Starting With the Oceans, Single-Celled Organisms Will Re-Inherit the Earth

I'll be the first to cop to being guilty of multi-celled chauvinism: Having complex cells with organelles, which form complex systems allowing you to breathe, achieve consciousness, play volleyball, etc, is pretty much as good as it gets. While we enjoy all these advantages now, though, single-celled, simple organisms are just biding their time. More readily adaptable than us multi-celled organisms, it's really a simple, single-celled world, and we're just passing through.

Case in point: the oceans. A team of German researchers just published a paper in the journal Global Change Biology that found that the more simple an organism is, the better off it's going to be as the oceans warm. Trout will die out, whales will fail, but unicellular bacteria and archaea (a type of microorganism) are going to flourish.

Animals can only develop and reproduce up to a temperature threshold in the water of about 41 degrees Celsius, or 105 degrees Fahrenheit. Beyond this, the cardiovascular system can't deliver necessary oxygen throughout the body. Even as individual animal species can develop smaller bodies or generate more hemoglobin to survive in warmer and oxygen deficient water, the highly developed metabolic systems that allow for things like eyeballs can't get over the temperature threshold and the other hurdles it brings, like decreasing oxygen.

"The adaptation limit of an organism is not only dependent on its upper temperature threshold, but also on its ability to cope with small amounts of oxygen,” said Daniela Storch, the study's lead author . “While many of the bacteria and archaea can survive at low oxygen concentrations or even without oxygen, most animals and plants require a higher minimum concentration.”

That's part of the reason that unicellular organisms are found in the most dramatic settings that Earth has to offer: from Antarctic lakes that were buried under glaciers for 100,000 years, to super-hot hydrothermal vents on the ocean floor, acidic pools in Yellowstone, and the Atacama desert in Chile. When we look around the solar system, we see environments that can't support complex, multicellular life, but still hold out hope that unicellular life has found a way in Europa's unseen seas, or below the surface of Mars.

But as the Earth's climate changes, and the ocean gets warmer and more acidic, complexity goes from an asset to a liability, and simplicity reigns.

“Communities of species in the ocean change as a result of this shift in living conditions. In the future animals and plants will have problems to survive in the warmest marine regions and archaea, bacteria as well as protozoa will spread in these areas,” said Dr. Hans-Otto Pörtner, one of the study's co-authors. “There are already studies showing that unicellular algae will be replaced by other unicellular organisms in the warmest regions of the ocean.”

The story of life on Earth is, if nothing else, symmetrical. Three and a half billion years ago, prokaryotic cells showed up, without a nucleus or other organelles. Complex, multicellular life emerged with an increase in biomass and decrease in global surface temperature half a billion years ago. In another billion and a half years that complex multicellular life died back out, leaving the planet to the so-called simpler forms of life, as they basked in the light of a much brighter Sun. The best-case scenario is that life lasts until the Sun runs out of fuel, swells into a red giant, and vaporizes whatever is left of our planet in 7.6 billion years.

Multicellular life will have just been a two billion year flicker against a backdrop of adaptable single-celled life. But hey, we had a good run.