During the era known as “Earth-Snow” around 700 million years ago, when glaciers reached the equator, multicellular organisms emerged on Earth after more than a billion years of only unicellular eukaryotes inhabiting the planet.
The question of why multicellular organisms arose at that particular time has important implications not only for understanding the diversity of life on Earth but also for potentially identifying life forms on other planets.
While it has been commonly believed that a threshold level of oxygen was necessary for unicellular organisms to evolve into multicellular colonies, this hypothesis fails to explain why multicellular ancestors of animals, plants, and fungi appeared simultaneously and why the transition to multicellularity took such a long time.
A recent study published in the journal Proceedings of the Royal Society B presents a new theory proposing that specific physical conditions during a period of “land-drying,” such as high ocean viscosity and limited resources, may have prompted eukaryotes to evolve into multicellular forms.
By utilizing scaling theories, researchers suggest that early ancestors of animals, resembling floating algae that consumed prey rather than photosynthesizing, could have increased in size and complexity in response to the challenges posed by Earth-Snow. Conversely, unicellular organisms relying on diffusion for movement and nutrition, like bacteria, may have decreased in size.
The study highlights how the ice-covered oceans during the Earth-Snow period blocked sunlight, diminishing photosynthesis and depleting nutrients in the sea. Larger organisms capable of processing more water had a survival advantage. As the glaciers receded, these organisms were able to further develop and thrive.
Based on the latest paleontological data, the study offers insights into potential ancestral organisms that could be preserved in the fossil record. Additionally, it introduces new methodologies for investigating the physical influences on organism physiology, which could prove valuable for future research in this field.