WI: Humans develop four arms (with hands)

For whatever reason during evolution Humans develop an extra set of arms (we are the only primates to develop them)
No ASBs please.

I'm under the impression that you have no understanding of how evolution works. To have humans (or, vertebrates in general - if humans have four arms, every terrestrial vertebrate will have four arms) would require a POD dating back ~500-600 million years into the past, and the butterflies from that would probably prevent evolution the evolution of hunanity in a recognizable form, altogether. In other words, i's pretty ASB.
 
It's also worth noting in virtually all human and animal cases supernumerary limbs are merely caused by a partially absorbed identical twin. Something like that can't be passed through the gene pool, even if it is a selective advantage.
 

NomadicSky

Banned
In order to have six limbs that would mean that you'd need to go really far back, beyond reptiles. Six limbs would have to just be the norm for no insect life on this world, and it isn't. From the reptiles to us four is the norm.
 
In order to have six limbs that would mean that you'd need to go really far back, beyond reptiles. Six limbs would have to just be the norm for no insect life on this world, and it isn't. From the reptiles to us four is the norm.

Actually, more or less, from the sharks to us. They do have a few more additional fins, but they basically already have arms/legs in the shape of pelvic fins and pectoral fins.
 
Difficult but not necessarily ASB

The most probable avenue would be a duplication of the hox genes.

Beat me to it. It's hox gene duplication. Gotta fool around with what segments go where.
Hopefully, this won't screw up the rib cage too much, or cause irreprable damage to internal organs.
Let's think of a plausible way this could happen without risking that kind of a problem.

It's been too many years since developmental biology. If I remember right, limb buds are a matter of a dorsal-ventral signal gradient interacting with the segment identifier. Maybe the promoter for the limb bud signal gets a segment tacked on that responds to the d/v gradient at the wrong segment. This could be due to wacky transposons, poor recombination during meiosis 1 or a retrovirus. (oh those wacky transposons!) Radiation-induced mutation is... more unlikely. That should do the job without directly harming internal organ development.

There you have it gentlemen: Non-asb 4-armed humans. Now get them to reproduce consistently. They'll have poorer health and coordination. Maybe they're considered sacred, like albinos are some places in Africa. Will 4-armed priests conduct sacrifices at the Great Pyramid of Mexico? (The Unconquered Sun FTW!)

The longer you keep these fellows going, the healthier their 'special attribute' will be. The rest of the genome will adapt, just through natural selection, to cause the extra arms to have less negative impact on health/survivability. EDIT: Just looked it up- the vertebrate limb inducer is a dash of FGF-10 at the right time.
 
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Stephen

Banned
Even if you did manage to mutate a human to look like that strange thing in Mortal Kombat most of the yime your extra limbs wont have much use or get in the way of each other. Reducing your ability to use the rest of your torso mucles in your arm movements would make them weaker.
 
ASB. Completely.

Mammals draw on our chordate lineage, going back to the very first fish to wander on to land. We are programmed to have 4 limbs. All the vertebrate organisms have 4 limbs at some part of their life.

The only way a particular human could have 4 limbs is a random mutation or birth defect, which would render it too ugly to breed, keeping the genes from passing on.
 
ASB. Completely.

Mammals draw on our chordate lineage, going back to the very first fish to wander on to land. We are programmed to have 4 limbs. All the vertebrate organisms have 4 limbs at some part of their life.

The only way a particular human could have 4 limbs is a random mutation or birth defect, which would render it too ugly to breed, keeping the genes from passing on.

Well the too ugly to breed keeps until culture starts to happen, after that estethics get more and more divorced from instincts (just wathc all forms of body modification) if 4 arms happens as a hereditable mutation artificial selection will do the rest... just as it happened with dogs.
On the other hand stability of said mutation is an issue, this will be a "luky monster" scenario at best...
 
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