The earliest animals on Earth may have inadvertently slowed the evolution of biodiversity, a new study suggests. According to researchers at the University of Cambridge, the way these ancient organisms reproduced limited competition and slowed evolution remarkably for millions of years.
Conclusion, published in Nature Ecology and EvolutionOffers a possible solution to a long-standing paleontological mystery. Although animals first appeared during the Ediacaran period, their diversity remained relatively limited for a surprisingly long time before a subsequent burst of evolutionary innovation dramatically expanded life on Earth.
early experiment of life
After billions of years of dominance by microbes, the first animals emerged in the Ediacaran period, which lasted from about 635 million to 539 million years ago. Some of these creatures also include fructofususReached heights of up to two metres, although most were much smaller.
These early animals looked very different from animals living today. Many were more similar to ferns than modern animals and lacked mouths, limbs, and the ability to move. 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, about 540 million years ago. Their disappearance has made it difficult for researchers to determine whether they are related to any animals alive today.
Previous studies have shown that many of these organisms reproduce asexually. They spread by producing genetically identical offspring attached to stolons, or runners, like modern strawberry plants. In the nutrient-rich oceans of the Ediacaran, this strategy worked very well.
“Life was very good during the Ediacaran, so the need for sex was limited,” said lead author Dr Emily Mitchell, from Cambridge’s Department 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 appeared to slow during this period, Mitchell and co-author Professor Andrea Manica studied fossils from Mistaken Point in Newfoundland, one of the most important Ediacaran fossil sites in the world.
The researchers combined laser scanning, spatial analysis and artificial intelligence to investigate how these ancient communities were organized and how they may have interacted.
He first demonstrated that asexual reproduction through runners reduced competition between neighboring organisms. The team then created computer simulations to test how early animal communities might have evolved under different reproductive strategies.
Thousands of simulations were run while a simple neural network identified which scenarios most matched the fossil evidence. Using a technique called approximate Bayesian calculations, researchers worked backwards from the fossil record to estimate how far organisms spread and how intensely they competed for resources.
Why competition matters
The results showed that limited dispersal due to asexual reproduction could explain why there were relatively few species in early animal ecosystems. Later, when organisms began to spread over greater distances and reproduce sexually, diversity increased dramatically.
Competition has long been one of the most important drivers of growth. However, the runner-based lifestyle of many Ediacaran organisms obviated the need to compete.
“If you are connected to your neighbor through these runners, you are sharing nutrients and you don’t need to compete with them,” Manika said.
As life gradually expanded from deep water into shallow marine environments, conditions became much more challenging. Tides, storms, changes in temperature and changing availability of nutrients introduced new pressures that made survival less predictable and increased competition for resources.
Stress and the rise of sexual reproduction
Researchers argue that these harsh conditions may have encouraged the transition toward sexual reproduction.
“If you’re suddenly in an environment where you’re essentially getting killed a few times per year, it changes everything,” Mitchell said. “Stress inevitably leads to sexual reproduction, and when that happens, we may see huge increases in dispersal distances as animals attempt to colonize new areas due to increased competition.”
As early animals adapted to new habitats and new reproductive strategies, species diversification accelerated. 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 far more complex.
The 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.