Why five digits?

I don’t think the Staff Report provides much of an answer. The question was why we have five rather than four or six, and the answer was essentially “Because natural selection favored it.” This doesn’t answer why five is best.

There has to be a distinction between the result of natural selection (five fingers) and judging the adaptiveness of a mutation (why five instead of four), otherwise natural selection becomes tautological: “Which features are the most adaptive? The ones favored by natural selection. Which features does natural selection favor? The ones that are most adaptive.” This is a classic creationist critique of natural selection as an explanation for evolution.

I’d like to know, from a “biological engineering” standpoint, why five digits are better than four or six. Would six digits make us clumsy, or be too much for our nervous systems to deal with? Would four digits be too few to perform some vital function, like fighting or making tools?

The Staff Report being references is: Why do we have five digits on each limb?

It’s a good thing we weren’t asked about why we have two sets of two limbs, because then I woulda had to say something about forewarned being forearmed, and so an octopus is twice as well off.

It may not mean much, but take notice that most animated and cartoon characters have less than five fingers - four being preferred. Funny, despite this, they still use base-10 math…

  • b

The answer is one word: chance. Our twig on the evolutionary tree hung on better than the others.

In the past there were animals with eight or nine digits (Cambrian explosion and after). But pentadactyls survived the extinctions, the Ice Ages.

Because the part in our genetic code for more than five digits is still there from our distant ancestors, every so often anomalies turn up.

That was answered in the column.

In other words: Five’s not better than four or six.

It just wasn’t a disadvantage, so there was no reason for it to be selcted against, and those who had it also had other features that DID give an adaptive advantage.

Simple as that.

Chance is clearly a factor, and I don’t want to downplay it. But implies a level of randomness that isn’t supremely significant in natural selection on a species level. Randomness and chance may be huge at the level of gene mutation, but at the level of gene expression, other factors come into play, such as the applicability of an adaptation to the conditions a critter lives in. If a chance variation isn’t just outright fatal, then those other factors get to work on it.

So obviously 4 or 5 or 6 digits isn’t outright fatal. You can certainly construct a theory explaining the advantages and disadvantages of any number of digits. And clearly, for the time being, 5 has had edge in humans and related beasts. The question remains, however, why 5? We have the geneology explained in the original column. But I’d like to see some informed discussion about the relative advantages and disadvantages of the number of digits we have, vs., say, the number that the Simpsons and Bugs Bunny have.

I don’t expect a definitive answer, since one probably isn’t possible. But the original column lacked a certain explanatory gusto, and chance alone doesn’t cut it.

I remember shaking the hand of a politician (Don Beyer, then Lt. Gov. of Virginia) and thinking he had adopted a peculiar grip that must have helped him through the long hours of handshaking endemic to the profession. Later I found out he had been born with only four fingers (OK, three fingers and a thumb) on his right hand. The missing digit didn’t help him when he ran for the Governorship against now-Sen. Goerge Allen but that had more to do with his position on whether autos should have been taxed as personal property than how many fingers he had, or hadn’t.

So at least in the modern world, four’s just as good as five. Sometimes the explanation is that there is no explanation.

BTW he didn’t use base nine either.

Well, sure. But where’s the fun in that?

C’mon, how about some wanton theorizing here? Please?

Glad I’m not the only one who thought the Staff Report basically dodged the question.

I have no theorizing, wanton or otherwise, on why our ridiculously ancient ancestors evolved five digits instead of four or six. Yet clearly, there must be some sort of evolutionary advantage to five. Otherwise, the four- and six-digit mutations never would have been selected against in the first place, and we’d see a mix of numbers in the general population.

I agree that the question was dodged - sort of.

He says that five is not at a disadvantage and then
chance made five win out.

But generally, when traits are not a disadvantage, nor an
advantage, both traits remain in the population.

Like hair color, eye color, and junk.

Why did five digits prevail, and other digitally-numbered
beasts not make it?

Well, I’m not sure that I care much as to why we have 5 digits. We do. They work. Fine with me.
What I want to know is why Octal would be a “more sensible arrangement” as opposed to what we use now. If horses could count, what would they use? Binary?

Hey! Whadda ya mean, if ? I seen 'em do it on tv.

Well, apparently there’s a better chance of a species having one default number of digits-per-appendage. Ancient horse-like skeletons sometimes have three-toed feet, but modern horses and asses uniformly have single-toed feet (solid hooves).

Given that…
I. For any vertebrate species, there is one normal number of digits, and 99% of the individuals of that species have that exact number of digits.
II. This normal number is not constant for all vertebrates, or even all mammals. (E.g., horses have one toe per hoof, but goats have two.)

Therefore, we assume that there is little, if any, evolutionary advantage to any particular number of digits, but something in the process of speciation has left each species with a single, rarely varying, number of digits, which number is arrived at arbitrarily. That the normal number for humans is five, then, as far as we can tell, and in all probability, is merely chance, and not terribly surprising.

Well, I’m glad I’m not the only confused one. Basically, the answer replied that five is not worse than four so it prevailed. But that doesn’t explain why four (or three) digits didn’t survive. Why were they worse than five? (this is basically rephrasing what bup said, sorry 'bout that)
Well, here’s some illegitimate theorizing. If you look at our pinkies, they don’t do a whole lot. But if you move each of your fingers independently, you’ll see that the ring finger and the pinkie finger seem connected somehow. (Since I’m a clarinetist, this makes it harder to trill with the ring finger than any other finger.)
So, my conclusion is that the pinkie finger is useful to coordination, though not in trilling, and that this was somehow useful in making tools. Maybe when the ring and pinkie finger work together, they’re more precise. (???)
Feel free to correct me, anyone. :slight_smile:

A bit of light reading, everyone:


So far, I’ve found out that in land vertebrates there are only five basic patterns to the Hox genes that control the numbers of digits. While the earliest land vertebrates had eight digits (similar to fish fin structure), this “fanned” method of locomotion became less and less necessary further on in amphibian/reptile evolution.

Maybe, just maybe, a five-toed creature was able to move faster than one with more toes. Dunno. My bet’s still on Lady Luck.

Well, chance or not, if humans had had 8 fingers we probably would have had computer a few years earlier and the octal counting systems wouldn’t be just a footnote.

On a related note, a computer professor once pointed out that Christmas was the same as Halloween since the value of “25” in decimal was equal to “31” in octal. In other words:

31 Oct = 25 Dec

I am a physical education/kinesiology student, and in my human development and biomechanics courses (and a course I took in anthropology) point out that the pinky is useful in tasks requiring dexterity, but the great advantage of having a pinky finger is that it greatly increases the strength with which we can grip. (Try grasping something really tightly without using a pinky). When something is grasped between the palm and the fingers, it is called the power grip. In this position, much force can be applied.

Presumably, this characteristic was selected for during our evolution, as pentadactylism is an primitive trait in placental mammals, and an evolutionary trend in all primates. Strong grip strength is important for arboreal living, which all primates originally did. Tree-dwelling animals would not be able to climb vertically or cling from branches, let alone brachiate, without a very powerful grasp. Primates have evolved a deree of prehension of the hand and foot (except humans) wherby they are able to grasp; a monkey walking along a branch grasps that branch.

The presence of five digits in primates is called a primitive trait - it is primitive since it was originally present in all placental mammals. In many modern placental mammals, this feature has disappeared. Using the example of a horse, it is suited to running on hard ground at high-speed. This results in jarring and does not require forelimb flexibility. Therefore many primitive skeletal elements were selected against and have been lost in the horse, including the clavicle, decreased shoulder flexibility, the two bones of the lower arm have been fused, and the five fingers have been reduced to one, the hoof. Therefore, horses have lost those primitive traits. Horses have a highly specialized locomotor pattern, whereas primates are relatively generalized, we can carry out many forms of locomotion (quadrupedal, bipedal, and several variation thereof).
Also, on the issue of a base-10 numbering system due to having 10 digits - this is not necessarily so. Although the great majority of human societies use a base-10 system for this reason, not all do. In an anthropology class, the professor said that there was a group of people (can’t remember the name now) who used a base-8 system because there are eight spaces between our ten digits.

Why? And what are the advantages of base 8?

Yeah… That’s what I originaly wanted to know in the first place! :slight_smile:
And here’s another odd question…
Why are our thumbs on the inside as opposed to outside?

I know this is a nitpick, but in the anatomical position (the standard position when used to describe the relative locations of body parts), the thumb is on the outside!

FYI - anatomical position is standing, facing forward, feet pointed forward, arms hanging at the side with palms facing forward. In this position, none of your bones “overlap” when viewed from the front. When your thumbs point “in”, your radius and ulna (forearm bones) are crossed. When your thumbs point “out” then are lined up straight next to each other.