Sharks were never far from our minds as we grew up on the beach in Adelaide. Although attacks were rare, they were real. My grandfather was witness to the fatal mauling of a swimming instructor in the 1930s, and later a friend from university was killed while scuba diving off Port Noarlunga. Yet for the most part sharks were more an idea than a living presence. Other than an unsettlingly close encounter with a bronze whaler when I was 20, my interactions with the creatures as a young person were mostly confined to observing gentle Port Jackson sharks, wobbegongs and grey nurses while snorkelling and diving.
This tendency to see sharks only through the prism of their vastly overestimated threat to human life obscures the many wonders of these remarkable animals. As John Long, professor of Palaeontology at Flinders University, explains in his wonderfully rich book, sharks are one of our planet’s great success stories. Incredibly ancient, enormously various in their design and possessed of a range of remarkable adaptations, they also play a vital role in our ecosystems. They regulate food chains and maintain the health of sea grass beds, coral reefs and kelp forests by controlling fish populations; and they contribute to the carbon cycle by transporting nutrients between different parts of the ocean.
The Secret History of Sharks is both an evolutionary and a personal history. It is an account of the immensely long process of adaptation and change that has given rise to more than 1,200 species of sharks, rays, skates, sawfish and chimaeras that populate the world’s oceans and waterways today; and it’s the story of Long’s lifelong fascination with sharks, ancient and modern.
We begin almost half a billion years ago, when the first traces of sharks appear in the fossil record. These early creatures are fugitive presences, known only from a handful of scales and fin spines. They seem to have borne little resemblance to the ones we know today. Perhaps a foot in length, they had the tiny, pointed placoid scales that distinguish sharks from other fish (and grant them the extraordinary efficiency in the water that makes them so formidable); but they may have lacked teeth. It took another 50 million years before shark teeth start to become abundant in sedimentary rocks. From that point on, sharks began to proliferate through the oceans and freshwater environments, developing feeding strategies and expanding into new ecological niches.
Long’s narrative also offers a deep-time history of the planet. As the continents drifted across its surface, creating and eventually unmaking ancient oceans such as the Tethys and the vast Panthalassic, marine environments changed as well. The shallow, trilobite-infested seas of the Silurian gave way to oceans crowded with terrifying marine dinosaurs in the Jurassic and Cretaceous; and again to seas populated by human-sized penguins, whales and the ‘ultimate apex predator’, the more than 50ft long Megalodon, that endured until only a few million years ago.
This perspective makes both the vulnerability and remarkable resilience of life in general, and sharks in particular, startlingly apparent. Over and over again, waves of extinction rippled out across the planet, and each time sharks found ways to adapt. Some of these adaptations are truly remarkable: the Helicoprions that ruled the oceans of the Permian boasted teeth on their bottom jaw that grew in a spiral like a buzzsaw that they used to eat squid and nautiloids.
Equally striking is the role sharks have played as drivers of evolution. The rise of the massive cardabiodontidae, one of the first modern sharks, in the Cretaceous, may have contributed to the extinction of ichthyosaurs and pliosaurs. Similarly, Long argues that the immense size of whales may have come about as part of an arms race with the enormous Otodus lineage that culminated in Megalodon. (But he also suggests that the size of Megalodon and its ancestors may have been a side effect of their taste for intrauterine cannibalism, in which the baby sharks consume each other while still inside their mothers.)
While sharks have helped shape marine environments for hundreds of millions of years, it is we who are now reshaping their world, with catastrophic consequences. One recent study found that a third of all shark, ray and chimaerid species are at risk of extinction; other studies suggest that more than half of all pelagic shark species and nearly two-thirds of coral reef sharks are in danger of being wiped out. The major driver of this calamity is overfishing; but habitat loss, rising temperatures, pollution, plastics and unutterably cruel practices such as shark-finning also play a part.
Long’s solutions to these challenges – including encouraging shark ecotourism – tend to emphasise the local over the global. While effective conservation policies frequently depend on empowering disadvantaged communities, it would have been nice to see more space devoted to the urgent need to reduce carbon emissions and prevent destructive fishing practices. Nevertheless, the author’s insistence that the long story of sharks and their capacity for adaptation and change holds valuable lessons for us is salutary.
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