A Lack Of Sex Held Back Life’s Diversity For Millions Of Years
The way that Earth’s first animals reproduced held back life’s diversity for millions of years, until stress and competition led to the development of sexual reproduction, which in turn accelerated the pace of evolution.
Researchers from the University of Cambridge studied fossils from the oldest-known animals on Earth, dating from 574 million years ago, and found that asexual reproduction slowed the pace of evolution to a crawl, since it limited competition between different groups.
Their results, reported in the journal Nature Ecology and Evolution, could help explain a longstanding puzzle in palaeontology: why animal life appeared on Earth but then barely changed for millions of years, before a second wave of diversification gave evolutionary progress a major boost.
After billions of years of microbial life, during the Ediacaran period, between 635 and 539 million years ago, life exploded in size and the first animals appeared. Some of these earliest animals, such as Fractofusus, could grow as tall as two metres, although most were much smaller.
These animals looked more like ferns than any animal we would recognise today: they do not appear to have mouths, organs or means of movement, so they are thought to have absorbed nutrients from the water around them. And like most Ediacaran life forms, they disappeared from the fossil record at the beginning of the Cambrian period 540 million years ago, making it difficult for scientists to link them to any modern life forms.
Researchers have previously determined that these early animals reproduced asexually, by sending out clones via stolons or runners, like modern strawberry plants. In the rich waters of the Ediacaran, they thrived.
Fossils of Fractofusus, an animal from the Ediacaran period. Credit Emily Mitchell
“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.”
Mitchell and her co-author Professor Andrea Manica used a combination of laser scanning, spatial analysis and artificial intelligence to study fossils from Mistaken Point in Newfoundland, one of the world’s richest sources of fossils from the Ediacaran, to help determine why early animal evolution slowed down, and why it might have sped up again.
The researchers first showed that asexual, stolon-based reproduction limited competition, then built a computer model to simulate how early animal communities might behave under different reproductive strategies. They ran the model thousands of times while a simple neural network helped narrow down which simulations best matched the diversity patterns seen in the fossil record. This approach, known as Approximate Bayesian Computation, allowed the researchers to work backwards from the real data to estimate how far organisms spread and how strongly they competed for resources.
Using this method, the researchers showed that limited dispersal linked to asexual, runner-based reproduction could explain why early animal communities had relatively few species, and why a later shift toward wider dispersal and sexual reproduction coincided with a sudden burst of evolutionary diversity.
Dr Emily Mitchell at Mistaken Point, Newfoundland, Canada. Credit Emily Mitchell
Competition and stress have been prime drivers of evolution for billions of years, but in the deep waters of the Ediacaran, asexual reproduction meant that competition was limited. “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.
However, as life in the Ediacaran slowly spread from the deep ocean to shallower waters, early animals faced more pressures: tides, storms, changing temperatures and nutrient levels all would have made life more precarious, leading to increased competition for resources.
“If you’re suddenly in an environment where you’re essentially getting killed a couple of times per year, then that changes everything,” said Mitchell. “Stress essentially leads to sexual reproduction, and when that happens, we can see a massive increase in dispersal distances as animals attempt to colonise new areas due to an increase in competition.”
As these early animals adapted to both a new mode of reproduction and new habitats, there was a corresponding increase in diversification, leading to the Ediacaran ‘second wave’ of animal evolution, a process that accelerated further in the Cambrian, once animals became mobile.
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.
The influence of reproductive mode on resource competition and diversity patterns in Ediacaran early animal communities, Nature Ecology & Evolution (open access)
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