Author: BBC World Service

This week on the show that brings you the science behind the news, we’re looking at news that China’s population has fallen for the second year running. Worrying news for China’s economy, but would a declining population be a good thing for the planet?

The Unexpected Elements team on three continents meet the musical frogs who are having to climb a mountain to keep their populations stable, and dig deep to explore the earth’s declining microbiome and the hope scientists have for the future.

As the Africa Cup of Nations continues, we’ll be wondering how you might date a footballer. Not in a romantic sense… we hear about some suspiciously mature youth players and how science can help when the age on a passport isn’t reliable.

Marnie will be wondering why Japanese men are shouting their love from a hilltop, and unpicking the recipe for a truly satisfying hug.

All that plus a postbag bursting with multilingual puns, and the reason Portuguese speakers have trouble with English doors.

Presented by Marnie Chesterton Produced by Ben Motley, with Alex Mansfield, Dan Welsh, Katy Tomsett and Jack Lee

What evidence is there for a semi-aquatic period in human evolutionary history? That’s the question that’s been bothering listener Dave in Thailand. He thinks our lack of hair and love of water might indicate that, at some point, we were more water-based than we are now. But what does science have to say on the matter?

The theory that our ape ancestors returned to the water for a phase in our evolutionary history is a controversial idea that most scientists disagree with.

Anand Jagatia chats to Dr Melissa Ilardo, assistant professor at the University of Utah, about our dive reflex - a physiological response we display when submerged underwater, which helps direct oxygen towards vital organs. But this is not a response that is unique to humans - it is found in all mammals. Experts say it developed long before all apes split off in the evolutionary tree.

To find out more about the theory itself Anand hears from John Langdon, emeritus professor at the University of Indianapolis. He explains why the aquatic ape theory is not generally accepted by anthropologists, what the fossil record can tell us about our evolutionary path and why evolution is much more complex than the aquatic ape hypothesis suggests.

While there may be little evidence of a semi-aquatic period in our evolutionary past, there are some communities around the world that have adapted to utilising their watery environments in more recent evolutionary history.

Anand speaks to Dr Nicole Smith-Guzman at the Smithsonian Tropical Research Institute who has found evidence that ancient populations in Panama were habitually diving in the sea for shells and seafood. She explains how she can piece together evidence from different sources to detect the activity of ancient populations. And Dr Melissa Ilardo explains how evolutionary pressure can cause physical changes in isolated communities, as our bodies ultimately adapt to help us thrive in more watery environments.

Producer: Hannah Fisher Presenter: Anand Jagatia Editor: Richard Collings Production co-ordinator: Jonathan Harris Sound engineer: Jackie Margerum

(Photo: Woman swimming underwater. Credit: Petrelos/Getty Images)

African swine fever has now reached Southeast Asia.This part of the world has high diversity in wild pigs, some of which are endemic to their native islands. In Borneo, pigs are a hugely important food source for indigenous populations, and are a vital prey species for many big cats. In some regions, the pig populations have now dropped by 90 to 100 per cent due to swine fever. Conservationist Dr Erik Meijaard explains what this could mean for Borneo.

Sticking with diseases, the World Mosquito Program breeds mosquitoes infected with Wolbachia. Why? Because mosquitoes infected with this bacteria are unable to become carriers of dengue, Zika and other mosquito-borne diseases. Dr Scott O'Neill of the World Mosquito Program talks about their latest research and the massive mosquito factory they're building in Brazil this year.

Finally this week, Dr Nitzan Gonen discusses her new study, where mouse testes organoids have been grown in the lab. She tells us about the potential applications for this research.

Presenter: Roland Pease Producer: Alice Lipscombe-Southwell Production co-ordinator: Jana Bennett-Holesworth

(Photo: Bornean bearded pig. Credit: Sylvain Cordier/Getty Images)