Science news top 25.

M A R T IN O E G G E R LI We are not alone. Humans’ vast inner and outer spaces teem with a menagerie of microbes that stand poised to alter conceptions of what and who we are. Traditionally, microbes have been viewed as insidious invaders that make people sick or as freeloaders in the human gut. That view is beginning to change. In 2013, scientists amassed substantial evidence that people and other animals form a unit with their resident bacteria, archaea, fungi and viruses — the collection of microbes known as the microbiome. In fact, only about 10 percent of a person’s cells are human; microbes make up the other 90 percent. Many researchers point out that ultimately, every species is out for itself. Nevertheless several new studies argue in favor of considering animals as superorganisms composed of host and microbes. Some scientists even advocate lumping a host organism’s genes with those of its microbes into one “hologenome.” Treating a host, such as the human body, and its resident bacteria as a unit — or at least as an ecosystem with intimately interconnected parts — offers various benefi ts, scientists say. The superorganism approach may help researchers better understand how diet, chemicals and other environmental By Tina Hesman Saey Microbiome results argue for new view of animals as superorganisms L ast year it was easy to choose a story to lead our annual Top 25 list. The discovery of the Higgs boson was a watershed moment, ending a decades-long quest by thousands of physicists to fully describe the subatomic realm. This year, nothing so momentous came to pass. But science isn’t just about dramatic announcements and tremendous technical feats. Anyone who reads Science News regularly appreciates that great new insights often arise from countless little bits and pieces of new knowledge. This year, careful readers may have noticed a steady accumulation of revelations about the bacterial communities that call the human body home. It has long been known that those microbes are essential to processes like extracting nutrients from food and fi ghting off their less benign brethren. But this year a growing body of research demonstrated that bacteria engage their hosts so vigorously that in some situations, scientists are left wondering which party is the tail and which is the dog. Human evolution has also produced an impressive body of new knowledge, though some of it only deepens existing mysteries. For example, the oldest hominid DNA ever analyzed linked 400,000-year-old bones from Spain not to the Neandertals that later dominated the region, but to mysterious early hominids known from sites thousands of kilometers to the east. It will probably be a few more years before anyone can explain what is becoming an increasingly controversial era of human evolution. This year also demonstrated that big fi ndings can be big letdowns. After a spectacular landing on Mars in August 2012, the Curiosity rover looked for elevated atmospheric methane concentrations that would have been telltale evidence for the presence of microbial life. Anything over a few parts per billion would have given us a clear choice for 2013’s top story. But Curiosity detected an average methane concentration of only 0.18 ppb, a fi nding that landed it in 17th place. — Matt Crenson, Managing Editor Microbiome insights add up