I always liked Mr. Spock. I don’t know how much of it, if any, had to do with his physical resemblance to Barnaby, a host of a local children’s show of my youth, who also had pointy ears. Spock’s pointy ears are probably the most remembered of his alien traits. Trekkies will also remember his greenish hue due to his copper-based blood, his seven hearts, his extra human-strength and preference for heat.
None of these character traits is implausible.  Basically all of them can be found among creatures currently alive on earth.  Many mollusks and arthropods have hemocyanin which uses copper to transport oxygen (and turns blue when oxygenated).  Earthworms have five hearts. Grasshoppers have six “hearts,” and many insects have a large general heart and a series of smaller accessory hearts at the bases of their wings and legs. Hemoglobin (our iron-based oxygen-transporting pigment) is capable of being dissolved in cells (rather than just plasma like hemocyanin) and therefore more oxygen can be carried and capacity increased, which is a decided advantage for large animals.
The only thing improbable about Mr. Spock is not his pointed ears, green blood, or seven hearts, is his very existence, not as a Vulcan but as a Vulcan/Earth cross. I’m not arguing that “Vulcans” (representing extraterrestrial life) do not or cannot exist but rather that any extraterrestrial life form could breed (produce viable life let alone one that could survive to maturity) with any earth life form is extremely unlikely, unlikely to the point of impossibility. Given their separate origin and individual evolutionary history, even forms so “similar” as Vulcans and earthlings, it is so unlikely that they are physically compatible. The similarities are not even skin deep.
Issac Asimov wrote, in something I read, that no matter where the extraterrestrial being came from in science fiction stories it was always after the shapely blond, as if Euro-America’s standards of beauty are universal, really universe-wide. Even in the movie The Creature from the Black Lagoon, the creature was attracted to the female co-star. However there is much more involved with reproduction than mutual attraction.
Life on Earth is based on carbon and uses DNA to direct cellular activity and to transmit information to the next generation. Carbon-based life is not the only way for life. Theoretically there could be other “life systems” based on different atoms (silicon for instance, as in one Star Trek episode). Carbon is a very versatile chemical, capable of forming many compounds. Water is a nearly universal solvent, with some important properties of its own that make it essential for life. Carbon-based chemistry, organic chemistry, has many properties (energy released by reactions, temperatures at which they occur, etc.) that make it more efficient, more practicable than a chemistry based on some other atom. Carbon is also reasonably abundant in the universe. Although non-carbon-based life is possible I feel that the advantages of carbon (mixed, as in our case, with water) are so great that the vast majority of life forms in the Universe are carbon-based, require water and, maybe, use DNA. Proteins are complex molecules that take complex forms, some can exist in two forms, mirror images of each other. They can be differentiated by the direction that they rotate polarized light,1 either to left or right. If you mix up a batch of one of the proteins that can exist in mirror image forms in a test tube, you get a mixture that is one half levorotary and one half dextrorotary. On Earth all optically active chemicals made by organic processes are levorotary. It is a 50/50 chance probability that an alien life form might use only dextrorotary proteins. The two different forms of the same molecule are not compatible, they cannot be interchanged one for the other.
However, even if alien life forms use DNA (with the same sugar/phosphate backbone with the same four nitrogenous bases (cytosine, thymine, adenine, and guanine) that doesn’t mean they can successfully reproduce with any other life form. Information in DNA is organized in genes, the genes are collected in “strings” called chromosomes. The chromosomes exist in pairs, each cell contains a certain number of the pairs. The “certain number” depends on the species, and varies from species to species. Humans have 22 pairs plus a mismatched “pair:” the X and Y chromosomes that determine sex  XX for female and XY for male), a “diploid number” of 46. The great apes have 48 (24 pairs), baboons, mangabeys, and macaques have 42, others (talapoin, guenon, and patas) have between 48 and 72 (36 pairs). The variation in number is even greater among the rest of life here on Earth.
Sexual reproduction begins with the formation of sex cells (gametes, eggs and sperm). By the process of meiosis, the chromosomes in a cell are divided between the daughter cells so that each has one half of each pair (in humans 23 of 46 chromosomes). During fertilization, two gametes combine and each chromosome “lines up” with its coresponding one from the other gamete forming one chromosome and then they replicate to produce a pair. The new cell again has two of each pair of chromosomes. Development then proceeds according to the “instructions” contained in the DNA. If the half pairs are not a reasonable match they fail to replicate and the fertilization also fails to continue.
We’ll just ignore the question of whether it is even physically possible for two alien species to actually mate. Their genitalia may be so different as to be incompatible. External physical similarity does not ensure that the genitalia are similar (there are species of insects that look identical to other species but their genitalia are different enough, like keys to locks, to prevent cross-breeding).  Similarity certainly does not ensure that gestation is similar or that the chromosome number is the same, the genes are organized in the same manner, or even “spelled” the same.
There are so many places where incompatible differences can occur that it is not surprising that animals so similar, sharing a long common ancestry like horses and donkeys, after a relatively short separation, cannot produce fertile offspring. What is the possibility that two species with no common ancestry, with totally separate evolutionary histories (billions of years long), are cross-fertile? Not very likely, but I still enjoy Star Trek.
The first time I saw the movie Alien I had shiver of recognition. Aficionados of the movie (and its sequels) may know and etymologists should have recognized the same thing I saw. The alien’s reproductive cycle was not invented by the writer(s). It was borrowed from natural history here on Earth.
There is a group of species known as tarantula hawks or digger wasps, among other names. Tarantula hawks are actually a species of wasps, the females of which hunt and capture tarantulas to provide a source of food for their larvae. When a female is about to lay a fertilized egg, she digs a burrow and then goes looking for a spider. When she finds an appropriate spider, she attacks it, stinging it in a particular spot so it is paralyzed and not killed. Still alive, but immobile and capable of living for a long while, the wasp drags the spider into the burrow. Here she lays the egg in or on the paralyzed spider, then she covers the burrow and leaves. The larva is on its own.
After a few days, the egg hatches, the larva emerges and begins to feed, on the spider. The larva eats the spider in such a way that the spider remains alive (the freshness of the food apparently being very important) until the larva is nearly finished eating and growing. The vital organs are the last to be eaten and once the spider is consumed the larva pupates and transforms itself into an adult wasp and emerges from the burrow.
In the movie Alien, some giant life form from outer space attacks some humans at a remote outpost. They are, in someway, immobilized and implanted with one of the alien’s offspring. The humans are alive until the alien child bursts out of their chest cavity, a scene graphically and repeatedly shown in the movies.
This may make a great plot for a sci-fi movie, however there are some things that make it an unlikely event in the real world. My objections fall into two categories either of which would make the success of the event so unlikely as to be practically impossible. First, the behavior of the wasps (and presumably the “alien”) is instinctive and not readily adaptable to a different host species, and secondly, a vertebrate like a human is unlikely to survive a sufficient length of time while being eaten to provide a suitable nursery. Vertebrate bodies are different in fundamental ways from invertebrates and less tolerant to being “abused.”
Digger wasp and tarantula hawk behavior is instinctive. They do not learn which spider or caterpillar to use or where to sting it to paralyze it. The larva is not taught how to eat the insect so that it doesn’t die until the last moment. The larva “knows” because it is in its genes. Instinctive behavior is not flexible. A tarantula hawk will not substitute a caterpillar if it finds one first rather than a tarantula. Each species of wasp preys on a particular insect. It has to find the right prey to “release” the behavior. The larva is programmed to eat its living food in a particular way, eat the food insect the wrong way and it may die too soon. If it dies too soon the larva may not complete its development and fail to survive to adulthood.
Evolution/natural selection is capable of some amazing things. It is within the realm of possibility that an organism could evolve the ability to successfully attack and subdue unknown alien species. That the larva would know what and how to do what they need to with an alien body is even more unlikely, but theoretically possible. The real problem (to me) is that humans would make a poor choice as a host in the first place.
I was taught “three minutes without air, three days without water, three weeks without food.” In a disaster situation remembering this would help me prioritized needs, etc. Breathable air in sufficient volume was more important than water to drink. You had several days to find drinkable water. The same way for a medical emergency, setting a broken arm of a victim who wasn’t breathing was useless. You had to get them breathing first before anything else.
Human anatomy and physiology are very different from spiders. We are “hot-blooded” and need a lot of energy from food to stay alive, without it we die relatively quickly. Shrews will starve to death in hours. We need a lot of oxygen to help burn the fuel so our heart and lungs operate to transfer oxygen from the air to our blood and then throughout the body. Our circulation system also removes “waste products,” carrying them to our kidneys for removal. Water keeps the blood liquid, helps metabolize protein, fills our cells, etc.
Spiders are “cold-blooded,” they use very little energy when not active, their requirements for food, oxygen, and water are greatly reduced. Oxygen is passively transported by trachea (hollow tubes) and not actually pumped. With the hard exoskeleton, and not respirating, water loss is greatly reduced.
A human, bound and immobile, will die in days. You start destroying parts of the body and death will come sooner. The heart, lungs, brain, and kidneys are critical, maybe the liver too. It is true that you can do a lot of damage to the human body without killing the person, if you know what you’re doing. You have to prevent excessive blood loss and collapse of the system, “shock.” Perhaps, in a fashion, the human body is much more “integrated” than the spider’s. Damage to a part of the body can impair the functioning of other parts (including vital organs). Otherwise minor injury can lead to shock and death. Blood loss, reduction in volume leading to inadequate pressure, is a major factor in shock. I don’t see how a larva could eat a human in such a way that would not cause continuous bleeding and concomitant loss of blood pressure that,no matter how slow the bleeding would cause death within days. Long before the larva is big enough to fill our chest cavity (having eaten our heart,lungs, kidneys, etc.). Puncture an artery and death will quickly follow.
While it is in the realm of possibility, a very remote possibility, that some alien life form could use us in a way similar to the way tarantula hawks use tarantulas, I’m not going to worry about it. It is far more likely an alien life form would use for dinner. They might, like spiders, inject a “poison” that would kill us and dissolve our insides so they can suck the juice out. That is what I would worry about.
Even here on Earth you cannot just substitute one species for another and have things continue on as before.
I was in Africa for the first time years ago, just when AIDS was really becoming a big concern for the majority of the population. I was traveling with my parents. My father, who was a doctor, was asked by one of the other people on the tour, if it was possible to get AIDS, actually HIV, from mosquitos. The fear was that since mosquitos transmit malaria, a blood-born disease, couldn’t they also transmit HIV, another blood-born disease. My father said no, it was extremely unlikely. I had a “gut-feeling” that he was right, it was years later while I was reviewing malaria and its life cycle (or “their” life cycles, since there are four species of malaria plasmodia) that I figured out why he was right to not be concerned that HIV would be transmittable by mosquito.
Malaria is caused by a protozoa, not by a virus. There are mosquito transmitted viruses (e.g. yellow fever, dengue fever, West Nile, equine encephalitises, etc.) so it is possible for a virus to be transmitted by mosquitos (“vector”). However it is a complex system that requires several parts to be properly coordinated. IT is not as simple as substituting HIV for malaria, or yellow fever. While we do not for certain how HIV spreads in nature, we do know that it spreads directly human-to-human by direct contact with infected bodily fluids. Malaria, yellow fever, dengue fever, etc. do not naturally spread human-to-human. We can do it artificially with blood transfusions. Malaria must pass through a mosquito before it can infect a human. It is not like a mosquito bites an infected human, sucks up some blood, then goes bites another human and the contaminated proboscis? (“needle”) infects the new human, or even that some of the blood from the first human is injected. In is more complex than that.
When a female Anopheles mosquito bites (only female mosquitos bite) an infected human, the blood with the Plasmodium sporozoite (the asexual spore of the malaria protozoa) is stored in the mosquito’s stomach. The blood is digested to provide enough protein to produce eggs. The conditions of being in the mosquito’s stomach cause the malaria to undergo sexual reproduction. The zygote exits the stomach, produce new spores which migrate to the mosquito’s salivary glands. Here they wait. This all takes some time, during which the mosquito, assuming she survives, lays the eggs and then goes looking for another blood meal to produce another cluster of eggs. When she bites, a small amount of saliva is injected. It prevents blood clotting and later causes the itching, an allergic reaction to the foreign protein. Along with the saliva is the malaria spore. If the organism bitten by a malaria-infected mosquito is a human or maybe another primate, the malaria spore will begin reproducing, asexually, and be ready to infect another mosquito. If an infected mosquito bites anything other than a primate, the transmission is broken.
Mosquito-born diseases are not spread because some blood is removed from one infected human and injected into another. No blood is transferred, the disease agent has to “infect” the mosquito and get into the salivary glands before it can reenter a human’s blood stream to cause another infection. Just because a disease is carried in blood does not automatically mean it can be transmitted by mosquito bites. It’s simply not that simple.
1The proper terms are: levorotary—rotation of the plane of polarized light to the left, that is counter-clockwise, and dextrorotary—rotation of the plane of polarized light to the right, that is clockwise.