In this world, bodies are more durable than intelligence.

Imagine mineral creatures who accumulate bodies of crystals, metals and minerals. It takes a really long time to grow a good body and lots of immatures die before their bodies get big enough to withstand the abuse of their environment.

But once those bodies are big, they are super durable. More durable than the intelligence of the creatures. It turns out dementia is the shadow of intelligence, and over time the structures that allow intelligence gradually weather and degenerate. The stone creatures can repair a toe or an eye but the history of a structure conferring intelligence is integral to its function. To repair it is to wipe it clean, and destroy it.

So a new head begins to grow. This junior head buds up alongside the original, and is initially just along for the ride. But it learns, and as it learns and grows it participates more in the control of their joint body. Somewhere along the line the young head begins taking charge more and more (it can be a rough time, this transition when both heads are reasonably competent). A creature with two competent heads can also be more formidable, the young head providing creativity and initiative and the old providing wisdom and insight.

When the young head has become mature the old head is senescent, sleeping much of the time and mumbling complaints as it is slowly resorbed in a process the reverse of how it arose. Sometimes the baby head of such an organism buds out before the oldest head is gone and a creature has three heads at once. The body is good for many cycles of heads, and bears the marks and scars of use by the intelligences who lived in it before.

Evolution isn't goal oriented; it moves away from stimuli. In a population, most mutations are neutral or mildly deleterious. Not all change is driven by selection after all... genetic drift is also important and you can get fixation of a potentially undesirable trait in a small population just by random chance. Stupid evolutionary dead ends happen all the time, ultimately rendering a species unsuitable for long term survival

All that just to say that there doesn't need to be an advantage, and the species doesn't need to be viable in the long term... eg. when the dumb, evil two headed giants meet an adventuring species, their long term future is very much in doubt. Canis Cerberus may be swiftly outcompeted by Canis Lupus when a change in climate allows the latter to invade the territory of the former.

A species finds itself with a tendency to produce two-headed, single-bodied (or dicephalic parapagous) conjoined twins, and this turns out to not be bad enough to kill them, perhaps because it came along with some other mutations that meant that the resulting children didn't have major movement issues. The mutation would have to be heritable so their children are likely (or guaranteed) to be two-headed too. That's all it takes. Perhaps they arose in a particularly benign environment. They'd probably have to be from a species that doesn't produce a large litter of juveniles as the odds are good that the unmutated children would be stronger and fitter and the twins may not thrive. Some species, like polar bears, habitually give birth to one set of twins and that might be a good sort of starting point.

For human coinjoined twins of this type, each head usually seems to control half the body; one arm and one leg. I can't find any information on how hard it would be for them to learn to walk but I suspect that it would be harder than for normal children.

Once they've survived to adulthood, there may be a number of possible advantages.

• Better field of vision and hearing... consider things like being able to have one head underwater and one head above, not just the obvious looking in two directions at once.


• Maybe one head could sleep whilst the other is able to do something useful like eat or care for children. It is remotely possible that they could move around whilst one half is asleep, especially if they were fairly buoyant swimmers.


• Twice as hard to choke! but again with the underwater theme, the underwater head needn't come up for air whilst searching or fishing. Doing a yard of ale would be trivially easy.


• For intelligent species, two brains can be useful, but there is perhaps a risk that the two might grow too similar to usefully react as two fully distinct individuals, but they'd definitely work well as a team. This might extend to the species being well suited for co-operative behaviour, as they've had to make use of it since they could walk.


• If they used their mouth/teeth for fighting, that's twice as many weapons, though the heads might get in each other's way.


• If vocal abilities were important (like birdsong, or howler monkey, er, howling) they've got twice the volume can always manage a duet, though they'll still only have the one set of lungs.

I'm sure there are many more, but this will be a start.

Polycephaly

Polycephaly is the condition of having more than one head. The term is
derived from the Greek stems poly meaning "many" and kephalē meaning
"head". https://en.wikipedia.org/wiki/Polycephaly

Although the above Wiki article is flagged as having 'multiple issues', it gives a good overview.

Both animals and people have been born with extra limbs and even heads. Over millennia there has been no apparent advantage or such people/animals would have increased their frequency in the population.

As has been pointed out, human conjoined twins exist in 2019 who effectively share a body (I won't give a link but the information is available online.

In the animal world, extra limbs and even heads crop up rarely but regularly. Snakes are particularly susceptible to this.

If you search Google Images for two-headed, there are some obvious Photo-shopped fakes but it's pretty easy to tell the real from the fake.

Here's a YouTube video showing a two-headed snake being handled. https://youtu.be/P2a_smk8IJk

Here's a video showing a two-headed snake being fed. https://youtu.be/kEsDjjvmy3c?t=445

In conclusion: If there had been an evolutionary advantage then we would see two-headed creatures everywhere.

The classic example of 'two headed aliens' is two aliens with side by side heads. Probably not that, seeing as direct competition in a single organism isn't great. If a two-headed intelligent species did exist, I'd imagine it'd be akin to a split between higher function and lower function. The two headed alien would have the heads vertical, rather than horizontal.

One head would be the menial head, a simple-ish intelligence good for tasks like rote chores, learning physical oriented skills such as combat, crafting, etc., while the other would be the 'superego' head, a head tasked with higher functions, such as advanced sciences, the arts, philosophy, etc.

If I were tasked with designing such an alien, I'd make it humanoid, with an extra set of smaller arms. One head would be one a taller neck, the other would be set into the chest, which would be wider to accommodate. The menial brain would control the main body, legs, main arms, neck, all from the higher head, given that he would be responsible for the day-to-day function.

The 'superego' would be set in the chest, with access to the smaller arms. It wouldn't need to control the body, because it's only really concerned with higher pursuits. It's personality would be such that it doesn't care about autonomy, and views itself as the mentor, while the 'menial' would view itself as the provider and caretaker.

The advantage of having one body with two heads and personalities instead of one body with one head and one personality is obvious from the point of view of the personalities. One of the persons would not be alive if not for the two headed condition. And the other personality might miss them, or be glad to get rid of them, or have mixed feelings about it.

Would this cause the body with two heads and two personalities to be more successful in survival than bodies with one head and one personality, so that it/they would reproduce and spread the mutated genes? Or would they/it be less successful in reproducing and thus the mutated genes would be lost?

Studies of the success of various two-headed animals might help you decide.

Since a two headed human with two personalities would have about 1.25 times the physical needs of a single human (the human brain uses a lot of energy and needs a lot of food), but about twice the ability to think per unit of time, in an advanced society it/they could have two separate work at home jobs at the same time and make money faster, thus possibly have a financial advantage.

There is a two-headed human, Joe-Jim Gregory, in the classic Robert A. Heinlein novel Orphans of the Sky.

Suppose that a two headed person (or persons) commits murder. If it is unsure whether both the heads cooperated in the murder, what should be done with him/them? I have read that was an actual legal dilemma in Paris, France about 400 years ago.

Since there is/are an example of two persons with one shared body alive, born in 1990, you could theoretically ask them about their decision making process, and whether they think that wild animals with their condition would be more or less likely to survive, or primitive cave person(s), or member(s) of an advanced futuristic society.

Picking up on the idea of @Halfthawed of a torso- or stomach-mounted face/head I could imagine a post-modern species, the enteric nervous system of which was forced to improve upon its means of communicating with the brain in order to prevent self-poisoning during situations of increased social pressure.

The formerly single-headed species would suffer huge death counts due to people ignoring their revolting stomachs and binging themselves to death, poisoning themselves with intolerable amounts of sugar or alcohol or destroying their livers, kidneys and cardiovascular systems in the long term. Maybe the only available food they needed to sustain themselves was also toxic to them and they needed to very carefully avoid poisoning themselves.

Maybe they reach maturity quite late in life and fall victim to those longterm effects before getting the opportunity to procreate.

Over extended time periods, only those whose gut-brains were smart and aware enough to pursue or trick their head-brains into abstaining from those excesses, would reach maturity and pass on their tendency for developing smarter and more self-aware gut-brains.

After a while, a state was reached, where some peoples unusually smart guts where sufficiently self-aware to project complex imagery into their bodies' master brain, but a large portion of those cases failed due to the master brains being hospitalized as hallucinating or just dying because of their gut-brains committing suicide (shutting down digestive functions) out of frustration. After all their limited means of communicating their problems did not suffice to reach the ones in charge.

Only the most devoted, intelligent and persistent gut-brains managed to carry their bodies to survival and an arms race began between head- and gut-brains. While the head-brains optimized their ability in dealing with that alien entity inside their bodies, the likes of which they had never -- in their evolutionary history -- encountered, those gut-brains became more successful, who appeared less alien to their head-brains.

By chance, the gut brains adapted faster and developed facial features and means of vocal communication which enabled them to communicate with their head-brains in the way they were used to. They also developed means of visually perceiving the environment so they could more easily relate to their head-brains impression of the world.

Nowadays most members of society are either gut- or head-brain of a shared body and in most cases, they cooperate in deciding upon food-intake and matters of bodily-health.

Keeping watch!

One of the key traits for evolution is survival of the fittest. One of the most dangerous times of day for any species attempting to survive is when they are asleep!

Humans "solved" this issue by becoming social creatures - by clustering into communities, we were able to rotate shifts and have some members awake while the others slept safely. (We also "un-solved" it for other animals, by becoming endurance hunters - wearing them down and striking while they slept)

Your species did not play quite so nicely with each other - in fact, gathering into social groups was a good way for the watchmen to "help themselves" to the resources (and lives) of the sleepers, and the only person you could trust to keep watch was yourself! As such, they have evolved a second head, so that while one slept the other could keep watch.

Eventually, once brain-power reached a certain level, the advantages of social cohesion overcame baser instincts, and civilisation emerged. The two heads now mostly wake and sleep together (much like how modern humans sleep in one long block, instead of 2 short sleeps with an active-period in the middle) - however, some eccentrics (workaholics, gaming addicts, and students cramming for exams or rushing that dissertation) still maintain the "always on" lifestyle

They’re Just for Show

The alien species (which might not resemble vertebrates except in the superficial sense of having at least one head) might not have its brains in its heads at all. They’re just mouths that can attack and eat. The advantage is that twice as many teeth are useful in a fight, or perhaps to gobble a bigger share of food in a pack.

They Work in Shifts

Maybe the personalities take turns sleeping.

They’re Complements

The right and left sides of the human brain aren’t as specialized as most people think, but maybe this is how the species developed two different, equally-important, sides of themselves with different abilities.

Body Part Replacement/Repair in case of temporary or permanent failure

Imagine a world that is volatile and hostile from an environmental point of view. While it does not affect the creatures externally, it affects them internally (like smoking, for example). These creatures have 2 parts (okay, now maybe I am slightly deviating from the actual premise, but let us imagine anyway) - one side of their body is active, the other side - while the brain etc. is active, the important internal organs (like lungs for example) are in a hibernation inside the body in a mucus layer or some other protective coating. These will be active once the primary organs fail.

Now to develop on this premise, maybe, these organisms have an high organ failure rate and much like 2 engines in an airplane, maybe it can also function as an replacement/security feature of the biological body. The premise is with infinite possibilities.

While OP is only considering the brain, I slightly increased the scope.

Probably not, since there do not seem to be any notable examples of such things occurring, and the term "evolutionary advantage" usually implies that a trait gets preserved.

With that said, one might imagine that having two heads could provide better range estimation at longer ranges, given the greater occular separation which would occur. The same could be said for localization of audible signals.

The neurological issues, though, would be severe. For the advantages postulated above would require the presence of long, large pathways for sharing sensory data between the two heads. These would presumably be vulnerable to various forms of damage.

Much worse, I suspect, would be the development of appropriate priority mechanisms to determine which head would be in charge, although it does seem possible that the two heads would develop specializations similar to that between the two hemispheres of ordinary brains.

In an omnivore, you might (possibly) get away with teeth on each head which are optimized for different purposes. One head might be an efficient meat-eater, while the other a better vegetarian processor. This would seem unlikely, and would take considerable thought to justify.

See "Saturn Rukh" by Robert Foreward. Flying creature that cannot land. It has two heads because each must sleep but the creature must be able to actively fly continuously.

Note that Ducks and Dolphins solve a similar problem by having half of their single brain sleep at a time while the other half watches for predators or continues swimming.

• You can see behind you.


• One head (brain) can sleep while the other keeps running things.


• If one head grew later in life, and therefore was younger, it could reduce issues associated with dementia (the older head may eventually 'die' but the younger one can keep going).


• If it's not useful, it could be a form of 'peacocking', whereby a male being able to survive despite being encumbered by a useless additional head demonstrates to the ladies just how fit everything else about him is.