Multicellularity Long-Term Evolution Experiment
Welcome to the MuLTEEverse
A long-term evolution experiment on the origin of multicellularity, run with snowflake yeast in the Ratcliff Lab at Georgia Tech.
- Generations
- 9,000+
- Populations
- 15
- Treatments
- 3
- Frozen samples
- 3,000+
- First transfer
- 2018
The origin of multicellular life was one of the most profound transitions in the history of life on Earth, allowing for the evolution of the large, complex organisms we see today. Understanding how it happened has been extremely challenging, largely because the earliest steps occurred hundreds of millions of years ago and left almost no direct record. Here, we take the opposite approach: rather than reconstructing the transition from fossils and phylogenies, we run it forward under conditions we control. Each day, fifteen populations of snowflake yeast grow for 24 hours and are then selected for how fast they settle through liquid, which depends mainly on cluster size. The growth phase is also selective, and it favours the opposite: small clusters that divide quickly. Everything that has evolved since, from millimetre-scale bodies visible to the naked eye to whole-genome duplication, has come out of the tension between these two phases.
What the experiment has shown
Key results
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01
Evolution of macroscopic size through cell elongation and branch entanglement
After 600 rounds of daily settling selection, roughly 3,000 generations, all five anaerobic populations evolved millimetre-scale bodies. Mean cluster radius rose from 16 µm to 434 µm, about a 20,000-fold increase in volume. As a material, these organisms went from weaker than gelatin to as tough as wood.
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02
Convergent whole-genome duplication driven by size selection
Diploid snowflake yeast under size selection became tetraploid within the first 50 days and fixed tetraploidy by day 100 in all ten anaerobic and mixotrophic populations, then held it through day 1,000. Select against large size and the evolved tetraploids readily shed ploidy, so it is selection rather than genome stability holding the state in place.
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03
Stable coexistence of small and large lineages through frequency-dependent selection
In the obligately aerobic treatment, three of five populations split from a monomorphic ancestor into coexisting small and large lineages that persisted together for about 4,300 generations. The two lineages compete over dissolved oxygen: small clusters compete better during growth and large clusters survive settling better. Group size alone, with no differentiated tissues, was enough to partition a niche.
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04
Metabolically driven fluid flows sustain exponential growth at macroscopic scale
Diffusion should limit exponential growth to bodies about 50 µm across, yet evolved anaerobic clusters keep growing exponentially in static liquid at millimetre scale, because their own metabolism drives fluid flows through them. Transport arrives before any transport organ evolves, providing a clear case of biophysical scaffolding.
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05
Convergent Hsp90 down-regulation underlies the evolution of cell elongation
All five anaerobic lineages that became macroscopic converged on reduced Hsp90 expression; the mixotrophic line that stayed microscopic did not. A protein-folding system conserved across eukaryotes was modified by selection acting on a group-level trait.
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06
Loss of the first-division delay and the emergence of coordinated cell division
The anaerobic ancestor divides asynchronously because daughter cells take about 25% longer to complete their first division than later ones. That delay builds more branched topologies, which concentrate mechanical stress and fracture sooner. Anaerobic populations had lost the delay by day 200 and stayed synchronous through day 1,000.
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07
Morphological entanglement is generic to growing branched organisms
Segmented 3-D electron-microscopy reconstructions showed evolved branches caged by two or more neighbours, in configurations that rigid-body translation and rotation cannot undo. Simulations found that the narrow geometric window familiar from non-living entangled materials applies only at short times; given continued growth, most permissible geometries eventually entangle. Timescale, not geometry, is the control variable, which means tough, fracture-resistant bodies can arise early and generically, before any developmental control evolves.
Full findings, population by population, are on the Populations page →
Before the MuLTEE
The MuLTEE grew out of two earlier projects. In 2012, Ratcliff and colleagues showed that ten populations of unicellular S. cerevisiae all evolved snowflake clusters within 60 transfers of settling selection; the genetic basis was later traced to loss of a single transcription factor, ACE2. In 2018, Bozdag established twenty populations to test how oxygen affects size evolution. Results from the first 145 transfers, published in 2021, showed that intermediate oxygen suppressed size evolution while anaerobic and high-oxygen conditions permitted it. Fifteen of those twenty populations were kept running and became the MuLTEE. See the full story →
Enter the MuLTEEverse yourself
We are committed to open, transparent science, to collaboration, and to education.
Protocols
All our protocols for working with snowflake yeast, from daily growth and settling selection to phenotyping, posted as they are written up.
Genomes & phenotypesData
All our published genome sequences and phenotype measurements, ready to download and analyse.
EducationOutreach
Classroom kits, lab guides and an interactive simulator for young future researchers.
Further reading
Publications
Every paper that uses MuLTEE populations, plus the snowflake yeast work the experiment is built on.
24 termsGlossary
Entanglement, ratcheting traits, the unicellular bottleneck and the rest of the vocabulary, defined.
In the mediaPress
The experiment in the New York Times, The Atlantic, Quanta, National Geographic and Smithsonian.