You Decide: What's the Greatest Mystery in Science?

A couple years back, we asked several scientists from various fields what they thought were the greatest mysteries, resulting in the list below. None of them has been solved since.

Now we'd like you to decide which is the single greatest mystery of science. Voting continues through Sunday, May 3.

We'll enshrine the whole Top 10 of your chosing in a new list. Cast your vote! (Write-in candidates welcome in the comments section below.)

How Does the Brain Work? / How Many Species Exist on Earth? / Does Alien Life Exist? / Who Are You? / What Causes Mass Extinctions? / How Did Human Culture Evolve? / What Causes Gravity? / How Did the Universe Begin? / Why Do We Desire Things? / What Drives Evolution? / What Happens Inside an Earthquake? / Where is the Rest of the Universe? / Is There a Theory of Everything? / How Did Life Arise on Earth?