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A witty romp through evolution reveals a trove of curiosities in mammalian biology

I, Mammal: The Story of What Makes Us Mammals

Liam Drew
Bloomsbury Sigma
2018
336 pp.
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In today’s world of alternative facts and attenuating funding of research, scientists need to communicate with the public as we never have before. Joining the ranks of a burgeoning number of professional scientists turned professional writers, Liam Drew, a practicing neuroscientist for 12 years, has put down his microfuge tubes and taken up the charge. His first book, I, Mammal: The Story of What Makes Us Mammals, stands out as a clear, conversational (sometimes to a fault), and engaging work that is especially good at explaining how evolutionary biology works.

Drew was drawn to the topic of mammalogy through personal experience. In the book’s introduction, “My family and other mammals,” he describes witnessing his wife give birth, watching his newborn daughter suckling in the neonatal intensive care unit, and the painful encounter with an errant football that led him to question the wisdom of the external placement of the testes.

Beyond the descent of man’s gonads, Drew’s tour of Class Mammalia ranges into early mammalian evolution, sex determination, reproduction, lactation, parental investment, warm-bloodedness, and the complexity of the brain.

Drew is an enthusiastic advocate for the field of “evo-devo,” the enthralling combination of developmental and evolutionary biology. He is particularly captivated by August Weismann’s 19th-century conceptualization of “germ plasm” (the idea that heritable information is transmitted only by germ cells), writing that it should have been a “Copernicus moment” for biology. “[A]ppreciating the germline takes our everyday notions that we own our sperm or eggs and turns it back to front.”

In a cleverly titled chapter on sex determination (“Y, I’m male.”), Drew credits the American geneticist Nettie Stevens with the discovery of the X and Y chromosomes in 1906. Stevens’s work, as he describes, not only explained chromosomal sex determination but also provided incontrovertible evidence that the chromosomes played a role in hereditary traits. (The discovery of the genetic basis of male sex determination would take nearly another century.)

Drew’s narratives of the essential characters of mammals—and his discussion of the evolution of marsupials and monotremes in particular—are quite engaging and entertaining. “Famously,” he writes “the first academic description of a platypus questions whether it was a hoax … an outright assault on the neat categories Linnaeus had spent years establishing.”

Drew is sensitive to the tension that the past half-century has seen between the molecular/cellular and the whole-organism approaches to taxonomy. He ultimately comes down in favor of molecular approaches, observing that “Nothing about limbs and genitalia makes their construction especially useful for inferring their evolutionary history.” However, in a nod to the value of morphology, he does acknowledge that “appreciating their development can help increase evolutionary understanding.”

Some fascinating aspects of mammalian genetics were decidedly missing from I, Mammal, including the silencing of one of the two X chromosomes that occurs in most cells of most female mammals (X-chromosome inactivation), the expression of only one of the two copies of a substantial number of genes inherited (genomic imprinting), and the phenomenon of epigenetics.

As a developmental biologist, I would also have had trouble telling the story of mammals without mention of reproductive cloning and Dolly the sheep. However, selected references for each chapter, as well as an index, allow the reader to follow up on interesting topics.

KEVIN SCHAFER/ALAMY STOCK PHOTO

The author’s encounter with an errant football led him to question the wisdom of external testes, as seen here on a stump-tailed macaque.

Writes Drew, near the end of the book: “I thought at one point my big take-home on mammals was going to be about how the traits that define them are so wondrously adaptable. … Instead, what’s important is the combination of individual traits.” It’s hard to argue with his conclusion, but I think the emphasis on adaptability is misplaced; it’s an obvious consequence of natural selection. The same, after all, could have been written about almost any other group of organisms as well.

Ultimately, however, I, Mammal is just the sort of book that can spark a love of nature and an appreciation for the ever-changing, eternally correcting march of science. It brought this reviewer back to a time spent wondering about the birds chirping outside and the creatures lurking in tide pools, rather than dealing with the more tedious tasks required of a practicing biologist.

The book is not a plea to preserve the enormous diversity of mammals, although it does bring up curiosities of mammalian biology that will no doubt delight and surprise even practicing scientists. I hope it will give every reader a taste for more.

In the end, I, Mammal serves more as a reminder of what we owe to the cataclysmic extinctions of the past. The idea that our species is likely to have only a short tenure on Earth is one that we should always keep in mind.

About the author

The reviewer is at the Department of Biology, San Francisco State University, San Francisco, CA 94132, USA.