Read on for some eye-opening facts about the human eye.

1. The first “eyes” evolved approximately 550 million years ago. These primitive eyes were actually clusters of photoreceptive proteins in single-celled organisms.

2. The eye muscles are the most active muscles in the body — more active, even than the heart. The eye muscles move about 100,000 times per day, with a lot of these movements happening during the rapid-eye movement (REM) part of sleep.

3. Spending a lot of time in the sun? Sunglasses can help you avoid an eye sunburn. (Yes, your eyes can get a sunburn.)

4. Impressed by the megapixel count on the latest iPhone? If your eyes were digital cameras, they’d have an estimated 576 megapixels.

5. The average person spends about 10 percent of their waking hours with their eyes closed.

6. You’ll shed about 98 feet of eyelashes in your lifetime. Human lashes have a lifespan of about five months.

7. Within two weeks of conception, fetuses begin to develop eyeballs. At birth, eyes are about 2/3rds of their total size.

8. Ten thousand years ago, all humans had brown eyes.

9. Do you have blue eyes? You share a single common ancestor with all blue-eyed people around the world. Blue eyes were a genetic mutation that appeared between 6,000 and 10,000 years ago near the Black Sea.

10. Blind people who were born with sight still “see” when they dream.

11. Your retinas perceive visual stimuli upside-down. Your brain flips the image.

12. The average person blinks between 15 and 20 times in a single minute.

13. The cornea is composed of cells and proteins—it doesn’t have any blood vessels, unlike most other tissues in the human body.

14. Researchers say seeing color is the single most important part of our visual experience as human beings. When you see something in color, you’re more likely to remember it.

15. After the brain, eyes are the second most complex organ.

16. Fingerprints have about 40 unique characteristics, while irises have 256, making them a more secure identity marker.

17. Your iris expands and contracts according to lighting conditions, causing pigments to either spread apart or compress. That’s why some people have eyes that appear to change color. Certain emotions can also cause the iris to contract or expand.

18. The human eye can distinguish approximately 10 million different colors.

19. A study of more than 10,0000 Americans found that people with blue eyes showed the highest frequency of alcoholism.

20. Besides humans, black lemurs are the only primates with blue eyes.

21. About 1/6th of the human eyeball is exposed.

22. The average eyeball is about an inch across and weighs about a quarter of an ounce.

23. Eyes cannot be transplanted. With an estimated one million nerve fibers connecting each eye to the brain, the eye is currently too complex to transplant.

24. The eye heals faster than most tissues. A scratched cornea, for instance, will heal completely within two or three days.

25. Blindness affects about 39 million people around the world. An estimated 285 million people are affected by visual impairments.

26. About 80 percent of vision problems are either avoidable or curable.

27. Contrary to popular belief, some people actually can keep their eyes open when they sneeze.

28. Afraid of eyeballs? You have ommatophobia.

29. Humans can differentiate more shades of green than any other color.

30. Green is the rarest eye color on the planet. About two percent of people have green eyes.

31. The eye can process images seen for as little as 13 milliseconds.

32. The eye muscle in the fastest in the human body.

33. Newborns can wail, but they can’t actually cry. That is, their eyes don’t actually produce tears they’re between four and 13 weeks old.

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