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Mammalian threads

A treatise on how science came to grasp evolution in mammals

By Akshit Sangomla
Published: Thursday 28 March 2019

Humans seldom think of themselves as mammals. But we share a significant part of our identity with more than 5,500 mammalian species currently living on the planet. Features like giving birth to live young ones, nurturing them with milk, having warm blood and large brain sizes have evolved in almost all mammals since they first popped up in the evolutionary chain around 210 million years ago. In I, Mammal: The Story of What Makes us Mammals, former neurobiologist Liam Drew takes the reader on a spectacular journey of how these features evolved over time, each of them with their own complicated story.

Mammals can be broadly divided into monotremes or egg laying mammals; marsupials or animals that give birth to premature live young ones and nurture them in pouches outside their bodies; and, placental mammals which give birth to fully developed young ones after nurturing them inside their bodies. The last category makes up most of the living mammals, including humans. The group of mammals that intrigued Drew the most are monotremes.

One animal among monotremes that is fascinating is the duck billed platypus—a mysterious-looking mammal that lives in the warm waters of eastern Australia. The platypus has been a puzzle for biologists ever since it was first discovered by travelers in 1797. It was subsequently identified and named by George Shaw in 1799. The book delves into the anatomy and behaviour of the platypus in considerable detail in order to understand how they evolved their characteristics, many of which are uniquely mammal-like, while others are far from it. For example there are five pairs of sex determining chromosomes in a platypus, while all of the other mammals have just one pair.

The study of the evolution of the platypus and other monotremes also throws light on how other mammals perfected their features. For instance, an efficient system of milk production and secretion to feed infants through mammary glands evolved from the antiquated ways of the platypus, which feeds its young by sweating milk out onto its skin.

The duck billed platypus has five pairs of sex determining chromosomes

Drew also finds out how placental mammals branched away from monotremes to evolve other characteristics like birthing of young ones (instead of laying eggs) and developing large brains with a connecting structure between the left and right hemispheres that helped them to communicate better. For instance, a mammalian brain is six to seven times larger than a reptilian brain of similar size. The main reason for the large brain size is the evolution of the cerebral hemisphere. These unique characteristics not only aided their survival but also made mammals thrive and flourish, making them the dominant group of animals on the planet.

The significance of the book lies in the fact that Drew considers himself as a mammal first and a human second, and that has enabled him to explore ancient biological history to understand what it means to be a mammalian.

(This article was first published in Down To Earth's print edition dated March 16-31, 2019)

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