February 08, 2012

Big eye sockets compensate for low ambient light levels in humans

Biology Letters doi: 10.1098/rsbl.2011.0570

Latitudinal variation in light levels drives human visual system size

Eiluned Pearce and Robin Dunbar

Ambient light levels influence visual system size in birds and primates. Here, we argue that the same is true for humans. Light levels, in terms of both the amount of light hitting the Earth's surface and day length, decrease with increasing latitude. We demonstrate a significant positive relationship between absolute latitude and human orbital volume, an index of eyeball size. Owing to tight scaling between visual system components, this will translate into enlarged visual cortices at higher latitudes. We also show that visual acuity measured under full-daylight conditions is constant across latitudes, indicating that selection for larger visual systems has mitigated the effect of reduced ambient light levels. This provides, to our knowledge, the first support that light levels drive intraspecific variation in visual system size in the human population.



Orang More said...

Neanderthals had bigger eye sockets, could that be an adaptation to high latitudes and "weaker" light?

Stephen said...

If my laymans understanding of photography is corect the way to improve the light gathering ability of an eye in poor light would be to increase the lens and apetuture size. Increasing the total eyeball size would just spread the same amount of light over a larger retinal area decreasing the amount of photons hitting each individual rod or cone unless the lens and apeture scaled up too. So increasing size would mainly increase acuity rather than light gathering ability.

fraherjsalz said...

It does make sense. However, it does not seem to be the case in Saharian and/or Middle Eastern tribes, where eye sockets are remarkably more developed than in latitudes where light is dimmer. Perhaps, another incidious fact could be day/night habits.

Bob Dunning said...

"Mad dogs and Englishmen go out in the midday sun".

"Thus mellowed by that tender light, which heaven to gaudy day denies."