Palmaris longus

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Palmaris longus is a muscle found in the forearm of many mammals, including humans. It is a slender, flat muscle, and if you held your arm parallel to the ground with your palm facing up, and then decided to allow someone to dissect your forearm, it would appear as a little flat surfboard not too far below the skin, with a tendon passing through your wrist into your hand.

The palmaris longus performs different functions in various animals. In some animals it pulls back a skinfold to expose a claw. It is more prevalent in animals which use their forelimbs to grab things while climbing, such as orangutans.

Here's where it gets interesting: Palmaris longus serves no apparent function in humans. For this reason it is actually very popular with reconstructive surgeons because they can "harvest" the tissue or the tendon and use it to rebuild other, useful muscles.[1] What is even more interesting is the fact that the muscle is completely or partially absent in around one-sixth of humanity.[2]

Contents

[edit] Missing muscles

Palmaris longus isn't the only "missing" muscle in the human body, there are plenty of others. The Plantaris muscle is in the leg and is absent in about 10% of people. Pyramidalis is in the rectal area and is absent in 20% of people. Levator claviculae is a real no-show, being absent 97% of the time. Occipitalis minor in the head plays the race card, one (somewhat dated) study suggests is always present in indigenous Malays, in 50% of Japanese, only 36% of Europeans, and is nonexistent in some regions of southwestern Africa and in Melanesians[3]

For the purposes of this discussion Palmaris longus has a major advantage over these other muscles, because its presence or absence can be determined without using a scalpel. Hold your hand out palm up, form a fist and then angle your fist toward your face. You'll see either one or two tendons crossing your wrist. If there is only one, then you are missing Palmaris longus. It can be missing in either arm, or in both.

[edit] Vestigiality and creationism

Palmaris longus is generally accepted by the medical community as a "vestigial" muscle.[4] Vestigiality refers to "evolutionary hangovers". In evolutionary theory there is selection pressure to encourage beneficial mutations, and to discourage harmful mutations. There is also selection pressure to dispense with originally useful traits that have since become a drawback (eg. thick fur coats after a climate has warmed). However if a feature has simply become irrelevant, and exerts no evolutionary pressure (either positive or negative) their fate just becomes a bit random.

Vestigiality in humans is disputed by many creationists, who categorically deny its existence.[5]. The issue is obviously this: as man was created as a 'perfect being' by God, there can't be any "mistakes". Creationists thus often suggest that what science regards as a "vestigial" and useless object in the human form is simply not properly understood.

[edit] The Palmaris longus puzzle

However Palmaris longus poses a very different problem from other allegedly vestigial features. The vestigial status of the appendix can be argued about, but it is only absent in roughly 1 in 100,000 people.[6]

Palmaris longus is different. While it would be unscientific to categorically insist that Palmaris longus has no useful function (while unlikely, it is plausible that some as-yet-unknown purpose may eventually be identified), there's no denying the fact that a lot of people (roughly 50 million in the USA) simply don't have the muscle in both arms. Roughly 20 million people don't have it at all. And to make things worse, not all of those people are Jesus-hating pinko-liberals; some of them are actually God-fearing, gun-toting Christians.

Explaining this variable absence from a creationist viewpoint thus becomes a challenge. Most sites ignore it. For those that do address it, the main thrust is to insist the muscle has a function and simply not talk about the absence.[7] However at least one site[8] has a novel approach - attributing it to falldidit in the guise of "mutational loss". Apparently we needed this muscle way back when we needed to grasp spears, but since the industrial age it has been "deselected" at a simply astonishing rate. Mutational loss occurs because of the "Fall", apparently prior to that our DNA was stable. Why this mutation was so specific to poor old palmaris longus has yet to be explained. Also, Palmaris longus demonstrates high ethnic variability without any apparent correspondence to social activity, eg. the roughly 40% absence among Nigerian tribal groups chiefly engaged in semi-nomadic hunter/farmer lifestyles.[9]

So, while formal studies have not been able to identify any function for the muscle, or any consequences from the muscle's absence,[10][11] and there may even be an unidentified purpose [12], we are still left with no plausible reason for its variable absence.

[edit] The evolutionary viewpoint

The evolutionary interpretation of the muscle's absence is fairly simple. We inherited the muscle through common descent, and numerous animals that we share a common ancestor with (such as the orangutan) still actively employ the muscle. Close primate relatives (such as the chimpanzee and gorilla) also do not actively employ the muscle, and hence they also demonstrate the same variability[13]. The common descent principle suggests that at some stage our ancestor employed the muscle actively. The primate branch then began to evolve the thumb apparatus (and particularly the thenar muscle group) and consequently the palmaris longus became vestigial. As there is no apparent pressure (positive or negative) concerning the muscle, evolution has largely left it alone[14]. Its persistence might possibly be due to the fact that the necessary changes to the gene(s) involved in losing the muscle may have other consequences, but this is only speculation.

[edit] In summary

What does it all mean? Well, if you are a rational person, then your possession or absence of Palmaris longus can make for a fascinating dinner discussion. But if you are a creationist and you are missing this muscle, this means that God won't be taking you up for the Rapture. Sorry.

[edit] Footnotes

  1. Wheeless' Textbook of Orthopaedics
  2. Absence of the palmaris longus muscle: a population study. Ulster Med J. 2001 May; 70(1): 22–24.
  3. Macalister A (1875). "Observations on muscular anomalies in the human anatomy. Third series with a catalogue of the principal muscular variations hitherto published". Trans. Roy. Irish Acad Sci 25: 1–130.
  4. Accessory Muscles: Sookur et al, Anatomy, Symptoms, and Radiologic Evaluation March 2008 RadioGraphics, 28, 481-499.
  5. eg. Creation Insights
  6. http://www.sma.org.sg/UploadedImg/files/SMJ/5309/5309cr4.pdf
  7. eg. True Authority
  8. Creation.com ‘Selfish gene’ theory, the ENCODE project and ‘vestigial’ muscles
  9. Mbaka: Prevalence of palmaris longus absence – a study in the Yoruba population Ulster Med J 2009;78(2):90-93
  10. Sebastin et al, Does the absence of the palmaris longus affect grip and pinch strength?, J Hand Surg Br. 2005 Aug;30(4):406-8
  11. Werner RA, Spiegelberg T Does the presence of the palmaris longus tendon influence median nerve function? Muscle Nerve. 2012 Jun;45(6):895-6
  12. There is at least one medical reference which actually does attribute a (weak) function to palmaris longus General Practice Notebook (UK) palmaris longus muscle (anatomy). This single entry is heavily referenced by creationist websites.
  13. Thejodhar et al, Indian J Plast Surg Vol 41.1 (2008)
  14. Maughan et al: The Roles of Mutation Accumulation and Selection in Loss of Sporulation in Experimental Populations of Bacillus subtilis Genetics. 2007 Oct;177(2):937-48
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