Over 99% of all species that have ever existed on Earth are now extinct. As we confront a growing number of species currently under threat, it's crucial to reflect on the incredible creatures we've already lost — some at the hands of humankind. From the gigantic land mammals of the ancient past to the smallest creatures lost to climate change, each entry reminds us of the urgency of conservation.

Click through the gallery to find 40 extinct animals we'll never see again...