[/caption]
One of the great puzzles in science has been the evolution of single-celled organisms into the incredibly wide variety of flora and fauna that we see today. How did Earth make the transition from an initially lifeless ball of rock to one populated only by single-celled organisms to a world teeming with more complex life?
As scientists understand it, single-celled organisms first began evolving into more complex forms more than 500 million years ago, as they began to form multi-cellular clusters. What isn’t understood is just how that process happened. But now, biologists are another step closer figuring out this puzzle, by successfully replicating this key step – using an ingredient common in the making of bread and beer – ordinary Brewer’s yeast (Saccharomyces cerevisiae). While helping to solve evolutionary riddles here on Earth, it also by extension has bearing on the question of biological evolution on other planets or moons as well.
The results were published in last week’s issue of the Journal Proceedings of the National Academy of Sciences (PNAS).
Yeasts are a microscopic form of fungi; they are uni-cellular but can become multi-cellular through the formation of a string of connected budding cells, like in molds. The experiments were based on this fact, and were surprisingly simple, they just hadn’t been done before, according to Will Ratcliff, a scientist at the University of Minnesota (UMN) and a co-author of the paper. “I don’t think anyone had ever tried it before,” he said, adding: “There aren’t many scientists doing experimental evolution, and they’re trying to answer questions about evolution, not recreate it.”
Sam Scheiner, program director in NSF’s Division of Environmental Biology, also adds: “To understand why the world is full of plants and animals, including humans, we need to know how one-celled organisms made the switch to living as a group, as multi-celled organisms. This study is the first to experimentally observe that transition, providing a look at an event that took place hundreds of millions of years ago.”
It’s been thought that the step toward multi-cellular complexity was a difficult one, an evolutionary hurdle that would be very hard to overcome. The new research however, suggests it may not be that difficult after all.
It took the first experiment only 60 days to produce results. The yeast was first added to a nutrient-rich culture, then the cells were allowed to grow for one day. They were then stratified by weight using a centrifuge. Clusters of yeast cells landed on the bottom of the test tubes. The process was then repeated, taking the cell clusters and re-adding them to fresh cultures. After sixty cycles of this, the cell clusters started to look like spherical snowflakes, composed of hundreds of cells.
The most significant finding was that the cells were not just clustering and sticking together randomly; the clusters were composed of cells that were genetically related to each other and remained attached after cell division. When clusters reached “critical mass,” some cells died, a process known as apoptosis, which allows the offspring to separate.
This then, simply put, is the process toward multi-cellular life. As described by Ratcliff, “A cluster alone isn’t multi-cellular. But when cells in a cluster cooperate, make sacrifices for the common good, and adapt to change, that’s an evolutionary transition to multi-cellularity.”
So next time you are baking bread or brewing your own beer, consider the fact that those lowly little yeast cells hold a lot more importance than just a useful role in your kitchen – they are also helping to solve some of the biggest mysteries of how life started, both here and perhaps elsewhere.
Did they name the snowflakes? I’d love to hear how they’re doing in a few more months. Any of them have light sensitive cells? Might they grow cilia or maybe muttonchops? Just saying…Elvis had to come from SOMEWHERE!
The cynical part of me wants to say that the machinery in modern cells is so much more complex than that of cells 500mya that we’re gonna have a tough time knowing for sure how the ancients did it. If modern cells ever go full-on multicellular, like these promising yeast cultures might do, they might be using tricks (biochemical pathways) that were not available to their ancient ancestors. But I’m no biochemist so I’m just speculating.
Not that this is anything less than an amazing discovery, of course!
Beer… isn’t there anything it can’t do?
Replicate directly in the fridge?
So from now on, drinking beer will be ingesting multicellular yeast cultures?
Ok, beer is tasty enough for me to do that sacrifice…
Heineken…refreshes the “parts” other beers cant reach!
This is a really interesting model for the evolution of multicellularity.
Note that there are many caveats:
– Multicellularity has evolved ~ 20 – 40 times depending on which biologist you ask.
– True multicellularity (clonal binding; controlled regulation; cell fate specialization) evolved early. Cyanobacteria did this anytime up to ~ 3.5 billion years ago, with sheathed clone’s regulation which of the neighbors would be fated to be nitrification cells.
The “snowflake” genotype seems to lack controlled regulation, so is not true multicellular as of yet.
– Complex multicellularity (large scale differentiated) evolved independently ~ 6 times. That evolves out of true multicellular ability, but need the eukaryote ability to harness a much higher energy density than bacteria to support many more traits and cell specializations.
– Yeast have a multicellular ancestor. The apoptosis (programmed cell death) used by the “snowflake” genotype to bud off propagules (daughter flakes) is likely a trait evolved back then.
– These cells evolve by “inclusive fitness”, but the researchers seem to diagnose non-existent “group selection”.
For an interesting follow up discussion with the researchers including their analysis of “between-cluster selection” and “division of labor” group selection, see here.
For the problem with “group selection” as opposed to evolutionary biologists observation of inclusive fitness, see here.
To sum up, according to Lane’s energy theory the difficult step for eventually evolving complex multicellularity is the eukaryote endosymbiosis between cells. In our case likely a relatively complex ancestor to eukaryotes/archaea and a simple alphaproteobacteria.
The nuclear ancestor must have had endomembranes for phagocytosis (nutrient particle uptake) to become the enslaver, and the mitochondria energy factory organelle must be dependent on a scavenging metabolism to become the enslaved. This crucial combination have only happened once, whether it is rare or the existing eukaryotes hog the niche.
I think a key part of Torbjörn’s reply is: Yeast have a multicellular ancestor.
Cool experiment, but this is analogous to regulating the development of a chicken embryo to grow teeth or a tail.
The ability still existed in the yeast’s DNA and machinery, it was just activated by the selective pressures. I don’t think this was a valid example of observed evolution of multi-cellular life.
… just trying to say it first in case a pack of rabid creationists appear, but I think this will drown in all the Beer comments hehe.
There are four lines of eukaryotes which are multicellular. There are of course the three famous ones, animals, plants and fungus, but there is also the often over looked slim mold. These evolved from amoebas, and spend half of their life cycle as unicellular protistans. They then come together to form a hard body that produces fruiting bodies and spores. Slime molds appear then to represent a multicellular line which never evolved beyond this particular stage, for one reason or the other.
Yeast is not likely descended from multicellular organisms, but has some genes which are employed by fungis to become multicellular. So yeast has genes, or genotypes, which can express some aspect of multicellular properties. This may reflect some early lineage of fungi which were then similar to the slim mold. The laboratory procedure is a straight forwards selection procedure.
LC
Note though that “plants” are paraphyletic, so aren’t an evolutionary line. They comprise two or three entirely separate lines evolving complex multicellular traits:
– brown algae, with protist ancestry.
– green algae.
– and possibly separately, land plants.
This is why you see biologists claim roughly that “Complex multicellularity (large scale differentiated) evolved independently ~ 6 times.”
The link I gave references the claim that yeast derives from multicellular ancestors. The primary author:
“Saccharomyces cerevisiae lost multicellularity hundreds of millions of years ago.”
I agree on the slime molds, they use some of the same proteins that we do to latch cells together in the fruiting stalk when they sporulate.
I had been under the impression that plants all shared the same multicellular clade or origin.
If Saccharomyces cerevisiae is a multicellular fungi which “went rogue” as single celled organism, then is there a cladistic map of what multicellular fungi it is related to?
LC
I don’t know if they have sorted the apoptosis out, it could be de novo.
The rest (insufficient separation of daughter cells) is de novo, or at least it doesn’t need to be the animal way of hooking cell’s cytoskeletons together with protein bridges.
So this could be anything from reusing old abilities to de novo, but the likely explanation is in between.
“Coo-Roo-Coo-Coo-Coo-Coo-Coo-Coo! Take Off, Eh?”