All of us have evolved from single cell organisms. The first step to multicellularity is a mystery, but now the lowly brewer's yeast is giving us a hint of how it could have happened, by "evolving" into a "primitive body" in just two weeks. University of Minnesota microbiologists have designed one of the cleverest experiments ever: they let yeast grow in agitated tissue culture flasks and then stop the agitation and let the yeast cells settle; then they took settled from the bottom of the flask, re-seeded them in another flask, and repeated the experiment. Very quickly, cells evolved to settle quickly (the ones that did not settle quickly were never passed on to the new flask). But the fast-sinking cells were not single cells anymore: they held on together and formed a snowflake-like structure. The structure was formed by daughter cells not fully separating after cell division. What was even more amazing, is that after the clumps reached a certain size, some cells "committed suicide" (named apoptosis, or programmed cell death) so some daughter cell clumps could separate into their own clumps. This suggests that the cells are no longer acting as individuals since some of them "sacrificed" for the good of the new "organism".
Given these lab results, it is no surprise that multicellularity has arisen more than once in the history of life on Earth. At some points, conditions must have favored multicellularity over single cells. The closest living unicellular relatives of multicellular organisms, the choanoflagelates, are known to sometimes form clumps of cells that stay together, like this Sphaeroeca colony in the photo here.
Take that, creationists!
Chrstie Wilcox at Science Sushi (Scientific American blog) has a good article on this story!
January 16, 2012 |
Let’s rewind time back about 3.5 billion years. Our beloved planet looks nothing like the lush home we know today – it is a turbulent place, still undergoing the process of formation. Land is a fluid concept, consisting of molten lava flows being created and destroyed by massive volcanoes. The air is thick with toxic gasses like methane and ammonia which spew from the eruptions. Over time, water vapor collects, creating our first weather events, though on this early Earth there is no such thing as a light drizzle. Boiling hot acid rain pours down on the barren land for millions of years, slowly forming bubbling oceans and seas. Yet in this unwelcoming, violent landscape, life begins.
The creatures which dared to arise are called cyanobacteria, or blue-green algae. They were the pioneers of photosynthesis, transforming the toxic atmosphere by producing oxygen and eventually paving the way for the plants and animals of today. But what is even more incredible is that they were the first to do something extraordinary – they were the first cells to join forces and create multicellular life.
It’s a big step for evolution, going from a single cell focused solely on its own survival to a multicellular organism where cells coordinate and work together. Creationists often cite this jump as evidence of God’s influence, because it seems impossible that creatures could make such a brazen leap unaided. But scientists have shown that multicellularity can arise in the lab, given strong enough selective pressure.
Just ask William Ratcliff and his colleagues at the University of Minnesota. In a PNAS paper published online this week, they show how multicellular yeast can arise in less than two months in the lab. To achieve this leap, they took brewer’s yeast – a common, single celled lab organism – and grew them in a liquid medium. Once a day, they gently spun the yeast in the culture, starting the next batch with whichever cells ended up at the bottom of the tube. Because the force of spinning pulls larger things down first, clumps of cells were more likely to be at the bottom than single ones, thus setting up a strong selective pressure for multicellularity.
Read the rest here.
Carl Zimmer writes this story for the New York Times:
Our ancestors were single-celled microbes for about three billion years before they evolved bodies. But in a laboratory at the University of Minnesota, brewer’s yeast cells can evolve primitive bodies in about two weeks.
The transition to multicellular life has long intrigued evolutionary biologists. The cells in our bodies have evolved to cooperate with exquisite precision. The human body has more than 200 types of cells, each dedicated to a different job. And a vast majority of the 100 trillion cells in our bodies sacrifice their own long-term legacy: Only eggs and sperm have a chance to survive our own death.
These demands for cooperation and sacrifice ought to make it hard for single-celled life to become multicellular. Yet animals, plants and other life forms have evolved bodies. “We know that multicellularity has evolved in different lineages at least 25 times in the history of life,” said William Ratcliff, a postdoctoral researcher at the University of Minnesota.
Dr. Ratcliff and his adviser, Michael Travisano, are experts in experimental evolution. They design experiments in which microbes can evolve interesting new traits within weeks.
“We were sitting in his office drinking coffee, talking about what would be the coolest thing you could do in the lab,” Dr. Ratcliff said. “O.K., the origin of life would be too hard. But other than the origin of life, what would be the coolest thing?” They decided it would be observing single-celled microbes evolving a primitive form of multicellularity.
The scientists designed an experiment with brewer’s yeast, which normally lives as single cells, feeding on sugar and budding off daughter cells to reproduce.
Read the rest here.
A result that will grow and add volume to our knowledge.
ScienceDaily (Jan. 8, 2012) — Much of what living cells do is carried out by "molecular machines" -- physical complexes of specialized proteins working together to carry out some biological function. How the minute steps of evolution produced these constructions has long puzzled scientists, and provided a favorite target for creationists.
Great piece on this story, by Ed Yong, in Nature (with video):
Single-celled organism can evolve multicellularity within months.
The origin of multicellular life, one of the most important developments in Earth’s history, could have occurred with surprising speed, US researchers have shown. In the lab, a single-celled yeast (Saccharomyces cerevisiae) took less than 60 days to evolve into many-celled clusters that behaved as individuals. The clusters even developed a primitive division of labour, with some cells dying so that others could grow and reproduce.
The study, by William Ratcliff and his colleagues at the University of Minnesota in St Paul, is published online today in the Proceedings of the National Academy of Sciences1. Referring to the origin of multicellularity, Richard Lenski, an evolutionary biologist from Michigan State University in East Lansing who was not involved in the study, says: “This has long been viewed as difficult transition, but these experiments show it might not be quite as difficult as assumed.”
Ratcliff came up with the concept for the experiment with his colleague Michael Travisano. “We were talking about the coolest work that we could do,” says Ratcliff. “We ruled out the origin of life as too difficult, but thought that evolving multicellularity would be feasible.”
Multicellular life has evolved independently at least 25 times, but these transitions are so ancient that they have been hard to study. Ratcliff adds, “It’s nearly impossible to look at living multicellular organisms and infer the ecology of their very different, very deep ancestors.”
Instead, Ratcliff and Travisano wanted to see if they could evolve multicellularity in a single-celled organism. They used gravity as the selective pressure. In a tube of liquid, clusters of yeast cells settle at the bottom more quickly than single cells. By culturing only the cells that sank, Ratcliff selected for those that stick together. After many rounds of selection over 60 days, the yeast had evolved into 'snowflakes' comprising dozens of cells (see video, above).
Read the rest here.