

We see so well because of snakes? - dctoedt
http://www.theatlantic.com/magazine/archive/2010/04/cover-to-cover/7977

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isleyaardvark
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...](http://entertainment.timesonline.co.uk/tol/arts_and_entertainment/the_tls/article6864376.ece)

"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.

~~~
bonsaitree
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.

------
bonsaitree
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.

~~~
edj
In what way is vergence different in men and women? Do you have a source? I'd
love to read up on that.

~~~
bonsaitree
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:

<http://tinyurl.com/yhbwjde>

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.

------
maxklein
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?

~~~
ericb
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.

~~~
maxklein
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.

------
cschneid
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.

------
indiejade
_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).

[edit] message for the downmodder: wtf?

~~~
dazzawazza
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.

~~~
indiejade
_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.

------
Arubis
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!

------
jeromec
_Furthermore, as Isbell points out, of all the species, only humans point
declaratively,_

Out of all species there are a lot of things humans do which are unique to
humans. That doesn't mean such things evolved from snakes.

