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The transition took place over the course of 50 weeks and was caused simply by the introduction of a predator to the environment. Time-lapse videos are available in the supplementary info.
The transition from unicellular to multicellular life was one of a few major events in the history of life that created new opportunities for more complex biological systems to evolve. Predation is hypothesized as one selective pressure that may have driven the evolution of multicellularity. Here we show that de novo origins of simple multicellularity can evolve in response to predation. We subjected outcrossed populations of the unicellular green alga Chlamydomonas reinhardtii to selection by the filter-feeding predator Paramecium tetraurelia. Two of five experimental populations evolved multicellular structures not observed in unselected control populations within ~750 asexual generations.
De novo origins of multicellularity in response to predation
(Score: 2) by ledow on Saturday February 23 2019, @11:10AM (1 child)
People never seem to consider:
Just because one out of a billion algae evolved to become multi-cellular "in response" (not really, it's random) to a particular threat does not mean that that evolution was otherwise advantageous. They may have survived the predation, but now they are not necessarily a "superior" organism.
You'll probably find that over the billions of year necessary, such evolution occurred billions of times. It may have been *worse* for the organism. It may have, by chance, then got eaten by something else entirely. It might have been in a bit of the ocean that meant it never really took off, so ended up dying off before it could spread. It could have evolved, thrived for thousands of years, and then been wiped out by something else.
There is *so much* left to chance that just proof of a single event being possible (which is what this is... a necessary step, sure, but not everything) doesn't mean it would have ever happened, or that it did ever happen like that, or that it wasn't happening every week but other things were getting in the way of it being a positive for the organism.
There are likely organisms that lived on this planet, owned the entire ocean for millennia, were inescapably prevalent and successful, were the bane of every other lifeform on the planet, but then by random chance were a victim of their own success, became a food source for something and died off entirely almost overnight. And that if they evolved again might well do the same... or might just get picked off by a lucky shrimp and digested before they could take hold.
Evolution is a planetary-scale process. Not individual actions like this, they are only the trigger and the method. It's like saying the cause of war is adrenaline. True in many respects, but just one tiny part of the chain.
And evolution results in a billion times more failure than success. But, like waitresses who moved to Hollywood, you don't hear of the failures. You only ever hear of the successes, precisely because they are a success.
It's the same on all scales. You can train in the gym every day for 30 years, become on the verge of becoming the world's most successful *whatever* and then get run over by a runaway bus. Though the potential was there, there are a million times more people who *could* do the job who never make it because of all kinds of reasons. Fluke, accident, other factors, weakness in other areas, just not being in the right place to do it, a competitor being a millimetre closer than the rivals, all sorts.
All this study proves is that you *can* spontaneously go from single- to multi-cellular. That probably happened more times in history than there are people on the planet. And probably (but by no means certainly) only once did it ever actually work to provide enough advantages to generate a whole tree of multi-cellular life. (P.S. convergent evolution is also a thing - where multiple independent organisms that have NEVER met evolve the same modifications to the same problem by sheer chance).
So it's possible that millions of algae all over the planet were evolving into multi-cellular organisms - and then even back into single-cell organisms - overnight, every night for a billion years, and nobody was around to notice. It's only the one/two/thousand/billion that did so in a way that - by luck, chance, good placement, or whatever - grew to sit at the base of our own evolutionary tree that anyone ever cares about.
Hell, there could have easily been sea mammals so far advanced of everything else on the planet, and they got beached by an unfortunate tsunami at their returned-to-every-year breeding site, and we've never found their fossils. We just can't know.
All we can do is prove that the steps taken are possible, and the overall mechanism averages out over billions of years to do dramatic things. At the cost of a billion times less dramatic or unfortunate things that also had the potential but never were.
(Score: 2) by Immerman on Sunday February 24 2019, @04:59PM
That is a good point.
Another thing to consider, especially in the context of the evolution of early life, is that any sort of sophisticated multicellularism tends to dramatically slow down the reproduction cycle, which simultaneously slows down the rate of evolution. So some multicelluar organism evolves, might even prosper for a while, but its internal cell processes are still primitive, and are evolving much more slowly than in the unicellular life. Which means that within a relatively short time the primitive multicellular life is competing against far more efficient and resilient unicellular life.
It seems to me that multicellular life just wouldn't become a long-term viable proposition until the evolution of single-cellular metabolic processes and other basic "cellular biotechnologies" had slowed down, globally, to the point that the cellular technologies of slower-evolving multi-cellular organisms would continue to remain "good enough" over the long term.
Once multicellular organisms were able to evolve a big enough advantage before they began to become "obsolete" at the cellular level, the field would be ripe for mutualism to evolve as fast-evolving "parasites" evolved to "farm" some of the excess calories instead, while protecting us from more aggressive invaders.