OK, this is just wild theorizing and I am definitely not an engineer, but how about this.
Each limb in a tetrapod has to be able to support body weight in a stable manner. The minimum number of contact points with the ground to provide stable support is three to form a tripod. Therefore the first terrestrial tetrapods had to have at the bare minimum of three toes. Since nature seems to always select for some redundancy where possible four toes would have become the norm to compensate for accidental amputations etc. The dew claw/toe in tetrapods is located quite a way behind the other digits and is designed primarily to give sideways support to prevent sliding. It doesn’t contribute a lot to stability during forwards movement. Once the dewclaw had moved to its current position it couldn’t contribute to stability and so had to be either replaced by another digit or more likely another digit was retained to make up the four necessary for stability +redundancy.
Now I know the heel can be used as the third leg of a tripod when standing motionless, but during movement the heel is lifted from the ground, so three really is the bare minimum number of toes.
Why only one redundant toe might have been selected rather than 2 or three I can’t hypothesise except that obviously the use-it-or lose it principle applies. If accidental amputations of two toes was uncommon enough it didn’t confer significant advantage then only one redundant digit would be needed.
Yes I concede this is a WAG based on some science and made up entirely to fit the facts.
Catmandu42
Why are our thumbs on the inside as opposed to outside?
Simple, because our thumbs are modified dewclaws. Dew claws are designed to allow an animal to apply sideways grip. This is applied in carnivores when hunting to allow them to hold their prey, and in arboreal mammals to allow them to grip tree limbs. It is also used by males to grip females during copulation. In other tetrapods the dew claw is held much closer to the ground and is used to provide traction that prevents the legs from moving on unstable surfaces.
Amongst these animals the legs are splayed, making it easy for the legs to fly out but almost impossible for them to collapse in, hence the dew claw developed on the inside. As a result of all these pressures the dew claws are located on the inside of the limbs and since thumbs are modified dew claws the thumbs are there as well.
“Why are our thumbs on the inside as opposed to outside? Simple, because our thumbs are modified dewclaws.”
Look at the ancestral tree tracing back from primates. You’ll notice that none of our ancestors all the way back to the amphibians had dewclaws. None. That doesn’t lend much credence to your theory. To say that the dewclaw is HOMOLOGOUS with the thumb is fine (i.e., have a shared ancestry), but not to say that the thumb is derived from a dewclaw. There is simply no evidence for that.
This is kind of an aside but seems to be an issue here as well. I’m surprised that no one’s brought this up but I recall learning in three seperate biology classes that five digits is actually a recessive trait in mammals. Six digits is a dominant trait. The reason we don’t all have six digits is because of a genetic mutation that causes the recessive trait to manifest.
Okay, I’ve realised I’m using the word dew claw completely incorrectly here.
I’m referring to the ‘thumb’ digits that diverge from the feet at a distinctly more proximal location than the other digits.
Anyone know the name, I’m sure I did know it but I’ve obviously forgotten.
Well, the “thumb” digit does not “diverge” from the feet at a more proximal position than the other digits relative to the carpals/tarsals (wrist/ankle bones). It may look that way in some taxa, like in humans. The digits in vertebrates are numbered I-V, with the innermost, corresponding to the human thumb, being digit I. In most vertebrates, it differs from the other digits in having fewer than 3 phalanges (joints). The “primitive” condition in tetrapods (at least those beyond the more-than-five-digits-stages) is a phalangeal fomula of 2-3-4-5-3. In mammals that has been reduced to 2-3-3-3-3 as a primitive condition, as is found in humans.
“Dewclaws” properly refers to the reduced digits that do not support weight (and often don’t touch the ground) in many artiodactyls and carnivores. In artiodactyls, IIRC they represent digits II and V (I being lost almost completely), while in dogs I think it is digit I.
blokhead, traits don’t spread in a population depending on whether they are dominant or recessive, they spread depending on whether they are advantageous or deleterious. There are lots of deleterious alleles that are dominant.
You said: blokhead, traits don’t spread in a population depending on whether they are dominant or recessive, they spread depending on whether they are advantageous or deleterious. There are lots of deleterious alleles that are dominant.
Well I’m not sure I agree with you there unless you also think that more people have Brown eyes(a dominant trait in humans) because its advantageous. I can’t see why brown eyes would be more advantageous than blue.
Although traits also manifest based on whether they are advantageous or deleterious(that’s just typical of evolution) they also do spread based on whether they’re dominant or recessive. If that weren’t so then we’d all have 6 fingers and brown eyes. And I’ve never had six fingers so I can’t say that it would be deleterious. You might think that but it may not be true.
Sigh. blokhead, I wan’t to insult you but you clearly do not have even a basic understanding of either genetics or evolution. The statements I made are so fundamental to the subject I don’t really feel it is necessary to defend them. I would suggest you do some reading in the subject before you decide you “think you disagree” with me.
I DON’T SIMPLY THINK IT…I KNOW I DISAGREE. Is that any more clear? And actually I might say the same to you. I am not a geneticist nor do I pretend to be but I do know a thing or two about it. I did not simply state that you were wrong merely that it’s not entirely correct. So let’s just agree to disagree and leave it at that.
Sorry, blokhead, the mission of this board is to fight ignorance, and your statements are based on ignorance. It is not merely that I disagree with you, the facts disagree with you. My statements are based on information that can be found in any elementary discussion of evolutionary genetics, while yours are evidently based on half-remembered or misinterpreted information from high-school or undergraduate biology classes.
To clarify: dominance/recessivness can influence the rate of spread (or elimination) of an allele in a population, but not whether or not it spreads. The main factor influencing whether a trait spreads or not (in large populations) is natural selection (including sexual selection) although other things such as mutation pressure can come into play in rare instances.
Now go do some homework on the subject before I bury you in Hardy-Weinburg equations.
Sorry to interrupt this little war you guys are having, but you’re both a little off base. Allow me to rephrase so things are technically correct:
For an allele that is NOT selectively neutral, dominance/recessiveness can influence the rate of spread (or elimination) of an allele in a population, but not whether or not it spreads; that is determined by selection. The rate is faster is the allele is dominant. For an allele that IS selectively neutral, dominance/recessiveness is unimportant, because genetic drift is indeed an essentially random process (assuming no pleiotropy, etc.)
If things like brown eyes or six fingers are dominant but selectively neutral, their spread or elimination can only occur by chance.
C’mon Doug, we’re both “a little off base?” Gimme a break.
I wasn’t trying to give blokhead a complete course in genetics. Admittedly I was simplifying a bit. And since the argument was about the comparative roles of dominance vs. selection, I didn’t feel it was necessary to get into such details as neutral mutations, genetic drift, etc.
If you want to get into a scientific nit-picking contest, I’ll have a crack at some of your posts. (I mean this in a friendly way. Certainly it would be much more entertaining to have a “little war” with you than with blokhead.)
And thanks for the correction on the spelling of Hardy-Weinberg. It just shows how long it’s been since I actually had to teach undergraduates about it. :o
I doubt you could bury me in Hardy-WeinBERG equations as you couldn’t even spell it correctly but thanks for trying. Now really I don’t much care who’s wrong or right (but I think Mr. Yanega was more on the mark than either of us)and this is all getting a bit silly so if you could not reply to this and just let’s be fair to everyone else that’s posting. This is my last post for this particular subject.
