Breastfeeding may have led to a common gene mutation found among the ancestors of Native Americans and East Asians who have survived the last Ice Age.
The gene mutation may have also affected the shapes of the teeth of ancient East Asians and Native Americans.
Most importantly, the gene mutation could be the first evidence of natural selection on the human maternal-infant bond and how indispensable this relationship was for human survival through times.
The finding, published in the journal Proceedings of the National Academy of Sciences, could also be used in understanding the origins of dense breast tissue and its impact in breast cancer.
Breastfeeding And Genetic Mutation
Between 18,000 and 28,000 years ago, the ancestors of Native Americans journeyed across the Bering land bridge in what is known today as Siberia going to Alaska. Living in the far north would mean that they lack exposure to the sun, hence, they are not getting enough vitamin D. Sun does not shine at all for some parts of the year above the Arctic Circle.
Vitamin D is important to develop a strong immune system. It also regulates the production of fat and promotes proper calcium absorption.
These ancestors, therefore, might have suffered health problems but the population survived for another 10,000 years.
Leslea Hlusko, lead author and an associate professor of integrative biology at the University of California, Berkeley, proposed that the ancestors of Native Americans survived their harsh environment because of a genetic mutation that enhanced the development of milk ducts in the mother's breast during breastfeeding. This gene, identified as EDAR, may have helped the mother to pass the needed fats and vitamin D to their babies, enabling them to thrive in the environment.
The gene EDAR is also responsible for a particular shape of teeth, as well as thicker hair and development of more sweat glands.
The ancient Native Americans have a pronounced teeth shape called "shoveled incisors." This is when the cutting teeth in front of the mouth, the four on top and four on the bottom, have ridges along the sides and biting area.
Shoveled incisors were also common among populations in East Asia: Korea, Japan, and northern China.
Previous studies have long proposed that incisors are another form of natural selection for ancient generations to soften animal skins. Hlusko, however, was not satisfied about this hypothesis.
A variant of the gene EDAR, identified as V370A, arose about 30,000 years ago and was seen in some people living in humid areas in Asia. While it can easily be concluded that sweat glands are essential for this environment, the question remains as to why incisors would also be found among Native Americans and Asian populations living in the extremely colder environment.
Specifically, incisors were common to the more than 5,000 teeth from 54 archeological sites in Europe, Asia, and North and South America that Hlusko and her team examined for their study.
Their examination suggests that some members of the ancient people who arrived at the Beringia have first carried the gene EDAR from Asia. From then, it mutated to another variant and spread among the populations that settled in the region between 18,000 and 28,000 years ago.
The incisors were only incidental to this genetic mutation and not a part of natural selection among the ancient Native Americans and East Asians — unlike with how mother-infant relationship brought by breastfeeding can be an evidence of natural selection.
"Consequently, as the genetic mutation was selected for in an ancestral population living in the far north during the last Ice Age, shovel-shaped incisors became more frequent too," writes, Hlusko.
Furthermore, in the case of Native Americans and East Asians, breastfeeding proved to have significantly aided the survival of their population rather than their incisors which have gotten more attention in previous studies.