A new study suggests that Earth’s early animals may have unintentionally slowed the rise in biodiversity. The way these ancient organisms reproduced limited competition and allowed them to evolve at a significantly slower pace for millions of years, according to researchers at the University of Cambridge.
The survey results are natural ecology and evolutionoffers a possible solution to a long-standing paleontological mystery. Animals first appeared during the Ediacaran period, but their diversity remained relatively limited for an astonishingly long time, until a subsequent burst of evolutionary innovation dramatically expanded life on Earth.
early life experiments
After billions of years of microbial dominance, the first animals appeared during the Ediacaran period, which lasted from about 635 to 539 million years ago. Some of these creatures include; fructophususreached a height of up to 2 meters, but most were much smaller.
These early animals were very different from those living today. Many were more fern-like than modern animals, and likely lacked mouths, organs, and locomotion. Scientists believe they absorbed nutrients directly from the surrounding seawater.
Like many other Ediacaran organisms, they disappeared from the fossil record at the beginning of the Cambrian period, approximately 540 million years ago. Their disappearance has made it difficult for researchers to determine whether they are related to currently living animals.
Previous studies have shown that many of these organisms reproduce asexually. They spread by producing genetically identical offspring connected by stolons or runners, much like modern strawberry plants. In Ediacaran’s nutrient-rich waters, this strategy worked extremely well.
“Life in the Ediacaran period was quite good, so the need for sex was quite limited,” says lead author Dr Emily Mitchell from the Cambridge School of Zoology. “There was relatively little competition, so there was no real pressure to change anything.”
Fossils, AI, and ancient ecosystems
To investigate why evolution appears to have slowed during this period, Mitchell and co-author Professor Andrea Manica studied fossils from Mistaken Point in Newfoundland, one of the world’s most important Ediacaran fossil sites.
The researchers combined laser scanning, spatial analysis, and artificial intelligence to investigate how these ancient communities organized and interacted.
They were the first to demonstrate that asexual reproduction by runners reduces competition between neighboring organisms. The team then created computer simulations to test how early animal communities developed under different reproductive strategies.
Thousands of simulations were run while a simple neural network identified the scenario that best matched the fossil evidence. The researchers used a technique known as approximate Bayesian calculations to work backwards from the fossil record to estimate how far organisms spread and how intensely they competed for resources.
Why is competition important?
The results show that limited dispersal caused by asexual reproduction may explain why early animal ecosystems contained relatively few species. Diversity then increased dramatically as organisms spread farther and began to reproduce sexually.
Competition has long been one of the most important drivers of evolution. However, many Ediacaran creatures had a runner-based lifestyle, which reduced the need for competition.
“If you connect with your neighbors through these runners, you’re sharing nutrients and you don’t have to compete with them,” Manika said.
The situation became more difficult as organisms gradually expanded from deep to shallow marine environments. Tides, storms, changes in temperature, and changes in nutrient availability created new pressures that made survival less predictable and increased competition for resources.
Increased stress and sexual reproduction
Researchers argue that this harsh environment may have encouraged the transition to sexual reproduction.
“When you suddenly find yourself in an environment where you’re basically getting killed several times a year, everything changes,” Mitchell said. “Stress inherently leads to sexual reproduction, and when that happens we see a significant increase in dispersal distances as animals try to establish themselves in new areas due to increased competition.”
Species diversification accelerated as early animals adapted to new habitats and new reproductive strategies. This period marked the second major wave of Ediacaran evolution and helped lay the groundwork for the even more dramatic evolutionary expansion of the Cambrian period, when animals became mobile and ecosystems became much more complex.
This research was supported by the Natural Environment Research Council (NERC), part of UK Research and Innovation (UKRI). Emily Mitchell is a Fellow of Newnham College, Cambridge. Andrea Manica is a Fellow of Clare College, Cambridge.
