The first animals on Earth took millions of years to evolve, a slowness that surprises paleontologists. How to explain it when life had already emerged? A team from the University of Cambridge provides an answer: asexual reproduction long slowed down diversity.
During the Ediacaran, 574 million years ago, the first animals looked like ferns. They had neither mouth nor organs, and fed by absorbing nutrients from the water. To reproduce, they used stolons, a kind of stem that gave rise to clones, like today's strawberry plants.
Illustration of an Ediacaran animal community.
Credit: Hugo Salais
In the rich waters of the Ediacaran, life was peaceful. Without predators or competitors, the animals had no need to change. The researchers showed that asexual reproduction limited competition between individuals, because the clones connected by stolons shared resources. Result: evolution stagnated.
To understand this phenomenon, scientists analyzed fossils from Newfoundland using a laser, artificial intelligence, and a computer model. This model, called Approximate Bayesian Computation, made it possible to simulate thousands of scenarios to see how different reproductive strategies affected diversity.
Their results are consistent with fossil data: the limited dispersal due to stolons explains why the first animal communities had few species. Then, when some animals migrated to shallower waters, they experienced stresses such as tides, storms, and temperature variations. This pressure favored the emergence of sexual reproduction.
Fossils of Fractofusus, an Ediacaran animal.
Credit: Emily Mitchell
With sexual reproduction, dispersal distances increased, allowing animals to colonize new territories. Competition intensified, speeding up evolution. This led to the "second wave" of the Ediacaran, then to the Cambrian explosion where life became mobile and diverse.