Nurturing Infants: The Evolution of Breastfeeding and Changing Health Recommendations

For the last 65 million years, mammals have reigned as Earth's predominant terrestrial inhabitants. Except for insects and arachnids (spiders), mammals boast the broadest global distribution across all terrestrial and aquatic biomes. Their remarkable ecological success is largely attributed to their ability to regulate body temperature. Ranging from the bumblebee bat, measuring just 1.2 to 1.6 inches (30 to 40 millimeters), to the colossal blue whale, surpassing 100 feet (30 meters) in length, the 5,500–5,700 mammalian species exhibit an extraordinary range of sizes.

The lineage of true mammals emerged approximately 200 million years ago and, over tens of millions of years, diverged into three main branches. Monotremes, such as the duckbilled platypus, are egg-laying mammals exclusively found in Australia and New Guinea. Marsupials, including kangaroos and opossums, are located in Australia and the Americas. They give birth to underdeveloped offspring that continue to grow outside the womb within a pouch called a marsupium. The majority of mammals, around 90 percent, are placental mammals (eutherians). In this group, the fetus develops in the womb until birth, at which point it is well-advanced in its development. An intriguing discovery in 2013 revealed the oldest known placental mammal, Juramaia sinensis, a shrew-sized creature dating back 160 million years. Humans, as placental mammals, are classified within the primate order.

Mammals possess several distinctive traits not found in other vertebrates. Mammary glands, originally coined as "mammals" by Linnaeus in 1758 from the Latin word for "breast," enable females to nourish their offspring with milk, the primary source of nutrition. Hair or fur, present at some stage of their life, provides insulation against extreme cold. The middle ear is equipped with three bones that transmit sound vibrations into nerve impulses. The lower jaw in mammals consists of a single bone on each side. Additionally, mammals are typically warm-blooded (endothermic), possess specialized and differentiated teeth, have a relatively larger brain, particularly in the neocortex (the most advanced brain region), feature a diaphragm that separates the heart and lungs from the abdominal cavity, and have an efficient four-chambered heart.

A poster from 1937 advocated for infant nursing, highlighting the age-old tradition of breastfeeding. However, during the period spanning 1900 to 1960, breastfeeding experienced a significant decline due to prevailing negative social attitudes and the growing popularity of infant formulas. Since that era, there has been a resurgence in the practice, with experts now endorsing it for the first six months of an infant's life and beyond.