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.”
Recent 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.
However, new studies reveal a different, often overlooked factor. Daily swings in oxygen levels on the shallow seafloor may have stressed early animals, pushing them to adapt in ways that fueled diversification.
A computer model that mimics conditions on the sunlit seafloor in the Cambrian period has shown that in warm, shallow waters, oxygen levels could fluctuate dramatically between day and night. 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 rapidly consumed by the algae, 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. Species that could deal with these fluctuations had a competitive edge in accessing the nutrients in the vast, shallow habitat.
Physiological stress is often seen as an obstacle to survival, but it can be a catalyst for evolutionary innovation. Animals evolved ways to cope with the stress of fluctuating oxygen levels on the shallow seafloor shelves. One key adaptation could have been the ability to efficiently sense and respond to oxygen fluctuations.
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 use a molecular pathway known as HIF to maintain homeostasis and build tissues.
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 could have been just as important in shaping the course of evolution.
