I am not sure there has to be an evolutionary explanation or advantage.  Is it a spandrel?

The evolution is a consequence of random mutations, but the natural selection is not totally blind and stupid. The composition of milk (which is a perfect food for babies) is genetically programmed. The "blind watchmaker knows" the evolutionary benefits of his work. Must to be an explanation.

For example this:

 Impact of Early Sodium Supplementation on Hyponatremia and Growth in Premature Infants: A Randomized Controlled Trial

http://www.ncbi.nlm.nih.gov/pubmed/25406227

https://www.researchgate.net/publication/268795554

From the abstract: "No increase in common prematurity-related morbidities was detected in infants who received supplemental sodium chloride. Conclusion: Sodium supplementation of enteral feedings in very premature infants averts hyponatremia and enhances weight gain."

suggests some existing explanation.

Article Impact of Early Sodium Supplementation on Hyponatremia and G...

And this equivalency law:

https://www.researchgate.net/publication/262067162

also has (scientific) evolutionary explanation - the simple rules of ion exchange process, and the maximum economy principle in biology and physiology.

Article Equivalency law in the metal requirement of the living organisms

"The evolution is a consequence of random mutations"

This is one mechanism, but I am not sure it is even the most important one.  What about genetic drift?  I think the rapid increase of human intelligence was due to selective infant mortality, with whole genomes being dumped via generally weak and unfit babies, in other words, rapid genomic evolution, independent of mutations.  So looking at individual genes or traits may be uninformative.

Yes, mutations and also genetic drift - along with natural selection are important mechanisms of evolution. Rapid or slow the change (mutations and/or genetic drifts) - the natural selection merciless. At all mammals - if the composition of the breast milk is not good (not perfect), the baby is dropped to a disadvantage already at the start of life. Because of this always big the selection pressure (compulsion) on the milk. Thus, the milk (naturally - including colostrum) is perfect food for babies from every viewpoint. But why is good the higher sodium content in colostrum? I would dare to bet - this has an explanation certainly. I look for this explanation.

"if the composition of the breast milk is not good (not perfect), the baby is dropped to a disadvantage already at the start of life."

I have looked in detail at the old literature when infant mortality was high in developed countries.  The main reason for this was faulty infant feeding, but I cannot find any evidence that the high mortality in non-breastfed infants was due to the type or quality of any alternative milks or formulae.  What was crucial was the way the infant was fed, and the skill and care devoted in this.

Let us see some facts. It is known, the developing foetus regularly drinks from the amniotic fluid (gastrointestinal adaptation of the fetus). The taste of colostrum and the taste of the amniotic fluid are similar (the sodium content of amniotic fluid is high, similar to the blood). Thus the high sodium content of colostrum is beneficial, because it's taste isn't unusual for the new-born babies, and the sodium content gradually decreases to the optimal level (colostrum, transitional milk, mature milk). Perhaps this is a possible explanation.

Anthony,

We do indeed have to take into consideration the effect of faulty infant feeding on infant survival, expecially at times and in places with low breastfeeding rates or virtually zero breastfeeding rates. Iceland till the 1770s was a case in point, with 10 generations of abandonment of breastfeeding by the mothers and thus no one to act as wet-nurses to provide human milk. Breastfeeding was not valued and cow's milk was an important part of the marginal economiy and was therefore highly valued. Cream particularly. Mothers gave their babies what they believed was best, and didn't draw conclusions that babies were not surviving because of the lack of appropriate nourishment and biolgoical protective mechanisms. Large numbers of infants were born and consequently a few survived - the hardiest. most robust infants. Iceland is one example, but breastfeedfing was abandoned or discouraged for various reasons in cold, dairying regions during a similar period, and later.

The tradition of avoiding breastfeeding in Iceland changed because of outside influences, and mothers took up this new practice suckling their infants.

It is commonly believed that the human race only survived because women "usually" breastfed.  This wasn't true during some periods in some regions. When breastfeeding wasn't practised, and babies were therefore NOT protected from infection by the several anti-infective processes it provides, the human race survived because of *another* biological process. This was the *absence* of lactational amenorrhoea, i.e. the delayed return of fertility provided by frequent on-demand breastfeeding. In the absence of lactational amenorrhoea, women had short intervals between each birth and so potentially had larger numbers of children. Out of these, some survived the poor nutrition, the lack of hygeinic preparation of food, and poor living conditions. According to Fildes, the surviving children in non-breastfeeding villages of Bavaria were considered particularly sturdy.

I'll put some references in a separate message.

To continue. Survival of the fittest as an evolutionary process was certainly affected by infant-feeding practices and fashions, especially in situations where hygeine and even the most basic understandeing of macro nutrients (let alone micro nutrients) was lacking. When a high percentage of infants died, their genetic material was of course lost to future generations. The most physically resilient infants survived and - if they survived to reproductive age and partnered - they were able to pass on their genes.

Here are some references relevant to my privious post.

Iceland:

Harstrup K. A question of reason: breast-feeding patterns in seventeenth- and eighteenth-century Iceland. In V. Maher (ed.) The anthropology of breast-feeding and natural law of social cconstruct. Oxford: Berg, pp. 91-108.

Gundarsdottir O. Saving the child. Regional, cultural and social aspects of the infanht mortality decline in Iceland, 1770-1920. Report No. 19, Demographic Data Base, Umea University, Sweden.

Bavaria:

Fildes V. Breasts, bottles and babies: a history of infant feeding. Edinburgh: Edinburgh University Press, 1988. [Sorry, I don't have the page numbers in front of me.]

Finland (regions)

Moring B. Motherhood, milk, and money: infant mortality in pre-industrial Finland. Social History of Medicine 1998; 11(2): 177-196.

Some authors suggest that newborns lack the ability to detect salty stimulus or believe to be equally palatable for pure water.
Here are a few references on the subject.

Food and Nutrition Bulletin Volume 17, Number 4, 1996 (UNU, 1996, 163 p.)

Allen JC, Keller PR, Archer P & Neville M (1991): Studies in human lactation: milk composition and daily secretion rates of macronutrients in the first year of lactation. Am. J. Clin. Nutr. 54, 69–80. | PubMed |

AOCS official method Ce-2-66 (1997): Preparation of methyl esters of fatty acids. American Oil Chemists Society, Champaign Illinnois, USA.

Barry-Lawrence P (1994): Breast milk. Pediatr. Clin. North Am. 41, 925–941. | PubMed |

Boersma ER, Offringa JP, Muskict AJ, Chase MW & Simmons JI (1991): Vitamin E, lipid fractions, and fatty acid composition of colostrum, transitional milk, and mature milk: an international comparative study. Am. J.Clin. Nutr. 53, 1197–1204.

Bondia-Martinez E, López-Sabater MC, Castellote-Bargalló AI, Rodriguez-Palmero M, Gonzalez-Corbella MJ, Rivero-Urgell M, Campoy-Folgoso C & Bayés-Garcia R (1998): Fatty acid composition of plasma and erythrocytes in term infants fed human milk and formulae with and without docosahexaenoic and arachidonic acids from egg yolk lecithin. Early Hum. Dev. 53(Suppl), S109–S119. | PubMed |

Brown KH, Black RE, MD, MPH, Robertson AD, MS, Akhtar NA, MBBS, Ahmed MdG & Becker S (1982): Clinical and field studies of human lactation: methodological considerations. Am. J. Clin. Nutr. 35, 745–756. | PubMed | ChemPort |

Brühl L, Schulte E & Thier HP (1994): Composition and structures of triglycerides of human milk and some base components for infant milk formulas. Fat Sci. Technol. 96, 147–154.

Byrdwell WC, Emken EA, Neff WE & Adlof RO (1996): Quantitative analysis of triglycerides using atmospheric pressure chemical ionization-mass spectrometry. Lipids 31, 919–935. |  1. Allen JC, Keller PR, Archer P & Neville M (1991): Studies in human lactation: milk composition and daily secretion rates of macronutrients in the first year of lactation. Am. J. Clin. Nutr. 54, 69–80. | PubMed  |

Dear Adriano, I doubt if authors "suggest" and "believe".

Every cell can detect the changes in extracellular sodium concentration. Note: energy expenditure (ATP consumption) of the sodium/potassium pumps. And:

Facial expressions develop before birth, researchers show

http://www.sciencedaily.com/releases/2011/09/110913090927.htm

The original article: Nadja Reissland, Brian Francis, James Mason, Karen Lincoln. Do Facial Expressions Develop before Birth? PLoS ONE, 2011; 6 (8): e24081 

http://dx.doi.org/10.1371/journal.pone.0024081

https://www.researchgate.net/publication/51636212

And:

High-definition scans suggest effects of smoking may be seen in unborn babies

http://www.sciencedaily.com/releases/2015/03/150323142837.htm

The original article: Nadja Reissland, Brian Francis, Kumar Kumarendran, James Mason. Ultrasound observations of subtle movements: a pilot study comparing fetuses of smoking and non-smoking mothers. Acta Paediatrica, 2015

http://dx.doi.org/10.1111/apa.13001

https://www.researchgate.net/publication/273467987

Article Ultrasound observations of subtle movements: A pilot study c...

Article Do Facial Expressions Develop before Birth?

 "Large numbers of infants were born and consequently a few survived - the hardiest. most robust infants. Iceland is one example,"

The dominant factor in the survival of infants (though probably not for older children) was the skill and competence of the mothers.  All this could explain why the national IQ of Iceland is higher than might be expected.

I have a further part of the explanation. But need some more knowledge from the (real) scientific literature of salt.

Saulo Klahr and Neal S. Bricker: Energetics of Anaerobic Sodium Transport by the Fresh Water Turtle Bladder

J Gen Physiol. 1965 March 1; 48(4): 571-580

 http://europepmc.org/articles/PMC2195440/pdf/571.pdf

 https://www.researchgate.net/publication/9259232

 From this: "The rate of anaerobic glycolysis, as determined by lactate formation, correlates well with the rate as determined by glycogen utilization. Using lactate formation as the index of anaerobic glycolysis, a linear relationship was observed between glycolysis and net anaerobic sodium transport."

40 years later:

 Christopher B Scott: Review, Contribution of anaerobic energy expenditure to whole body thermogenesis.  Nutrition & Metabolism 2005, 2:14

http://www.nutritionandmetabolism.com/content/2/1/14  

https://www.researchgate.net/publication/7785036

 From this: "Entropy represents energy that is not available to perform work so that simply put, energy transfer is inefficient. ... Brisk activity of the sodium pump necessitates a rapid rate of ATP re-synthesis. If this is true then it is important to recognize that in some cells lactate with presumed heat production is better correlated with sodium and potassium pumping than is oxygen uptake. ... It seems logical to conclude that most mammalian energy expenditure does come from aerobic metabolism but the evolution of a metabolic acceleration with concomitant heat production comes from both anaerobic and aerobic pathways. The relative contributions of each pathway to whole-body thermogenesis are not known."

This already something (entropy, sodium-potassium pump, anaerobic glycolysis and lactic acid, in one article) but unfortunately not too much. The explanation of the correlations and the consequences are missing. Would this be the "progress" of the health science over 40 years? Professor Scott presumably did not read the next article:

Toshimasa Osaka, Akiko Kobayashi, and Shuji Inoue:

Thermogenesis induced by osmotic stimulation of the intestines in the rat

J Physiol. 2001 April 1; 532(Pt 1): 261–269.

 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2278526/

 https://www.researchgate.net/publication/12050801

 From this: "The energy expenditure induced by 20 % glucose was 2.79 +/- 0.45 kJ kg-0.75 for 3 h (Fig. 4). The RER (respiratory exchange ratio) increased from 0.82 +/- 0.01 to 0.92 +/- 0.01 at 115 min (Fig. 1B), suggesting the oxidation of carbohydrate ... The metabolic rate rose during the 10 min infusion period of 3.6 % NaCl, stayed at a plateau level of ~ 205 J kg-0.75 min-1 between 35 and 120 min and then slowly declined but was still significantly higher than the baseline level at 3 h. The energy expenditure induced by 3.6 % NaCl was 3.49 +/- 0.33 kJ kg-0.75, ... The RER did not change after infusion of any of the NaCl solutions (Fig. 2B)."

This is a very valuable and important experimental research article about the Sodium-Induced Cellular Anaerobic Glycolysis (SICAG) or Sodium-Induced Cellular Anaerobic Thermogenesis (SICAT), but nothing was written about this - in the paper.

Unfortunately this article was censored (and is ignored), it must to read very carefully - to understand the effect of high sodium intakes.

And, 9 years later - unfortunately, Mr. Mathur don't understand it.

Ram K. Mathur: Role of diabetes, hypertension, and cigarette smoking on atherosclerosis   J Cardiovasc Dis Res. 2010 Apr-Jun; 1(2): 64–68.

 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2945206/

 https://www.researchgate.net/publication/46579781

 From this: "To determine the mechanism of thermogenesis, Osaka et al.[7-9] infused hypertonic solution of glucose, NaCl, ... The mechanism of thermogenesis is not clear. However, it may involve intestinal osmoreceptors. ... It is this thermogenesis that is responsible for the generation of atherosclerotic plaque."

Back to the original question (the explanation):

 It is well known that in the new-born babies the thermoregulation system does not work (properly). It cannot develop under constant temperature, during pregnancy, and the lung of new-born babies is "beginner" in the supply of his task. Also well known the cooling (decreasing core temperature = hypothermia) can be lethal for babies. But the blind watchmaker (the natural selection) "learned" well the fundamental natural laws of physics, chemistry, biochemistry and physiology, and he uses these knowledges (note - the biochemistry and the physiology were his own works). The higher sodium content in colostrum and excess diffusion of Na ions switches on the anaerobic glycolysis in cells of babies automatically. This is the SICAT or SICAG presented by Osaka et al. - despite that is concealed and the article was censored. This excess "entropy-transfer" into the babies, and the excess heat production is necessary, because has evolutionary benefit by decreasing the risk of lethal cooling. But naturally, after the development of the thermoregulation is not need excess (anaerobic) thermogenesis and isn’t need excess sodium (and the sodium content decreases to the optimal level - day by day gradually). The blind watchmaker "knows" the optimal ratios (and quantities) of metals and other components of milk (Na/K ratio, ratio between sum of alkaline metals and sum of polyvalent metals etc.) and makes these ratios in mature milks. The wrong recipes of milks fall out continuously from the genes - the natural selection works so.

 These two parts of the explanation are not controversial, and are strengthened each other. Perhaps exist some additional explanations. I don't know - the question is still open.

Article Contribution of anaerobic energy expenditure to whole body t...

Article Energetics of Anaerobic Sodium Transport by the Fresh Water ...

Article Thermogenesis induced by osmotic stimulation of the intestin...

Article Role of diabetes, hypertension, and cigarette smoking on ath...

De Luise M., Blackburn G.L., Flier J. S.: Reduced activity of the red-cell sodium-potassium pump in human obesity.

The New England Journal of Medicine 1980 Oct 30; 303(18): 1017-22. DOI: 10.1056/NEJM198010303031801 http://www.ncbi.nlm.nih.gov/pubmed/6252462 http://www.nejm.org/doi/full/10.1056/NEJM198010303031801

Abstract: “Looking for evidence of reduced energy use in the cells of obese persons, we measured the numbers of sodium-potassium-pump units in erythrocytes from a group of 21 obese human subjects and found them to be reduced by 22 per cent as compared with those of nonobese controls (P