For nearly two million years, the human story has been defined by dispersal and migration. Increasing behavioral plasticity and cultural buffering allowed hominins to travel across land into new and unfamiliar environments outside of Africa. Less well understood is whether, how, and when premodern humans navigated water. Although the earliest preserved watercraft dates to the Holocene, a growing body of evidence indicates the presence of early humans and even earlier hominin species in island locations only reachable by over-water travel during the Pleistocene. This evidence challenges a long-standing consensus in paleoanthropology that held that the world’s oceans and seas were passive barriers that only recent populations of modern Homo sapiens could traverse.

In Human Dispersal, Human Evolution, and the Sea: The Palaeolithic Seafaring Debate, John F. Cherry and Thomas P. Leppard deliver a long-overdue critique of this terrestrial bias. Although they argue that the sea was not necessarily the impassible barrier we have often thought it to be, evidence of hominin dispersal over water should be subject to standards of evidence that have not always been consistently applied. Cherry and Leppard argue for a rigorous “Gold Standard” of evidence of hominin dispersal over water. This standard requires, first, that paleogeographic reconstructions prove that a given island environment was insular at the time hominins arrived. Second, the standard demands incontrovertible archaeological evidence of hominin occupation on the island. This evidence should ideally include all the following: diagnostic artifacts within undisturbed strata, reliable absolute dates of those strata, and associated hominin fossil remains. As the authors note, these standards are seldom met in island archaeology.

The book includes a preface and six chapters. In the opening chapter, Cherry and Leppard frame the debate by highlighting the cognitive, behavioral, and technological requirements for our genus to expand its range size beyond Africa. In the second chapter, they explore methodological problems and interpretive frameworks, providing a comprehensive critique of Paleolithic archaeological interpretation, and they detail the rigor required to meet their Gold Standard. Chapters 3 and 4 provide meticulous descriptions of the evidence from two regions of greatest interest: Island Southeast Asia (ISEA) and the Mediterranean Sea. Cherry and Leppard carefully gauge this evidence against the Gold Standard.

In chapter 5, the authors examine modes and intentionality of over-water dispersal, the cognitive and behavioral requirements thereof, and even physiological implications of over-water travel for premodern Homo. Particularly interesting is their discussion of the application of the adaptive flexibility hypothesis to hominin water crossings that David Gaffney discussed in his 2021 article “Pleistocene Water Crossings and Adaptive Flexibility within the Homo Genus,” published in the Journal of Archaeological Research (29[2]:255–326). Finally, Cherry and Leppard conclude in chapter 6 that whereas limited forays of short distances over water occurred in the Mediterranean after roughly 200 KYA, evidence from ISEA suggests waif dispersal events leading to allopatric speciation beginning as early as one million years ago. This mode of dispersal does not rely on sophisticated seafaring technology, but it was sufficient to bring hominins to several islands in the Lower Paleolithic. Modern Homo sapiens seem to have been the only hominin capable of intentional large-scale and long-distance maritime travel.

The text is supplemented by clear figures and tables, including maps, photos, and illustrations of fossils and chipped stone artifacts. Although the illustrations are perfectly crisp and easily interpreted, some of the photos of lithic artifacts, shorelines, and people involved in maritime activities lack the same level of clarity—though this is a minor issue and does not detract from the overall work.

For a concise book, Cherry and Leppard are surprisingly detailed in their analyses. They craft and compress an interdisciplinary synthesis into a succinct volume. Although the authors hold a clear position and defend it effectively, they also outline differing opinions and respond to them in good faith. For example, several sites on islands in the Mediterranean fail to reach the authors’ threshold for the Gold Standard, but they explain their reservations clearly and objectively.

Exhaustive, but far from exhausting, this book is a rare combination of academic rigor and accessibility. By seamlessly interweaving paleontology, archaeology, evolutionary biology, and even physiology, Cherry and Leppard have produced what will undoubtedly become the definitive text on the Paleolithic seafaring debate. Central to their analysis is the “Sagan Standard”—extraordinary claims require extraordinary evidence. This is a standard we would be well served in applying to other areas of paleoanthropological research beyond this topic. I commend the authors for their meticulous attention to detail and believe this book should be required reading for anyone seeking a more nuanced understanding of hominin dispersal.