Where d'you get those peepers
Dawkins, Richard, Where d'you get those peepers?., Vol. 8, New Statesman & Society, 06-16-1995, pp 29.
Creationist claims that organs like eyes are too complex to have evolved naturally
are way wide of the mark, says Richard Dawkins.
In fact, eyes have evolved many times, often in little more than a blink of geological
Creationism has enduring appeal, and the reason is not far to seek. It is not, at least
for most of the people I encounter, because of a commitment to the literal truth of
Genesis or some other tribal origin story. Rather, it is that people discover for
themselves the beauty and complexity of the living world and conclude that it
"obviously" must have been designed. Those creationists who recognise that
Darwinian evolution provides at least some sort of alternative to their scriptural theory
often resort to a slightly more sophisticated objection. They deny the possibility of
evolutionary intermediates. "X must have been designed by a Creator," people
say, "because half an X would not work at all. All the parts of X must have been put
together simultaneously; they could not have evolved gradually."
Thus the creationist's favourite question "What is the use of half an eye?"
Actually, this is a lightweight question, a doddle to answer. Half an eye is just 1 per
cent better than 49 per cent of an eye, which is already better than 48 per cent, and the
difference is significant. A more ponderous show of weight seems to lie behind the
inevitable supplementary: "Speaking as a physicist, I cannot believe that there has
been enough time for an organ as complicated as the eye to have evolved from nothing. Do
you really think there has been enough time?" Both questions stem from the Argument
from Personal Incredulity. Audiences nevertheless appreciate an answer, and I have usually
fallen back on the sheer magnitude of geological time.
It now appears that the shattering enormity of geological time is a steam hammer to
crack a peanut. A recent study by a pair of Swedish scientists, Dan Nilson and Susanne
Pelger, suggests that a ludicrously small fraction of that time would have been plenty.
When one says "the" eye, by the way, one implicitly means the vertebrate eye,
but serviceable image-forming eyes have evolved between 40 and 60 times, independently
from scratch, in many different invertebrate groups. Among these 40-plus independent
evolutions, at least nine distinct design principles have been discovered, including
pinhole eyes, two kinds of camera-lens eyes, curved-reflector ("satellite dish")
eyes, and several kinds of compound eyes. Nilsson and Pelger have concentrated on camera
eyes with lenses, such as are well developed in vertebrates and octopuses.
How do you set about estimating the time required for a given amount of evolutionary
change? We have to find a unit to measure the size of each evolutionary step, and it is
sensible to express it as a percentage change in what is already there. Nilsson and Pelger
used the number of successive changes of x per cent as their unit for measuring changes of
Their task was to set up computer models of evolving eyes to answer two questions. The
first was: is there a smooth gradient of change, from flat skin to full camera eye, such
that every intermediate is an improvement? (Unlike human designers, natural selection
can't go downhill not even if there is a tempting higher hill on the other side of the
valley.) Second, how long would the necessary quantity of evolutionary change take?
In their computer models, Nilsson and Pelger made no attempt to simulate the internal
workings of cells. They started their story after the invention of a single
light-sensitive cell--it does no harm to call it a photocell. It would be nice, in the
future, to do another computer model, this time at the level of the inside of the cell. to
show how the first living photocell came into being by step-by-step modification of an
earlier, more general-purpose cell. But you have to start somewhere, and Nilsson and
Pelger started after the invention of the photocell.
They worked at the level of tissues: the level of stuff made of cells rather than the
level of individual cells. Skin is a tissue, so is the lining of the intestine, so is
muscle and liver. Tissues can change in various ways under the influence of random
mutation. Sheets of tissue can become larger or smaller in area. They can become thicker
or thinner. In the special case of transparent tissues like lens tissue, they can change
the refractive index (the light-bending power) of local parts of the tissue.
The beauty of simulating an eye, as distinct from, say, the leg of a running cheetah,
is that its efficiency can be easily mea-optics. The eye is represented as a
two-dimensional cross-section, and the computer can easily calculate its visual acuity, or
spatial resolution, as a single real number. It would be much harder to come up with an
equivalent numerical expression for the efficacy of a cheetah's leg or backbone. Nilsson
and Pelger began with a flat retina atop a flat pigment layer and surmounted by a flat,
protective transparent layer. The transparent layer was allowed to undergo localised
random mutations of its refractive index. They then let the model deform itself at random,
constrained only by the requirement that any change must be small and must be an
improvement on what went before.
The results were swift and decisive. A trajectory of steadily mounting acuity led
unhesitatingly from the flat beginning through a shallow indentation to a steadily
deepening cup, as the shape of the model eye deformed itself on the computer screen. The
transparent layer thickened to fill the cup and smoothly bulged its outer surface in a
curve. And then, almost like a conjuring trick, a portion of this transparent filling
condensed into a local, spherical subregion of higher refractive index. Not uniformly
higher, but a gradient of refractive index such that the spherical region functioned as an
excellent graded- index lens.
Graded-index lenses are unfamiliar to human lens-makers, but they are common in living
eyes. Humans make lenses by grinding glass to a particular shape. We make a compound lens.
like the expensive violet- tinted lenses of modern cameras. by mounting several lenses
together, but each one of those individual lenses is made of uniform glass through its
whole thickness. A graded-index lens, by contrast, has a continuously varying refractive
index with in its own substance. Typically, it has a high refractive index near the centre
of the lens. Fish eyes have graded-index lenses. Now it has long been known that, for a
graded-index lens, the most aberration-free results are obtained when you achieve a
particular theoretical optimum value for the ratio between the focal length of the lens
and the radius. This ratio is called Mattiessen's ratio. Nilsson and Pelger's computer
model homed in unerringly on Mattiessen's ratio.
And so to the question of how long all this evolutionary change might have taken. In
order to answer this, Nilsson and Pelger had to make some assumptions about genetics in
natural populations. They needed to feed their model plausible values of quantities such
as "heritability" . Heritability is a measure of how far variation is governed
by heredity. The favoured way of measuring it is to see how much monozygotic (that is,
"identical") twins resemble each other compared with ordinary twins. One study
found the heritability of leg length in male humans to be 77 per cent. A heritability of
too per cent would mean that you could measure one identical twin's leg to obtain perfect
knowledge of the other twin's leg length, even if the twins were reared apart. A
heritability of 0 per cent would mean that the legs of monozygotic twins are no more
similar to each other than to the legs of random members of a specified population in a
given environment. Some other heritabilities measured for humans are 95 per cent for head
breadth, 85 per cent for sitting height. 80 percent for arm length and 79 per cent for
Heritabilities are frequently more than 50 percent, and Nilsson and Pelger therefore
felt safe in plugging a heritability of 50 per cent into their eye model. This was a
conservative, or "pessimistic", assumption. Compared with a more realistic
assumption of, say, 70 per cent, a pessimistic assumption tends to increase their final
estimate of the time taken for the eye to evolve. They wanted to err on the side of
overestimation because we are intuitively skeptical of short estimates of the time taken
to evolve something as complicated as an eye.
For the same reason, they chose pessimistic values for the coefficient of variation
(that is, for how much variation there typically is in the population) and the intensity
of selection (the amount of survival advantage improved eyesight confers). They even went
so far as to assume that any new generation differed in only one part of the eye at a
time: simultaneous changes in different parts of the eye, which would have greatly speeded
up evolution, were outlawed. But even with these conservative assumptions, the time taken
to evolve a fish eye from fiat skin was minuscule: fewer than 400,000 generations. For the
kinds of small animals we are talking about, we can assume one generation per year, so it
seems that it would take less than half a million years to evolve a good camera eye.
In the light of Nilsson and Pelger's results, it is no wonder "the" eye has
evolved at least 40 times independently around the animal kingdom. There has been enough
time for it to evolve from scratch 1,500 times in succession within any one lineage.
Assuming typical generation lengths for small animals, the time needed for the evolution
of the eye, far from stretching credulity with its vastness, turns out to be too short for
geologists to measure! It is a geological blink.
By Richard Dawkins Richard Dawkins' latest book, on which this
is based, is "River Out of Eden ", published by Weidenfeld & Nicolson, price