implications for medicine. But there’s a catch. This year, Kitai Kim found that the cells retain a memory of their past specialities. A blood cell, for example, can be reverted back into a stem cell, but it carries a record of its history. It keeps molecular Post-it notes that annotate their DNA and affect how it’s used. This constrains its future, making it easier to turn this converted stem cell back into a blood cell than, say, a brain cell. A different technique for creating stem cells, which controversially involves destroying embryos, doesn’t share this problem.
A natural experiment in Nicaragua clearly shows that language has the ability to affect the way we think. In the 1970s, deaf Nicaraguan schoolchildren spontaneously invented a new sign language with its own grammar and vocabulary. The language has become more complex over time, and so have the thoughts of its users. Jennie Pyers found that people who learned NSL before it developed specific gestures for left and right perform more poorly on a spatial awareness test than children who grew up knowing how to sign those terms.
Imagine taking a course of antibiotics and suddenly finding that your sexual desires have changed. That’s exactly what happened to some Israeli fruit flies, which were raised on diets of either maltose or starch. They developed gut bacteria that specialised on these foods and as a result, they preferred to mate with other flies with similar food biases. A dose of antibiotics, and their mating preferences vanished. The study suggests that you can only understand an animal’s evolution by considering the genes of the bacteria and other passengers that live inside it. This is the hologenome – the combined genes of a host and all the microbes it contains.
Two studies this year showed that humans often have nothing to do with the diseases that plague us; we’re just collateral damage in an invisible war. When faced with predatory amoebas, E.coli (a mostly harmless gut bacteria) turns nasty, evolving into a resistant strain that kills the predators from within. And by coincidence, these same strains are also better at infecting human cells. Meanwhile, the normally harmless nose bacteria Streptococcus pneumonia becomes infectious when it battles against another species called Haemophilius influenzae. This competitor summons white blood cells to do away with Streptococcus, which can defend itself by producing a thicker coat. But this armour also allows Streptococcus to evade our own immune system, resulting in pneumonia, meningitis and other diseases.
This crossfire effect explains why some bacteria cause potentially fatal diseases, even though they’re not contagious. They go down with their host, because they are not really targeting their host at all. Many human diseases really have nothing to do with us at all. However, some scientists are turning these bacterial wars to our advantage, harnessing the weapons that they use against one another.