According to a new study, a rare mutation in breastfeeding-related genes may play a key role in this transition.
About 20,000 years ago, when the glaciers had just peaked, the ultraviolet radiation levels of sunlight in the Amerindian and East Asians were very low.
Studies have shown that the production of vitamin D in the newborn’s skin is affected, affecting their growth and survival, but this mutation counteracts this effect and helps the baby grow and thrive.
Basically, genetic alterations increase the branching density of breast mammary ducts and help mothers provide infants with adequate vitamin D and fat through breastfeeding.
“This highlights the importance of the maternal-infant relationship and its importance for human survival,” said Leslea Hlusko, associate professor of integrated biology at the University of California, Berkeley, in a statement.
People living near the equator can get enough vitamin D to regulate the absorption of fat and calcium, but those in high latitudes will miss it because the sun does not shine for a whole year.
To solve this problem, people have been looking for fat centuries ago, but babies rely only on breastfeeding.
Another interesting fact is that this rare mutation has an effect on the shape of the population’s teeth.
The researchers noticed almost at the same time that the sudden spread occurred that Native Americans and Northeast Asians began to develop shovel-shaped incisors (alongside the ridges and undercuts on both sides), only to be found among those belonging to these two groups.
Changes in tooth shape and branching of mammary ducts are associated with the same genes in humans.
Hlusko and colleagues proposed this theory after studying population records. In fact, they noticed that almost 100% of early American Indians and 40% of modern East Asians had shovel-shaped front teeth.
Then, they used the genetic effects associated with tooth changes to understand the evolution of the breast during the last ice age.
Previous studies have shown that shovel-shaped incisors may have become a way to soften animal skin, but the new theory is completely different.
“People have always believed that this type of shovel is so powerful that there must be evolutionary choices that tend to be such qualities, but why is there such a strong choice in the shape of your front teeth?” added Hlusko.
“When you share the genetic effects on your body, choosing one feature will cause other things to happen.”
The study, titled “Environmental selection during the last ice age on the mother-to-infant transmission of vitamin D and fatty acids through breast milk”, was published April 23 in Proceedings of the National Academy of Sciences.