Here on earth there are three barriers to interspecific hybridisation: behavioural, functional, and genetic. Behavioural is rather obvious; in the vast majority of cases different species are not sexually attracted to each other, and do not attempt to interbreed. If they do, it is usually in captivity, when they lack possible mates of their own species.
Functional BarriersIn point of fact, closely related species can interbreed, and sometimes the offspring are fertile. However, often there is some defect present. Crosses between lions and tigers, for example, tend to be huge. It is like a mechanic replacing one part of an automobile engine with the analogous part of another make. If the two makes are similar, the result will be reasonably successful. If they are different, the engine will perform poorly. If they are too different, it will not run at all. But the real analogy would be the replacement, not of a single part, but half of them.
Consider, now, the situation where the parents come from different planets, and their body plans have a completely different evolutionary history. It is possible to conceive of a humanoid alien which looks just the same as a human being (an unlikely event, I will admit, but theoretically possible) while being completely different on the inside, because it possible to construct the same body plan with totally different components. The examples I am about to provide are by no means exhaustive.
First of all, we might start with something which is utterly fundamental, but of which hardly anyone is aware: optical rotation. Living organisms are carbon-based because carbon has a valence of four, meaning that each atom can bond with a maximum of four other atoms. Since two of those atoms can also be carbon, long chains of carbon atoms are possible. Now, let us visualise one carbon atom in the middle of a chain. It is bonded to another carbon atom at each end, and two other atoms - let us call them "A" and "B" sticking out of the side. It is obvious that they can be attached in two different manners: with A to the right, or with B to the right. The molecule can thus exist in two forms, which are mirror images of each other. In biochemistry, they are classified as "dextro" (D) or right-handed, and "laevo" (L) or left-handed. They often possess very different chemical properties.
The point is that, although artificially synthesized drugs typically contain more or less equal quantities of D an L molecules, living organisms produce only L molecules. That is, here on earth. It is a result of the way life first developed at the dawn of time. But that was just a fluke. On another planet everything might well be constructed on D molecules. Indeed, I cannot see any reason why half the living planets shouldn't contain L-life and the other half D-life.
Let's look at a few other features. Many birds, such as parrots, can copy human speak and so, if they were clever enough, they would be able to talk. However, their voice boxes are quite different from ours. Instead of vibrating a pair of vocal chords, they vibrate a type of drum in an organ called a syrinx.
The eyes of a mollusc, such as a giant squid, and those of a vertebrate, such as us, look very similar, and they function just as effectively. However, the rear of the eye is structured quite differently in the two groups. And while we are on the subject of eyesight, we might mention colour vision. The back part of our eye contains cells called cones, with pigments in three different colours: red, green, and blue. We can distinguish a million separate colours by matching the incoming light against the proportion absorbed by each of the pigments. Colour blind humans usually have one of the pigments out of kilter with the others, and less often lack one of the pigments. It would be incredible if an alien had exactly the same pigments as us. But most mammals possess only two pigments. Contrary to what you may have heard, they do not see the world in black and white, but they do lack the red pigment. But most birds, reptiles, and amphibians, not to mention most insects, have four pigments. Some, especially among the insects, can see in the ultraviolet.
Much of our anatomy is the result of evolutionary accidents. Why, for example, should an alien's organ of balance be attached to the ear, as ours is (the inner ear)? Why should it possess only seven vertebrae in its neck, as almost all mammals do? Why twelve pairs of ribs? Our skulls consist of a mosaic of bones, which need not be the same in an alien humanoid. Some of these bones are normal, run-of-the-mill internal bones, and some develop by ossifications in the skin.
It is the endocrine system which is the most arbitrary in position and formation. Because these glands feed hormones directly into the bloodstream, they could logically be placed anywhere in the body cavity. Our thymus, for instance, is where it is because it evolved from a mucus-secreting gland in the pharynx of an ancient ancestor in the days before backbones were invented. There is no reason, other than evolutionary history, why the pituitary gland should be placed under the braincase, or the adrenals on top of the kidneys. Indeed, there is no reason why their functions ie the hormones they produce, should not been spread out among a whole lot of different glands placed all over the body.
Sex. I caught list many more examples, but since sex will always catch people's attention, let's discuss the "naughty bits". Essentially, they start off in the embryo as a combination of a pair of sex glands (testes or ovaries), and a pair of tubes which allows the products (sperm, eggs or, in the case of mammals, babies) to pass to the outside world. In men, it is the Wolffian duct, in women the Müllerian duct. However, a tube, called the urethra is also required to pass urine from the bladder to outside world, which is why we urinate and copulate through the same orifice. But marsupials (mammals with a pouch) and placentals (mammals like us) differ in the placement of the sex ducts and the urethra. And this leads to some interesting results.
In male placentals, the urethra comes in front of the Wolffian duct, in marsupials it comes in behind. That means that in marsupials the testicles descend in front of the penis, rather than behind it, as we assume is normal. (The testicles descend to allow them to be cool enough for the sperm to grow.)
In females, the Müllerian ducts swell up in sections to become vaginas, uteri (wombs), and Fallopian tubes. Among placentals, the Müllerian ducts come in between the urethras, a position which allows the ducts to fuse. First they fuse at the near end, resulting in a single vagina. Then the sections which will later form a uterus get a chance to fuse. In some species which give birth to large litters, this doesn't happen; from the cervix, two separate uteri diverge, each capable of implanting several embryos. In others, the near ends fuse, resulting in a single uterus with two horns. The final result is that found in humans, with the ducts completely fused to produce a single uterus.
The situation is quite different, however, in marsupials. There, the urethras come in between the Müllerian ducts, making fusion impossible. Marsupials therefore possess a double internal vagina. The inevitable result is that the male's penis is forked.
Bats, carnivores, insectivores, rodents, and other primates (monkeys and apes) have a bone in the penis. In many species the penis is decorated with spines and/or various folds and frills. (If you are wondering why, they increase sensitivity, allowing a speedy ejaculation, something not necessary, and even counterproductive in a monogamous species.) Mammals display three different types of penis: a vascular one, as in humans and horses, a fibrous one, as in bulls, and an intermediate type, as in rats. Reptiles possess two half-penes, and most birds don't have any at all.
Genetic BarriersBy now you should have gathered than interplanetary hybridisation is as likely as splicing together a motorcycle engine and a diesel engine, and expecting it to work. But the real problem is genetic. As has been explained before, closely related species can interbreed, but the offspring tend to be non-adaptive. Often they are also infertile. Probably most of you know that genetic information is carried on genes, and that the genes are carried on bodies called chromosomes, which each of us possess in duplicate, one set from each parent. The genes of a horses and a donkey are similar enough to produce viable offspring, a mule. However, the horse's genes are carried on 32 pairs of chromosomes, and the very similar genes on the donkey's 31 pairs. The poor old mule thus received an odd number of chromosomes. If it tries to mate with another mule, or with a donkey or horse, its offspring will have either one too many or one too few chromosomes, and it will not develop.
But even that implies that the species are closely related. It is well known that humans share more than 98% of our genes with chimpanzees. (We also share about a third of them with the cabbage, but that's seldom mentioned.) But the number of genes shared with an alien from Alpha Centauri is zero, zip, zilch. They cannot mix. Full stop. Exclamation mark.
But there's more. You probably also know that genes are made up of DNA. DNA is a string of four types of nucleotides, labelled A, C, G, and T. Proteins, on the other hand, are strings of twenty different types of amino acids. The DNA in your genes is responsible for the manufacture of proteins, each amino acid being represented by one or more codons of three nucleotides. Since the possible number of combinations of three nucleotides is 4 x 4 x 4 = 64, and there are only 20 amino acids, plus a start sign and stop sign, a certain amount of redundancy exists. For example, both AAT and AAC code for the amino acid, asparagine.
The important thing to understand, however, is that this genetic code is completely arbitrary. There is no good reason by any set of three nucleotides should code for any particular amino acid. The code we have is to one that arose on earth when life first developed from the primordial soup, and it will continue until the last days of earthly life. However, by the laws of chance, the genetic code on any other planet will be different - and that's assuming that the unearthly life forms use DNA at all. Reproduction between the two would be totally impossible. Even splicing an earthly gene into an alien microbe to produce a genetically modified microbe would be impossible.
Nevertheless, there is one last objection. If you follow the literature on UFOs, and particularly alien abductions, you will be aware of claims of alien-human hybrids being encountered. Well, all I can say is that people claim to have seen beings which they interpret as alien-human hybrids. That's not the same thing.
Now, do you think it is possible for two intelligent, technological species to evolve on the same planet, as many scifi stories suggest? If so, click here.
Otherwise, go back to the Index.