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on Snakes and Us

In The Fruit, the Tree, and the Serpent, Lynne A. Isbell weaves together facts from anthropology, neuroscience, palaeontology and psychology to explain that our emotional connection to snakes has a long evolutionary history. This history, Isbell says, is responsible not only for snake fear – the serpent in the garden of Eden, the worldcreating Rainbow Serpent of Australian aboriginal myth and B-grade cinema fare – but also for our keen primate vision and perhaps even our facility with language.
The crux of Isbell’s Snake Detection Theory focuses on vision and is straightforward: “Visual systems are more developed in those primates that have shared the longest evolutionary time with venomous snakes and least developed in those primates that have had no exposure at all to venomous snakes”. In a cool, knowledgeable voice, Isbell unpacks this statement, supplies it with evidence, and counters possible objections. Her fascination with the snake-and-primate nexus, the product of a “ten-year near obsession to solve what gripped me as no murder mystery had ever done before”, propels the book.
Primates, especially monkeys, apes and humans, see better than most mammals; our vision boasts depth perception and the ability to detect colour. Brain areas important in enabling these specializations are the P and K pathways. The P pathway gives us acuity and colour, while the K pathway aids in preconscious detection of objects. When, walking along, we freeze, and only then notice a snake on our path, we are using our K and P pathways respectively.
Snakes matter to primates because they eat us, or constrict us to death. Because of this, our primate ancestors “uniquely benefited from clearly seeing and identifying objects that were close by and in front of them”. Monkeys tested in the lab were nonchalant when shown digitally altered videos of other monkeys responding fearfully to flowers. When instead they were shown real tapes of monkeys who were scared of snakes, even if snake-naive themselves, they too became afraid. Primates are evolutionarily prepared to fear, detect and respond to snakes.
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Isbell’s detective work in support of the Snake Detection Theory involves chronology and biogeography. Snakes and primates co-evolved at just the right times and in just the right places to make sense of the evolving vision changes that Isbell describes. Comparative neurological analysis nicely tracks the animal co-evolution: New World monkeys, who had less exposure to venomous snakes, exhibit more varied visual systems than do their Old World counterparts.

Why did other snake-prey species not experience similar increases in visual acuity? For many, their smell-based foraging styles didn’t allow it: “Animals that were insect or plant predators simply could not afford weaker olfactory systems, even if they could benefit from visual expansion”. Primates are to a large extent fruit-eaters, and fruit is strongly scented, so that a reduction in smell capacity was not dangerous for them. Importantly, fruit-eating involves glucose ingestion, and glucose prepares the brain in certain ways that, energy-wise, underwrite the development of the P and K pathways. In sum, snakes are the key selection pressure, and frugivorousness enables the primate brain to respond.

Isbell’s case is convincing. She cleanly distinguishes between evidence-rich facts and evidence-poor speculation, so that readers come to trust her scholarship. But the book is neither well-suited to nor intended for the casual reader. Though Isbell offers an elegant graphic summary of her key points near the book’s end, readers who are not well versed in neuroscience will need fierce concentration to grasp her theory. The book is always rewarding, however. Her snake tales from long years in the bush are informative and often funny. Isbell writes solid evolutionary science and also takes calculated risks. Aware that she swaddles nearly everything of interest about primate and human evolution in snake theorizing, she embraces a single-factor explanation. Always the marks of good science, testable predictions stud the text and may productively occupy a new generation of researchers.

And what of a link between snakes and the origins of language? Humans are the only species to point declaratively, Isbell says, though some ape researchers will disagree on this. Studies show that we’re much better at following a point in our visual periphery than our visual centre, and while looking down rather than up. Isbell asks, “What was it outside central vision and in the lower visual field that was so urgent for our ancestors to see that it caused neurological changes to enable us to turn automatically in the direction of a gaze and a pointing finger?”. Anyone with a glucose-rich primate brain who has read this far can supply the one-word answer to Isbell’s query. Thanks to snakes, declarative pointing emerged, and led, she suggests, to a cascade of events that equipped Homo sapiens with language abilities. In Genesis, the serpent tells Eve that if she eats fruit from the Garden, her eyes will be opened and she will see. Smart snake.

Lynne A. Isbell
THE FRUIT, THE TREE, AND THE SERPENT
Why we see so well
207pp. Harvard University Press. £33.95 (US $45).
978 0 674 03301 6


Barbara J. King is Chancellor Professor of Anthropology at the College of William and Mary in Williamsburg, Virginia. Her book Being with Animals: Why we are obsessed with the furry, scaly, feathered creatures who populate our world will be published next year. She has a column at www.bookslut.com