The transition to multicellularity was one of a few major events in life’s history that created new opportunities for more complex biological systems to evolve. As this transition fundamentally changes what constitutes an individual, dissecting the steps in this transition remains a major challenge within evolutionary biology.

Compared with other major transitions in evolution that occurred just once (for example, the origin of eukaryotes), multicellularity has evolved repeatedly. Most origins of multicellularity are ancient and transitional forms have been lost to extinction, so little is known about the potential for multicellularity to evolve from unicellular lineages, or the route through which a multicellular life history arises.

In a new paper from NASA Astrobiology-funded researchers at the University of Montana and their colleagues, published this week in Nature Communications, new light is shed on the evolutionary problem of how life transitioned from unicellularity to multicellularity.

The team shows how a single-celled alga can evolve a crude form of multicellularity in the lab – a configuration it never adopts in nature: a chance to replay one of life’s most important evolutionary leaps in real time.

This is the second time a single-celled organism has done this in the lab – two years ago, the same was done with brewers yeast. But the alga is an entirely different organism, and comparing the two could explain how the transition to multicellular life happened a billion years ago.

Multicellularity has evolved at least 20 times since life first began, but no organisms have made the leap in the past 200 million years, so the process is difficult to study. To replicate the step in the lab, the team grew 10 cultures of a single-celled alga. Every three days, they centrifuged each culture gently and used the bottom tenth to found the next generation. Since clusters of cells settle faster than single ones, this meant that they effectively selected for algal cells that had a tendency to clump together.

Sure enough, after about 50 generations, algal cells in one of the 10 cultures began to form clusters. To the researchers’ surprise, these clusters – the first step towards true multicellularity – seemed to pass through a coordinated life cycle. Cells stuck together for hours while they settled, then quickly broke apart into single cells again each of which then divided to form new multicellular colonies.

The team used a similar technique to evolve multicellularity from a single cell of yeast. However, critics noted that although modern yeasts are single-celled, they have descended from a multicellular ancestor, so the yeast may have merely been exhibiting an ancestral hangover. The alga, on the other hand, has always been unicellular.

Another difference between the two organisms is that they become multicellular in different ways. Individual yeast cells remain attached to one another after cell division to form multicellular “snowflakes” that reproduce by breaking off arms. The algal cells, in contrast, divide fully but the cells remain embedded in a jelly-like sheath. This multicellular mass later releases individual cells to reproduce.

Further study of the differences could shed light on why multicellularity has developed differently in the various lineages of life.

The study suggests that multicellularity itself is not necessarily a difficult evolutionary hurdle.