Joanna Bourke begins What it Means to be Human with a curious letter from "An Earnest Englishwoman" to The Times in 1872, enquiring whether women might be classified as animals. It transpired that violence against animals was more severely dealt with in the courts than violence against women. This is just one of many ironic twists in a wide-ranging textual quest to discover what "being human" has meant from 1791 to the present. Bourke's book reveals the extent to which the term "human" has been politically loaded, historically complex and theoretically ambivalent.

Humanistic and Enlightenment theory was, of course, conceived primarily with men in mind. Consequently, when Mary Wollstonecraft and other radical women asserted the rights of women, they contested masculinist definitions of "humanity". Bourke notes too how Wollstonecraft made the connection between the exclusion of women and the exclusion of slaves. This empathetic link would be extremely important in radical thinking in the 18th and early 19th centuries and would help to bring white women into the anti-slavery movement. By the late 19th century, however, sections of the women's movement were prepared to argue the spiritual superiority of women as a basis for a redefinition of the terms in which humanity was conceived. Bourke does not explore the implications of this claim to be distinguished from men as a group; instead she is concerned with the tortuous course of the demarcations between "animals" and humans. Her book charts the ways hierarchies among humans and between humans and animals have been socially created and politically contested.

Bourke rescues many forgotten contestants from relative obscurity, including the American diet reformer Sylvester Graham, the esoteric mystic Anna Kingsford and her devotee Edward Maitland, whose concern for animals led him to challenge the UK's scientific establishment. Bourke observes wryly that some advocates of animals preferred them to human beings. In contrast, she notes, the socialist Henry Salt, who influenced Mahatma Gandhi, sought to humanise the conditions of animals and human beings alike.

Reaching beyond history's customary scope, the book traces not only shifts in the boundaries of how being human has been conceived in the past, but looks at the bewildering implications of medical science. Taking us into recent developments that involve transplanting animal organs (xenotransplantation), and the transference of human genetic material into animals in order to make transplants more feasible, Bourke looks at how physical changes might interact with ways of seeing in the future. Towards the end of her book, she enquires when the consequent "chimeras" might be regarded as human. We have entered unknown territory.

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What it Means to be Human ingeniously subverts assumptions of a clear-cut notion of "humanity". Bourke successfully undermines any complacency about absolute distinctions. It is a case that has perhaps been strengthened by the possibility of the existence of differing human species. She contrives to handle abstract arguments with clarity and skill, and her extraordinary knowledge of curious ideas and bizarre habits makes for a fascinating read.

Yet I was left with a feeling of unease when I reached the end. Bourke's emphasis on unpicking meanings, combined with the kaleidoscopic perspectives that flicker into view only to fade out, left her arguments suspended. The awkward contradiction in chronicling fluidity is that in order to understand any particular facet it is always necessary to freeze the ongoing moving scenario in order to capture it and analyse. It is unfortunately the case that when we hold the momentary still, it inevitably interrupts the flow. While this is annoying, we require such pauses if we are to ponder specificities and follow through their implications. How to do both at the same time is surely one of the most challenging dilemmas of history.

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Bourke is impressive in demonstrating that "being human" is far more problematic than it might appear; however, her effort at resolution by asserting uniqueness and the desire for connection, while commendable, is not completely convincing. Moreover, although she asks what is to be done, her conclusion is not entirely clear. Nevertheless, What it Means to be Human is intellectually stimulating in scope, and it is a delight to read such an ambitious book. Bourke deserves congratulations for bravely going where many historians would fear to tread. She also deserves many readers prepared to engage critically with the important issues raised by her quest to deconstruct "being human".