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Chaos would happen. Here is a chronological order of events:
The size of the earth does not allow for such a hole to exist for very long. Gravity would compact the crust down and push the magma up, filling the hole. The compacting would cause massive earthquakes along every fault line as the tectonic plates compress together. Mountain ranges would ...
11
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 ...
11
Have you ever heard of the Naked Mole Rat?
They’re a subterranean, matriarchal species. They are, as the name suggests, naked and ratlike.
They also have the distinction of being the only eusocial (swarming) mammal EDIT: There are in fact two, the naked mole rat and the closely related (and much furrier) Damaraland mole rat, though their social structure ...
5
I'm pretty sure that the minimum population required to avoid inbreeding is drastically less than 5000 people. This question might be helpful in that regard, the selected answer indicates the minimum to be around 160 people I believe. That said, I know that some theories state we would need about 10,000 to effectively represent the ethnic diversity of Earth ...
5
A few adaptations.
1: No heavy exoskeleton. The old square-cube law you know. Exoskeletons get heavy. These big ones have just enough to hold themselves in - a minimal flexible cuticle like that of a maggot.
Apneustic respirations. Oxygen dissolves directly through the skin. Some maggots can do this, especially water dwelling ones. Amphibians and ...
5
Not much bigger than Arthropleura, which was large but also flat.
The other major problem arthropods have besides oxygen is molting, how exoskeletons have to grow. Sooner or later they have to molt to grow larger and when that happens they have no hard exoskeleton to support their body until the new one expands and hardens. Get too large and they will ...
5
If you follow my answers, I really love to answer with "yes, nature loves to do all sorts of awesome things," and then link to some strange creature that does things you'd never expect possible.
This is not one of those answers.
The temperatures you describe are simply not accessible in the way you seek. They can't be achieved as an organ.
The first ...
5
Two possibilities: If no ocean empties itself into that hole (it's in the middle of a continent), you just get a few hundred years or so of decreasingly interesting (=catastrophic) climate, because of the huge heat source. The hole gradually fills itself with magma over time, the surrounding continent sinks, and in the end you get a new ocean. Add a few ...
4
Short Answer: we don't know, but we have a few ideas.
we know a a few things that could encourage it and a few things that might discourage it but those a a few points on a giant see of possibilities. in all likelihood it was the interaction of many positive forces that caused human like intelligence to evolve. We just don't have a good idea of why ...
4
They are visually and biologically similar to ‘normal’ rats (such as rattus rattus or
rattus norvegicus) though they are typically larger and have a higher intelligence.
No problems here, that's a very easy to believe change and requires little justification.
Most often their fur is black, though it can be brown and, very
rarely, white.
It's the ...
4
The main problem with inbreeding is that a child may get the same recessive gene from both parents, turning it active. Recessive genes are genes that may carry a disorder, but are only active if the child gets the same gene from both parents; otherwise, the healthy version will be the active one. With three genetic parents, the child would have to have the ...
3
Oxygen consumption isn't the only problem.
Their biology is optimized for their current size. With a significant change in it, they should rework almost every major system.
Heat management
With increase in size, their insides (which generate heat) increase more than their surface area (which loses heat). It either requires a better cooling system or a ...
3
Yes, this should be possible. Nature is capable of creating and withstanding acid fluids (most obvious example: the stomach), so having acid blood developed as a defense mechanism sounds not that strange. The blood would most likely still be red if not alternative molecules for the transport of oxygen are used.
Only problem I see here is that the whole body ...
3
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 ...
3
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 ...
3
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.
...
3
The other answers are correct that the population size will need to be larger - but they're wrong in their math. It's exponentially more.
Your question is about genetic stochasity in relation to minimum viable population sizes. When tracing/predicting genetic effects, we typically use punnett squares to measure the effects of this. Wikipedia has a good ...
2
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 ...
2
Define "intelligence." No really. I mean it. What is it defined as?
The question of what causes intelligent life to evolve is somewhat stymed by the reality that we have trouble even defining what intelligence means. So that's the real problem here... and an interesting one it is.
If I dig at what is similar among the varied attempts at definitions, I ...
2
Generally speaking, intelligence will increase over time if there's a reproductive advantage to being smarter.
This is clearly not always the case; we've still got a myriad of species with tiny brains (there's a planarian with a number of brain cells in the mere thousands, fully mapped by function and all identical, yet it survives, reproduces, and can even ...
2
To amplify StarfishPrime's comment, a natural defense biomechanism which releases (and possibly generates at the time of injury) acid or alkali fluid is a lot easier to justify than the blood itself.
Related capabilities in Terran animals: snakes which spit venom, frogs whose epidermis contains toxins.
2
Re: the question on how big an arthropod can get, you have a real example that reached an estimated $2.6\,m$ in the extinct Jaekelopterus. Some relevant data:
Not an insect, yet an arthropod as you asked.
Aquatic. This probably made growth easier. But I think with a bit of genetic engineering you can solve this, as per the other replies.
Apparently, oxygen ...
2
Theoretically, the oceans of that world would diminish. Sea life would be interrupted drastically, potentially making many animal species go extinct. Due to this loss in sea level, the volume of sea water at the polar ends of the planet would recede, which could potentially cause the ice caps to melt. This would begin to raise the sea level again, but at the ...
2
You increased one side of the equation by 50%, so you must increase the other side of the equation by 50%. So 7500 of the species.
Since you now need an extra parent for every family, and you need 3 children instead of 2 (one to replace each parent) you have therefore increased all the requirements by 50%. Allowing some variation for how inbreeding is ...
2
Limbs can be used either for manipulation or for locomotion.
Manipulation is better carried out near eyes and mouth, as it serves for either inspection/interaction or alimentation. This poses a constraints on having the manipulation appendices in the front, if with front are identify the part where the head is.
From this it follows that the locomotion is ...
1
The most logical order (assuming the limbs are roughly the same size and are analogous to the limbs of current animals) is to have the manipulation capable limbs near the mouth and the intermediate (capable of water propulsion) limbs at the back, leaving the leg analogues in the middle.
The reasons for this are simple: flippers are most efficiently used ...
1
Stable environment
While intelligence undoubtedly existed independently of climate, I propose humans were really able to start using it to their advantage only after this happened:
This shows temperature graph for past 50 thousand years, it can be seen the climate was exceptionally stable for most recent 10000 years. Which neatly coincides with invention ...
1
Intelligence is useful in a fairly narrow range of environmental complexities and rates of change.
Consider the following:
If your environment is sufficiently predictable that you can come up with a single best-case strategy that always works (or works with a consistently high-enough probability) all the time, then you don't need intelligence. Evolution ...
1
That is not really how multicellular evolution works on earth... but... who is to say that cross breeding/genetic sharing between vastly different groups is not a thing on your world. If it is then yes this is totally plausable, you just have to change how genetics work on your world.
Bacteria indecriminatley swap genetic material on plasmids, allowig ...
1
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 ...
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