Multicellularity may have evolved without direct benefits

The environment plays an important role in the emergence of evolutionary innovations

To the Point

• Mathematical model: The study shows that multicellularity can evolve without direct benefits over unicellular ancestors. In spatially heterogeneous environments, indirect ecological effects may support this transition, including escape from competition and more effective use of resource-rich habitats.
• Depending on the conditions: The model predicts either replacement of the ancestral unicellular life cycle or the coexistence of multiple life cycles.

How multicellular life arose from unicellular ancestors is one of the central questions in evolutionary biology. It is often assumed that early multicellular groups must have enjoyed direct advantages over single cells. So far, however, there is little evidence that the first such groups already had traits such as faster division, lower mortality or a developed division of labour.

A new theoretical study involving the Max Planck Institute for Evolutionary Biology now asks whether multicellularity could become established even without such immediate benefits. The answer, the authors suggest, is yes – under certain ecological conditions.

Living in different environments

To explore this question, the researchers developed a mathematical model in which a rare multicellular life cycle competes with an abundant unicellular ancestor across two different but connected environments. Within this framework, multicellularity can spread even when group living is locally disadvantageous.

The model identifies two possible mechanisms. In the first, multicellular groups escape competition by occupying locations that are less favoured by their unicellular ancestors. In the second, they benefit because they tend to exploit resource-rich environments more effectively.

The relevant advantage therefore does not necessarily lie in groups outperforming single cells under identical local conditions. Instead, it may emerge because groups experience different environments on average. In this way, the study highlights the role of ecology in shaping major evolutionary transitions.

In the Proterozoic Ocean

As a case study, the researchers apply their framework to the Proterozoic Ocean, a period in Earth’s history during which several multicellular eukaryotic lineages are thought to have originated. In that setting, too, the model suggests that ecological context may have helped early multicellular forms to become established.

The study does not directly reconstruct the past. Rather, it provides a theoretical framework for understanding the ecological conditions under which multicellularity might evolve.