Ah, space aliens. They're exotic, they're mesmerizing, they let the Science Fiction writer explore all sorts of themes and plots that just wouldn't be possible with plain old human beings.
But if you are attempting to write a work of hard science fiction, your aliens have to be realistic. The sophisticated reader, who is knowledgeable in physics, chemistry, and biology, must believe that these otherworldly creatures could actually evolve on real planets in the real universe.
This is not an easy task. George R. R. Martin said as much in his 1976 essay "First, Sew On a Tentacle (Recipes for Believable Aliens)". Your alien species will come from a world with its own evolutionary history, its own flora and fauna — if distinctions like "flora" and "fauna" even make sense in that world's biosphere — and must occupy some evolutionary niche on that world, or by all rights it shouldn't exist at all.
The only case study we know of
Thus far, we know of only one planet where life actually arose. But just in our one biosphere alone, we've seen an amazing diversity, an astonishing spectrum of bizarre and wondrous organisms that all fall under the umbrella of "life." Much has been made of how alien the humble starfish is when compared with a human being, but even a starfish still has cell nuclei and mitochondria and shares the same genetic code with us. There are life forms on, and in, the Earth whose very chemistry is different from ours, and that show us unequivocably that the route from microbes to man didn't have to play out anything like it actually did.
The first organisms on Earth that could reasonably be called "alive" were probably short, self-replicating nucleic acid chains. It's now known that some RNA strands can act like catalysts, helping other chemical reactions take place simply by virtue of the strand's shape; a few of these "ribozimes" can even catalyze the creation of other RNA strands, including copies of themselves. They may have "lived" inside phospholipid membranes that became the ancestors of modern cell membranes, and they would have even been subject to a very primitive kind of natural selection. This video shows one possible chain of events that could have occurred on ancient, pre-biotic Earth and led to the first living organisms.
But it didn't have to go that way. There could have been other catalytic, self-replicating molecules that made their way onto the planetary stage instead. Or, at least, we have no reason to think that something else could not have worked. Our cells have nucleic acid chains and phospholipid membranes because the life forms that eventually made their foothold on ancient Earth had nucleic acid chains and phospholipid membranes. But on another planet, with conditions similar to but not identical to the primordial Earth, it might very well be the case that some other, similar molecule would have "won the race" instead, and the biosphere on that world might rest upon genetic material that isn't made up of chains of nucleic acids.
Ribozymes, like modern protein enzymes, sometimes need "cofactor" molecules to function properly. Some of these cofactors are individual amino acid molecules, or very short molecular chains (2-3 long) of amino acids. Amino acids occur naturally in some comets and asteroids, and were doubtlessly present on the ancient Earth. This was likely how proteins first started becoming bound up in living organisms: Those Ribozymes that had the necessary chemistry to capture free-floating amino acids had access to more cofactors than those ribozymes that didn't, and could thus out-compete them. The ability for two amino-acid-carrying ribozymes to join their amino acids together in a chain would also have been useful; indeed, such behavior can be (and has been) "evolved" in a laboratory. Eventually, one specific short RNA strand can become associated with one specific amino acid throughout a given protocell, forming what we in the modern world would call Transfer RNA. From there it's a short step to associating a specific string of RNA nucleotides with a specific piece of tRNA, and thus with a specific amino acid. This is the origin of the genetic code. This video explains this process in greater detail.
But the genetic code we have today — a mapping of 64 different nucleotide combinations to 20 different amino acids — is not universal to all life forms on Earth. The mitochondria inside your cells, for example, have their own DNA and replicate themselves according to their own drummer, but their genetic code is slightly different from the genetic code in your cell nuclei. It's mostly the same, but not entirely the same. An RNA/DNA using organism that evolved on another planet could —and, indeed, almost certainly would — have an entirely different genetic code. Maybe they only make use of 16 amino acids, not 20, and get away with having codons that are only 2 nucleotides long instead of 3. Maybe they don't use amino acids to build their bodies but something else, and that something else has 10,000 variants instead of 20; if they use 4-nucleotide RNA/DNA like we do, each codon would have to be at least 14 nucleotides long.
For the first billion or so years of life on Earth, the only life forms were bacteria, bacteria-like archaea, and viruses. Bacteria are small, surounded by a peptidoglycan-based cell wall, and keep their main DNA in a single loose strand anchored on one end to the cell membrane. Then, some 2.7 billion years ago, a new player entered the stage: the eukaryotes. Eukaryotic cells are considerably larger than bacteria, have a motile surface (lacking a peptidoglycan-based cell wall), and most importantly carry their DNA in a separate interior bubble called a nucleus. The large-scale structure of eukaryotic DNA is different from bacterial DNA, allowing the cell to carry its genetic material in several different strands, or chromosomes. With the exception of a few weird bacterial corner-cases, all multicellular life on Earth descended from these early eukaryotes.
Then, some 2.4 billion years ago, Earth experienced the worst case of air pollution in its history. A very successful bacterium called cyanobacteria hit upon a way to synthesize the glucose it needed to survive, by using plain old carbon dioxide and water in the presence of sunlight. Unfortunately, this process gave off a deadly byproduct: oxygen gas. Oxygen is highly reactive, and spells instant death for any organism that isn't aerotolerant. It's thought that most extant species of bacteria simply died out in this Oxygen Holocaust. Many others retreated to places where oxygen couldn't reach, such as stinking mud pits. A few, though, managed to evolve aerotolerance and survive. Eukaryotes were among the lucky few who evolved aerotolerance.
And once all this oxygen was lying around, it didn't take long for another organism called purple bacteria to hit upon a way to use it. With the right metabolic pathway, a purple bacterium could get a lot more useful energy out of a single glucose molecule by combusting it with oxygen than it could by simply fermenting the glucose. They were not merely aerotolerant, they were full-blown aerobic. They now produced more energy than they could possibly use, and eventually, they and the eukaryotes hit upon a wonderful mutually-beneficial deal: The purple bacteria could live inside the eukaryote, protected from small predators and given access to all the nutrients at the eukaryote's disposal, in exchange for which the purple bacteria would turn glucose and oxygen into energy that the eukaryote could use. The descendants of these original, symbiotic purple bacteria evolved into modern mitochondria, which exist as organelles inside every eukaryotic organism on the Earth today.
The organization of DNA into chromosomes had an additional benefit: Virus protection. A virus can destroy its host cell, but only if it can find a specific pattern in the host cell's DNA to graft itself onto. If an exact match isn't found, the virus fails and the host organism is safe. Viruses are always evolving, though, and it only takes one to hit upon that lucky combination that can kill you and every "daughter cell" related to you. One way to significantly reduce the odds that a virus can latch onto your DNA is to shuffle the genetic deck whenever you reproduce — change a nucleotide here, a gene ordering there, etc., and a virus that might kill you will be harmless to your offspring. The problem? Changing your DNA willy-nilly is dangerous. You don't know which parts of your DNA will be fine if you shuffle them around, and which parts will (say) take away your ability to digest food if you tamper with them. The solution? Shuffle your genetic deck with another member of your own species. Your genes produced a survivor (you), and that other organism's genes also produced a survivor. So if you mix-and-match between your two genomes to produce a joint offspring, this new progeny should also have a good chance of surviving, and will be better able to dodge existing viruses. We call this technique sexual reproduction. These videos show one possible chain of events that could have occurred on ancient Earth which led to sexual reproduction.
As a totally unintended consequence of sexual reproduction, the rate of evolutionary adaptation skyrocketed.
Normally, when a single-celled organism (or its zygote) divides, the two daughter cells separate and go on their merry way. But in some circumstances, the daughter cells can stay clumped together — and sometimes, banding together with your siblings gives you advantages over all the loner organisms out there. Eventually, the group sticking together could evolve so that it always sticks together, and then the members of that cellular colony have a golden opportunity: They can specialize, so that (for example) a few cells become gametes while others stay "normal" for their entire lifetime. In such a case, where most cells never become gametes, the colony is similar to a beehive: A few reproducing organisms (queens/drones) supported by a non-reproductive mass of organisms with the same genome (workers). Ever-greater cell specialization is possible from this point on, so long as the colony sticks together. The colony can even move and act as a unit, assumining intercellular communication is possible. At that point, the colony has become a multicellular organism. More than a dozen times in the history of life on Earth, in both eukaryotes and prokaryotes, multicellularity has evolved; even fossilized cyanobacteria shows some evidence of multicellular organization. This video discusses probable ways that it might have evolved.
As mentioned above, purple bacteria eventually moved in to the interiors of eukaryotes and entered into a symbiotic relationship with them, eventually evolving into mitochondria. All modern eukaryotes contain mitochondria. However, purple bacteria weren't the only organisms to do this. The same cyanobacteria that caused the Oxygen Holocaust also found a home in some — but not all — eukaryotes, after the point at which multicellularity had emerged, and eventually evolved into endosymbiontic organelles called chloroplasts. The eukaryotes lucky enough to harbor these chloroplasts could now use both photosynthesis (which generates oxygen) and respiration (which consumes oxygen), thereby never having to get up off the couch. These eukaryotes eventually evolved rigid cell walls — very different in chemical composition from bacterial cell walls — and became the Algae and the Plants.
The rest of the multicellular eukaryotes were stuck foraging for food for their entire lives. One group ended up specializing in exploiting decaying dead matter; they became the Fungi. But another group chose to kill and eat other living organisms, or at best subsist on organisms that had only recently died. They became the animals. All macroscopic, rapidly-moving organisms on the planet belong to the animal kingdom.
Again, it didn't have to happen this way. What if choloroplasts had moved into all eukaryotes, but some of them couldn't get enough energy from sunlight and stayed mobile to supplement their "sunlight diet" by eating other organisms? They could have traits like plants and animals. The difference between plantlike and animal-like organisms in such a biosphere would be a question of degree (how much does this organism depend on photosynthesis for is sustenance), not a question of kind.
The simplest animals on Earth are the sponges. They're basically filter-feeders; they just sit there and let water pass through their bodies, and feed on any plankton that happen to come drifting through. ("Plankton" is a broad term that covers all tiny water-bound organisms that can't swim against the current; some plankton are bacteria, some are algae, some are plants, some are even small animals.) Sponges have no central nervous system, and only the most primitive of means for distributing nutrients throughout their tissues (i.e. no blood, and certainly no circulatory system). A central nervous system, or something like it, probably first evolved with the anemonies; touch one part of an anemony, and other parts of the organism will contract in response. The development of a nervous system marked the beginnings of behavior, rules governing how a large multicellular organism responds to stimuli.
Of course, you can't respond to a stimulus unless you can detect that stimulus, hence the evolution of senses. Of great benefit was the sense of sight. Vision evolved independently no less than sixty-four different times within different animal classes. The first "eyes" were nothing more than a light-sensitive patch of skin, attached to a nerve that sent different signals depending on how much light was received. Just the ability to detect the difference between being in light and darkness had a huge survival advantage; if the lights suddenly went out, it likely meant that a predator was casting its shadow on you, and by running in a random direction you might be able to escape. The Other Wiki has an article on the evolution of the eye. What's important to note is the convergence of eye evolution in different species. Human and octopus eyes, for example, evolved totally independently of one another, yet their similarity in structure is striking — in some ways, an octopus eye is better than a human eye, in that there are no blood vessels cluttering the area in front of the octopus' retina. Many insects have "compound eyes", which are basically a retina turned inside-out — each facet on a fly's eye can see only one "pixel" of the world around it, and it assembles a vague, low-resolution picture of its surroundings from these pixels.
For hundreds of millions of years, there were only a few taxa of animals in the Earth's oceans, all of whom were relatively simple in structure. Then, 530 million years ago, an enormous number of animal species appeared in the fossil record, an event called the Cambrian Explosion. (This "explosion" occurred over the course of several million years, and is only "rapid" when compared to the rate at which new species appeared before and since.) Nearly every one of the 30+ animal phyla that still exist on the Earth today first appeared during the Cambrian explosion. Many of the phyla that appeared in the Cambrian explosion are now extinct, but curiously, no new animal phyla have appeared since then. All animal evolution since that time has been variation within existing phyla.
One of the phyla that appeared within the Cambrian explosion, exemplified by species like Pikaia and Amphioxis, was the Chordates. These creatures had a notochord running down the length of their backs, which could carry nerve signals from one end of the body to the other. Within certain species of this phylum Cordata, this notochord would eventually evolve into a full-blown spinal cord. All modern vertebrate classes — fish, birds, amphibians, reptiles, and mammals — are descended from these chordates.
Eventually, some of these chordates found a new niche to inhabit, that no animals had inhabited before: The brackish, less-salty-than-the-ocean waters at the mouths of river deltas. They of course had to evolve a kidney to expel all the excess water they took on in these low-salinity environments, but they also faced another problem. Sodium salts aren't the only salts dissovled in ocean water. There are a lot of minerals in sea water, including calcium, and calcium had long ago become a mineral that much of their biology depended on. So, now, they needed a way to store calcium inside their bodies, for those times when the brackish waters didn't have enough calcium dissolved in them for their daily needs. Big lumps of calcium have the approximate consistency of rocks, so they needed a place in their bodies to store these "calcium rocks" which wouldn't interfere with their breathing, eating, mobility, etc.. What better place to store them than hanging in little bundles off of their notochords! This is how the notochord gradually became a true backbone. Eventually these lumps of calcium started getting formed into deliberate, interlocking shapes which could flex between the segments without wasting space or pinching the notochord (now called the spinal cord) they were wrapped around.
And once calcium started being used for "bone" in one place in the body, it didn't take long for it to start appearing as bone elsewhere. This was the beginning of the endoskeleton.
These chordates with backbones and other bones were called fish. Once they had the equipment to survive the brackish waters around river deltas, new species soon began inhabiting true fresh water environments farther inland. The old, jointed exoskeleton legs of their ancestors gave way to muscled fins with bones in them. Some of these eventually evolved joints too. And in at least one lineage, the gill slits which filtered oxygen from the water formed into internal sacs that required the creature to breathe in and out — lungs.
Much is made about the "first animals to leave the oceans", with most folks usually pointing to either lizard-like amphibians that used their arms for brachiation under water, or lungfish that could pull themselves across land for short distances. The people that think these were the first animals to leave the oceans are hopelessly vertebrate-centric in their thinking. The first land animals weren't fish. They weren't amphibians. They were insects. Insects evolved the ability to breathe air and survive on land 40 million years before vertebrates did. With no vertebrate predators to threaten them, some of these insects grew to nearly 3 feet long. They couldn't get any bigger than this, though; without an endoskeleton, all the squishy guts inside their bodies have to be anchored to the inside of their chitinous exoskeletons. The bigger the insect, the more those guts will inevitably "sag" toward the bottom of their body cavity, and the thicker (relative to the overall length of the insect) the exoskeleton needs to be. Compounding this problem was the need to extract oxygen from the air. Insects have neither lungs nor oxygen-carrying blood; they have to draw air in from spiracle valves on their skin, and carry it directly to the tissues through networks of tiny tubes. This means no point inside an insect's body can be more than a couple of centimeters from its exterior.note These twin problems form the insect version of the Square-Cube Law, and it piles up a lot quicker than the square-cube law does for us bones-on-the-inside vertebrates.
Incidentally, while insects were the first land animals, they weren't the first land eukaryotes. Plants beat the insects onto land by 75 million years.
Nevertheless, when amphibians did move onto the land, they completely wiped out all the giant insects. A giant, thick exoskeleton full of sagging guts just could not compete with an endoskeleton. Today, the largest surviving insects are creatures like the rhinoceros beetle, less than half a foot long. These early land vertebrates still had to lay their eggs in the water; the evolution of the hard-shelled egg that could survive out in the open air came later.
Many "What If?" scenarios come to mind when imagining the timing of the moves onto land. What if the plants and insects had not colonized the land before the vertebrates did? Then the vertebrates would still have needed to forage for food in the water. The ability to walk on land would have been a way to avoid predators rather than a way to find new food sources, and would thus have evolved in the smaller prey animals first. While the oceans and waterways would have been be filled with the full gamut of vertebrate sizes, the land would have been home to only the smaller critters.
Eventually, reptiles emerged, who produced hard-shelled eggs that could be safely stowed away on land. They out-competed the amphibians in much the same way that the amphibians had out-competed the insects. But like the insects, the fish, and the amphibians — in fact, like every other organism living on Earth at this point in history — reptiles were cold-blooded. Their body temperature depended entirely on the temperature of their surroundings. Since a lot of biological processes depend on chemical reactions that only happen within a certain narrow range of temperatures, they had to evolve all sorts of tricks to keep warm. Some got large enough that they wouldn't lose heat very quickly in cold weather, a trick known as gigantothermy. Some evolved enormous sails on their backs which they could turn toward the sun for gathering heat, and then fold up at night to prevent heat loss.
But pretty soon, one sub-group of the reptiles, called the therapsids, hit upon another strategy for thermal regulation. They produced extra heat inside their own bodies via chemical reactions. The glucose respiration their cells relied upon for energy also gave off some heat, so by "burning" extra glucose they could keep themselves warm even when the environment got cold. We call it endothermy (not to be confused with an endothermic chemical reaction, which is a reaction that absorbs heat rather than giving it off; animal endothermy requires exothermic chemical reactions). Endothermy allowed them to keep active on cold nights, when other animals could barely move, and to survive in colder climates.
Warm bloodedness came at a price, however. To produce all that chemical heat, you needed much more glucose than you'd use otherwise. Over 75% of the glucose consumed by a human's metabolism goes solely into producing heat, for example. That means needing a lot more food. A warm blooded creature has to eat, and eat, and eat, nearly all the time. The 3 meals a day we humans take for granted as normal is in stark contrast to the one meal every week, or every month, that a reptile needs. As a consequence, a much smaller percentage of warm blooded creatures could afford to be carnivores. While 1 in 5 cold-blooded fish or reptiles is carnivorous, only about 1 in 100 warm-blooded creatures are carnivorous, because a warm-blooded carnivore has to kill and eat a lot more prey than a cold-blooded one does. (Kinda backwards from the notion of the "cold-blooded killer," isn't it?)
On an alien world, there's no guarantee that endothermy would have evolved as the dominant strategy for thermal regulation. Sails, fins, or gigantothermy could have taken over. So could the strategy of the Galapagos diving iguanas, who sunbathe on rocks and then immerse themselves in the cold oceans for a short time to make use of their stored heat. Other strategies that never appeared on Earth are also possible.
By the end of the Paleozoic era, therapsids had diversified until they filled nearly the same ecological niches as the mammals of today. There were great grazing herds that roamed the grasslands, there were predators who fed on the grazing herds, there were tree-dwelling arboreals, there were coastal swimmers; nearly every niche filled by a mammal today had an analog among the therapsids. An alien visitor would find little difference between Earth at the end of the paleozoic and Earth in the modern era.
And then, disaster struck.
Or rather, several disasters all struck at the same time. Land masses slowly drifted together to form the supercontinent of Pangaea, squeezing out oceans that used to harbor life between them. Volcanic activity caused devastating global warming, and exposed underground coal beds which caught fire and caused more global warming; and all this warming melted the frozen methane hydrate under the oceans, which caused even more global warming. Oxygen levels fell, and hydrogen sulfide levels soared. A giant asteroid may have struck the earth too. With all these catastrophes adding together, 96% of marine species and 70% of land vertebrates vanished. Over half the Earth's taxonomic families went extinct. This event, known as the Permian-Triassic Mass Extinction, is the worst mass extinction that still survives in the fossil record.
As a result of the Permian mass extinction, the modern-mammal-like domination of the therapsids evaporated. But what if the Permian mass extinction hadn't happened, or had been milder? We still to this day might be living in a world dominated not by furry mammals, but by scaly-skinned warm-blooded lizardlike therapsids.
(Insert further discussion of terrestrial biology here, with an eye for how it might have gone differently somewhere else.)
See also the Useful Notes article on Prehistoric Life.
Your world's biodiversity
An excellent source for thoughts on the different forms that aliens might take, from their metabolism to their art, is Robert Freitas' Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization
Unless they've embarked on a systematic process of mass extermination on their homeworld, your aliens will not be the only species on their home planet. They will be part of an enormously diverse biosphere, containing things as simple as the first organisms to have evolved there (like Earth still has bacteria) to things as complex as themselves, and every level of complexity in between.
Evolution is your best friend (and your worst enemy)
Evolution — random variation coupled with natural selection — will be the shaping force of any biosphere, whether the life in that bisophere is based around proteins and nucleic acids, or silicon crystals, or self-organizing superheated plasma. The basic selection criterion of evolution is reproductive success: that is, for any given organism, how many copies of its genes can it create in a given time period which are themselves capable of reproduction, before that organism dies?
If something kills the organism before it gets a chance to reproduce, no copies of its genes will get made. If it reproduces but its offspring are sterile, the copies of its genes that got created in those offspring will die with them. If, on the other hand, the organism dies while saving the lives of its siblings, those siblings likely contain copies or near-copies of its own genes, and will thus be preserved. To be successful on the evolutionary stage, an organism must be able to survive to its reproductive age and then actually produce fertile offspring.
Thus, every feature of an organism — its senses (if any), its means of locomotion (if any), its robustness in the face of environmental adversity, and even its psychology — must serve a purpose (or have served a purpose in its evolutionary past) that either increases its odds of survival, or increases the number of offspring it creates, or both.
Many lifeforms have a lot of biological quirks that don't provide much of an evolutionary advantage, but also don't provide enough of a disadvantage that it's been weeded out by natural selection. However, most of these are so widespread because they used to be advantageous. For instance, goosebumps; they come from your body trying to fluff up fur it doesn't have, either to keep in heat better or to look bigger and scarier. Handy for lots of mammals, including our distant ancestors, not so handy for modern humans. Unless your aliens have genetically engineered themselves to remove these sorts of things, they'll probably have a few strange little traits like this.
You may have heard of the term "convergent evolution": that when something works very well in a certain environment, it may develop independently in multiple, mostly-unrelated species. For instance, this is the reason dolphins and sharks have similar body shapes and coloring, despite the fact that one is a mammal and one is a fish. This can be a handy concept to draw from: for instance, perhaps your aliens have two ears for the same reason many Earth animals do- it allows one to judge directions and distances much more accurately. However, don't overdo it. There's a difference between "similar" and "identical," and random chance has a significant enough role in evolution that even if you had a planet completely identical to Earth and let life evolve there, it would likely take a very different form from ours.
The bad news is, forcing yourself to apply evolutionary principles to every aspect of your alien species creates an enormous burden for you as the writer. The good news is, if you pull it off, your readers will appreciate the effort.
Believable space-faring aliens
If you want your alien species to be space-farers — that is, tool-makers who can build vehicles capable of crossing interstellar distances — they will need to have some traits in common with humans. Such as:
Living in air
To build spacecraft — or a technological civilization anywhere past the stone age — your aliens will need to be able to smelt metal. This means that they need access to a heat source powerful enough to raise metals to their melting point. Ancient humans accomplished this by building fires, that is, by combusting plant matter and/or coal (which used to be plant matter) in air. If humans lived under water, like porpoises or fiddler crabs, we wouldn't be able to build fires.
Hypothetically, other metal-smelting heat sources could be available to an underwater species. There are volcanic vents on the ocean floor, for example, but these only exist where you happen to find them, are surrounded by boiling hot water and toxic (to us) sulfur compounds, and eventually shut themselves down.
A species on a planet that lacked an oxidizing atmosphere would face a similar problem. If you want to build a fire in, say, a methane atmosphere, you'd have to bring your own oxidizer. Without high-tech equipment to make oxygen (or chlorine, or some other substance that will combust with methane), there must be some organisms in your planet's ecosystem that produce, and store, oxidizing chemicals so that your aliens can build smelting fires with them.
To build spacecraft, the aliens will need to be intelligent. This means that whatever they have that passes for a "brain" will need to house billions of neuron-like switching elements for abstract thought. There is probably a certain minimum size that any biological neuron-analog will need to be, so the aliens' brain itself will need to be at least, oh, several cubic centimers in size. This puts a lower limit to how small the aliens can be. Little green men may be feasible, but microscopic green men are impossible so long as we limit ourselves to life based around organic molecules.
Conversely, there are probably upper limits on such an organism's size. Any organism that moves and thinks as an organized whole (as opposed to an agglomeration of semi-autonomous cells like a slime mold or a sponge) is going to need something akin to a central nervous system to relay messages quickly from one part of its body to another. It's also going to need a means of distributing whatever nutrients its cells require and carting away the cellular waste products, akin to blood. The bigger these systems become, the more problems they incur. The apatosaurus, for example, had a very tall neck, so the blood pressure its heart had to produce in order to drive blood all the way up to its head was enormous. It adapted to this problem by having a very small brain, so that its head didn't need a lot of blood. If your space aliens have their brains in their heads, and come from a world with a surface gravity on par with Earth's, they cannot be much taller than humans unless they evolved some solution to this problem, such as multiple hearts (one down near the feet and one up near the head), or vein-like anti-backflow valves in their upper arteries. The Square-Cube Law also plays havoc with an organism's developmental needs, since growing enormous leg bones requires you to gather more food from your environment just to build the leg bones out of.
You can't make tools without something you can use for "hands". An elephant's trunk, an octopus' tentacles, a monkey's prehensile tail, a dog's mouth, or even a bird's beak can be used to pick things up, but performing fine work requires either fingers or tools that you can shape to use like fingers.
Smarts, and the ability to make tools, isn't enough to get you into space. You need to be able to create a cultural knowledge base that knows how to deal with its existing technology, and allows an individual to innovate new technology by building on what the culture already knows. As our own space programs have demonstrated, leaving our home planet requires both a highly advanced technological infrastructure and a remarkable degree of cooperation, and that requires the ability to communicate knowledge — sometimes very complex knowledge — from one individual to another.
This communication of knowledge must encompass both long-term learning, such as "Here is how you build a transistor", and short-term coordination, such as "Okay, Bob, when I lift my end of this heavy object, you lift yours. Ready? One, two, three, lift!"
Just because a tool-making, social, linguistic species has the ability to make spacecraft doesn't mean they'll have the desire to make them. There must be some quality about their basic psychology that drives them to explore, and to cooperate in their exploration, and this psychological quality must make sense from an evolutionary standpoint. In the case of humans, our desire to explore space probably came out of a desire to explore the horizons of our surroundings, to see if there were any wildebeasts to hunt or any other human tribes to trade or fight or mate with.
Perhaps a species of herbivores, such as Larry Niven's Puppeteers, would be motivated to see what's over the horizon by a simple desire to ensure the safety of the herd — if they discovered a leopard, they could prepare for it and thus decrease their odds of getting eaten. Or perhaps the grass that the Puppeteers graze on (or whatever ground-covering organism passes for grass on their planet) only grows in random patches that last a few weeks, so they have to find the next grass patch or starve to death.
If your aliens are cold-blooded carnivores that only need one meal a month, they won't be motivated by the manic desire to acquire ever more resources that we humans are so intimately familiar with. Would such creatures even be tool-makers in the first place? Perhaps they need the tools to defend themselves against other predators that are warm blooded, which would lend them a certain degree of paranoia (even xenophobia). These creatures would be motivated by fears similar to those that drive the herbivores, and if us warm-blooded predatory humans ever stumbled across them they'd almost certainly see us as a threat and try to eliminate us. If they are space explorers, they are space explorer-exterminators.
Of course, distinctions like "warm blooded" or "cold blooded", and "herbivore" or "carnivore", are terrestrial ones. The alien biosphere might not have the sharp dichotomy between plants and animals that exists on Earth — it may have mobile creatures with central nervous systems like animals, who subsist on photosynthesis (and a bit of decaying dead organic matter) like plants. But however they live, there must be something in their basic survival psychology that pushes them to explore, or they're never going to build space ships in the first place.
Once you've got the basics of your species' psychology figured out, consider checking out Design an Alien Mind to flesh out the rest of the thought processes of your new aliens.