I think this short review does a disservice to Isbell's work. It seems to use a very minor point about pointing and peripheral vision, when it looks like she did much more research to support her views. From this review: http://entertainment.timesonline.co.uk/tol/arts_and_entertai...
"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'."
Edited: I see a lot of comments about snakes not preying on humans. But humans evolved over millions of years from much smaller primates. That's where Isbell thinks the evolutionary arms race took place. You could argue the reason snakes don't prey on humans is because our ancestors won that arms race.
Thanks very much for the additional background material. I agree wholeheartedly that The Atlantic article fails utterly to capture the essence of her thesis, but I also think she's seeing a co-evolutionary causation where it's merely a strong correlation.
Those same environments with higher density of vipers also play host to the widest variety of plant life. In many fruits and leaves, the differences between positive identification and edible vs. inedible have more to do with color and texture than scent.
Additionally, in a canopy environment fraught with the hazards and complexity of 3 dimensional navigation, and lack of a constant "ground scent-base", food source identification via sight would seem to be a more caloric-efficient approach. That's the strategy employed by most tropical birds and they have an even greater freedom of movement amongst the branches. It's also worth pointing out that exclusively ground-dwelling tropical birds such as Kiwi, Kakapo, Cassowary, and the Ocellated turkey all forage by scent and all have undergone reverse evolutionary trade-offs in visual acuity.
From the pop science summaries I thought she was full of crap. But the details are actually quite interesting, and I can see why she has other scientists excited.
I haven't seen the data, but at first blush, I really question the strength of this hypothesis.
Most (all?) animal senses have evolved firstly for finding and obtaining food.
From antelopes to bees, the spectral and polarization sensitivity (if any) of the eyes is perfectly matched to discriminate their primary food source against the background of their visual environment.
Predators evolved binocular vision (for distance estimation), and vergence tracking to estimate future prey pathing movement. In fact, amongst humans, the extremely wide margin in vergence tracking performance between the sexes is indicative of a very early specialization for hunting by men and gathering by women.
Evolving defensively against such an extremely rare event (no known extinct or extant species of snake even actively preyed against humans) would seem counter to the daily duty cycle demands of the human sensorium.
I recall a few studies from colleage and an old friend is currently studying non-human vergence tracking sexual dimorphism. I'll take a look, but here's a quick Google search on the relevant terms:
One of the interesting results is that women are better at short-range vergence skills (e.g. obscured fast facial recognition & foreground-background discrimination) while men are better overall and absolutely excel at long range pathing prediction.
Wait a minute. Snakes don't eat humans and are not predators for humans. Snakes have no interest in attacking humans, they just don't want to be attacked by humans. Snakes also avoid going where there are many humans as they are likely to get killed quickly.
So when there are two predatory species that try to avoid each other as much as possibly, how can there be such a strong effect by one to the other?
As someone with 20/20 vision whose main interest growing up was snake-catching, I have trouble with this hypothesis. Unless a snake is basking or is already moving, you generally won't see it until it is underfoot. In other words, the proposed adaptations don't actually help you see a snake before it is underfoot. And as you mention, snakes are mainly interested in avoiding people.
Wouldn't the dangers of uneven terrain, tripping, and falling be a better explanation of why we are good at looking down? Peripheral vision makes sense if you're also watching out for the family as you walk.
Yes, I've seen the situation where you have a group of people looking at a close-by tree, and one person is shouting: look there is a snake. And everyone is saying "where, where"? I've seen this go on for 10 minutes, and people just don't see the snake. It's like some type of puzzle.
Most cases of snakes biting humans occur when the snake is stepped on. Most cases of snakes eating other animals is when the snake can get its jaws (unhinge-able jaws) around a warm-blooded critter.
Tangent: one of my favorite programming patterns is to roughly mimic the human visual processing. This is super simplified, but humans make snap judgements based on small amounts of data, then progressively make better judgements as better data comes in. (ie, you'll jump away from that brown stick instantly before you even think about if it's really a snake).
In programming terms, it's a narrowing funnel of accuracy. You start off vague but very very quick to catch the important stuff early, then you filter it better and better as you have time.
Furthermore, as Isbell points out, of all the species, only humans point declaratively, and we are much better at following the pointing of others to our visual periphery than to our visual center, and while looking down rather than up. “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?” Isbell asks.
Her answer: snakes. “I cannot think of any other object in the lower visual field that would have been more difficult to see and more unforgiving if missed. The relationship between declarative pointing and the evolution of language is so strong, neurological and cognitive studies find, that the two are, to a degree, interchangeable. And so, Isbell avers, Genesis has it right: the snake made us human.”
I disagree with the assertion that only humans point declaratively. Other methods of "pointing" could include a variety of behaviors apparent in the animal kingdom: perked ears (cats), nose / "catching the scent" (dogs / esp. bloodhounds), flickering tongue (snake).
A cat pricks it's ears to increase fidelity of it's hearing, dogs point with their nose as a learned trait they don't do it in the wild and a snakes 'sees' with it's tongue as it picks up scent trails.
None of these behaviours are naturally evolved communication methods, two are evolved traits that better adapt the animal to it's environment and the other is a learned behaviour.
If creationism ever compliments evolution it is purely by accident.
A cat pricks it's ears to increase fidelity of it's hearing, dogs point with their nose as a learned trait they don't do it in the wild. None of these behaviours are naturally evolved communication methods
This is pure speculation. Who is to say that learned traits can or cannot evolve naturally, over time?
Besides, your use of "it's" is wrong.
An underlying assumption of the statement "pointing declaratively" is that there's some one or some thing to be declarative to (as in an audience, or partner, or fellow pack member). I liked the article, but it did go about assuming that humans are the only creatures who know how to be declarative.
We're probably more verbally declarative than any other species, but I highly doubt we're the only "declarative" species out there.
Not immediately related to the topic at hand, but this is why reading things aloud is so different than silently to yourself.
Right from the second sentence: "Coolly testing hypotheses and assessing evidence across an impressive range of disciplines[…]." Read aloud, that's a lot of 's's in an article about snakes. Fun!
"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'."
There's also a very interesting and much longer review I just submitted: http://news.ycombinator.com/item?id=1195869
Edited: I see a lot of comments about snakes not preying on humans. But humans evolved over millions of years from much smaller primates. That's where Isbell thinks the evolutionary arms race took place. You could argue the reason snakes don't prey on humans is because our ancestors won that arms race.