As a professor, I am always on the lookout for themes to engage students in courses about biodiversity and the evolution of life. So far sex, reproduction, levels of organisation (from molecules to ecosystems) and energy transfer have been useful leitmotifs for my courses. And, now, skeletons ¨C almost as good as sex, one could argue.

I have no idea why this did not occur to me long ago because skeletons have, for decades, been right under my nose. Every day I walk to work through Skeleton Park, the burial ground of thousands of immigrant workers who died here in Kingston, Ontario during the early 1800s. And in the 1960s, I had a job preparing skeletons for the Royal Ontario Museum ¨C mostly tiny bats but once a huge grizzly bear. But for me, at least, skeletons always had a supporting role ¨C I had missed the main event so wonderfully described in this book.

What an engaging story those skeletons ¨C and the authors of this book ¨C have to tell about the history of life, a story woven together here by tales of discovery and discoverers. Skeletons are a recent development, first appearing in the fossil record ¡°only¡± about 550 million years ago, during the most recent 15 per cent of the Earth¡¯s history. Those first skeletons mark the start of the Phanerozoic aeon, which continues to the present, when most of the life forms we now recognise evolved. We know so little about life before the Phanerozoic because that life, lacking skeletons, left few visible fossils. Now even we humans alone contain more than 70 million tonnes of skeleton, structures that arguably shape our lives.

It seems likely that skeletons first appeared as external shields that evolved in response to predation. From that start, the evolution of skeletons from outside to inside, and from small to large, can be characterised as arms races between predators and their prey. In species with external (exo-)skeletons, the evolution of size was limited by the need to shed that covering periodically, making the animal vulnerable to attack, and by the constraints of mechanical design. Inside, however, a skeleton can grow with the animal (or plant), with many of the design constraints relaxed. Internal skeletons also provided scope for new modes of locomotion, including flight. The largest marine, terrestrial and aerial animals that ever lived all had internal skeletons.

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Skeletons have had an immense influence on life as we know it. Thus the tiniest shells that accumulated en masse in ancient seas changed the Earth¡¯s chemical cycles and eventually allowed the flourishing of life forms. And coral skeletons together formed the largest reefs, the study of which was one of the building blocks of Darwin¡¯s ideas about the changing Earth. As the authors sadly note, we are now in danger of losing those reefs to climate change, leaving the Earth but a skeleton of its former self.

Robert Montgomerie is professor and research chair in evolutionary biology at Queen¡¯s University in Canada.

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Skeletons: The Frame of Life
By Jan Zalasiewicz and Mark Williams
Oxford University Press?
320pp, ?18.99
ISBN 9780198802105
Published 22 March 2018