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Disaster had befallen the tribe of Mitochondrial Eve, their population had been reduced to less than a dozen individuals, siblings and cousins all.

Although this population bottleneck was eventually overcome, the genetic damage was done. A congenital defect caused by the inevitable inbreeding had robbed all the descendents of that tribe of one of their external senses and only 5 senses remained. The physical structure of that sense remained, just as a deaf person has ears, but it created no conscious sensation.

What was that lost sense, and what is the organ responsible for it?

The purpose of this question is to inform a speculative fiction story in which the sense is reactivated via a simple piece of genetic engineering to living adult humans. Therefore the sense should biologically feasible and the physical structure necessary to use it already present.

The sense doesn't have to already exist in nature but, if it does, a citation indicating a creature that has that sense would be very welcome.

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    $\begingroup$ This seems really open-ended and opinion-based, but as something not long enough to befit an answer I shall point into the direction of 'gaze detection'. It's not a missing sense, but it's basically what has come to being referred to as the 6th sense. $\endgroup$
    – Lemming
    Dec 21, 2021 at 10:37
  • $\begingroup$ how about smision? $\endgroup$ Dec 21, 2021 at 10:43
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    $\begingroup$ People have more than 5 senses : perception of pain and the one of our own body are others than the classic ones. $\endgroup$
    – Tortliena
    Dec 21, 2021 at 13:14
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    $\begingroup$ I voted to close because we lack intentions. And because we lack any direction to follow and therefore better answers, it's more a question fishing for ideas or just gathering opinions. What tells us which answer is better? The most plausible thing? The "coolest" one? The one that could give the most advantage in the old environment? What are we restrained by, actually? $\endgroup$
    – Tortliena
    Dec 21, 2021 at 13:19
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    $\begingroup$ I posted a comment opposing the closure of this question, and I remember someone upvoted it this morning, but I don't see it above. Is that how this site is supposed to work? $\endgroup$ Dec 22, 2021 at 1:51

5 Answers 5

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Tetrachromacy

The original humans had a fourth cone cell, one sensitive to ultraviolet light. This let them see the hidden ultraviolet markers on plants and birds.

enter image description here

enter image description here

This let the tribes of humans with it have a decisive advantage in resource collection, getting superior food by their advanced vision, and improving their ability to spot camouflaged predators.

Unfortunately for them, this also meant that they chose the most open and hospitable of territories, which fared poorly in the disasters that killed other humans. The ones with broken senses lived, and spread and became modern humans.

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    $\begingroup$ Fish had a fourth cone cell. Mammals lost two of these early in their development - it had to be re-evolved by various monkey types. $\endgroup$
    – alamar
    Dec 21, 2021 at 11:18
  • $\begingroup$ Yep. And in this fictional world, apparently they evolved two extra cones. $\endgroup$
    – Nepene Nep
    Dec 21, 2021 at 11:22
  • $\begingroup$ This seems to be a higher developed sense and not a 6th sense. $\endgroup$
    – Trioxidane
    Dec 21, 2021 at 12:02
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    $\begingroup$ What is the difference between a sixth sense and a developed sense? Humans can't see ultraviolet, so it's not just an expansion of ability. $\endgroup$
    – Nepene Nep
    Dec 21, 2021 at 13:16
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    $\begingroup$ Notably, the other answer isn't actually a new sense. The Magnetoception receptors are in the skin, so it's just a variation of feeling, which can now sense magnetism as well as temperature and pressure. Unless you intend to use this critique on every reply that uses existing senses, it's not that useful. Senses tend to rely on existing channels to report new things. $\endgroup$
    – Nepene Nep
    Dec 21, 2021 at 13:34
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Magnetoception

enter image description here

Magnetoception is a general description for the ability to detect the Earth's magnetic field and use it for navigation. To quote the 2010 review from which the image above is taken:

...The magnetic sense must therefore be addressed as an additional and stand-alone sense, besides vision, hearing, olfaction, taste, electroreception and mechanosensation.

Experimental studies of magnetic field guided behavior have focused on migratory animals like birds (for review: [6], reptiles [11] or fish [12]), but evidence also exists from experiments with mammals (mole rat [13]; bat [14]) and several invertebrate species (crustaceans [15]; honey bees [16]; cockroaches [17]; marine molluscs [18]). Despite the overwhelming behavioral evidence, the molecular, physiological, and cognitive mechanisms enabling animals to sense and extract useful information from the geomagnetic field remain obscure.

The cells shown above detect magnetic fields in the upper beaks of birds by having particles of iron in the dendrites (receptive areas) of nerves, and return sensations by the trigeminal nerve that relays sensation from the face. Human magnetic sense has been proposed to be mediated by CRY2, a cryptochrome in the eye (optical pigment like protein) that senses magnetism a different way using light.

"Mitochondrial Eve" did not actually live alone, but was among at least tens of thousands. Her children never needed to resort to Noah's Ark Family Hour to produce the next generation. She is merely the lucky winner - chosen long after the fact - whose mitochondria won out in a long game of evolution. But in the spirit of the question, her offspring still suffered a major trauma - the Laschamp event 42,000 years ago, which probably would have been a geomagnetic reversal except the coin came up heads twice in a row. "Auroras blazed near the equator", and magnetic perception became if anything a disadvantage.

For this story, let's suppose that ancient humans from H. erectus onward, being long distance runners, had been busily evolving a magnetic sense from some weak vestigial mechanisms. By 42k years ago, many humans could easily navigate by magnetic fields. However, the talent remained rare, because once a tribe had a couple of shamans who could guide them on long trips, the selective advantage for further perfection of the mechanism weakened. Then the Laschamp Event hit and the shamans lost their magic powers for 250 years. The other humans rebelled against the shamans' privileges, put forward their own chiefs, and held witch hunts for centuries even after the geomagnetic field had gotten back to normal. The genetic basis of the sense involves multiple genes, and people with the right forms of all of them are rare, so there are only a very few oddballs here and there claiming to have this talent, who are dismissed by the rest as charlatans.

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  • $\begingroup$ Nice! And a good hook for lots of imaginative storytelling. $\endgroup$
    – DrMcCleod
    Dec 21, 2021 at 13:01
  • $\begingroup$ Experiments have proven that most humans actually have good magnetoception, we are just not as consciously aware of it the way we are with our main 5 senses. $\endgroup$
    – Nosajimiki
    Jan 3 at 15:17
  • $\begingroup$ @Nosajimiki - very interesting! I think I found your study: biorxiv.org/content/10.1101/448449v1.full.pdf (later published). I'm not convinced - it only shows the subjects reacted to the field reversal by a change in alpha waves. Any tiny cue, such as clicking from a coil under strain, or a shift in the buzz of current, or subsonic taps of the footsteps of the experimenter 20 feet away, might cause this. Also, detecting a full field reversal may not imply ability to meaningfully sense direction. Nonetheless ... would like to see further study! $\endgroup$ Jan 3 at 15:35
  • $\begingroup$ @MikeSerfas That is only one study. Another study found that a person could be blindfolded and spun around and still have a good since of what direction things were in... but if you put them in a faraday cage they could not. Some other studies seem to prove that people don't have, magnetoception but then a meta study showed that experiments done in major cities showed no magnetoception, but those done in more isolated places showed that we do . So, if you live in a city, you magnetoception is not nearly as good as someone who lives in the country thanks to all the Wifi/radio noise. $\endgroup$
    – Nosajimiki
    Jan 3 at 16:34
  • $\begingroup$ Which then raises the issue: if we were more consciously aware of our magnetoception would modern living be unbearable? The constant background noise of all of our electrical devices would be like a being in a room full of strobe lights for our eyes, or running motors for our ears. $\endgroup$
    – Nosajimiki
    Jan 3 at 16:39
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Empathy -- Head Hair

The original humans had a much stronger sense of empathy. They could sense the emotional state of other nearby humans. Modern humans would be sociopaths by their standards.

The organ for controlling this was the head hair. Obviously there is something suspicious about the head hair. A hairless ape having hair on the armpits and genitals makes sense as a way to reduce friction. But why the top of the head and nowhere else?

It's because the head hair was originally biologically active. It was simultaneously a transmitter and receiver for emotional state. It also hurt to cut your hair.

All that remains in modern humans is the eyebrows. Their position indicates emotional state. But the effect is much weaker.

The result of this extra sense is that the original humans were more herd oriented. They had stronger emotional ties to their family and tribe. On the other hand they were less inventive (though no less intelligent) since the first human to invent a bronze axe, or a new way of gathering seeds, was immediately met with a psychic wave of suspicion from their friends.

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Echolocation.

Humans are capable of some limited echolocation (barely), as demonstrated by some visually impaired persons. And we still have ears and vocal cords.

Originally, your humans had ears more sensitive to higher frequencies (something that is negatively correlated with age in modern humans) and vocal cords capable of producing near ultrasound. This disappeared and gave the way to the abillity to produce fine grained sounds - AKA speech.

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  • $\begingroup$ Interestingly enough, humans are perfectly capable of learning echolocation. It is just not in our biosocial or social norms to do so. $\endgroup$
    – Nosajimiki
    Jan 3 at 15:22
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The ability to taste

A Super taster is someone for whom the following chemicals (from some common foods) taste bitter, but other people don't taste them at all. "It is thought to be related to the presence of the TAS2R38 gene"[1]

  • Phenylthiocarbamide
  • Propylthiouracil

These chemical compounds can be found in uncooked broccoli and other common foods. One could hypothesize that many more tasting genes could have been lost or damaged by inbreeding, causing the remaining population to be unable to taste anything with their tongue.

[1] https://en.wikipedia.org/wiki/Supertaster

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