One of the biggest evolutionary hurdles for life on Earth was the jump from single-celled to multi-cellular organisms...or at least, that's what we thought. Scientists set out to replicate this evolutionary leap in laboratory conditions. It took them two months.
There almost certainly wasn't one single leap to multicellularity, and scientists suspect there were about twenty distinct instances in which single-celled organisms evolved into multicellular creatures. But since the most recent documented case of this happened 200 million years ago, it's difficult for us to say a lot about its specific mechanisms. But now, thanks to researchers at the University of Minnesota, there's now been a 21st multicellular evolution.
The researchers took brewer's yeast, a single-celled organism often used in lab experiments. They placed the yeast in a liquid and, at the end of each day, centrifuged the various yeast cultures. They then took the yeast that settled at the bottom of the container and used that for the next day's experiments. The heavier yeast would tend to sink to the bottom, which means the scientists were artificially selecting for yeast that would clump together with other cells.
After 60 days and 350 generations, all ten of the yeast cultures had evolved into what the researchers dub a "snowflake" form. But these weren't different yeast organisms clumping together - all the cells in these snowflakes shared identical DNA, meaning they were formed from yeast cells that had stayed connected after cellular division. These snowflakes can grow bigger through continued cell division, and eventually a portion of the organism breaks off to form a new snowflake, which is a pretty good approximation of reproduction in other simple multicellular organisms.
Researcher William Ratcliff explains:
"The key step in the evolution of multicellularity is a shift in the level of selection from unicells to groups. Once that occurs, you can consider the clumps to be primitive multicellular organisms."
There is, however, one caveat that should be kept in mind. Brewer's yeast has evolutionary organisms that were multicellular, and critics of this research suggest that these yeast simply activated a vestigial ability to become multicelllular, rather than evolving into something entirely new.
To answer these criticisms, Ratcliff and the rest of the team plans to repeat their experiments with Chlamydomonas, another single-celled organism with no multicellular ancestry. It might well take a lot longer than 60 days to make it multicellular, but Ratcliff is optimistic that this result can be repeated, and that these experiments can help us understand one of the most crucial phases in our evolutionary history.