The notion that nothing good happens after midnight does not seem to apply to times of birth.Around the world the peak hours for births fall between I am and 7 am;the numbers then decrease throughout the rest of the day. This has led many scientists to believe that giving birth during the early morning offers some sort of evolutionaryadvantage,perhaps gained long ago when mothers and their infants would benefit from having their group reunited during the small hours to help with care and to defend them against animals.
The problem with this theory is that almost all the information on the timing of human births comes from modern,urban settings,such as clinics and hospitals,which could produce artificial conditions that affect the variation in timings. Not so, it turns out.As Carlye Chaney shows in her research,early-morning births are common to communities with both modern and traditional lifestyles.
The mothers that Ms Chaney chose to study live in a rural province in Argentina.They were divided into two groups that were considered to be both culturally and genetically distinct.One group consisted of 1,278 women from the Criollo population,a people of mixed Spanish and indigenous（本地的）heritage who live relatively modern lives and typically engage in farming.The second group was made up of 1,110 women who belonged to the Wichi and Toba populations，two of the traditionally nomadic（游牧生活的）peoples who also live there.
If a modern way of life plays a part in favoring early-morning births,Ms Chaney speculated that she would see such timings dominate in the resuilts for the Criollo women,but feature less prominently among the results for the Wichi and Toba.Not so.The data and additional analysis make it clear that there is no statistical difference in the average birth time
found between the two groups. Both showed a surge in births between 2 am and 3 am, and a big drop around 5 pm.More specifically,the average time of birth for the Criollo was 6.34 am and that for the Wichi and Toba was 4.18 am.
Ms Chaney believes that the mechanism driving the tendency for mothers to give birth during the early morning is likely to be melatonin , a hormone which is known to increase at the start of labor（分娩）and is predominantly produced by the body between midnight and 5 am.That mechanism may well have comeabout,Ms Chaney suspects,because it was advantageous to go into labor when most help wasto hand.
What do many scientists believe according to Paragraph 1?