Fear of Snakes Drove Primate Evolution (http://www.livescience.com/animals/060721_snake_primate.html)
This in interesting article and theory of primate evolution.
"An evolutionary arms race between early snakes and mammals triggered the development of improved vision and large brains in primates, a radical new theory suggests."
Thanks, that was interesting. I notice they didn't also mention the evolution of pit vipers (IR homing snakes) as part of the "arms race". The first reference I could find, http://www.app.pan.pl/archive/published/app44/app44-327.pdf , says the earliest pit viper fossils are from the Lower Miocene, about 23-15 million years ago. These are the probably the most evolved "mammal killers" among snakes--but they post-date much of primate evolution including great apes.
Thanks Elliot, that is interesting about the pit vipers as well. When I read this article, I couldn't help but think about the deep-rooted fears that humanity has of snakes- and the connection to mythological beasts like dragons. It seems that there is an ancient biological imperative at work that still lingers. I find this stuff absolutely fascinating.
How come fear of water hasn't lead cats to drink more booze ?
Facinating. This is very interesting for me since my kids are in the process of learning to see snakes and other camoflaged animals. When my firstborn was four months old, she would lose track of rabbits when they stopped moving. When she turned one, she started noticing alot of wildlife that I overlooked -- usualy birds that were very far away. At age two she started noticing snakes that were poorly hidden, yawning or sprawled in the open. Now at two and a half, she can reliably spot all the snakes at the pet store and at the zoo. Even when they're motionless, have their eyes turned away and are obscured by vegetation.
So yeah, seeing snakes came way after grasping.
After mulling this (and admittedly just a single read & with a casual layman's perspective) I'm more skeptical. Out of mammals binocular vision seems to be common among predators; eyes on sides of head typical of prey animals like deer and rabbits. If you want to see a predator in ambush, wide field of vision seems more important than stereoscopic vision although, yes, the latter might help a bit in discerning nearby camouflaged enemies.
Other mammals with binocular layouts I can think of: bats, sloths. I don't know if either are eaten by snakes, but sloths certainly couldn't do much to avoid them.
Bats and sloths have notoriously bad eyesight. They evolved other senses that have largely replaced their eyesight. Bats use echo location and sloths use vibration sense.
The deal with primates is they evolved outstanding optical convergence, the kind of eyesight that predators have, even though they don't hunt anything that's worthy of it. I mean seriously, the best vision on the planet should have evolved to hunt the world's most elusive prey. Does it make sense that the uber-prey is ...fruit?
What this new hypothesis does is rationalize why a non-predator has awesome predatory eyesight.
BEFORE: Everything that evolved optical convergence is a predator. Thererfore, bananas are highly elusive prey and only a highly evolved predator (like say, a monkey) could possibly bring one down.
AFTER: Optical convergence is an evolutionary solution to the problem of camouflage. Therefore, primates must have needed to detect something camouflaged, like say... snakes!
Fruit bats (flying foxes) don't use echolocation, do have good binocular eyesight, and eat fruits and nectar. Also in common with primates, they're arboreal (of course). Wikipedia notes in fact that there have been theories that fruit bats are more closely related to primates than to other bats, but the article says those theories have been disproved by genetic analysis.
While grasping fruits in the abstract may not require stereo vision, certain types of arboreality might, as might predation on insects. The article only calls the latter hypothesis "unproven" and is pretty vague on the neuroscience behind dismissing the connection between grasping branches and binocular vision. Perhaps fruit bat ancestors, like early primates, were preyed on by snakes? Could be.
Another point, binocular vision has advantages other than near depth perception; it also assists with low-light perception since two eyes gather twice as much light as one. The reason I mention this is that some primates move very slowly and don't leap from branch to branch--but these are also nocturnal. (In any case the initial adaptation--the "reason" binocular vision developed--doesn't necessarily have to apply to everybody who has that characteristic today.)
And another detail, although the article mentions "primates", binocular eyesight is actually common among the Euarchonta, which includes treeshrews and colugos along with primates. (With treeshrews, according to Wikipedia, this only applies to arboreal species.) So regardless of whether we accept the theory or not, it's probably more appropriate to apply it to that whole group
I dunno. The Wikipedia article says flying foxes have 'good' eyesight but it also says they rely on smell to find fruit, 'crash' into the tree and then feel around for it with their feet. The Encycopedia Brittanica only says that they use sight (possibly damning their eyesight with feint praise?) I'm inclined to believe they have good eyesight for a bat but I doubt that they have the level of eyesight in question.
The neuroscience, basicaly, is that the human brain is composed of structures that evolved at different times. Thus the so called 'reptilian brain' refers to the lizardly underpinnings of our minds and is, more or less, a reptile brain with a chimp brain bolted on top, with the human higher function stuff bolted on top of that. That's physiologicaly.
Developmentaly, there's a similar phenomenon where kids go through the various stages of human evolution sequentialy as they age. That's why I remarked up thread that my kids learned to grasp way before they learned to see snakes. It's not hard science but it's a a rough aproximation.
Anyway, assuming the Neuroscience is sound, it makes sense. And I especialy liked it because it brought to mind my kids when they knew how to grab but not how to guage distance. (It's the stage where kids make Jedi like grasping motions at far away objects and it's hillarious! ) :D
You might try googling fruit bats, which contrary to what I'd thought aren't 100% identical to flying foxes. I think you'll find the group as a whole does have good vision as evidenced by behavior and brain development.
Also I'd say that grasping per se should be looked at separately from grasping as a form of locomotion and food gathering in an arboreal context.
Ah, I didn't realize until just now that this is an old article. The publication by Isbell is available online at her website http://www.anthro.ucdavis.edu/lynneisbell/PDFs/isbell_snakes_2006.pdf
I haven't read it, but some of the reviews and glosses describe the neuroscientific detail it goes into. She's also published a book on her theory: http://www.hup.harvard.edu/catalog/ISBFRU.html?show=reviews
One thing that puzzles me, she herself talks about how prosimians (lemurs and the like in Madagascar) aren't scared of snakes. But they do have forward-facing eyes and many of them leap through trees. She attributes the highly-developed eyesight of old world monkeys to an "arms race" with poisonous snakes, but it's not clear if she might leave room for the initial adaptation of binocular eyesight to be due to other causes.
EDIT: Still browsing around, found this (http://query.nytimes.com/gst/fullpage.html?res=990CE5DB1F3EF930A3575AC0A9609C8B63&sec=&spon=&pagewanted=all) NYT article by Isbell. (She says tree shrews and fruit bats don't have especially good eyesight. Hmm, sources are all over the map on that.)