Re: Evolution question - the eye.

On May 17, 10:31 pm, William Morse <wdNOSPAmo...@xxxxxxxxxxxxxxxx>
wrote:
On Tue, 12 May 2009 08:07:57 +0100, Mike Dworetsky wrote:
"John Smith" <bobsyoung...@xxxxxxxxx> wrote in message
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"Mike Dworetsky" <platinum...@xxxxxxxxxxxxxxxxxxxx> wrote in message
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"John Smith" <bobsyoung...@xxxxxxxxx> wrote in message
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Just wondered this, this morning.

The evolution of the eye is amazing.
why, over so-so many generations, are we still having such a large
variety
of difficulties with seeing?

Many of the difficulties or problems of vision were not problems until
comparatively recently, for example, 600 years ago it was not usually
necessary to be able to read in order to survive and thrive.  But
these days, even small imperfections in visual performance create
difficulties.

Tens of thousands of years ago there were also some survival benefits
to being nearsighted (for some) as they could stay at home/in the cave
and make
fine flint tools.  As long as there were enough eagle-eyed hunters
with bows
and arrows the tribe would be able to eat.  And for a lot of the
hunting and
gathering, visual acuity was not a problem as spearing a mammoth or
collecting nuts and berries were activities done close up.  And the
women
(if stereotypes were actually true) stayed close to home and did
woman-things, many of which did not require keen eyesight.

Few humans in the "natural state" lived past around 40, so age-related
loss
of near-field vision that is common now in older people was rare.

So unlike birds of prey, visual acuity was not strongly selected for
as a survival trait.  The lifestyle and diet of an eagle or hawk
demands perfect
eyesight for hunting, and any such bird with imperfect vision would be
less
likely to reproduce.

Makes sense.
The thought was triggered by my old age eyes - but I know of kids as
young as four needing to wear coke bottle glasses.

Humans living is small social units (extended families, tribes, bands)
goes back a very long way, even the Neandertals lived in groups (as
evidenced by social customs like rituals at burial).  In such a group
eyesight is not 100% at a survival premium.

A hunter or warrior may need keen eyesight, but a flint-knapper can be
nearsighted yet valuable to the group.

Here I think you are onto something. There is likely to have been a range
in eyesight among humans for quite a while, with many tasks not being
affected, and perhaps even enhanced, by near-sightedness.

The reproductive peak among such peoples is likely to have been in the
late teens and early twenties, and most people would be dead by forty
before problems such as presbyopia (middle age inability to accommodate
focus to nearby objects), cateracts, retinal degeneration, etc could be
a problem.

You need to differentiate between life expectancy at birth and life
expectancy at say age 30 among hunter-gatherers. It is not unusual for
adults in hunter-gatherer societies to live to age 70. In fact the usual
explanation for menopause among women is that they did routinely live
past age 50 but were too valuable to the group at that age due to their
knowledge and assistance to their grandchildren to risk having them die
in childbirth. (I am phrasing this argument backwards - I doubt there was
much group selection - but you I hope you get my drift).

So it remains an open question as to why older humans should lose their
ability to focus on nearby objects. One would think that they would be
less involved in activities such as hunting game, and gathering food far
from the village where spotting say a berry patch from distance would be
valuable. They would seem to be more involved in caring for children and
teaching crafts, activities where one would expect them to lose their
ability to focus on far away objects.

You are thinking about the problem backwards. You are imagining that
evolution selected adults to lose their ability to focus on close
objects (to accomodate) and wonder why. Instead think about what age-
related changes to the physiology of vision cause that to happen where
evolution doesn't have any ability to change the fact. Many things
happen the way they do not because evolution selected for them but
because that is simply the way the machinery works.

The details are quite complex and still debated. Generally it has
been considered a form of rigidity in the lens so that it is unable to
change its shape. There is some new (2005) research that indicates
that human vision is rather different in this regard than non-human
primates and that lifelong continuation of growth of the lens
contributes to the change. See
The mechanism of presbyopia.
Strenk SA, Strenk LM, Koretz JF.
Prog Retin Eye Res. 2005 May;24(3):379-93.\
http://www.ncbi.nlm.nih.gov/pubmed/15708834

.

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