I came across a lengthy article published last month on Science that went deep into the biology of a question I’ve always been personally curious about: how did the rudimentary unicellular life of the past (and present) evolve into the complex multicellular life that now dominates the planet? I found the piece fascinating and thought you might as well, so staying true to the name of this blog, I’ve summarized it below.
Original Piece: "The momentous transition to multicellular life may not have been so hard after all" by Elizabeth Pennisi for Science.
Science Summarized: Until recently, we didn’t really know how single-celled organisms transitioned into multi-celled organisms, even though multicellularity has been in fashion for a long time (fossil evidence of multicellular algae dates back ~2-3 billion years).
Scientists have come to three main conclusions thanks to recent advances and discoveries in the fields of genetics, cell biology, animal development and evolutionary biology:
Organisms have been evolving for billions of years. Single- and multi-celled organisms have long shared the same basic cellular machinery, and can over time reprogram their genes to change that machinery to develop multicellular capabilities as needed.
Multicellularity can evolve quickly. The biological processes needed for multicellularity to flourish can evolve relatively quickly -- new experiments suggest only a few hundred generations (that’s quick in biological time!)
Change is not “one and done”. Some organisms have made the transition more than once in their history of being alive, for example, some fungi have unicellular and multicellular siblings.
The experiments that led to these conclusions are a little science-heavy, unless you can say “Chlamydomonas” five times fast. Some highlights include:
Genes previously thought to be exclusive to multicellular organisms are also found in unicellular ones (and vice versa); one scientist found that an ancient unicellular organism both looked similar to and had the same genes as the cells that make up the inside of a sea sponge, which regarded to be the oldest multicellular organism on Earth, suggesting that evolutionary link.
Single-celled organisms are capable of gene regulation -- meaning they can turn on and turn off genes as needed in order to survive-- something previously thought to be exclusive to those of us endowed with multiple cells.
A single gene mutation can lead to multicellularity; scientists saw it over and over again with individual yeast cells that were specifically bred to create multicellular structures with specialized cell roles, which went on to spur more multicellular yeast structures.
Cells get more complicated as time goes on; once you go multicellular, you don’t go back.
However, two major mysteries are still being debated.
The first unknown is why it took so long for multicellularity to become prevalent, given the new data. The answer might have to do with oxygen levels on a young Earth and the difficulty of evolving complex gene regulation. The second unknown is what was driving this transition. We still don’t know what caused life to make the multicellular jump in the first place.
All of this is evidence that shapes our understanding of how we, humans, got here.
And the evidence is pointing towards it being a series of small steps rather than a giant genetic leap towards multicellularity. We’re moving towards a more complete understanding about how life on our planet (and perhaps other planets?) evolved.
The image above is captioned and credited on Science as: “To study developing multicellular animals, students at the Marine Biological Laboratory in Woods Hole, Massachusetts, apply various stains. In juvenile squid (Loligo pealei) they reveal muscle (red), hairlike cilia (green), and cell nuclei (blue).” Wang Chi Lau/Embryology Course at the Marine BIological Laboratory.