Fossils of the oldest known animals on Earth, dating back 574 million years to the Ediacaran period, indicate that asexual reproduction dominated the oceans, stalling evolutionary progress until environmental pressures prompted the emergence of sexual reproduction and triggered a surge in biodiversity.
Artist reconstruction of the Fructofusus community, showcasing a large specimen surrounded by medium-sized ones, with smaller specimens forming clusters. Image credit: CG Kensington.
Following billions of years of microbial life, the Ediacaran period, occurring approximately 635 to 539 million years ago, saw the emergence of larger and more complex organisms, including the first animals.
Among these early animals, some specimens of fructophusus could reach heights of up to 2 meters (6.6 ft), although most were much smaller.
These ancient creatures resembled ferns more than modern animals. Lacking mouths, internal organs, or means of movement, they are believed to have absorbed nutrients directly from their surrounding water.
Most Ediacaran life forms vanished from the fossil record at the dawn of the Cambrian period, around 540 million years ago, complicating efforts to connect them to contemporary organisms.
Previous studies revealed that these primitive animals reproduced asexually, using clones that spread via stolons and runners, similar to how modern strawberries propagate. They thrived in the nutrient-rich waters of the Ediacaran ocean.
“Life in Ediacaran times was favorable, minimizing the need for sexual reproduction,” stated Dr. Emily Mitchell, a researcher at the University of Cambridge.
“There was limited competition, which reduced the pressure for evolutionary change.”
Dr. Mitchell and her colleague, Professor Andrea Manica, utilized advanced techniques such as laser scanning, spatial analysis, and artificial intelligence to investigate Ediacaran fossils found at Mistaken Point in Newfoundland, Canada.
They demonstrated that asexual reproduction via stolons decreases competition and then created a computational model to simulate how early animal communities operated under various reproductive strategies.
Testing this model thousands of times, they applied simple neural networks to identify simulations that aligned best with the diversity patterns seen in the fossil record.
This method, known as Approximate Bayesian computation, enabled researchers to analyze actual data and estimate organism dispersal and resource competition intensity.
Through this process, they determined that the restricted dispersal linked to asexual reproduction accounts for the limited species diversity in early animal communities, while the transition to sexual reproduction coincided with a dramatic increase in evolutionary diversity.
For billions of years, competition and environmental stress were the primary forces behind evolution, yet in the Ediacaran sea, asexual reproduction prevailed, and competition was minimal.
“When organisms are interconnected through runners, they share nutrients rather than compete for them,” explained Manica.
As Ediacaran life gradually migrated from deeper to shallower waters, early animals encountered increasing pressure. Factors such as tides, storms, and fluctuations in temperature and nutrient availability all contributed to a more unstable living environment, thus intensifying competition for resources.
“Suddenly finding oneself in an environment where threats to survival arise multiple times a year drastically changes everything,” Dr. Mitchell remarked.
“Stress inherently prompts a shift to sexual reproduction, resulting in significantly increased dispersal as animals seek to colonize new areas due to heightened competition.”
“As these early animals adapted to new reproductive strategies and habitats, a notable increase in diversification occurred, leading to a ‘second wave’ of animal evolution during the Ediacaran, a trend that was further amplified in the Cambrian as animals became more mobile.”
For more information, refer to the study published in this week’s edition of Nature Ecology and Evolution.
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E. G. Mitchell and A. Manica. The influence of reproductive mode on resource competition and diversity patterns in early Ediacaran animal communities. Nat Ecol Evol, published online June 9, 2026. doi: 10.1038/s41559-026-03094-2
Source: www.sci.news
