Palmaris longus

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'''''[[Palmaris longus]]''''' is a muscle found in the forearm of many mammals, including [[human]]s. 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 skin fold 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.<ref>[http://www.wheelessonline.com/ortho/palmaris_longus Wheeless' Textbook of Orthopaedics]</ref> What is even more interesting is the fact that the muscle is completely or partially absent in around one-sixth of humanity.<ref>[http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2449224/?tool=pmcentrez Absence of the palmaris longus muscle: a population study. Ulster Med J. 2001 May; 70(1): 22–24.]</ref>

==Missing muscles==
''Palmaris longus'' isn't the only "missing" muscle in the human body, there are plenty of others. The ''{{wpl|plantaris_muscle|Plantaris}}'' muscle is in the leg and is absent in about 10% of people. ''{{wpl|pyramidalis muscle|Pyramidalis}}'' is in the abdominal area and is absent in 20% of people. ''{{wpl|levator claviculae muscle|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<ref>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.</ref>

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.

==Vestigiality and creationism==
''Palmaris longus'' is generally accepted by the medical community as a "[[vestigial]]" muscle.<ref>[http://radiographics.rsna.org/content/28/2/481.full Accessory Muscles: Sookur ''et al'', Anatomy, Symptoms, and Radiologic Evaluation March 2008  RadioGraphics, 28,  481-499.]</ref>  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.<ref>[http://www.uark.edu/~cdm/creation/shame.htm eg. Creation Insights]</ref> 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.

==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.<ref>http://www.sma.org.sg/UploadedImg/files/SMJ/5309/5309cr4.pdf</ref> 

''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. 

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.<ref>[http://www.trueauthority.com/cvse/discover.htm eg. True Authority]</ref> However at least one site<ref>[http://creation.com/selfish-gene-theory-the-encode-project-and-vestigial-muscles Creation.com ‘Selfish gene’ theory, the ENCODE project and ‘vestigial’ muscles]</ref> 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.<ref>[http://www.ums.ac.uk/umj078/078(2)090.pdf Mbaka: ''Prevalence of palmaris longus absence – a study in the Yoruba population'' Ulster Med J 2009;78(2):90-93]</ref>

So, while formal studies have not been able to identify any function for the muscle, or any consequences from the muscle's absence,<ref>[http://www.ncbi.nlm.nih.gov/pubmed/15935531 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]</ref><ref>[http://www.ncbi.nlm.nih.gov/pubmed/22581546 Werner RA, Spiegelberg T ''Does the presence of the palmaris longus tendon influence median nerve function?'' Muscle Nerve. 2012 Jun;45(6):895-6]</ref> and there may even be an unidentified purpose,<ref>There is at least one medical reference which actually does attribute a (weak) function to ''Palmaris longus'' [http://www.gpnotebook.co.uk/simplepage.cfm?ID=-1019936742 General Practice Notebook (UK) Palmaris longus muscle (anatomy)]. This single entry is heavily referenced by creationist websites.</ref> we are still left with no plausible reason for its variable absence.

==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 {{wpl|gorilla}}) also do not actively employ the muscle, and hence they also demonstrate the same variability.<ref>[http://www.bioline.org.br/pdf?pl08024 Thejodhar ''et al'', Indian J Plast Surg Vol 41.1 (2008)]</ref> 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 {{wpl|Thenar_eminence|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.<ref>[http://www.genetics.org/content/177/2/937.full.pdf 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]</ref> 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.

==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.

==See also==
*[[Suboptimal design]]

==References==
{{reflist|2|80%}}

[[Category:Anatomy]]
[[Category:Evolution]]
[[Category:Suboptimal design]]

Categories: Anatomy, Evolution, Suboptimal design