Could Humanoid Aliens Exist?
One thing you'll notice if you watch a lot of sci-fi is that aliens often look similar to humans and communicate with the same vocal range we do, which seems kind of unlikely in real life.
This alien-human resemblance is mostly for the sake of filming, and for the sake of storytelling—people tend to relate better to other creatures that look and think sort of like humans. At the very least, it helps us relate to otherworldly beings if we can recognize some human traits in them. But what are the chances that we might meet a human-like being from another planet someday? Or maybe that they exist, but we'll never be able to meet them? We know there are probably other planets similar to Earth out there. But do they have the conditions necessary to cause a species to evolve the same way, or close to the same way, as we did? Well, what are characteristics of humans, and how did we get these traits?
Traits of Humans and Conditions That Caused Them:
Need to grasp small objects, used for climbing and tool use.
Helped hominids see longer distances when walking across plains, not needed in forest-dwelling, tree-climbing primates.
Nomadic predators or scavengers, traveling large distances and needing a good memory, social animals develop social intelligence and language use.
Tool making and use
Materials, Intelligence, Need for Tools (Hunting, Agriculture)
Use of fire
Eating Meat, Which Carries Infection Risks, Having Stuff to Burn on Planet (Wood, Coal, etc.)
Warm-bloodedness is suited to a colder climate, living on land is suited to planet with a lot of dry land, mammalian reproduction slower, but it is more protected from predators. Mammalian reproduction and maternal care of infants causes social bonding, relationships, family structures, gendered division of labor, attachment of parents to their children.
Humans evolved hairlessness to thwart body lice.
Urbanization and Agriculture
Necessary political and economic basis for society, large-scale agriculture requires a lot of social cooperation, which requires a legal code and written record-keeping.
Advanced Agricultural Methods, Peace, High Population, Innovative Freedom, Trade Across Long Distances
Humans Invented Clothing for Warmth and to show Social Position, form of Communication
Large Social Structure, Social Cooperation for Survival (Not Solitary), Does Not Communicate Primarily By Scents or Gestures, Vocal Chords, Sharp Hearing to Distinguish Voices
So, if we think about it, we are just one species, out of billions of species on this planet, with ALL of the above traits; but very few of those traits are unique to humans (clothes, industrialization, written laws as the basis of social order, fire). But one could argue that it certainly may have been possible for several hominid species to coexist on this Earth simultaneously, since that's what happened thousands of years ago. They couldn't survive the Ice Age, or were killed or out-bred by Homo sapiens, but it's interesting to imagine a world where human-like others of a different species shared the planet with us. But what are the chances of hominid life forms evolving on other planets?
Space and the Question of Earth-Like Planets
There are two schools of thought on this question of whether life could evolve on other planets similar to us. The "Debby Downer" position is called the Rare Earth hypothesis; the belief that the circumstances of life on Earth are so crazy that there's probably nothing like this place anywhere else. Other scientists, including Carl Sagan, however, propose a contrary view to that, believing that Earth is a typical planet orbiting a typical star in a typical galaxy. Since our size, star, position, galaxy, etc. are all not anomalous, they reason that there are probably many Earth-like planets floating around out there. This is the view I prefer, not just because it lets us imagine that really cool sci-fi things could really be happening out in space, but it just makes sense to me because there are many planets and many stars out there, and if our planet exists, it seems reasonable to think others similar enough to it to support life must be too.
But hominid life, specifically, gets a little more tricky. It's true that there was a very specific set of conditions that led to human life happening when it did and to humans looking and behaving the way they do now. I think there are probably a vast number of planets with rudimentary life forms similar to our simplest of plants, fungi, and unicellular organisms. But it did take a long time for Earth to go from that to having human civilization, and many planets may be seemingly stuck somewhere in Earth's past, biologically speaking. Still others, though, might have had human life on them for hundreds of thousands, or millions of years. It's fun to speculate about, but we may never really know.
(Source/Further Reading: Rare Earth Hypothesis, Wikipedia.)