Author: BBC World Service

Why do we have two eyes? Two ears? Two arms and two legs? Why is one side of the human body – externally at least – pretty much a mirror image of the other side?

CrowdScience listener Kevin from Trinidad and Tobago is intrigued. He wants to know why human beings – and indeed most animals - have a line of symmetry in their bodies. Yet, beyond their flowers and fruits, plants don’t seem to have any obvious symmetry. It seems that they can branch in any direction.

Anand Jagatia sets out to find out why the animal kingdom settled on bilateral symmetry as the ideal body plan. And it takes him into the deep oceans of 570 million years ago. Paleobiologist Dr. Frankie Dunn is his guide to a time when animal life was experimenting with all sorts of different body plans and symmetries.

Frankie shows Anand a fossil of the animals which changed everything. When creatures with bilateral symmetry emerged they began to re-engineer their environment, outcompeting everything else and dooming them to extinction.

Well... nearly everything else. One very successful group of animals which have an utterly different symmetry are the echinoderms. That includes animals with pentaradial - or five-fold - symmetry like starfish and sea urchins. And that body shape poses some intriguing questions... like “where’s a starfish’s head?” Dr. Imran Rahman introduces us to the extraordinary, weird world of echinoderms.

To answer the second part of Kevin’s question - why plants don’t seem to have symmetry – Anand turns to botanist Prof. Sophie Nadot. She tells him that there is symmetry in plants... you just have to know where to look! Beyond flowers and fruits, there’s also symmetry in a plants leaves and stem. The overall shape of a plant might start out symmetrical but environmental factors like wind, the direction of the sun and grazing by animals throws it off-kilter.

And, while the human body may be symmetrical on the outside, when you look inside, it’s a very different story. As listener Kevin says, “our internal organs are a bit all over the place!” Prof. Mike Levin studies the mechanisms which control biological asymmetry. He tells Anand why asymmetry is so important... and also why it’s so difficult to achieve consistently.

Contributors: Dr. Frankie Dunn, Oxford University Museum of Natural History, UK Dr. Imran Rahman, Natural History Museum, London, UK Prof. Sophie Nadot, Université Paris-Saclay, France Prof. Mike Levin, Tufts University, Massachusetts, USA

Presenter: Anand Jagatia Producer: Jeremy Grange Editor: Cathy Edwards Production Co-ordinator: Ishmael Soriano Studio Manager: Andrew Garratt

(Image: Orange oakleaf butterfly (Kallima inachus) on tropical flower, Credit: Darrell Gulin/The Image Bank via Getty Images)

The US astronauts Butch Wilmore and Suni Williams are currently stranded on the ISS. They arrived on the Boeing Starliner, which was meant to bring them home after eight days. Unfortunately, it has run into tech issues, meaning that the astronauts may be stuck up there for up to eight months.

We started to ponder, what could an extended period of being stuck in space do to your body?

Next we look to the world of psychedelics research, which has currently got itself a little bit stuck.

We also find out more about the Haraldskær Woman, discovered preserved in a Danish bog in the 1800s. Mads Ravn, head of archaeology, research and collections at the Vejle Museums in Denmark, reveals the stories behind the bog bodies and explains how they ended up stuck in the mud.

And staying with the theme of stickiness, we find out what Neanderthals used as glue.

That, plus many more Unexpected Elements.

Presenter: Marnie Chesterton, with Camilla Mota and Kai Kupferschmidt Producer: Harrison Lewis, with Alice Lipscombe-Southwell and Noa Dowling. Sound engineer: Mike Mallen

Fishery assessment models – the “backbone” of fisheries management – overestimate the sustainability of the world’s fisheries, according to a study of 230 fisheries worldwide, and populations of many overfished species are in far worse condition than has been reported. We hear from Rainer Froese of GEOMAR - Helmholtz Centre for Ocean Research Kiel.

The lunar south pole contains evidence of ancient magma ocean. An analysis of lunar soil in the Moon’s southern high-latitude regions, performed using data from India’s Chandrayaan-3 mission, suggests the presence of remnants of a former ocean of magma. Roland speaks to Santosh Vadawale of India’s Physical Research Laboratory.

Stefanie Haustein of the School of Information Studies at Ottawa University discusses the article processing costs for open access of the journal publishing giants in science.

The risk of a banana apocalypse could be near, but biologists, including Li-Jun Ma of the University of Massachusetts, might have found a key to their survival. Bananas are facing functional extinction due to the disease Fusarium wilt of banana caused by a fungal pathogen. Discovery of molecular mechanisms used by the banana-destroying microbe brings hope.

Presenter: Roland Pease Producer: Jonathan Blackwell Production Coordinator: Andrew Rhys Lewis

(Image: Fisherman emptying net full of fish into hold on trawler. Credit: Monty Rakusen via Getty Images)