Why am I symmetrical?

Aug 23, 2024

Anand Jagatia, a curious host exploring biological symmetry, dives into why animals, including humans, exhibit bilateral symmetry, while plants do not. He travels back 570 million years to the Ediacaran period with paleobiologist Dr. Frankie Dunn, uncovering ancient life forms that began the symmetry trend in the animal kingdom. The discussion extends to the strange world of echinoderms with Dr. Imran Rahman, raising questions about their unique penta-radial symmetry and its evolutionary implications.

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INSIGHT

Bilateral Symmetry Sparked Animal Dominance

Bilateral symmetry originated in early worm-like animals around 570 million years ago and became dominant because it enabled directed locomotion and a through-gut.
Frankie Dunn shows trace fossils where a head-tail body axis let animals move toward food and away from waste, driving competition and ecological change.

ANECDOTE

Charnia Shows Ediacaran Glide Symmetry

Charnia fossils from the Ediacaran look plant-like but are animals with glide symmetry rather than true bilateral symmetry.
Dr. Frankie Dunn holds a Charnia cast and explains its branching and glide-offset pattern that defines many Ediacaran taxa.

INSIGHT

Bilaterians Reengineered Early Ecosystems

The rise of bilaterians re-engineered environments by burrowing and oxygenating microbial mats, causing many Ediacaran forms to decline.
Frankie Dunn explains bilaterian activity destroyed anoxic mats, creating a tipping point in Earth's ecosystems.

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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)