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Posted on 01/17/2012 6:18:07 AM PST by LibWhacker
By watching evolution in progress, scientists reveal key developments in the evolution of complex life and put evolutionary theories to the test
The transition from single-celled to multicellular organisms was one of the most significant developments in the history of life on Earth. Without it, all living things would still be microscopic and simple; there would be no such thing as a plant or a brain or a human. How exactly multicellularity arose is still a mystery, but a new study, published January 16 in Proceedings of the National Academy of Sciences, found that it may have been quicker and easier than many scientists expected.
"This is a significant paper that addresses one of the most fundamental questions in evolutionary and developmental biology," says Rick Grosberg, an evolutionary biologist at the University of California at Davis, who was not involved with the research.
Since evolution acts on individual cells, it pays off for a cell to be selfish. By hogging resources and hindering neighbors, a cell can increase the odds that more of its own genes get passed into the next generation. This logic is one of the reasons it has been challenging to imagine how multicellularity arose; it requires the subjugation of self-interest in favor of the group’s survival.
"Traditional theories make this out to be a difficult transition because you have to somehow turn off selection on the individual cells and turn it on for the collective," says Carl Simpson, a paleobiologist at the Museum für Naturkunde in Berlin, Germany, who also was not involved in the research. "The big result here is that these transitions can be super easy."
In the new paper, researchers at the University of Minnesota at Minneapolis used a simple but elegant technique to artificially select for multicellularity in yeast. They dumped unicellular yeast into a tube of liquid food and waited a few minutes for the cells to settle. Then they extracted the lowest fraction of the liquid and allowed whatever cells it contained to form the next generation. Because the cells had to cluster together in order to sink to the bottom and survive, the artificial selection made it more advantageous for yeast to cooperate than to be solitary.
After just 60 generations, all of the surviving yeast populations had formed snowflake-shaped multicellular clusters. "Hence we know that simple conditions are sufficient to select for multicellularity," says biologist Michael Travisano, who led the research.
But at what point do the yeast become something more than a cluster of cells? When do they begin behaving as one organism?
In a true multicellular organism, such as a rabbit, evolution acts upon the rabbit and not upon each of the billions of cells that build it. So the researchers set out to determine whether artificial selection would act upon the snowflake yeast as if they too were multicellular organisms. To test it, one batch of the multicellular yeast was allowed only five minutes to settle in a tube (representing a strong selection pressure), while another batch was given 25 minutes (a weaker selection pressure). After 35 generations, the yeast that were exposed to stronger selection evolved to have larger cluster sizes, while those in the weak selection group actually shrank in size. This indicated that each cluster of cells was evolving as one organism.
In addition, time-lapse photography [video below] revealed that, in order to reproduce, the multicellular yeast divides itself into branches that develop into the multicellular form as well. The daughter clusters did not create their own offspring until they had reached a similar size as their parents. The presence of this juvenile stage shows that the snowflake yeast had adopted a multicellular way of life, says William Ratcliff, a postdoctoral student in Travisano’s lab.
The researchers also found evidence of rudimentary division of labor, which is an essential characteristic for more complex multicellular life forms. In a human, for example, some cells may differentiate into blood cells, others may differentiate into immune cells, but only select egg or sperm cells help form the next generation.
In the multicellular yeast, the division of labor was more subtle. Although the experiment's artificial selection favored large clusters, a large cluster required more time to grow before it could reproduce. That meant that smaller clusters, which divide in half more quickly, could soon outnumber the larger clusters. But after many generations of selection, the large clusters evolved a solution: non-reproductive cells which served as points where offspring could break away from the parent cluster. By providing more break points, these specialized cells allowed the clusters to break into more pieces, to produce a greater number offspring quickly.
“The discovery that there are cells specialized to die in order for the structure to reproduce is suggestive of the first steps toward cellular differentiation,” Grosberg says.
Although researchers agree that the yeast clusters could indeed be considered multicellular organisms, they remain relatively simple. "The researchers are not going to evolve sponges with this approach, but it's amazing what they’re able to do so quickly," Simpson says.
The fast evolution was not all that surprising to Grosberg, who has written papers arguing that multicellularity should be relatively easy to evolve; other researchers have estimated that multicellularity has arisen independently on at least 25 different occasions throughout the history of life. Yet nobody really knew how it originated, or what steps were involved in the process. By watching evolution in progress, the new research uncovered experimental evidence for these theories and revealed one possible scenario of how multicellularity may have evolved.
"We had hypotheses about how multicellularity could evolve, but until now, no one has really been able to test them,” Ratcliff says. "Now that we have this experimental system, we can ask lots of really exciting questions."
They must dis-prove Adam in order to dis-prove the creator. That is the ultimate goal.
Creator.
is it ‘evolution’ when you breed a dog for centuries until it is a good hunter or tall or short or... something? what makes a new ‘breed’
It’s still a DOG- that is what made me think about the evolution argument.
I think Darwin’s claim that a giraffe’s neck grew so he could reach the trees to eat leaves ignores a whole slew of generations unable to reach the leaves (where are those bodies?)
The same can be said for the brain, the liver, kidneys, the heart, ears, nose, sinuses.
How is it the the ears, nose, throat and sinuses all work together?
How did eyes "evolve"? Especially considering if one "devolves" an eye, it quits working. So at what point does the body/dna decide to evolve an eye, and then goes about developing an eye that over millenia finally becomes a WORKING eye.
How does the body then know to develop neuro pathways that connect the eye to the brain. How does the brain know to develop an area that can receive information?
How did the body know to develop ribs to protect the vital organs?
I’m just a layman, but multicellularity has always seemed to be the big miracle to me, not so much life itself. I mean, afterall, life evolved very rapidly as soon as the planet cooled down, but it took another three to four billion years to get complex organisms.
Now, along comes this experiment.
Like your dog and giraffe comments.
The Christian Man's Evolution: How Darwinism and Faith Can Coexist
Family Giraffidae ~ while there are only two extant members ~ giraffes and okapi, there have been others. The okapi has the form most nearly resembling that of the earlier geraffids. See Paleotragus
New democrat voters!!
Evolution......in a lab?
"We hold these truths to be self-evident, that all men are created..."
"We hold these truths to be self-evident, that all men are created..."
I get it. The multi-cellular yeast was created by researchers selecting for that trait just like farmers select for leaner hogs or cows with higher milk production.
i have thought the same thing-
how does an eye evolve into an eye incase you know thats what you want-
someone told me a bump evolves light sensitibvity raises to a cone, cone become ‘pinhole camera’ ligh sensitivity make it further evolve into an eye.. but this seems logically ridiculous- it makes sense... only if you dont think about it too long.
A cone would trap dirt and become infected way more times than it would grow light sensitivity
Lol, omg, it’s a real beer/ale. Never heard of ‘em, but I want to try it now.
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yeast with multicell differentiation?
I’m not of a scientific bent. Does this mean you could have sour dough biscuits with chlamydia?
I can’t wait for the day the multicellular yeast evolves into a new species of elk or lizard.
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