There is only one Earth, one Homo sapiens, in all the cosmos after all, it is hard to believe that billions of evolutionary steps could be perfectly reproduced elsewhere. In that sense, our species is unique. But where does one draw the line between similarity and dissimilarity?
We cannot completely discount the possibility that alien life will look or act like us, not least of all given the statistical probabilities. Humans are no longer governed strictly by the laws of natural selection; however, whilst cloning and gene manipulation may allow human enhancement and the curing of disease, we are still constrained by the physiological limitations hard-coded into our genes. Other carbon-based life forms might utilise the same chemical structures (DNA or even RNA) as information carriers, especially with these molecules found in abundance throughout the interstellar medium.
We are bound to discover extrasolar Earth-like planets, revolving in habitable zones' around main sequence stars as our instruments become more powerful. That's not to say the sequence of events that gave rise to complex life on Earth aren't fortunate, but the ingredients' aren't as rare as one might think, with evidence that amino acids exist in interstellar clouds. Whilst proponents of the Rare Earth hypothesis argue that the universe is naturally unfriendly towards the development of complex life, the principle of mediocrity' insists that our circumstances are not uncommon within our own Solar System, Saturn's moon Titan has been described as possessing many of the atmospheric conditions present on a primordial Earth. There is even evidence of liquid water, past or present, on Enceladus, Europa and Mars.
However, the main focus is on behavioural traits we cannot assume that characteristics we observe amongst humans are exclusive to our species. In any case, so-called human-like intelligence is difficult to define, particularly with so many variations between different people. Self-awareness, fundamental to definitions of our living', implies a consideration that other beings, with distinct identities, will also share self-awareness. Extend that further, and it is no stretch of the imagination to consider that any advanced life, no matter its biochemistry, will possess similar faculties of reasoning.
The existence of just one species with human-like intelligence on our planet is not surprising if we take into account Darwin's theory of evolution. Homo neanderthalensis, one of our "closest relatives", had an ever greater cranial capacity, and arguably, fossilised evidence indicates the physiology to develop complex language. One of the most established theories is that contact with Cro-Magnons ultimately led to its demise; after all, a prolonged co-existence between two competing species is unlikely.
It is entirely possible for convergent evolution to occur, for aliens to independently develop similar thoughts and behaviours which are essential to the functioning of an advanced society. Steven Dick's Intelligence Principle suggests that cultural evolution amongst all terrestrial and extraterrestrial species is governed by a universal goal to further knowledge and intelligence. In theory, the laws of natural selection apply throughout the universe, and while there will of course be great diversity, convergionism argues that nature will come up with the same base designs, conducive to survival in various environments. Frank Drake has even proposed that anthropomorphic extraterrestrial life might be the norm.
The potential for complex language is possibly one of the universe's greatest evolutionary advantages, utilising as it does higher brain functions. Following on from the previous section, it does seem as though language, including, but not limited to vocalisations, will be a common output for intelligent life. Studies analysing the sounds emitted by cetaceans such as dolphins and whales show that they have a complex system of communication and behaviours. In fact, it has been suggested that dolphin intelligence, with problem-solving ability beyond that of the great apes, is akin to an alien intelligence, potentially offering an insight into how we might communicate with extraterrestrials.
Hypothetically speaking, out of the million advanced civilisations in our galaxy (as predicted by Carl Sagan) we can expect at least a handful of them to have developed similar, or more advanced technologies to our own. Similarly, the Drake equation (using the values initially proposed by Drake himself) seems to infer the existence of civilisations in our galaxy with which we could communicate. Rapid technological progress, of the kind which aids the long-term continuation of a civilisation, intuitively necessitates a sense of cooperation and cohesiveness of society. We may come into contact with power-hungry aliens, united in a goal to conquer any hapless life forms they encounter, or peaceful beings who have decided to pursue a more spiritual existence and in doing so have completely overcome their primal instincts. Furthermore, there will probably be some variation within these populations.
The convenience of speculation on all matters astrobiological is that one's opinion can rarely be disproved. These are questions that will probably not be answered anytime in the near future, given the sheer scale of the universe. Knowing this, we should leave the door open for the possibility of alien life in many respects similar to our own.