What makes us mammal? Hair, milk, big brains, complex teeth, warm blood — all of these are part of it. But the defining feature of a mammal, the thing that separates us from everyone else, is all in our heads. Feel around with your hand where your jaw joins your skull, palaeontologist Steve Brusatte encourages in this lively history of our kin. For us, jaw meets skull directly in a hinge known as the dentary-squamosal connection; for reptiles and birds, the other groups of land living animals, this joint is made up of three other bones. Fascinatingly, we still have those three bones, they’ve just shrunk and migrated to become the tiny bones of the inner ear (the ones that pop up every now and then in trivia questions; “What’s the smallest bone in the human body?”), enabling sophisticated hearing.
Brusatte started out a dinosaur expert, a topic he explored in his first book. But he was turned by the puzzles of the origins of mammals. The story starts 325 million years ago, in the coal swamps of the Carboniferous period, when the lineage that would eventually spawn mammals separated from the lineage that would give us reptiles and dinosaurs (and birds). That moment is identified by the loss of a hole — a synapse — in the skull. Mammals are synapsids; they have one hole behind the eyes. He traces a tangled evolutionary history, through creatures known as pelycosaurs and cynodonts, multituberculates and morganucodontids, and onwards through the dinosaur extinction to the flourishing of mammals over the past 66 million years. He follows the rise of modern mammals, once again distinguished by happenings in the skull, this time the evolution of tribosphenic teeth, or teeth with classic, three-pronged shape that enables us to chew a variety of different foods.
As you can probably tell, Brusatte doesn’t shy away from the technical concepts, as happy delving into anatomy and obscure branches of the mammal tree as he is talking about superstars like mammoths and sabretooths. The result is a thorough and enriching history, a portrait of the current state of the science. Nor is it confined to fossils and evolution; Brusatte also conjures the excitement of fossil hunting, such as the discovery of tiny, fingernail sized skulls, and the characters who have made some of the most important discoveries, such as the Polish palaeontologist Zofia Kielan-Jaworowska, who led all-female expeditions to the Gobi dessert on the hunt for mammal fossils in the 1950s. And he doesn’t overlook the dark side of science. Take, for instance, William Caldwell, an officer in New South Wales in 1884 who conscripted 150 Aboriginal people to kill 1,400 platypus and echidnas to find out what makes them so strange; “imperialistic science at its worst,” Brusatte notes.
A story about mammals is also a story about the world. Some of the bits I found most fascinating were side stories on how the earth’s climate has changed, and particularly what caused it. For instance, the ice world of the Carboniferous period was caused by newly-evolved land plants taking up so much carbon dioxide that they cooled the atmosphere. The greatest extinction ever, at the end of the Permian, was driven by vast outpourings of lava in Siberia, when temperatures rose 5-8 degrees over tens of thousands of years — chillingly, a much slower rate than the earth is warming today. Climate change has been driven by the opening of seas, or, as in our most recent ice ages, the weathering of the Himalayas (weathering rock removes carbon dioxide from the atmosphere) combined with solar cycles. Each fluctuation is like a sieve; some creatures don’t pass through, other survive to flourish and reshape the world. It’s a pressing message when we’re now sifting the world’s ecosystems once more.
Gay rating: not gay.