An ancient stone tool unearthed at the excavation site near Kenya's Lake Turkana. It's not just the shape and sharp edges that suggest it was deliberately crafted, the researchers say, but also the dozens of stone flakes next to it that were part of the same kit. MPK-WTAP hide caption

itoggle caption MPK-WTAP

The most recent common ancestor of all today's snakes likely lived 120 million years ago. Scientists believe it used needle-like hooked teeth to grab rodent-like creatures that it then swallowed whole. Julius Csotonyi/BMC Evolutionary Biology hide caption

itoggle caption Julius Csotonyi/BMC Evolutionary Biology

The skull of a chicken embryo (left) has a recognizable beak. But when scientists block the expression of two particular genes, the embryo develops a rounded "snout" (center) that looks something like an alligator's skull (right). Bhart-Anjan S. Bhullar hide caption

itoggle caption Bhart-Anjan S. Bhullar

Reconstruction of the giant filter feeder, scooping up a plankton cloud. Aegirocassis benmoulae was one of the biggest arthropods that ever lived. Family members include today's insects, spiders and lobsters. Marianne Collins/ArtofFact hide caption

itoggle caption Marianne Collins/ArtofFact

An artist's rendering of what Dearcmhara shawcrossi probably looked like in dinosaur times. Todd Marshall/University of Edinburgh hide caption

itoggle caption Todd Marshall/University of Edinburgh

Workers at the National Geographic Museum in Washington grind the rough edges off a life-size replica of a spinosaurus skeleton. Mike Hettwer/National Geographic hide caption

itoggle caption Mike Hettwer/National Geographic

Love your hair. Artists' depictions of a Neanderthal man and woman at the Neanderthal Museum in Mettmann, Germany. Martin Meissner/AP hide caption

itoggle caption Martin Meissner/AP

A family of elephants in Kenya's Maasai Mara game reserve. Tony Karumba/AFP/Getty Images hide caption

itoggle caption Tony Karumba/AFP/Getty Images