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The rise of multicellularity was a major transition in evolution and set the stage for unprecedented increases in complexity, especially in land plants and animals. However, despite its nearly unrivaled importance, scientists have never been able to come up with a clear answer to why it actually happened in the first place.
How and why organismal complexity increases are central questions in evolutionary biology. Both the maximum and the average levels of complexity have increased exponentially from the origin of life to the present day (although the vast majority of life forms remain simple).
It’s almost unfathomable to compare an amoeba with an elephant and appreciate that somewhere down the evolutionary tree they both have a common ancestor. These large increases in organismal complexity have been theorized to result from a series of events in which existing individuals combined to become components of a new kind of individual with parts specialized to play various roles.
At least that was the prevailing theory, but there was no strong evidence to prove it… until now.
New research from a group working out of the University of Montana has finally provided proof for at least one potential cause of this pivotal transition. Using a relatively simple experimental setup, the scientists were able to observe the transition of single-celled algae into multicellular colonies when isolated under intense selective pressure from a predator.
This is a startling and potentially invaluable development to the field of evolutionary biology, and can greatly influence our understanding of how life actually works as well as our place among it.
What’s the big deal with multicellularity?
Nearly all macroscopic life is multicellular. But the existence of this state of being is an outcome of evolutionary processes, not a starting condition.
It seems, in fact, to be a common outcome, because as it turns out, multicellular organisms have actually evolved from unicellular ancestors dozens of times! Animals, plants, fungi, and a host of other beings all descended from different unicellular ancestors.
But despite the process being so apparently favorable in nature, we couldn’t really tell how the transitions actually took place. The two primary approaches to the study of this, comparative methods and fossil records, were entirely retrospective by nature. And although both approaches have been critical to our understanding of early multicellular evolution, each has its limitations.
For one, little or no fossil evidence exists that is relevant to the first steps in the transition from unicellular to multicellular life for most multicellular groups. And comparative methods suffer from a lack of intermediate forms between the multicellular organisms we are interested in and their existing unicellular relatives.
And there is no real way of knowing if these relatives are more than just a poor approximation of their common ancestor as both groups have been evolving independently since they diverged.
It was always important to improve on these approaches to the study because knowing more about this fundamental transition in evolution would help us to understand the greater intricacies within the greater building blocks of life. The problem is, there wasn’t a clear way of doing that. Until now…
What makes this study different?
The new research is so potentially impactful because it sought out a new solution that had only ever been theorized to studying the multicellular transition—replicate it.
In theory, if one was able to turn single-celled organisms into multi-celled organisms under laboratory conditions, it would enable real-time observations of morphological, developmental, and genetic changes that attend the transition to multicellular life. All data that could prove to be invaluable to ongoing research.
It’s hard to overstate just how impossible everyone in the field thought something like this was. The problem was just too complex, with far too many variables that were all thought to be acting together over exceptionally long periods of time to give rise to multicellularity.
But the scientists working on this study had a theory that a carefully selected unicellular candidate and just a single type of selective pressure could give them the results they were looking for.
But what kind of evolutionary force could be powerful enough to do this all on its own? The answer as it turns out, comes down to the simplest and most ancient struggle on Earth:
Predation.
The Multicellular How-To:
Predation has long been hypothesized as a plausible selective pressure that could explain at least some origins of multicellularity, as most predators can only consume prey within a narrow range of sizes (IE becoming multicellular make them too big to be eaten).
Furthermore, since filter-feeding predators are common in aquatic ecosystems, and algae that are larger than a threshold size are largely immune to them, the researchers proposed that this ‘predation threshold’ could explain the evolution of multicellularity in ancient, single-celled algae.
Of course, more importantly, they believed that they could make it happen again.
They devised an experiment in which they used a ciliate predator (a single-celled blob that moves around using little "hair" motors called cilia) known as Paramecium Tetraurelia to select for spontaneous evolution of multicellularity in populations of single-celled algae.
The made 5 control and 5 predator-selected populations of the algae, then allowed them to grow unhindered, transferring portions of each sample to fresh environments weekly. They did this for just under a year, which equated to ~750 generations for the algae, and made observations as they went.
Their results were undeniable.
The incredible results:
By the end of their testing, two of the five predator-selected populations had evolved simple multicellular structures, while none of the control populations had anything even resembling this.
Even more incredibly, they were able to show that these changes were more than just merely cells aggregating together, but were, in fact, part of new, heritable multicellular life cycles that had independently evolved in the algae strains.
Additionally, the scientists then go on to show that these new multicellular life cycles are stable over thousands of generations, meaning that beyond just protecting them from predators, the algae were experiencing a distinct fitness advantage because of their adaptation.
And most important of all, because the algae chosen for this test had no multicellular ancestors, these experiments represent a completely novel origin of obligate multicellularity. This is a genre-changing discovery for cellular biologists because these results show that the transition to a simple multicellular life cycle can happen rapidly in response to an ecologically relevant selective pressure.
While this is certainly a huge achievement for the field, there is still a lot to be learned and a whole lot of data left to be analyzed.
Were the genes for multicellularity already within the algae and just needed a change in expression to start working? Would the same results be seen in unicellular organisms from other phylums?
We may not know the answers to all of these questions yet, but for now, we can at least know that we are one step closer to learning them. As well as just that one bit more informed about how we came to be and our place in the world.
Until next time, enjoy your spot as a multicellular being on top of the food chain!