A study in Nature Ecology & Evolution suggests that local reproduction through offshoots limited competition and diversity in early Edyterian animal communities. The shift to reproduction that allowed for wider dispersal may explain the evolutionary acceleration before the Cambrian explosion.
Some of the earliest known animals appeared in the fossil record about 574 million years ago. They didn’t look like the animals we know today: many were soft, sedentary creatures that clung to the seabed and didn’t move from place to place. Despite their early appearance, their evolution was relatively slow for a long time, until a later stage when a “second wave” of faster diversification occurred.
A new study published inNature Ecology & Evolution suggests a possible explanation for this strange pace: The evolution of the first animals may have been limited not only by environmental conditions, but also by their mode of reproduction. According to the researchers, local reproduction through branches connecting cloned individuals reduced competition within the same species, possibly inhibiting natural selection that could have led to faster diversification. (Nature)
Animals that stayed connected
The study looked at ancient animal communities from the Ediacaran period, about 635 to 539 million years ago. During this period, fossils of relatively large multicellular creatures appeared, even before the Cambrian explosion, when animals became much more diverse, mobile, and complex.
The researchers, Dr Emily Mitchell and Prof Andrea Manica from the University of Cambridge, examined fossil communities from the Avalon region, including sites in Newfoundland, Canada, and the Charnwood Forest in England. Such fossils are often preserved “in situ” on the ancient seabed, so their location can be used to learn not only about their shape, but also about their growth, reproduction and competition patterns.
The study focused on reproduction through Stolons – Extensions or threads connecting cloned individuals. In this form of reproduction, new offspring do not disperse over great distances, but remain close to the parent and sometimes connected to it. This pattern is also familiar in certain modern creatures, such as colonies of marine animals or clonal plants, although this does not mean that the Adhyakian animals were identical to them.
Less competition within the species
It seems that local, connected reproduction can be advantageous. If several individuals are connected, they may act as a unit and share resources among themselves. In such a case, one individual in a relatively resource-poor area can benefit from connecting with other individuals in better areas.
But this comes at an evolutionary cost. When individuals of the same species are connected and dispersed in dense but limited groups, competition within the same species is weakened. If there is no strong competitive pressure between individuals of the same species, natural selection works differently. Fewer individuals are pushed out, and fewer new traits gain a clear advantage.
Researchers call this phenomenon heteromyopia – A situation in which competition between species occurs on a smaller spatial scale than competition within the same species. In simpler terms: species “see” and compete with neighbors of other species before they actually compete with their own species. Such a situation can allow less adapted species to continue to exist, because the dominant species is limited in its distribution and does not easily take over the entire habitat.
Why this can hinder diversity
The study combined spatial analyses of fossils with mechanistic models of competition and dispersal. The researchers examined how different reproductive patterns affect competition and the number of species in a community. According to the model, reproduction through offshoots limits dispersal, reduces competition within the species, and can maintain relatively stable but not very diverse communities.
In contrast, when reproduction emerges that allows for wider dispersal—for example, through offspring or water-borne reproductive cells—individuals of the same species can reach new areas, compete more directly for resources, and exert stronger selection pressure. Such a change can accelerate evolutionary diversification.
According to the paper, the shift from more limited, localized reproductive patterns to more widespread, widely dispersed reproduction may explain the sharper increase in diversity seen in the late Edyterian, the “second wave” of evolution. The researchers conclude that asexual reproduction via stolons likely limited the early evolution of animals until reproductive patterns emerged that allowed for wider dispersal and competition.
Another step towards the Cambrian explosion
The finding doesn't mean that reproduction alone explained the entire history of early animals. The evolution of life in Edikar was also influenced by oxygen, ocean chemistry, seafloor conditions, predation, movement, body size, and many other environmental factors. But the study highlights an important point: Sometimes the way creatures reproduce can change the pace of their evolution as much as environmental conditions.
This is particularly important for understanding the transition from the Edyterian to the Cambrian world. In the Edyterian, communities of relatively large creatures appeared, but many of them were sedentary, local, and very strange in terms of modern animal life. In the Cambrian, which began about 539 million years ago, fossils show more diverse animals, with hard bodies, locomotion, predation, and much more complex ecosystems.
The new study suggests that even before the Cambrian explosion, there was a profound change in the way ancient animals reproduced, dispersed and competed. This change may have been one of the conditions that allowed evolution to “accelerate.”
Short FAQ:
What does the study claim? That the way ancient animals in Edikar reproduced may have limited competition and diversity, and therefore slowed the pace of evolution.
What are stolons? Extensions or threads that connect cloned individuals and limit their dispersal distance.
Does this explain the whole Cambrian explosion? No. This is one possible mechanism within a large complex of environmental, ecological, and evolutionary changes.
More of the topic in Hayadan:
