Imagine a world where the oxygen you need changes dramatically between day and night. Your world shifts from being rich in oxygen (oxic) in the day, so you have energy to hunt for food, to suffocatingly oxygen-free (anoxic) at night, which slows you down.
Now, picture early animals trying to survive in such an extreme environment. This was the reality for early animal life in oceans and seas about half a billion years ago. This was also the time when animal diversity boomed, in what is known as the “Cambrian explosion”.
My team’s new research suggests that these drastic oxygen fluctuations played a crucial role in this dramatic period.
For decades, scientists have debated what triggered this evolutionary burst. Many scientists have pointed to long-term atmospheric changes, where increasing oxygen levels supposedly drove a variation in the number of animal life forms. Over the last couple of years, however, the view on increasing atmospheric oxygen as a simple trigger for the rise of animals has been questioned.
Our new study reveals a different, often overlooked factor. Daily swings in oxygen levels on the shallow seafloor may have stressed early animals (the ancestors of all animal life today), pushing them to adapt in ways that fuelled diversification. Rather than good conditions driving the change, we argue that harsh conditions triggered this.
We used a computer model that can mimic conditions on the sunlit seafloor today. This model takes into account what life can produce or consume, but also how temperature, sunlight, and different types of sediment or water affect the overall conditions. Using this so-called “biogeochemical model”, we have shown that in warm, shallow waters, oxygen levels could fluctuate dramatically between day and night in the Cambrian (when oxygen was generally lower than today).
During the day, photosynthesis by marine algae produced lots of oxygen, creating a fully oxygenated environment. But at night, when photosynthesis stopped because there was no light, oxygen was instead rapidly consumed by the algae as they respired (using energy and oxygen to perform cell functions), leading to anoxic conditions.
This daily feast-and-famine cycle in oxygen availability created an intense physiological challenge for early animals, forcing them to develop adaptations to handle fluctuations in nutrients. For those that could deal with these fluctuations, adaptation gave them a competitive edge.
The shallow, sandy beach-like shelf environments in oceans around the world also expanded dramatically at this time because the super-continent – known as Rodinia – broke up into smaller pieces. This increased the total circumference of continental crust, creating more continental edges where sun, nutrients and life could interact. These new continents were also flooded, so shallow, sunlit seafloor zones expanded even further.
Sunlit marine environments tend to be the richest in nutrients. Species that had adapted to cope with daily oxygen fluctuations could more easily access the nutrients in this vast, shallow habitat. The stress-tolerant species would win the race to food.
How stress drives evolution
Physiological stress is often seen as an obstacle to survival. But it can be a catalyst for evolutionary innovation. Even today, species that endure extreme environments often develop specialist traits that make them more adaptable.
Our study suggests a similar pattern played out in the Cambrian. Animals evolved ways to cope with the stress of fluctuating oxygen levels on the smörgåsbord of the shallow seafloor shelves.
One key adaptation could have been the ability to efficiently sense and respond to oxygen fluctuations. This trait is regulated by a cellular control system – a molecular pathway that adapts how the cell responds to external conditions. The control system that may have emerged at the Cambrian explosion is known as HIF-1α (hypoxia-inducible factor 1).
In modern animals, this system helps cells detect and adapt to changes in oxygen conditions, controlling processes like energy metabolism and the coordination of a cell’s functions.
However, HIF-1α offers resistance to toxins such as hydrogen sulphide, a common byproduct of anoxic conditions. Our modelling suggests that animals with advanced oxygen-sensing mechanisms would have had a survival advantage in the fluctuating conditions of the Cambrian seafloor, allowing them to outcompete species without this capability.
From harsh environments to animal diversity
Today, biodiversity hotspots like tropical rainforests and coral reefs thrive under conditions of high biological competition and ecological complexity. However, in extreme environments where survival depends on withstanding harsh physical conditions rather than competing with other species, different evolutionary pressures come into play. Any adaptations against stress that led to increased survival would also be inherited efficiently, too.
Read more: Cancer tumours could help unravel the mystery of the Cambrian explosion
The ability to cope with these rapid changes may have allowed certain animal lineages to thrive over others, leading to the emergence of more complex and adaptable life forms.
Today, all animals with tissues as we know them (several layers of cells) use HIF to maintain regular maintenance or steady state (known as homeostasis). This molecular pathway is critical for building tissues and healing tissues. These “control knobs” in cells are even suggested to be essential for how animal life could get as large and old as giraffes, elephants and humans.
This new model challenges traditional views that focus solely on large-scale geological changes as the primary drivers of early animal evolution. Local-scale challenges faced by individual organisms – such as surviving daily swings between oxygen-rich and oxygen-starved conditions – could have been just as important in shaping the course of evolution.
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