In Our Time: The Evolution of Lungs
BBC Radio 4 – July 10, 2025

Introduction

In this enlightening episode of In Our Time, host Melvyn Bragg delves into the intricate evolution of lungs, tracing their origin from ancient fish to the diverse respiratory systems observed in modern animals, including birds and dinosaurs. Joined by expert guests Steve Brusatte, Jonathan Codd, and Emily Rayfield, the discussion unpacks the biological, environmental, and evolutionary factors that have shaped lung development over millions of years.

1. The Fundamental Role of Lungs

The conversation begins with Emily Rayfield elucidating the primary function of lungs:

"[00:29] Emily Rayfield: ... the main function of lungs is to allow gaseous exchange... we bring oxygen into the body and expel carbon dioxide."

Chris Brusatte further elaborates on the biochemical necessity of oxygen:

"[03:29] Chris Brusatte: Oxygen is essential for making energy... it's needed for respiration, where glucose reacts with oxygen to produce carbon dioxide, water, and ATP."

Thus, lungs are indispensable for facilitating the critical exchange of gases that sustains cellular respiration and, by extension, life.

2. From Swim Bladders to Lungs: Evolutionary Origins

Jonathan Codd introduces the evolutionary transition from swim bladders in fish to lungs:

"[04:21] Jonathan Codd: ...some fish used lungs as a secondary method to obtain oxygen by gulping air, which likely led to the development of true lungs."

He emphasizes that the first lungs appeared in aquatic environments, granting certain fish an advantage in low-oxygen waters by supplementing gill respiration. This adaptation set the stage for the eventual colonization of terrestrial habitats.

3. Environmental Pressures and the Silurian Period

Melvyn Bragg discusses the environmental factors during the Silurian period that spurred lung evolution:

"[08:00] Melvyn Bragg: Around 400 million years ago, fluctuations in rainfall and water availability reduced dissolved oxygen in aquatic environments, creating a selection pressure for air-breathing capabilities."

These conditions favored fish that could extract oxygen from the air, facilitating a gradual shift towards terrestrial living and the development of more efficient respiratory structures.

4. Mechanics of Breathing: From Buccal Pumping to the Diaphragm

The mechanics of different breathing systems are explored, starting with buccal pumping:

"[12:35] Emily Rayfield: Buccal pumping involves the depression of the mouth cavity to force air into and out of the lungs, as seen in frogs and some amphibians."

Melvyn Bragg adds insights into how the evolution of the diaphragm in mammals revolutionized breathing:

"[25:37] Melvyn Bragg: The diaphragm allows mammals to create pressure differences in the thoracic cavity, enabling efficient inhalation and exhalation."

This muscular adaptation distinguishes mammalian respiration from other mechanisms, providing greater control and efficiency in gas exchange.

5. The Role of Surfactants in Lung Function

Surfactants are highlighted as a universal component of respiratory systems:

"[11:23] Melvyn Bragg: Surfactants are complex mixtures that reduce surface tension in the lungs, preventing collapse and facilitating gas exchange across the thin membranes."

Emily Rayfield underscores their evolutionary significance:

"[12:35] Melvyn Bragg: Surfactants evolved early and are present in all air-breathing animals, indicating their fundamental role in effective respiration."

6. Diverse Respiratory Systems: Mammals, Birds, and Dinosaurs

Jonathan Codd and Steve Brusatte compare the different lung structures across species:

"[15:12] Jonathan Codd: Mammalian lungs use ribs and the diaphragm for breathing, contrasting with the unidirectional, multi-chambered lungs of birds."

"[20:15] Melvyn Bragg: Both birds and dinosaurs possess multi-chambered lungs with complex internal folding, indicative of efficient oxygen extraction akin to avian systems."

This diversity illustrates how different evolutionary paths have optimized lung function to meet varying environmental and physiological demands.

7. Fossil Evidence: Unraveling Dinosaur Lungs

The panel discusses how paleontologists infer lung structures in extinct species:

"[16:14] Jonathan Codd: Although soft tissues like lungs rarely fossilize, features such as hollow bones and foramina suggest the presence of air sacs similar to those in modern birds."

Emily Rayfield adds:

"[21:02] Chris: Fossilized foramina and pneumatized bones in dinosaurs like T. Rex indicate bird-like lungs, enabling efficient respiration necessary for large body sizes."

These findings bridge the gap between ancient dinosaurs and contemporary avian species, highlighting evolutionary continuity.

8. Implications for Dinosaur Physiology and Size

Steve Brusatte links respiratory efficiency to the gigantism observed in some dinosaurs:

"[23:08] Steve Brusatte: Enhanced oxygen extraction through bird-like lungs likely supported the massive sizes of sauropod dinosaurs, a feature unachievable with simpler lung structures."

Emily Rayfield concurs, noting:

"[25:27] Emily Rayfield: Air sacs not only improved respiration but also reduced skeletal weight, allowing dinosaurs to attain colossal sizes without compromising structural integrity."

Thus, advanced lung systems were pivotal in enabling some dinosaurs to become the largest land animals ever to exist.

9. Breathing and Locomotion: Overcoming Biological Constraints

The discussion turns to the interplay between breathing and movement:

"[28:45] Melvyn Bragg: The evolution of accessory breathing mechanisms, such as air sacs, allowed animals like birds and certain lizards to breathe efficiently without hindering locomotion."

Jonathan Codd introduces Carrier's Constraint:

"[29:38] Jonathan Codd: Carrier's Constraint refers to the challenge of breathing and moving simultaneously, which has been mitigated in species with specialized respiratory adaptations."

This adaptation is crucial for animals that require continuous movement and high metabolic rates.

10. Modern Variations and Adaptations in Breathing

The panel explores contemporary examples of diverse respiratory strategies:

"[27:34] Emily Rayfield: Some modern lizards, like monitor lizards, possess partial air sacs that supplement their breathing, demonstrating ongoing respiratory diversity."

"[32:05] Chris: Cutaneous respiration, or skin breathing, seen in amphibians and certain salamanders, supplements lung function, showcasing multiple respiratory pathways."

These adaptations reveal the ongoing evolutionary experimentation with different breathing mechanisms to meet ecological niches.

11. The Future Evolution of Lungs

Speculation on future respiratory adaptations addresses environmental changes:

"[33:26] Chris: Increasing atmospheric CO₂ levels may drive evolutionary changes in lung surface area and gas exchange efficiency to cope with altered oxygen availability."

Jonathan Codd reflects:

"[34:09] Jonathan Codd: While predicting exact evolutionary trajectories is challenging, current trends suggest biochemical adjustments in gas exchange interfaces in response to environmental pressures."

12. Human Hiccups: An Evolutionary Remnant

An unexpected yet fascinating conclusion links human physiology to ancient respiratory mechanisms:

"[38:41] Melvyn Bragg: Hiccups trace back to amphibian ancestors, where they facilitated rapid water movement across gills, a vestigial reflex preserved in humans today."

This underscores the deep evolutionary roots embedded within our own respiratory systems.

Conclusion

The episode of In Our Time provides a comprehensive exploration of lung evolution, highlighting the intricate adaptations that have enabled life to thrive in diverse environments. From the humble swim bladder to the complex lung systems of birds and dinosaurs, lungs have undergone remarkable transformations driven by environmental pressures and the relentless march of natural selection. The discussion not only illuminates the past but also invites contemplation on the future of respiratory evolution in a changing world.

Notable Quotes

• "[00:29] Emily Rayfield: The main function of lungs is to allow gaseous exchange... we bring oxygen into the body and expel carbon dioxide."

• "[04:21] Jonathan Codd: Some fish used lungs as a secondary method to obtain oxygen by gulping air, which likely led to the development of true lungs."

• "[08:00] Melvyn Bragg: Around 400 million years ago, fluctuations in rainfall and water availability reduced dissolved oxygen in aquatic environments, creating a selection pressure for air-breathing capabilities."

• "[20:15] Melvyn Bragg: Both birds and dinosaurs possess multi-chambered lungs with complex internal folding, indicative of efficient oxygen extraction akin to avian systems."

• "[38:41] Melvyn Bragg: Hiccups trace back to amphibian ancestors, where they facilitated rapid water movement across gills, a vestigial reflex preserved in humans today."

Further Readings

For those interested in a deeper dive into the topics discussed, a reading list accompanies the podcast episode on the BBC website, providing curated resources to expand your understanding of pulmonary evolution and its significance in the broader tapestry of life's history.