We aren't even sure if they are really alive, but, as Maryam Chaib de Mares explains, the distribution of viruses across the three domains of life (Bacteria, Archaea, and Eukarya) may reflect how viruses have shaped the evolution of cellular life.
You might have thought eusociality, an extreme form of cooperation that makes a colony of individuals verge on a superorganism, was just for ants and bees. As we learn more natural history, though, we see more and more examples. Here, Jack Boyle explains how scientists created artificial nests so they could observe the social habits of X. saxesenii, an ambrosia beetle normally concealed in tree trunks, to determine whether they meet the three eusociality criteria.
What makes a polar bear a polar bear? Allison Shultz explains that the difference between polar bears and their cousins, the brown bears, is more than skin deep.
Phil Grayson explains how improvements in mitochondrial and nuclear genomics have rewritten what seemed like an open-and-shut case in the evolution of flightless birds.