegraph has a silly article titled “Aliens are likely to look and behave like us” . It’s based on the ideas of Professor Simon Conway Morris  who is a big fan of evolutionary convergence. He seems to believe that humans evolved in a way that is close to optimal and that aliens would have to evolve in a similar manner. The article’s claim that aliens will look and behave like us has three flaws, one is assuming that humans are the ideal form for space travel, another is the significant possibilities for vast objective differences between species, and the final one is the human tendency to regard small differences as being really significant.
The Arrogance of Assuming that Humans are Ideal.
It seems extremely arrogant to assume that humans have the ideal form for performing tasks that we have not yet performed – such as traveling to another star system.
To assume that it is certain that humans will colonise the stars is also extremely arrogant and foolish as well, I think that a significant amount of money should be devoted to existential risks that we face. People who believe that humans will inevitably colonise the stars (probably because God told them – there’s no other reason for such certainty) will tend to oppose taking prudent measures to reduce the probability of failure.
We should also consider the possibility that a human society that colonises another star system at some future time might be so different from us that we regard them as alien.
How Might Aliens be Different?
It seems to me that one problem with the theory of convergent evolution (as applied to different planets) is that evolution doesn’t seem to work in big steps. It seems to me that for an increasing portion of the jobs in our technological society there would be a benefit in having four arms and two legs, and maybe having more than four fingers on two of the hands. But evolving a bone structure that makes four arms useful is not a minor tweak so it seems unlikely to happen naturally – we might be able to genetically modify humans to have four arms and two legs with some future technology.
I can’t believe that four limbs is the optimal number for every mammal, amphibian and reptile (apart from the legless ones), that two legs plus two wings is the optimal number for every bird, and that six legs is the optimal number for every insect. I also can’t believe that two arms and two legs is the optimum number for every primate with the only significant difference in limbs being the primates that possess a prehensile tail. Evolution doesn’t produce perfect creatures, what it does is to produce creatures that tend to be slightly fitter than their ancestors. So even if there was an optimal form for a species that develops interstellar-craft, there would be no reason to assume that humans have such a form even if we had developed interstellar travel.
I think it’s relatively safe to assume that aliens would have two eyes (stereoscopic vision is a significant advantage), but apart from that I don’t think we can assume much about their appearance. For example I don’t think that there’s any inherent reason why creatures as intelligent as current humans could not evolve from octopuses – 8 arms would be quite useful for operating machines.
Aliens that spread beyond their own home planet don’t have to be optimal for such tasks, they only need to become the dominant species on their planet, develop decent technology, and then want to leave their planet. A species that took 100,000 years to achieve what humans have achieved in the past 100 years might not be considered optimal – but it might still be able to eventually launch interstellar craft. I don’t think it’s reasonable to assume that aliens would have to be as intelligent as humans to do that. But it does seem most likely that any aliens who manage to get here would be more intelligent than us.
Guns, Germs, and Steel: The Fates of Human Societies by Jared Diamond has a really interesting analysis of how human history was determined by geography and the availability of plants and animals suitable for domestication. Part of the explanation for the apparent lack of intelligent aliens could be planets where the dominant species was stuck in a situation like that of the people of New Guinea, the lack of plants and animals suitable for domestication prevented their society from developing. A stone-age human civilisation could potentially be stable for millions of years.
The article says “Extra-terrestrials might not only resemble us but have our foibles, such as greed, violence and a tendency to exploit others’ resources“. Well for starters a species that can displace all other contenders to become the dominant species in their planet would have to be capable of greed and violence. So that is probably correct but is not really going out on a limb.
Physorg.com has a short article describing the benefits of sexual reproduction in terms of the rate of accumulation of harmful mutations . So it seems that sexual reproduction is most likely among intelligent aliens. But I don’t think that there is any reason to assume that aliens couldn’t be hermaphrodites. The Telegraph article claims that “Star Wars and Star Trek could be more accurate than they ever imagined in depicting alien life“, but given that Star Trek has avoided homosexuality even when demanded by the plot it seems that whatever aliens do in the bedroom would be way outside the range of activities alluded to in Star Trek. It seems that almost all sci-fi has aliens that are less “alien” than a significant portion of the human population. I think it’s safe to predict that if we should meet aliens they will not look like humans wearing cheap make-up or rubber suits – that has more to do with the budgets assigned to movie studios than any claim to potential realism.
It seems most likely that a society capable of developing space travel would need a great deal of training for it’s young before they become self-sufficient. Species of animal that abandon their eggs and hope for the best seem unlikely to succeed in doing that. I think that this implies that a pair-bonded species that spends an extended amount of time raising their young is most likely to succeed.
Perceptions of Aliens
Vulcans are widely regarded as being on the Autism spectrum. So it seems that having some similarities in personality to about 1% of the human population is enough to convince most human viewers that the character in question is an alien. A Google search for such things just turns up many references to people on the Autism spectrum who feel like aliens – which incidentally is the source of the alias ETBE  that I used for more than 10 years before being diagnosed as an Aspie. Star Trek aliens aren’t actually very alien by human standards!
Given the repeated demonstrations of the general human inability to recognise the psychological similarities in people from different countries or racial/ethnic/religious backgrounds it seems likely that few people would recognise aliens as being at all similar to us. As so many people are unable to recognise the fundamental similarities in people who happen to have a different skin color I can’t imagine many people immediately regarding beings that look like octopuses as having human traits.
Could an eusocial organism (such as bees or ants) develop space travel? It seems to me that most people wouldn’t regard humans who act in a manner that resembles ants (such as the Borg Collective) as being “like us”. The possibility of an eusocial race evolved from humans is explored in Coalescent: A Novel (Destiny’s Children, Bk. 1) by Stephen Baxter (and more briefly in the sequels).
The universe is a large place. Making a specific prediction about the first alien species that we might encounter might make some sense. But given the potentially billions of species that might be out there we should assume that some of the more improbable forms are represented. Claiming that anything in particular is impossible seems very rash.
Also anyone who makes claims about what evolution “should” do seems to have missed the point. What evolution does is to give rise to species that tend to be a better fit for the environment. Different environments result in different creatures. We can try and predict what creatures might evolve, but we don’t get to say that things “should” happen.