I just found this article this morning on new research conducted to better understand morning sickness. The prevailing opinion now seems to be that morning sickness is actually useful. (Great.) Scientists found that the things that most often triggered morning sickness in pregnant women were "the sight, smell or taste of meats and strong-tasting vegetables" likely to "contain foodborne microbes" or chemicals known to cause birth defects like smoke and alcohol. My experience in this area seems to support this research, I don't know about you Mamas.
Scientists also found that morning sickness "symptoms peak precisely when embryonic organ development is most susceptible to chemical disruption, between week six and week 18 of pregnancy," which is actually pretty fascinating.
Oh, and we humans are the only species on the planet fortunate enough to experience this rite of passage. There is no evidence of any other animal anywhere who experiences morning sickness. Lucky us.