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posted by hubie on Sunday May 12, @02:01PM   Printer-friendly
from the five-finger-discount dept.

Why do most mammals have 5 fingers?

The simple question of "why five" has puzzled scientists from multiple fields, and the answer still isn't entirely clear.

If you look at the paws of a cat, a dog or even a kangaroo, you'll notice they have something in common with our hands. Even if some might be shrunken or differently positioned, all of these mammals have five digits, or fingers.

[....] To answer the question of why mammals have five fingers, we must first understand why tetrapod (Greek for "four-footed") vertebrates have five fingers. Mammals belong to the superclass Tetrapoda, which also includes reptiles, amphibians and birds. Even members of this group without traditional limbs have five fingers in their skeleton — whales, seals and sea lions have five fingers in their flippers — even if they have four or fewer toes.

There is some variation: Horses have just one toe, and birds have one fused finger bone at the end of their wing. However, scientists have discovered that these animals start out with as many as five fingers as embryos, but they shrink away before they are born.

[....] Nobody is sure when this five-finger plan first evolved. The first known animals to develop fingers evolved from fish around 360 million years ago and had as many as eight fingers, Stewart said. However, the existence of the five-finger plan in most living tetrapods indicates that the trait is likely a "homology" — a gene or structure that is shared between organisms because they have a common ancestor. The common ancestor of all living tetrapods must have somehow evolved to have five fingers and passed that pattern down to its descendants.

A common ancestor explains how mammals got five fingers, but it doesn't tell us why. One theory is canalization — the idea that over time, a gene or trait becomes more stable and less likely to mutate. [....] If the number has worked for millions of years, there's no reason to change it, according to this theory.

[....] Some speculate it might be down to gene linkage: As genes evolve over millions of years, some become linked, meaning changing one gene (the amount of fingers) could lead to other more serious health issues. But as of yet, nobody has offered concrete proof, Stewart told Live Science.

"We can ask a very simple question of why don't we see more than five fingers, and it seems like we should arrive at a simple answer," he said. "But it's a really deep problem. That makes [this field] really exciting."


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  • (Score: 5, Interesting) by pTamok on Sunday May 12, @02:43PM (7 children)

    by pTamok (3042) on Sunday May 12, @02:43PM (#1356663)

    Evolution has no direction, but it does get 'stuck' in local maxima. I suspect that 5-fingers is a local maximum where the changes necessary to get to a different number of fingers render an organism significantly less fit for its local environment for a period before the necessary adaptations (re)occur, so its not a magic number, just a 'happenstance' number.

    That said, I'm sure someone can make an argument about the necessary brain complexity for motor control of (say) 6 fingers against 5, and that given the metabolic load incurred running brain tissue, there's a significant-enough hit to an organisms energy requirements to make 6 fingers less 'fit-for-purpose' than 5. Why pay for more data-centre resources than you need, if it is more likely to bankrupt your company?

    But it is all bit 'just so' story-like. It's difficult to run proper comparative experiments. The dimensional space an organism operates in means that isolating a single variable/dimension is extraordinarily difficult. I would guess there is pretty strong pressure to minimise the number of digits in order to minimise the amount of brain needed to run them, and the 'best' number is environment dependant. Horses in their own environment get by with one. Primates have 5. Humans have a rate of hexadactylity of the hand (gaining an 'extra' little finger') of about 1 in 1000 births, but it's usually associated with severe genetic defects, emphasizing that its difficult to move from the local maximum of 'fitness for purpose'.

    I've mused at times that replacing the little finger with a thumb might pay dividends, but it would likely have bad knock on effects I haven't thought of.

    • (Score: 1, Interesting) by Anonymous Coward on Sunday May 12, @03:17PM (4 children)

      by Anonymous Coward on Sunday May 12, @03:17PM (#1356665)

      I would guess there is pretty strong pressure to minimise the number of digits in order to minimise the amount of brain needed to run them

      I don't think brain is an issue - humans and many other animals can use tools. The extra fingers are just tools. As long as they're wired up well enough, the brain can learn to use them ( See: https://www.youtube.com/watch?v=LlfPIKQmPok [youtube.com]
      https://www.youtube.com/watch?v=by7kBtNmlGI [youtube.com]
        ).

      So mutants with functional 6 do appear but so far the advantages don't appear enough for that to start dominating. And it's probably harder to go to 6 since the default base has been 5 for hundreds of millions of years - so you need more "functional" mutations for 6 to work.

      For land dwellers reduction in fingers/toes seems to happen more often than increase. When you are running, extra finger bones in a leg either mean more weight or more fragile and thus easier to damage finger bones. More weight = slower movement of limbs = slower running speeds. Of course not all animals need to run fast.

      For ocean dwellers I think it doesn't matter as much? 8 or 10 fingers for a fin might work better than 5 for paddling etc.

      • (Score: 1) by pTamok on Sunday May 12, @08:27PM (2 children)

        by pTamok (3042) on Sunday May 12, @08:27PM (#1356700)

        I think you missed the point: 'wiring up' the additional finger requires additional nervous tissue, and it has to terminate somewhere, and the brain needs an expanded motor control centre (the motor cortex [wikipedia.org]) - if you look in the Wikipedia article, you see that a large part of the motor cortex is devoted to the fingers.

        The brain is extraordinarily metabolically expensive: See Metabolic costs and evolutionary implications of human brain development [nih.gov]

        the magnitude of brain glucose uptake, both in absolute terms and relative to the body’s metabolic budget, does not peak at birth but rather in childhood, when the glucose used by the brain comprises the equivalent of 66% of the body’s RMR, and roughly 43% of total expenditure.

        and
          Paying the brain's energy bill [doi.org]

        The brain is metabolically expensive. In humans, the brain consumes approximately 20% of our metabolic energy, despite comprising only 2% of our body mass, making it amongst the most energetically costly organs in the body

        (Note the difference between child age 5 (very roughly 40% of caloric intake) and adult (very roughly 20% of caloric intake).

        Expanding fingers from 5 to 6 would be a 20% increase in expenditure dedicated to the fingers, and that has to be paid for in metabolic energy (there's a cost per neuron). 6-fingered primates would need to eat more. That could make a difference at the margins. Maybe.

        I guess the point is that going from 5 to 6 fingers is not zero-cost, so if that extra cost can't pull its weight in evolutionary terms, it ain't gonna persist.

        • (Score: 1, Interesting) by Anonymous Coward on Monday May 13, @02:55AM (1 child)

          by Anonymous Coward on Monday May 13, @02:55AM (#1356752)
          My point is that for many land dwellers fewer fingers is more likely. Because moving about requires legs more than toes. The more toes you have the weaker the toes or the heavier the foot which tends to be disadvantageous when having the same amount of muscles to move the foot.

          Most of our ancestors had much smaller brains. The big expensive brain stuff is only a more recent thing.

          If you look at other animals from now till distant pre-history with "cheaper" brains you should realize you don't need that many calories to control an appendage (not talking about the actual physical movement).

          Yes it still costs something but I suggest it's not as big a deal compared to the costs of moving a limb that's heavier or repairing a limb with extra bits that damage more easily (plus risking death etc from disease etc).

          These costs are applicable and more significant for a lot longer over evolutionary history (at least for land tetrapods and even the flying ones - bird claws have to be functional, strong enough and light, extra heavy claws = higher flying costs, extra weaker claws = claws that get damaged more easily ).
          • (Score: 1) by pTamok on Monday May 13, @08:17AM

            by pTamok (3042) on Monday May 13, @08:17AM (#1356784)

            I think you make a valid point.

            The possibilities for discussion around this are many, but I'm going to stop here. Thanks for your contributions.

      • (Score: 2) by Reziac on Monday May 13, @03:29AM

        by Reziac (2489) on Monday May 13, @03:29AM (#1356754) Homepage

        And those six-fingered hands, per the incidence, appear due to a single autosomal dominant gene. With the stability of expression, unlikely to have negatives if homozygous (as is the case with some incomplete dominants), but it's something to look at.

        Perhaps more interesting than the appearance of an extra digit is that it appears to have the full set of nerves, tendons, and muscles to make it work properly, while being neither quite one of the regular four fingers nor a thumb.

        --
        And there is no Alkibiades to come back and save us from ourselves.
    • (Score: 2) by aafcac on Sunday May 12, @10:05PM

      by aafcac (17646) on Sunday May 12, @10:05PM (#1356711)

      That's a point that ought to be remembered more often.

      In this case, there are a few benefits to more fingers that rapidly diminish. More fingers means that you can cover more area with less lateral movement of the joints. You can also spread whatever forces you need across more smaller fingers for whatever amount you need to be able to hold while still maintaining dexterity. You can also fit 5 fingers onto an appropriately sized hand than 6 and cover a wider part of a circle than would likely work well with only 4 fingers.

      4 fingers and 6 fingers would probably be just fine, but probably not impact reproduction and with 4 fingers you can't really afford to lose any of those fingers without it impacting your ability to handle things.

    • (Score: 3, Informative) by cmdrklarg on Monday May 13, @07:41PM

      by cmdrklarg (5048) Subscriber Badge on Monday May 13, @07:41PM (#1356837)

      I've mused at times that replacing the little finger with a thumb might pay dividends, but it would likely have bad knock on effects I haven't thought of.

      The pinky finger has a large role in your overall grip strength, so that would be an issue.

      --
      The world is full of kings and queens who blind your eyes and steal your dreams.
  • (Score: 3, Funny) by SomeGuy on Sunday May 12, @02:46PM (1 child)

    by SomeGuy (5632) on Sunday May 12, @02:46PM (#1356664)

    They evolved with five fingers so they could flip off other species.

    • (Score: 2) by driverless on Monday May 13, @11:36AM

      by driverless (4770) on Monday May 13, @11:36AM (#1356797)

      Nah, it's just a natural consequence of the Law of Fives, "all things happen in fives, or are divisible by or are multiples of five, or are somehow directly or indirectly appropriate to 5".

  • (Score: 1, Touché) by Anonymous Coward on Sunday May 12, @06:25PM (5 children)

    by Anonymous Coward on Sunday May 12, @06:25PM (#1356678)

    If it wasn't, we would have six fingers

    • (Score: 5, Informative) by ElizabethGreene on Sunday May 12, @07:00PM

      by ElizabethGreene (6748) Subscriber Badge on Sunday May 12, @07:00PM (#1356688) Journal

      I know a guy looking for a man with 6 fingers on his right hand. He's a swordsman, do you know him? The name escapes me.

    • (Score: 2) by darkfeline on Monday May 13, @12:38AM (3 children)

      by darkfeline (1030) on Monday May 13, @12:38AM (#1356734) Homepage

      If we had six fingers, we'd use a 6 based numeric system, and metric prefixes would be 6 based.

      This falls into the category of topics which beg the converse.

      e.g., why did sentient life evolve on Earth? -> If it didn't evolve on Earth, we wouldn't be asking -> given that it happened, the chance it happened is 100%.

      what are the chances that the physical constants of this universe are so perfect? -> if it weren't perfect, no one would be asking

      --
      Join the SDF Public Access UNIX System today!
      • (Score: 0) by Anonymous Coward on Monday May 13, @04:23AM (1 child)

        by Anonymous Coward on Monday May 13, @04:23AM (#1356757)

        If we had six fingers, we'd use a 6 based numeric system

        Well, 12 base anyway, as in eggs and doughnuts, then there's the calendar, clock, distance in feet and inches. Now that we have gene editing, we should start producing more six fingered babies. They'll be wicked sax players

        • (Score: 1) by pTamok on Monday May 13, @07:16AM

          by pTamok (3042) on Monday May 13, @07:16AM (#1356776)

          If you use positional notation, then 5 fingers is base 6.

          Right digits count units, left digits count sixes, as follows:

          Left had - no fingers raised - zero sixes.
          Right hand
            - no fingers raised - zero units
            - thumb raised - 1 unit
            - index finger/forefinger raised - 2
            - middle/second/long finger raised - 3
            - ring-finger raised - 4
            - little finger/pinkie raised - 5
          Raise thumb of left hand and drop fingers of right hand - one 6
          Keep thumb of left hand raised and raise thumb and fingers of right hand as above to count from 7 to 11
          Raise forefinger of left hand and drop fingers of right hand - 12
          Keep thumb and forefinger of left hand raised and raise thumb and fingers of right hand as above to count from 13 to 17
          Raise middle finger of left hand and drop fingers of right hand - 18
          Keep thumb, forefinger, and middle finger of left hand raised and raise thumb and fingers of right hand as above to count from 19 to 24
          Keep going, so fingers of left hand count the number of sixes counted on the right hand - you get to a maximum of 35 (5 sixes on the left hand, 5 units on the right hand)

          If you use positional notation for all the fingers rather than just the hands, you can count (in binary) up to 1023.

          Another way of counting is to use your thumb as a pointer to the phalanges (fleshy bits of fingers between joints) - so using the thumb as a pointer, you can point to three positions on your index finger, three on your middle finger, three on your ring finger, and three on your little finger (I find it slightly uncomfortable to point to the lowest one on the little finger). This allows you to point to 12 positions. By using positional notation on the hands, you can count thirteens, so up to 132-1 = 168, or if you 'simply' count 12s, up to 12 twelves on the left hand, plus 12 more on the right hand.

          But, once you realise you can use the thumb as a pointer on the fingers, you realise you can use each finger as a pointer to the phalanges of the thumb, so the sequence goes
          1) Thumb points to base of index finger
          2) Thumb points to middle of index finger
          3) Thumb points to top of index finger
          4) Index finger points to top of thumb (which looks very similar to (3), but feels very different)
          5) Index finger points to middle of thumb
          6) Index finger points to base of thumb (or is simply curled so the pad is touching the palm)
          7) Thumb points to base of middle finger
          ...
          12) Middle finger points to base of thumb (or is simply curled so the pad is touching the palm)
          13) Thumb points to base of ring finger
          ...
          18) Ring finger points to base of thumb (or is simply curled so the pad is touching the palm)
          19) Thumb points to base of litttle finger
          ...
          24) Little finger points to base of thumb (or is simply curled so the pad is touching the palm)
          25) Thumb of left hand points to base of index finger of left hand, thumb of right hand points to nothing
          ...
          624) Little finger of left hand points to base of thumb (or is simply curled so the pad is touching the palm), little finger of right hand points to base of thumb (or is simply curled so the pad is touching the palm)

          If humans had more phalanges [wikipedia.org], we could count further using this system. Pilot whales have 10 phalanges [wikipedia.org] in the bit of their anatomy corresponding to the human index finger (it's hidden inside their flippers). Odd that the number of fingers is more highly conserved than the number of phalanges.

      • (Score: 0) by Anonymous Coward on Monday May 13, @05:28PM

        by Anonymous Coward on Monday May 13, @05:28PM (#1356824)

        Whoosh

  • (Score: 0) by Anonymous Coward on Sunday May 12, @06:30PM (1 child)

    by Anonymous Coward on Sunday May 12, @06:30PM (#1356680)

    Because if we didn't have 5-fingers on each hand, our numbering system wouldn't be base-10, which would be awful! :)

    • (Score: 3, Touché) by pTamok on Sunday May 12, @08:47PM

      by pTamok (3042) on Sunday May 12, @08:47PM (#1356703)

      The Sumerians and Babylonians take a minute to look smugly at their base 60 system.

  • (Score: 4, Interesting) by ElizabethGreene on Sunday May 12, @07:00PM (2 children)

    by ElizabethGreene (6748) Subscriber Badge on Sunday May 12, @07:00PM (#1356687) Journal

    I was ignorant of how broad this anatomical curiosity is, but I'd seen glimmers of it. A few years back I disassembled the lower legs of a deer to look at the bone structure and harvest sinew. I was flabbergasted to find small hand and finger-like structures. It was a "wow" moment for me. I've had a few moments like that where the evolution lightbulb really came on for me. Seeing the arms and shoulders of a Gorilla with alopecia (hair loss) was one. Another was when I was tanning a deer hide and I used a solution to allow the hair to slip. Underneath their course straight brown fur they have patches of short black curly hair indistinguishable, at a glance, from human body hair.

    We're all variations on a theme.

    • (Score: 0, Flamebait) by r1348 on Sunday May 12, @10:33PM (1 child)

      by r1348 (5988) on Sunday May 12, @10:33PM (#1356715)

      You also might be a psycho.

      • (Score: 2) by ElizabethGreene on Monday May 13, @04:31PM

        by ElizabethGreene (6748) Subscriber Badge on Monday May 13, @04:31PM (#1356822) Journal

        Psychopathy is characterized by a lack of empathy and regret for past actions. With the caveat that self-reflection is a terrible way to diagnose mental health conditions, like the blind leading the blind, I don't believe that fits.

  • (Score: 3, Interesting) by bzipitidoo on Sunday May 12, @07:53PM (1 child)

    by bzipitidoo (4388) on Sunday May 12, @07:53PM (#1356696) Journal

    Prime numbers arise in several areas for good reasons. There are the 13 and 17 year locusts. Gears and hashing functions use prime numbers, the former so that if a tooth breaks off, wear will still be distributed evenly and power delivered fairly evenly, and the latter to ensure the most complete and equal usage of all available room. If a prime number of digits is best, then it's only a matter of which prime. Perhaps 3 is too few. 3 digits is good for control and manipulation in 2D, but in 3D, another digit is very useful. 4 not being prime, 5 is therefore optimal.

    Now, how to explain having 4 limbs instead of 5 or 3? And why do insects have 6 legs, and spiders 8 legs? Doubtless has everything to do with the physics of locomotion and fighting.

    • (Score: 2) by krishnoid on Sunday May 12, @10:02PM

      by krishnoid (1156) on Sunday May 12, @10:02PM (#1356710)

      Blasphemy! We have 5 fingers because we were created in God's image [youtu.be]! Also, who says we don't have a fifth limb [youtu.be] (sorry about the quality)?

  • (Score: 1) by pTamok on Sunday May 12, @09:09PM (3 children)

    by pTamok (3042) on Sunday May 12, @09:09PM (#1356706)

    And a surprising one at that.

    I give you: the Elephant! [nature.com]

    And the Giant (and Red) Pandas have a thumb that opposes the normal five fingers, so six fingers in total.
    Why Do Pandas Have Thumbs? [jstor.org]

    • (Score: 0) by Anonymous Coward on Monday May 13, @06:11AM (2 children)

      by Anonymous Coward on Monday May 13, @06:11AM (#1356765)

      And the Giant (and Red) Pandas have a thumb that opposes the normal five fingers, so six fingers in total.
      Why Do Pandas Have Thumbs? [jstor.org]

      And why do men have nipples? [youtube.com]

      • (Score: 3, Informative) by ChrisMaple on Tuesday May 14, @04:53AM (1 child)

        by ChrisMaple (6964) on Tuesday May 14, @04:53AM (#1356889)

        I see at least 2 reasons for men having nipples.

        One is genetic economy; it's probably simpler to encode into the genome that both sexes have nipples than to encode that only females have nipples. Consider that the testes and the ovaries both grow from the same embryonic cells.

        Second, men's mammary glands are potentially functional and can be activated with the appropriate chemical and/or mechanical stimulation. See Milk: the Mammary Gland and its Secretion (1961) by S.K. Kon and A.T. Cowie.

        • (Score: 0) by Anonymous Coward on Wednesday May 15, @02:42AM

          by Anonymous Coward on Wednesday May 15, @02:42AM (#1356987)
          They're erogenous zones, that's a good enough reason for me.

          Maybe the virgins or the less fortunate/experienced need more reasons...
  • (Score: 3, Funny) by srobert on Sunday May 12, @11:28PM (3 children)

    by srobert (4803) on Sunday May 12, @11:28PM (#1356722)

    Prehistoric men and women had only four digits on each hand, three fingers and one thumb. I learned this by being observant while viewing an interesting television series entitled The Flintstones.

    • (Score: 2) by hendrikboom on Monday May 13, @02:39AM (1 child)

      by hendrikboom (1125) Subscriber Badge on Monday May 13, @02:39AM (#1356749) Homepage Journal

      As does Donald Duck.

      • (Score: 2) by ChrisMaple on Tuesday May 14, @04:59AM

        by ChrisMaple (6964) on Tuesday May 14, @04:59AM (#1356891)

        Consider Bugs Bunny, who has remarkable finger abilities. Although he usually has 4 fingers per limb, in at least 2 instances he has 5 fingers. Look for him sniffing around a magician's top hat.

    • (Score: 2) by Thexalon on Monday May 13, @03:35AM

      by Thexalon (636) on Monday May 13, @03:35AM (#1356755)

      True for modern man, too, at least in Springfield and Quahog R.I.

      --
      The only thing that stops a bad guy with a compiler is a good guy with a compiler.
  • (Score: 1) by shrewdsheep on Monday May 13, @02:34PM

    by shrewdsheep (5215) on Monday May 13, @02:34PM (#1356818)

    It is to allow one to loose two fingers and still be called three-finger-Joe.

  • (Score: 2) by Tork on Monday May 13, @08:09PM

    by Tork (3914) Subscriber Badge on Monday May 13, @08:09PM (#1356839)
    There have been something like 100 million distinct species on this planet. A large swathe of them have comparable skeletons. Okay their proportions are different, but the general hierarchy and arrangement is there. I know a lot of people believe alien life MUST be different from ours. I'm on the fence on if I actually believe that. I mean, if it turned out our shape was one likely to be landed on a zillion times throughout the galaxy I would NOT be surprised. Heck, we might even find out stuff like "species born with 8 fingers on each hand never make it to space.
    --
    🏳️‍🌈 Proud Ally 🏳️‍🌈
  • (Score: 2) by ChrisMaple on Tuesday May 14, @05:12AM

    by ChrisMaple (6964) on Tuesday May 14, @05:12AM (#1356892)

    As a point of reference, look at the Smithsonian Triceratops skeleton shown on the wikipedia page. Five toes on the front feet, 4 toes on the left rear foot. The right rear foot appears to be damaged.

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