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This is a classic of fantasy/sci-fi stories. A creature with two heads, two brains, two distinct personalities, sharing a single body. An entire race of Siamese twins. But the question is, what environment or constraints would cause this adaptation to come about? What would be the evolutionary advantage to two distinct personalities sharing a single body?

Also, if anyone can answer this one, how would their nervous systems be wired? Which head would control which part of the body? This is an important question, as it relates to how they would evolve.

Evolution isn't goal oriented; it moves away from stimuli. In a population, most mutations are neutral or mildly deleterious. 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 bouyant 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.

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.

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.