How to evolutionary explain menstrural cycles synchronization?
This is a myth which most likely came from methological flaws in the original study. It could never be reproduced. This paper ("Darwin’s Legacy: An Evolutionary View of Women’s Reproductive and Sexual Functioning") deals with it. The interesting part starts from page 30 and says (the references can be ressolved in the article):


  Critiques of MSH Studies
  
  Wilson (1992) and Yang and Schank (2006),
  among others, have criticized the study design, methods, and
  statistics used by McClintock (1971) and others who have claimed
  evidence of MS. For example, McClintock 
  incorrectly used the Page test for ordered hypotheses with multiple
  treatments (she used the same groups of women repeatedly instead of
  independent treatments), making it impossible to evaluate the true
  level of significance of her reported findings. Likewise, reports of
  greater estrous synchrony in chimpanzees caged together than in those
  caged apart (Wallis, 1985) and synchronization of estrogen peaks in a
  sample of five golden lion tamarins (French &amp; Stribley, 1987) are
  rendered moot by the use of unsuitable statistical tests (Schank,
  2001). Furthermore, computer simulations suggested that the null
  hypothesis of no synchronization could not be rejected in either the
  chimpanzee or tamarin samples (Schank, 2001).