The deep ocean was a stable refuge for Earth's first complex animals
In short: For billions of years, life was microscopic. Then, about 570 million years ago, something changed. Large, soft-bodied organisms appeared, but they didn't settle in the sunlit shallows. Instead, they thrived in the dark, deep ocean, using the depths to survive a world of fluctuating temperatures and scarce oxygen.
During the Ediacaran period, more than 570 million years ago, life underwent a dramatic shift. Following an era of predominantly unicellular life lasting billions of years, more intricate creatures—such as meter-long, soft-bodied sponges—appeared. For a period of 15 million years, these larger, more complex life forms existed exclusively in deep waters.
Scientists have long puzzled over why these creatures chose the deep ocean, an environment characterized by scarce food, low light, and a shortage of atmospheric oxygen (S2:p3, S/4:p3). A study published on December 12 in the Proceedings of the Royal Society B suggests that the deep ocean provided a vital sanctuary. Unlike the shallow waters, which experienced wild temperature swings, the depths offered stable temperatures that allowed organisms to use limited oxygen more efficiently.
To test this, researcher Tom Boag used sea anemones as biological models, as their ability to breathe through their skin mimics the biology of Ediacaran fossils. The study found that oxygen requirements increase when temperatures deviate from an optimum 'medium'. In colder waters, oxygen diffuses more slowly into tissues, forcing animals to expend more energy to move oxygenated water through their bodies. This stability was essential for the survival of the first complex lineages, which serve as the evolutionary missing links to all subsequent complex life.
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Sources
- Summary by Intellect · source: How Life Begins in the Deep Ocean
- Also: Why deep oceans gave life to the first big, complex organisms - Stanford Doerr School of Sustainability
- Also: Why deep oceans gave life to the first big, complex organisms
- Also: ed.ted.com — How life begins in the deep ocean - Tierney Thys
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