Why five digits?

Cecil's Columns/Staff Reports
21

The actual facts in this matter have only recently been reconsidered. Until lately, it was thought that five digits had always been the norm, and that early Tetrapod fossils with fewer were the exception. It now appears more likely that five was selected for.

It seems to me that five digits may be more or less the most practical number for supporting weight in the splayed postures of primitive amphibians and reptiles, but I’m just guessing. Ever since, it’s been the default, with fewer occurring chiefly in fast runners with one or two toes specialized as hooves.

Cartoon characters traditionally have four digits because it’s easier to animate, and close enough to five to look natural.

22

I’m not convinced by this example. It’s true that if I keep my pinky outstretched and try to grasp something with my other fingers, the grip isn’t as strong. But I think that’s because the pinky is actively acting against the rest of the fingers by me keeping it outstretched. If I immobilize the pinky in the closed position, I feel no difference between that and using it.

Short of amputating, I don’t think there’s anyway to pursue this example further.

23

Since I’m going to have to change the answer anyway (my background research was in error), I’ll address your concerns in the revised version; the short of it is that - as I tried to point out - five doesn’t HAVE to be “best” from a design standpoint. It just has to have no significant difference in reproductive value from any other heritable mutant variations. Under such conditions, the only way a four or six-digit mutant will spread is by genetic drift, which is an incredibly long shot.

24

I have to agree with onigame. That example just doesn’t cut it. If you extend your pinky, it is pulling against the other fingers. I’m not sure the percent increase in grip strength by the pinky, but I’m not convinced it is that much over a hand that doesn’t have a pinky. If we had evolved with only four fingers, you would be using all fingers to grip. If extra fingers are more valuable, then why not six, seven, or eight?

When discussing finger linkages, it seems to me that the 3rd and 4th fingers are stronger linked than the 4th and 5th. Try extending your pinky with all other fingers bent. Then try extending your ring finger with (a) pinky extended and middle finger bent; (b) pinky bent (trapped to palm) and middle finger extended. I find I have better range of motion moving with middle finger than pinky. I recall a comment by someone who does a lot of rock climbing who said something to the effect that on small hand holds, use the middle and 4th finger rather than pointer and middle, because those two fingers are linked.

25

This is an additional question, and more generically about genetics, but since there’s already three open threads, I thought I’d pose it here.

I’ve always heard that polydactyly is a dominant trait (or at least that the 6-finger allele is dominant). Since I know very little about genetics, my questions are :
Is this even correct?
Does the dominance of an allele provide any information about an organism’s evolution? Should the dominant trait win out over time in an otherwise equal situation?
If it should, perhaps the polydactyl mutation hasn’t been around long enough? Would it always have been dominant, or is it possible for that to change over time? I suppose in that case it might be considered a different mutation (recessive polydactyly vs. dominant polydactyly). Have two different versions like this possible/observed (on any gene)?

26

Well, in cats polydactylism is considered a dominant trait, or so I’ve been informed. Why there aren’t more of them if this is true is a matter of speculation. I think they just aren’t bred as fanaticaly as the “pure” breeds because “cat fancier” magazine or whatever says not to. In humans, I would suspect that having 6 digits on each hand would probably win you a lot of bar bets, but wouldn’t exactly endear you to the ladies.

27

Dominance and recessiveness don’t have any direct connection to the commonness of a neutral gene. Dominant just means that you’ll show the trait if you have the gene; it does not mean that you’re more likely to pass it on to your offspring. The textbook example in humans is hair color: Dark hair is dominant over light hair (there’s a slew of other genes responsible for hair color, as well, but this is a reasonable simplification). If a person has dark hair, they might have two dark genes, or one of each, and if they have one of each, then they can still pass on a light-haired gene and possibly have blonde kids. If a person is light-haired, then they must have two light genes, and so all of their kids will have at leastone light gene.

This does not apply if the gene is not neutral; that is, it has either a positive or negative effect. If a “bad” trait is carried on a dominant gene, it will very quickly die out, since anyone having the gene will have the trait, but genetic diseases can hide out in recessive genes for a very long time, where the organism won’t even notice them.

28

This response is part anecdotal, part random speculation. One of my roommates in college used to work cannery ships as a summer job. One year he had an accident on the ship and lost his right pinky. That was it for that year, of course, but he went back the next year . . . and was home again in a week. Even though he’d fully recovered (short of growing a new finger) and had done a lot of physical therapy, he struggled with controlling his knife. He definitely felt it was a grip strength problem. I don’t know that this proves anything, but I thought I’d throw it out there.

Now for random speculation. Let’s, at least for the moment, assume that pinkies ARE important from a grip strength standpoint. Why not more fingers? Well, additional anatomical features require additional energy to develop and support them. You gain one kind of benefit from adding features, but for a price. At some point you have diminishing returns. The benefit might not increase as fast as the cost. So maybe six fingers are slightly better, but more than slightly more expensive. Throw in catmandu42’s unenthralled ladies, and five fingers ends up winning. This is probably grossly oversimplified, but you can see what I’m driving at. (A brick wall?)

Though, finally, I have to say that I know at least two women who would have found a use for an extra finger or two, if I’d had them. I’d have won more than a bar bet.

29

We have five digits on each hand because otherwise, our gloves wouldn’t fit.

30

In re. Dr. Lecter, having six fingers obviously does not compromise mental ability, and if that does not aid in survival I don’t know what does.

:smiley:

Seriously now, my theory is chance with some coordination issues thrown in. Five fingers might not be much easier to control than six, or much harder than four, but the limited amount of neurons you can put to controlling the fingers on one hand would limit the amount of fingers per hand. Maybe this barrier would not be firm (few things are at this level of abstraction) but it would exits. So with maxiumum brainpower per hand to set the upper boundary, minimum dexterity and grip strength to set the lower boundary, and a liberal amount of blind chance, five is just the number of fingers most humans wound up with.

Besides, if we didn’t have an odd number, how could we flip someone the middle finger?

:smiley: :smiley:

31

Replace the sentence “Maybe this barrier would not be firm (few things are at this level of abstraction) but it would exits.” with “Maybe this barrier would not be firm (few things are at this level of abstraction) but it would exist.” and slap me upside the head.

32

What I wanna know is, why do birds only have FOUR digits on their feet, if they evolved from the same pentadactylic critters that we did? Huh? Hmm? Huh?

33

First I would like to remind sparta that we are talking about comparative anatomy i.e. the anatomy of organisms in general. In vertebrate comparative anatomy the thumb is considered to be on the inside. It is only human anatomy which considers it necessary to twist peoples’ limbs around before dissecting them. I don’t think there’s any good reason for this, it’s just a historical quirk of the medical profession.

More importantly, I take issue with several of Doug’s statements. I note in passing that he quite reasonably glosses over the conflicting definitions of Tetrapoda (traditionally vertebrates with digits but controversially defined by Gauthier as Amniota+Amphibia). But I can’t tell what he means by ‘land tetrapods’.

>the evolution went–as far as we can tell–very rapidly from fins to five toes, possibly with seven and/or six toed intermediates.

There is no evidence at all for a transition from fins to five toes. The best evidence suggests a move from fins with a fan of bones to fins containing many digits. An unnamed fish (http://sln.fi.edu/qa98/biology/journals/part14.html) has 8 in the forefin (hind fin unknown), but this may have been variable.

Closer relatives also had more than five digits per limb. See http://phylogeny.arizona.edu/tree/eukaryotes/animals/chordata/terrestrial_vertebrates.html for a cladogram of tetrapods and their relatives. Note that sucessively closer outgroups include Ichthyostega (unknown, 7), Acanthostega (8, 8) and Tulerpeton (fore 6, hind 7). Digit numbers are unknown for the other outgroups.

It is extremely unlikely that these early forms all independently acquired extra digits. It is much more parsimonious to suppose that the first toed animals had more than five, but the number may have been quite variable. Later tetrapods reduced the number to five - this may (speculation) have been associated with the move to land. Some, notably amphibians, later reduced the number further. Only one clade - ichthyosaurs - permanently increased its digit count.

>There simply haven’t been any such mutations on the line leading to primates.

Polydactyly is a very common mutation among tetrapods. It occurs fairly frequently in humans and many other mammals. It doubtless occurred millions of times in the early mammals which gave rise to primates.

>Under neutral selective conditions, the only way a four or six-digit mutant will spread is by genetic drift, which is an incredibly long shot.

Genetic drift is not an incredibly long shot. It is an extremely powerful evolutionary force. ‘Silent’ mutations, which are thought to be virtually unaffected by selection, can become fixed in populations extremely rapidly (in geological terms). It is not known whether polydactyly per se has any advantages or disadvantages. It can be caused by otherwise harmful mutations, but other forms appear benign.

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sagitta Holy snikes! Thanks for the post. Nice way to enter the board. Good info. Is this your field?

35

The offshoot which had six fingers was deselected by a swordsman who kept repeating the same line, “My name is Inigo Montoya, You killed my father, prepare to die.”
The movie “The Princess Bride” chronicles the first such incident. Thus having six fingers became an undesireable trait.

36

Wow, a Heinlein-inspired name, and a Princess Bride-inspired first post! Stick around, Mycroft.xxx, you’ll fit right in at this place :smiley:

37

Heinlein-inspired? Mycroft? … Somehow I think it goes back a wee bit further, old chap. If we were dicsussing oxygen, I’d say it was element-ary.

38

Thank you, samclem. you’re quite right, I am a professional annoying smartass. Also I used to study biology, and I was sufficiently interested to do a little web research.

tracer: the story of birds’ toes is quite interesting. Fossils suggest that it happened like this: early dinosaurs started walking on their hind legs. Theropod dinosaurs soon reduced the first and last toes to small claws, so they were walking on just 3, a bit like some early horses. Some theropods enlarged the first toe to make a strong claw (but not dromaeosaurids (‘raptors’), whose sickle claw is the 2nd toe). Later theropods also lost ‘fingers’ 4 and 5.

Birds evolved from theropods, probably but not certainly from dromaeosaurs. They reversed the first toe to point backwards-handy for perching, but by then the fifth toe had already disappeared. Most birds have 4 toes on their feet, and three digits fused together inside the wing. From studies of chick embryos the remaining fingers look more like 2, 3 and 4 than the 1, 2 and 3 of theropods, but I recommend siding with the fossils on this one.

A little polydactyly trivia: I remember reading of some graves of important families excavated in the Middle East. I think it was Egypt, but before the time of the Pharaohs. most of the skeletons had exrta fingers and/or toes, and the number tended to increase over time. The archaeologists thought that this family might have been revered for their polydactyly, marking them out as rulers.

Anne Boleyn, the 2nd wife of Henry VII of England, had 6 fingers on one hand, but they didn’t do her much good. It was seen as a sign of witchcraft, and court gossips whispered that she had seduced the king by unnatural means. When she became inconvenient to Henry, she was beheaded on trumped-up charges of adultery.

Any other famous personalities with extra digits?

39

Sagitta wrote:

>More importantly, I take issue with several of Doug’s statements. [snip] But I can’t tell what he means by ‘land tetrapods’.

It becomes clear below what I meant, and why I specified this.

>>the evolution went–as far as we can tell–very rapidly from fins to five toes, possibly with seven and/or six toed intermediates.
>There is no evidence at all for a transition from fins to five toes. The best evidence suggests a move from fins with a fan of bones to fins containing many digits.

And from fins with many digits, to limbs with many digits, to limbs with five digits, all in a brief geological interval is what I implied above. I didn’t say DIRECTLY from fins to five digits, I said RAPIDLY.

>It is much more parsimonious to suppose that the first toed animals had more than five, but the number may have been quite variable. Later tetrapods reduced the number to five - this may (speculation) have been associated with the move to land.

And in what way is this any different from what I said? If the first tetrapods living on land had more than five digits, then they represent the intermediates I referred to. The idea that they reduced the number to five in association with the move to land is exactly why I specified LAND tetrapods, as opposed to aquatic forms like Acanthostega and semi-aquatic forms like Ichthyostega.

>Polydactyly is a very common mutation among tetrapods. It occurs fairly frequently in humans and many other mammals. It doubtless occurred millions of times in the early mammals which gave rise to primates.

Doubtless it has, and now you’re just nit-picking, because I didn’t say “succesfully perpetuated mutation”.

>Genetic drift is not an incredibly long shot. It is an extremely powerful evolutionary force. ‘Silent’ mutations, which are thought to be virtually unaffected by selection, can become fixed in populations extremely rapidly (in geological terms).

Here we genuinely differ in opinion. Drift is a negligible evolutionary force in any population of respectable size. It only becomes “powerful” in populations composed of, say, less than 50 male/female pairs. An excellent analogy is trying to flip a coin so it comes up heads every time; the odds of flipping nothing but heads only become significant when you have a VERY small number of flips. Genetic drift is almost exactly the same: random shuffling of alleles in the population until one of the two alternative alleles manages to - at some point - eliminate the other. Moreover, it has to be the initially rare allele which wins in the end. This is NOT a powerful force, by any means. It is almost never invoked as being meaningful except in cases where one or a few individuals of a species become totally isolated from the remainder of the species, called “the founder effect” - a situation which may occur in many speciation events, but evolution and speciation are not synonyms; speciation is just one special case of evolution. Let’s put it this way: natural selection is a major force in evolution, and drift is a major force in speciation. The VAST majority of evolutionary change occurs via the action of natural selection on mutations, and does NOT involve speciation.

40

I’m just impressed with how many 1st and 2nd time posters this SR inspired.