No sex: Why early animal life on Earth was held back for millions of years

Scientists suggest Earth's earliest animals reproduced asexually, slowing evolution and delaying the biodiversity boom that transformed life.

In Short

• Earliest animals reproduced asexually, limiting evolution
• Fossils from 574 million years ago were analysed using AI
• Shift to sexual reproduction triggered biodiversity surge

Researchers have uncovered a possible reason why animal life on Earth remained relatively stagnant for millions of years after its first appearance: the earliest animals reproduced without sex.

A new study by scientists at the University of Cambridge suggests that asexual reproduction among Earth’s earliest animals limited competition and slowed evolutionary change, delaying the explosion of biodiversity that would later transform life on the planet.

The findings, published in the journal Nature Ecology & Evolution, help explain a long-standing mystery in palaeontology. While the first animals emerged during the Ediacaran Period, between 635 million and 539 million years ago, they showed remarkably little evolutionary change for millions of years before a dramatic diversification occurred.

The researchers focused on fossils dating back about 574 million years, including those of Fractofusus, one of the oldest known animals. These strange organisms looked more like fern-like structures than modern animals. They lacked mouths, organs and the ability to move, likely absorbing nutrients directly from surrounding seawater.

Previous studies had shown that these organisms reproduced asexually, sending out clones through stolons or runners, much like modern strawberry plants. According to the new research, this reproductive strategy may have been a key factor limiting evolutionary innovation.

“Life was pretty nice during the Ediacaran, so the need for sex was rather limited,” said lead author Emily Mitchell. With abundant resources and little competition, there was little pressure for organisms to adapt or diversify.

To investigate further, Mitchell and co-author Andrea Manica combined laser scanning, artificial intelligence and computer modelling to analyse fossils from Mistaken Point, one of the world's richest Ediacaran fossil sites.

Their simulations showed that asexual reproduction limited the distance offspring could spread, resulting in relatively isolated communities with low species diversity. Because neighbouring organisms were often genetically identical and physically connected, competition for resources remained weak.

However, conditions eventually changed. As life gradually expanded into shallower waters, organisms encountered new environmental stresses, including storms, tides, temperature fluctuations and changing nutrient availability. These harsher conditions increased competition and favoured the emergence of sexual reproduction.

The shift to sexual reproduction allowed offspring to disperse farther and generated greater genetic variation, giving species a better chance of adapting to changing environments. This, the researchers argue, triggered a second wave of diversification during the late Ediacaran and set the stage for the famous Cambrian Explosion about 540 million years ago.

The study highlights how environmental stress and competition may have played a crucial role in pushing early animals toward sex, a development that ultimately accelerated the evolution of complex life on Earth.

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