Scientists have found that a shift towards sexual reproduction did more than create genetic diversity – it may have been one of the key innovations that transformed life on Earth from a world of relatively static organisms into complex ecosystems.
The study, published in Nature Ecology & Evolution, examined some of the oldest known animal fossils on Earth and suggests that asexual reproduction limited competition and slowed evolutionary change during the Ediacaran Period, more than 540 million years ago.
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Researchers from the University of Cambridge focused on fossils from Mistaken Point in Newfoundland, Canada, one of the world’s most important fossil sites for preserving early animal life. Their findings could help explain a longstanding mystery in palaeontology: why animals first appeared on Earth but then remained relatively unchanged for millions of years before diversification suddenly accelerated.
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The Ediacaran Period, which lasted from around 635 to 539 million years ago, marked a turning point in the history of life. After billions of years dominated by microbes, large and complex organisms began to appear in the oceans. Many of these creatures looked unlike any animals alive today, with some resembling fronds or ferns, lacked mouths and organs, and are thought to have absorbed nutrients directly from seawater.
Among them was Fractofusus, an organism that could grow up to two metres long. Previous studies have shown that many Ediacaran species reproduced asexually, spreading across the seafloor by sending out clones connected by runner-like structures similar to those used by modern strawberry plants.
“Life was pretty nice during the Ediacaran, so the need for sex was rather 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.”
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To investigate how this reproductive strategy influenced evolution, Mitchell and co-author Professor Andrea Manica combined laser scanning, spatial analysis and artificial intelligence to study fossil communities preserved at Mistaken Point. The team then developed computer models to simulate how early animal populations behaved under different reproductive strategies.
Their results suggest that asexual reproduction restricted the dispersal of organisms and reduced competition between neighbouring individuals. Because many colonies remained physically connected, they effectively shared resources rather than competing for them.
“If you’re connected to your neighbour by these runners, then you’re sharing nutrients and you don’t need to compete with them,” said Manica.
Over time, however, environmental conditions began to change. As animal communities expanded into shallower waters, they encountered new challenges, including storms, tides, temperature fluctuations and shifting nutrient supplies. These stresses may have encouraged the evolution of sexual reproduction, which allowed offspring to spread farther afield and colonise new habitats.
According to the researchers, this increase in dispersal led to greater competition and, ultimately, faster evolutionary change.
“Stress essentially leads to sexual reproduction,” said Mitchell. “When that happens, we can see a massive increase in dispersal distances as animals attempt to colonise new areas.”
The shift appears to coincide with a second wave of diversification during the late Ediacaran, setting the stage for the Cambrian Explosion, when mobile animals rapidly evolved into a variety of forms.
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