Plankton collected in the Pacific Ocean with a 0.1mm mesh net. Seen here is a mix of multicellular organisms — small zooplanktonic animals, larvae and single protists (diatoms, dinoflagellates, radiolarians) — the nearly invisible universe at the bottom of the marine food chain. Christian Sardet/CNRS/Tara Expeditions hide caption

itoggle caption Christian Sardet/CNRS/Tara Expeditions

This fungus among us — baker's yeast, aka Saccharomyces cerevisiae — is useful for more than just making bread. iStockphoto hide caption

itoggle caption iStockphoto

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

Families harvest poppy bulbs in the Badakhshan province of Afghanistan. To collect the opium, they score the bulbs and let the milky substance ooze out. The dried residue contains about 10 percent morphine. David Guttenfelder/AP/National Geographic hide caption

itoggle caption David Guttenfelder/AP/National Geographic

More than 1 million people in Peru earn less than the equivalent of about $450 each year. Courtesy of Michael Rizzo/CGAP hide caption

itoggle caption Courtesy of Michael Rizzo/CGAP

The Allen Cell Types Database catalogs all sorts of details about each type of brain cell, including its shape and electrical activity. These cells, taken from the visual area of a mouse brain, are colored according to the patterns of electrical activity they produce. Courtesy of Allen Institute for Brain Science hide caption

itoggle caption Courtesy of Allen Institute for Brain Science

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

Psyched that it's finally spring? Shifts in the season may affect more than your mood. Corbis hide caption

itoggle caption Corbis