Fentanyl is a powerful morphine substitute, but it is also incredibly addictive – millions struggle with weaning themselves off it. And of the 600,000 drug deaths worldwide each year, the World Health Organisation estimates 80% are due to opioids in general, with synthetic opioids like fentanyl being a growing part of the problem. New work with genetically manipulated mice suggests that fentanyl affects two parts of the brain, one associated with the high, but also another that regulates fear. This knowledge could aid in the development of treatments to reduce addiction to the opioid.

Early developers: Long before a developing implants into a mother's uterus, in fact as the fertilised egg divides for the first time into a pair of cells, which line becomes the future baby and which will become the 'life support' system of the placenta has been decided. Embryologist Magdalena Zernicka-Goetz explains why this early unfolding of the genetic programme is important, and why it's taken so long to discover it.

Getting through pregnancy is only the first step in a person’s life. Surviving childhood, particularly for our old stone age ancestors, was the next challenge. And a new study looking at children’s teeth found at ancient archaeological sites gives clues as to why our ancestors fared better than the neanderthals around them during the last ice age.

Supersense: twitching hairs on some caterpillars turn out to be early-warning sensors feeling the electric field of an approaching wasp, giving the potential prey precious moments to hide or escape death. Biophysicist Daniel Robert explains the challenge of seeing the electric world of insect hunters and hunted.

Presenter: Roland Pease Producer: Jonathan Blackwell Production co-ordinator: Jana Bennett-Holesworth

(Image: Fentanyl. Credit: Isaac Lee via Getty Images.)