It seems that a key evolutionary step the step from unicellular organisms to multicellular ones is not as difficult as scientists thought. Researcher William Ratcliff and his team at the University of Minnesota in St. Paul now have concrete evidence that suggests that the key step to multicellularity takes about 60 days for a yeast species called brewer’s yeast. They ought to know; they saw it in their own labs.
Their experimental methodology mimicked that followed by Lenski and his team in their famous experiment on bacteria. Ratcliff et al grew yeast in a liquid and gently centrifuged out each culture daily. They then injected the next batch with the yeast collected at the bottom of each tube. Imagine a long line of test tubes, each subsequent test-tube having a culture one day more recent. Now imagine ten such lines. That’s basically how the experiment was done.
After only 60 days or 350 generations, each of the ten lines of test-tubes (technically called culture lines’) showed multicellularity. They evolved to form a so-called snow-flake’ form. Ratcliff and his team noticed that this was different from mere clumping together of cells. The cells were actually co-operating with each other and there was a basic division of labor. Further, these snowflake clumps break off after a certain size, giving rise to progenies, very much like many other asexually reproducing multicellular organisms like sponges.
Hints of Multi-cellular behavior
The researchers were really dumbfounded to find an elementary notion of division of labour at work. Further, certain adult cells would die off via a programmed cell death process called apoptosis. Apoptosis happens only in multi-cellular organisms. Dying is not an evolutionarily correct strategy for a unicellular organism, but is a good one for a multi-cellular organism. Programmed cell death of old cells helps other cells survive better, thus helping the individual. Further, for asexually reproducing species, dying cells can provide fissure lines, along with part of the organism can break off and give rise to progenies, increasing the chances of survival. This is a clear sign that the clumps are evolving as one.
No, we are not there at the origin of life yet, but we’re getting close.
Neil Blackstone, an evolutionary biologist at Northern Illinois University raises an important objection to the experiment. He argues that
yeast, having once been multicellular, never lost it completely,
He argues that something which has never achieved multi-cellularity would never have evolved it so fast.
Ratcliff and team hope to answer that question by repeating the experiment with Chlamydomonas, a single celled alga with no history of multi-cellularity. In the meantime, they will continue to monitor the yeast looking for signs of further division of labor.