For millions of years, some of Earth’s earliest animals barely changed.
They lived, grew, and spread across the seafloor, but evolution seemed stuck in slow motion.
Now, a new study suggests that the reason may have been surprisingly simple: these ancient creatures were reproducing without sex.
Researchers from the University of Cambridge have found evidence that asexual reproduction may have limited the diversity of early animal life, slowing evolution for millions of years.
Their findings, published in Nature Ecology & Evolution, suggest that the later rise of sexual reproduction helped trigger a burst of evolutionary innovation that transformed life on Earth.
The study focuses on animals from the Ediacaran Period, which lasted from about 635 million to 539 million years ago.
This was the time when the first large and complex animals appeared after billions of years of mostly microscopic life.
Many of these early creatures looked very different from animals alive today.
Some resembled giant fern-like structures rather than recognizable animals. One example, called Fractofusus, could grow up to two meters tall.
These organisms had no mouths, no organs, and no ability to move. Scientists believe they absorbed nutrients directly from the surrounding seawater.
Previous research showed that these ancient animals reproduced asexually. Instead of mating, they produced clones of themselves through structures similar to the runners used by modern strawberry plants.
New individuals grew from these runners while remaining connected to the parent organism.
According to the researchers, this lifestyle worked well in the stable and resource-rich environments of the deep ocean. There was little competition, plenty of nutrients, and few reasons for animals to change.
To better understand how this affected evolution, the research team studied fossils from Mistaken Point in Newfoundland, Canada, one of the world’s most important sites for Ediacaran fossils. Using laser scanning, spatial analysis, artificial intelligence, and computer simulations, they recreated how these ancient communities may have functioned.
The results suggested that asexual reproduction kept competition low because neighboring organisms were often genetically identical and physically connected. Instead of competing for resources, they could effectively share them.
This reduced pressure for adaptation and innovation. As a result, relatively few new species emerged, and evolutionary change remained slow.
Things began to change when some of these organisms spread into shallower waters. Life there was much less predictable. Tides, storms, temperature fluctuations, and changing nutrient supplies created a more stressful environment.
Under these harsher conditions, competition for resources increased. The researchers believe this stress may have encouraged the development of sexual reproduction.
Unlike asexual reproduction, sexual reproduction creates genetic variation. Offspring are genetically different from their parents, giving populations a greater chance of adapting to new challenges. It also allows organisms to spread farther and colonize new habitats.
The study found that this shift was linked to a dramatic increase in biodiversity during a second wave of Ediacaran evolution. Evolution accelerated even further during the Cambrian Period, when animals became mobile and competition intensified.
The findings suggest that sex played a crucial role in shaping the history of life on Earth. By increasing genetic diversity and competition, sexual reproduction may have helped transform a world of relatively simple organisms into the rich variety of animal life we see today.
Source: University of Cambridge.
