Classification: Dietary
Mnemonic: Iodine Deficiency
Specific Model: Iodine Deficiency
Original Proponent(s): de la Marrett 1936    
Assessment: Popularity: Not mentioned in any text surveyed.
Simple: #42 / 42 (last) 22%
Detailed: #42 / 52 (last) 16%
Basic Summary: Iodine deficiency led to human bipedalism    
Discussion: De La Marett’s book “Race, Sex and The Environment” included posssibly the most bizarre idea on bipedal origins ever published.
Its central theme was that environmental factors, especially those influencing nutrients such as Iodine, were responsible for major shifts in the attributes of populations and, consequently, the evolutionary history of many species, including bove the hitherto unbroken carpet of forest with which it was formerly covered.” Marett (1936a:127) which led to them living above the tree-line in a habitat that was severely Iodine deficient. This Iodine deficiency, according to the book’s argument, was the basis of many alterations of hominid phenotype, due mainly to mutation. In the case of mode of locomotion, the already hypertrophic forearms, it is claimed, were more vulnerable to this kind of mutation which resulted in them basically not being used at all, thereby forcing a change to bipedalism.

De la Marett is quite open about the lack of evidence for such a hypothesis.
He writes
“[E]even granting the sudden invasion of a mountain habitat by Man, it still remains difficult to explain his upright posture. . . . In view of the form and habits of the gibbon and of his three greatly enlarged relatives, it would be much more reasonable to expect a life above the tree line to select in favour of a return to a quadrupedal form of locomotion. Indeed it is necessary to look only as far as the baboon to witness a most successful realization of that possibility. Mechanical explanations seem thus to break down. The upright stance, in itself, confers no more survival value than did brachiation in the trees. Each trait seems much more probable to represent an ingenious escape from impending calamity. I suggest, therefore, that, just as failure of limb development favoured an arboreal specialisation of the arms at the expense of the anthropoid legs, so, at a later stage, some similar disadvantageous change of environment tended to wither the arm, and throw upon the Hominid legs the whole responsibility of bodily support. It must be admitted that the ecological and physiological basis of such a theory is slender. It will be assumed, however, that a lack of either Iodine, or of lime and vitamin D, is inclined to increase the tendency towards a failure of normal limb-development. If this is true, the proportion of cripples should show a positive correlation, both on the one hand with the goitre and deaf-mute figures, indicative of Iodine shortage, and with the incidences of rickets and other calcium deficiency diseases on the other. I have not yet, however, had an opportunity of investigating this point.”
de la Marett (1936:149)
 Strengths: It is difficult to think of a single strength of this idea.    
Weaknesses: This hypothesis can be criticised on several grounds.

Firstly, there is little evidence that early hominids lived in mountainous areas in Africa where, it is most commonly assumed, the human lineage first arose and if even they did it is difficult to understand why these apes, having been exposed to the extremely slow process of mountain uplifting would have left their arboreal lifestyle and began walking around on difficult, cold terrain above the tree-line when their former habitat was so close.

Secondly, there is the Iodine factor itself. There does appear to be good evidence that Iodine levels are significantly depleted in mountainous areas but, as de la Marett admits, there is no real evidence to suggest that such a mineral deficiency could result in such peculiar, limb specific, deformations as to convert a brachiating ape into a bipedal one.

Thirdly, missing from this argument is any consideration of how forces of natural selection might have come to bear upon this population. Mountainous terrain does not appear to be the sort of habitat where traits favouring bipedalism would be selected for. It seems much more likely that any such mutations would be strongly selected against and would rapidly disappear from the gene pool.
1.1 Survival Value 0 (Poor) The original causal factor is not postulated to be selection at all, but rather some chance mutation due to a deficiency of certain minerals in the diet. Furthermore, the model takes almost no account of post-mutation selection which would have been likely to act specifically against bipedalism in mountainous terrain.    
1.2 Sexual Selection 0 (Poor) No argument is offered in favour of sexual selection.    
1.3 Not Teleological 0 (Poor) This model does not offer a very compelling argument for intermediate steps but, rather, that it happened in one rather coincidental leap    
2.1 Improved Food Acquisition 0 (Poor) de la Marett's model would appear to make food acquisition more difficult.    
2.2 Accounts for Predation 4 (Fair) The model postulates a move to a habitat that is relatively predator free (compared to the open savannah) but early hominids, adjusting to anatomical changes due to mineral deficiencies would still be very vulnerable to predators adapted to mountainous niches.    
2.3 Why Apes are not Bipedal 7 (Good) If we accept the premise that severe Iodine deficiency was the primary cause of the evolution of bipedalism and if it is true that Pan/Gorilla habitats are not deficient then the model would meet this criterion    
2.4 Extant Analogues 0 (Poor) De la Marett does not offer any evidence of chimpanzees and/or gorillas exposed to Iodine deficiencies as models.    
2.5 Applies to Both Sexes 9 (Good) Applies equally to both sexes.    
3.1 Hominid Anomalies 0 (Poor) De la Marett’s book was published several years before any postcranial remains for Australopithecines were found.    
3.2 Fits Paleoecological Record 0 (Poor) The fossil evidence that has amassed since 1936 largely contradicts a highland model for early human evolution.    
3.3 Precursor to Strider and knuckle Walker 0 (Poor) The notion that the ancestral condition of hypertrophic forelimbs and reduced hind limbs, adapted to brachiation, was suddenly reversed by change of diet and Iodine deficiency specifically is counter-intuitive from a biomechanical point of view.    
4.1 Extended Explanatory Power 0 (Poor) De la Marett’s thesis is that mineral deficiency was responsible for many ape-human differences including hair loss, brain grown and changes in jaw morphology. However, most of these arguments are contradicted by what is now known about iodine deficiency (e.g. smaller brain size is a symprom of cretinism.)    
4.2 Complimentary 4 (Fair) The idea that the primary habitat for early bipeds was above the tree-line in mountainous areas runs counter to most other models of human evolution.    
4.3 Falsifiable or Testable 3 (Poor) The model, theoretically, should be easy to test: examine any anecdotal cases of Iodine deficiency in captive chimpanzees and/or gorillas. Unfortunately de la Marett offers no such test.    
References de la Marett, J. (1936). Race, Sex and Environment. Hutchinsons Scientific (London)