For many evolution deniers, eyeballs are a big talking point. The human eye is remarkably complex -- kind of perfect, especially compared to a lot of the weirdness going on in our bodies -- and it wouldn't work at all if any single one of its parts disappeared. Because evolution sort of works by building systems up step by step, some creationists argue that something as intricate as the eyeball couldn't have "evolved." What would the intermediary step be, exactly?

But this isn't how evolution actually works, and eyeballs are totally in the clear. In the latest installment in his "12 Days of Evolution" series, It's Okay To Be Smart's Joe Hanson debunks this old myth piece by piece.

As Hanson explains in the above video, eyes really did evolve step by step, if you don't try to oversimplify the process. The first light-sensitive cells showed up in the very earliest days of life, when single-celled organisms used this mechanism to swim towards the sunny side of the primordial ooze.

It takes 1,829 steps to go from a light-sensitive cell to a complex eye, according to computer simulations, but the steps are there. Plus it only takes about 350,000 generations for the transition to happen (though in reality it took a bit longer).

Eyes have actually developed independently over and over again, which is pretty cool. And there actually are uses for "half-evolved" eyes. Take, for example, this mollusk, which uses simple eyes made out of stone to sense predators swimming above it.

Besides: Human eyes aren't actually perfect. Other animals have evolved way more complex and interesting eyes than we have. If you don't believe me, look no further than the mantis shrimp, which makes our vision seem totally puny.

We can watch this evolution work in the other direction, too -- in real time. A recent study found that cave-dwelling species that have lost the ability to see over time (an energy-saving measure in a world where vision is utterly useless) are gradually losing the brain structures that support sight, too.

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