WASHINGTON | Scientists have traced the origin of eyes back to a transparent blob of living jelly floating in the sea about 600 million years ago.

That creature, the distant ancestor of a modern freshwater animal known as a hydra, could only distinguish light from dark.

But that simple trick was such an advantage that it was passed on from generation to generation of the hydra’s cousins and their myriad descendants. It was the precursor of the wildly different, ever more complex eyes of fish, ants, flies, giraffes and people.

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The discovery also helps to counter one of the principal arguments used by anti-evolutionists to discredit Charles Darwin’s theory and to support their belief in “intelligent design.”