Monday, August 1, 2011

Large eyes evolved to allow people to see better at higher latitudes? Just so.

People whose ancestors long lived at higher latitudes have larger eyes and larger brains than the rest of us, and a new paper, reported here, tells us why.
People who live at higher latitudes have larger eyes and more processing power in their brains to deal with visual information compared with those living nearer the equator, a study suggests.
"As you move away from the equator, there's less and less light available, so humans have had to evolve bigger and bigger eyes," said Eiluned Pearce from the Institute of Cognitive and Evolutionary Anthropology at Oxford University, a lead author on the study.
"Their brains also need to be bigger to deal with the extra visual input. Having bigger brains doesn't mean that higher-latitude humans are smarter, it just means they need bigger brains to be able to see well where they live."
This sounds quite plausible, and of course it could well be true.  But, as with most such evolutionary stories, it's easy to come up with competing explanations that may well be just as valid (or invalid).  And it's also easy to find reasons that the proposed explanation could be wrong.  Even the premise that we "had to have" bigger eyes is less than obvious.  Unfortunately, none of these propositions, including the original one, can be tested.  

So, what could be wrong with the big eyes/big brain theory?  First, let's keep in mind that the idea that there's selective pressure for larger eyes means that people with smaller eyes systematically had fewer children than people with larger eyes because they couldn't see well enough.  They were poorer hunters, or fishermen, or stumbled off the ice into the sea, or didn't see predators on time, or squinted unappealingly when they gazed at their beloved -- or whatever.  (And, the authors imply that food and predators are harder to see in the polar climes than in the dense foliage of the tropics -- also rather strange and less than obvious!)

And this in turn implies that those who couldn't see well got no help from those who could, but as far as is known, people who lived in the arctic 10,000 years ago, when, according to this theory, larger eyes were evolving, traveled in small bands of related people who hunted together and shared food.  It's not like, say, malaria as a selective pressure.  It's easy to see how malaria could reduce fitness, and resistance to the disease could raise it.  

But, for the sake of argument, let's imagine that people deprived their kinfolk of food if they couldn't see well enough to hunt it themselves, or that people with bad eyesight (related to the amount of light) couldn't find mates.  Is darkness likely to be a stronger selective force than other forces in the same environment?  There's just as much endless light as there is endless dark at the poles, not to mention the glare of the sun off the snow in winter months, so, why wasn't the need for retinal protection a strong enough selective force to keep eyes small so they would take in less light (e.g., to protect from retinal cancer or cataracts, or just to reduce glare)?  

And, at higher latitudes the landscape is rather barren most of the time, and there is a lot less to see than in the tropics: nothing but snow and sparse vegetation, and much less colorful life to discriminate.

And why waste nutritional needs to service higher energy big-eyes and the energy hungry neurons to support it?

And, if poor eyesight really did lead to lower fitness, near and farsightedness should also have been selected out of our lineage long ago, and optometrists would all be unemployed today.  We'd never even know bad eyesight once existed.  

Of course what it means to 'see well where they live' is something so open-ended that with this in mind one can probably always make up a story.  Large eyes should imply a larger visual cortex and so on  because there are more retinal cells sending messages and more integration among them needing to take place, which doesn't imply higher intelligence (as the authors do clearly say).  But other factors such as the nature of color vision or light sensitivity (fraction of rods in the retina) also would be relevant.  Bigger heads for reasons unrelated to light could lead to bigger eyes.

And so on.  

The fact, which we would have no reason to question, that some species have larger eyes towards the poles may be unexceptionable.  But what if not all species at high latitudes have large eyes?  Then do we cling to the hypothesis about light by making post hoc excuses for the exceptions, or do we give up on our "had to have" assertions?  

The problems are worth raising and the story worth critiquing because it reflects an important problem in evolutionary biology, which is assertions that far outdistance the data.  These kinds of evolutionary arguments are easy to make, crop up all the time, and often make the news. But it's bad science to propose a hypothesis that can't be tested, and then treat it as though it's true, or as though it's a deep new insight (so to speak).  Or to treat it as if it's newsworthy and a big discovery.  And it's bad journalism to report it as if it's true rather than just speculation.

1 comment:

James Goetz said...

"Grandmother, what big eyes you have!"

"My side of the family lived many generations in mountains, my child," said the big bad wolf. :)