Multicellular Life Evolves In Months, In a Lab 285
ananyo writes "The origin of multicellular life, one of the most important developments in Earth's history, could have occurred with surprising speed, U.S. 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 labor, with some cells dying so that others could grow and reproduce. Multicellular life has evolved independently at least 25 times, but these transitions are so ancient that they have been hard to study. The researchers wanted to see if they could evolve multicellularity in a single-celled organism, using 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, they selected for those that stick together. After many rounds of selection over 60 days, the yeast had evolved into 'snowflakes' comprising dozens of cells."
Re:I've always wondered... (Score:5, Interesting)
Now a related question -- is there any evidence (for or against) that life originated more than once on earth? Is the prevailing theory that a single reproducing organism came into being, from which all others were derived, or is it more likely that multiple instances of life happened over the course of time, and they all happen to take the same form? If this is the case, then it lends credence to life existing elsewhere in the universe, with much similarity to what we know. However, if it is unlikely for more than one independent instance of life to be similar, then we should be observing various non-related life types here on earth (i.e., some carbon based, some silicon based, etc).
Re:I've always wondered... (Score:5, Interesting)
In all probability new life (unrelated to current life) cannot evolve on Earth because current life either prevents the required conditions (eating the food before it gets concentrated enough for extremely primitive life to make use of it) or out competing the new life as soon as it arises.
If somehow the Earth were cleansed of all life but otherwise left unaffected, there is no great reason to believe it couldn't re-evolve life. However, as we don't understand the origin of life, there is a possibility that necessary conditions are no longer available - e.g. early life relies critically on the presence of a radioactive nucleotide with half-life of a few hundred million years, present in the early Earth but now decayed.
We find evidence of life in pretty much the oldest rocks on Earth which could contain evidence of life. So in the only instance we can study, life arose about as soon as it possibly could have. This suggests (but does not prove) that given the right conditions, evolution of life is an easy step, rather than one which requires a once-in-a-trillion-years fluke occurance.
However, unicellular life was around for some 2.5 to 3 billion years before multicellular life arose (or at least, multicellular life which left fossil evidence.) This suggests that the step from unicellular to multicellular is hard. Or so I've argued, until this result turned up...
So, we have this result, and the fact that multicellularity has arisen multiple times, and although only in Eukaryotes, it has arisen in very distantly related Eukaryotes (plants vs the fungi/animal clade) suggesting that multicellularity is fairly easy to evolve. So why did it take so long? Perhaps it required a certain level of atmospheric oxygen before multicellular life was viable (plot [wikipedia.org].)
(I have only tangential professional connection to these topics, so these are merely semi-educated ramblings.)
Re:I've always wondered... (Score:4, Interesting)
Re:I've always wondered... (Score:5, Interesting)
This has indeed been pondered! We're pretty sure that all life that presently exists all comes from one root, however. If there ever were alternative life-starting events, they didn't survive. The reason for this is that all extant organisms share a number of completely arbitrary decisions called chirality [wikipedia.org] (if you know any physics, that's left-handed vs. right-handed molecular symmetry.) Chirality is completely random in the chemical reactions that produce amino acids and nucleotides, but absolutely fixed, in the same way, in every living organism we've studied. A number of environmental tests have been conducted specifically to look for organisms of contrary chirality, but we haven't found anything yet.
There are two points here. As for the single root of life, I saw Carl Woese give a talk on this - see timely PNAS perspective here if you have institutional access: http://www.pnas.org/content/early/2012/01/13/1120749109.short?rss=1 [pnas.org]
(he's a giant in evolutionary biology and the one who proved archaea were a separate lineage using ribosomal RNA sequences, thus redefining our understanding of microbiology, so I'm inclined to give large weight to his views)
His view was that some events almost certainly happened to one unique organism, you can do the backwards projection on the endosymbiosis of mitochondria and a very distinct genetic profile emerges from multiple, independent lines of evidence. But when you try and project all the way back to the LUCA (last universal common ancestor of all three kingdoms) the uncertainty becomes so large and some of the contradictions so severe that it is in fact best explained by groups of highly similar (but not identical) universal ancestors over a window of time, not just literally one unique genome at a specific point in time. So he thinks that the "base" of the tree of life ends up being more like a collection of small shrubbery or bushes instead of a singular point of origin. Carrying that thought a bit further, if there were indeed multiple bushes of life at the start it seems probable there were also other bushes that completely vanished without a trace (no fossil record possible).
As for the universal chirality, that speaks to the origin of self-replicating macromolecules that would have preceeded the last universal common ancestor by quite a spell, so we can only speculate what happened based on our knowledge of organic chemistry. NASA funds some rather creative chemists to think about this question to help define what life might be like elsewhere, and last time I saw one of them speak they seemed to be of the opinion that it was probably just a random chance that gave us one hand and not the other and that there were pools of similar chemical species being selectively concentrated by some sort of clay catalyst. But that means it could have occurred multiple times and only one pool resulted in a proto-cell, or multiple proto-cells arose and the rest died off, or maybe all steps really did only happen once, there's absolutely no projection or record to build upon except geological models of what the earth might have been like then.
Re:Yes - sounds like "grant time" (Score:5, Interesting)
I'm betting they chose yeast because now they can get the interesting ones sequenced for a few thousand each, which is completely feasible even with a very modest grant compared to what it would cost for algae (or anything that isn't yeast or e. coli really).
Just skimmed through the paper and was almost surprised to see they haven't done the sequencing (yet?) - identifying the presumed mutations would have made this study much more interesting. A 12 Mb genome doesn't need much NGS capacity! Until then, I don't think we can rule out epigenetic inheritance, which has previously been demonstrated in yeast.
Re:Yes - sounds like "grant time" (Score:5, Interesting)
First. Thank you for a very interesting post which provides insight that is still understandable by those of us who are not molecular biophysicists. This is not always easy to do.
I may now be able to provide some insight into Slashdot's science discussions, which you may or may not have discovered yet....
A good scientist (which I'm sure you are) will read new research with an open mind combined with healthy scepticism. He/she won't automatically discard papers due to confirmation bias, and they won't shout "CORRELATION DOES NOT EQUAL CAUSATION" every single time they read reporting of a paper which suggests some correlation, because they realise that demonstrating correlation is often a necessary first step towards establishing causation and as such it is still novel and useful to publish papers that suggests correlation.
Slashdot, however, is home to some brilliant scientists who are completely drowned out by the masses of cynical, semi-clever "know-it-alls" who love to demonstrate their cleverness by shooting down any new research, often without bothering to read it first. They will shout "bad science" at the top of their lungs as a knee-jerk reaction to any perceived short coming, even if this short coming is just a limitation in scope of a paper or simply just ignorance on their own part. If the paper doesn't fully answer every possible question, it is worthless.
Re:Yes - sounds like "grant time" (Score:2, Interesting)
Is there some environment where sinkers get more nutrients and floaters get eaten or killed?
This is saccharomyces cerevisiae, the yeast used to make beer. Brewers have been selecting for floculent yeast since long before scientists started playing with them. The fact that this isn't mentioned once in the article invalidates the entire thing for me. This is not wild yeast learning a new trait. It's a well known trait being selected for. When I was brewing, I spent many hours watching yeast colonies, which vary wildly from strain to strain. Personally, I prefer the clearer taste that come from floculent yeast.
Re:Not so sure about this. (Score:4, Interesting)
Slashdot moderation simply hasn't evolved to the point where you can.
Good joke, but you've actually hit on a fundamental flaw with Slashdot's moderation system.
Once in a while, I have mod points. I dig really deep, and look really hard, for those comments that are truly insightful and informative. But I get punished for trying to do a really good job: many of my mod points expire before I can use them.
I've always wondered what the justification is for Slashdot mod points to have an arbitrary and artificial expiration date. Here's to hoping that, some day, the moderation system will evolve!