Evolution of the eye
Major stages in the evolution of the camera eye.
The evolution of the eye has been a subject of significant
study, as a distinctive example of a homologous organ
present in a wide variety of taxa. Certain components of
the eye, such as the visual pigments, appear to have a
common ancestry – that is, they evolved once, before
the animals radiated. However, complex, image-forming
eyes evolved some 50 to 100 times – using many of the
same proteins and genetic toolkits in their construction.
Complex eyes appear to have first evolved within a
few million years, in the rapid burst of evolution known
as the Cambrian explosion. There is no evidence of eyes
before the Cambrian, but a wide range of diversity is
evident in the Middle Cambrian Burgess shale.
Eyes show a wide range of adaptations to meet the
requirements of the organisms which bear them. Eyes
may vary in their acuity, the range of wavelengths they
can detect, their sensitivity in low light levels, their abil-
ity to detect motion or resolve objects, and whether they
can discriminate colours.
History of research
The human eye, demonstrating the iris.
Since 1802, the evolution of a structure as complex as
the projecting eye by natural selection has been said to
be difficult to explain. Charles Darwin himself wrote,
in his Origin of Species, that the evolution of the eye by
natural selection at first glance seemed "absurd in the
highest possible degree". However, he went on to ex-
plain that despite the difficulty in imagining it, it was
...if numerous gradations from a perfect and com-
plex eye to one very imperfect and simple, each
grade being useful to its possessor, can be shown to
exist; if further, the eye does vary ever so slightly,
and the variations be inherited, which is certainly
the case; and if any variation or modification in
the organ be ever useful to an animal under chan-
ging conditions of life, then the difficulty of believ-
ing that a perfect and complex eye could be formed
by natural sele