All the threads on evolution remind me of one of the most interesting evolutionary mysteries I know of, which is the two billion year gap between the original of unicellular organisms and the evolution of multicellular life about 600 MY ago.
I encountered that in Steve Gould’s book “Wonderful Life” about the Burgess Shales organisms. He points out that apparenlty all we had on Earth for two billion years, from the fossil record, is unicellular organisms, some colonial but still not multicellular animals akin to present-day salps, and floating mats of algae before the first multicellular life evolved in the Precambrian.
Why did it take so long? Gould speculated that making the move from unicellular organisms to multicellular organisms has to be MUCH tougher than simply going from free-floating amino acids to DNA, to judge from the fossil record.
In fact, given that it took SO FREAKING LONG to make the jump, Gould speculates that this may be the real sticking point for the evolution of life – that the universe may be full of worlds where the highest lifeform is algal mats, but with few or none other than ours where multicellular life evolved.
I mean , WHAT THE HELL WERE THOSE LAZY-ASS UNICELLULAR ANIMALS DOING FOR SO LONG? WHY DIDN’T THEY GET OFF THEIR SLIMY LITTLE ASSES AND DO A LITTLE EVOLVING SO THE REST OF US COULD COME ALONG? IS IT SO MUCH TO ASK?
Has there been any further speculation on this point in the scientific community? It sounds like a really juicy mystery to me, but Gould’s about the only person I know who’s mentioned it.
The question is when multicellular life did arise. The earliest ancesters of today’s animals were probably something like jellyfish or flatworms, so fossils of them would be extremely rare. There are fossils of what some people think might* be worm tracks over a billion years old, but there’s no certain way to tell.
At least one theory is that multicellular life couldn’t thrive until a significant amount of oxygen built up in the atmosphere. Even though photosynthesizing alge appeared billions of years ago, it might have taken over two billion years for buffering agents like dissolved iron in the oceans to become oxidized before a surplus of oxygen could accumulate.
Or who knows? Given the paucity of sedimentary rocks over a billion years old, maybe multicellular life arose a couple of times, only to be knocked back to the alge level by extinction-level events. In addition to planetesmal impacts, we know that the early earth suffered wild swings in temperature over the eons. Maybe the last billion years have been comparitively stable.
Lumpy has it – mostly. According to the book I’m reading (Life by Richard Fortey), fossil evidence for the earliest multicellular organisms – which are interpreted as plants – date as far back as 2 billion years. Multicelluar animals, however, did not appear for another 1.4 billion years, and (p. 80)
I was looking for hypothesis on why it took so long for multicellular life to arise and found this old thread. In the dozen years since this thread was fresh have any new ideas been put forward?
The transition from “free-floating amino acids to DNA” must have taken an incredibly long time, indeed, since it still hasn’t happened. Amino acids and DNA are completely different molecules and have nothing at all to do with each other. One certainly never turned into the other one.
And yes, I realize this is a zombie. Fighting long-dead ignorance.
The “invention of sex” (mixing genetic material from two parents) was a huge advance, and a costly and perhaps an unlikely one. Sex, with the doubling of genes adding robustness, increased the speed with which genetic variation, and hence evolution, could proceed. While many advances (flying, etc.) arose multiuple times in evolutionary history, I think it is supposed that the “invention of sex” was a unique event.
The wait for this invention coincides with part of OP’s “gap.”
The question presupposes that unicelluar organisms were static in the interim, and that they did not evolve considerable complexity - albeit complexity that didn’t involve multicellular developments. The Ciliata are not exactly simple. Indeed I often think that the complexity of the workings of a single cell are significantly greater than the workings of any large organism - if we exclude the cellular mechanisms. Only (perhaps) when we get to emergent complexities, such as a working brain do things catch up.
There are many, many different unrelated forms of “sex”, ranging from bacteria exchanging plasmids up through multicellular organisms like us mingling genes to produce offspring. And I find it hard to believe that vertebrate male-and-female sex has any evolutionary heritage at all in common with, say, the fungi that have hundreds of different sexes. Heck, I’d be surprised if animal male-and-female even have any connection to plant male-and-female.
This. There’s a big, big difference between bacteria and protists in size and complexity. The latter had to evolve before multicellular life could reasonably begin.
Many papers on evolutionary biology, e.g. the first link I clicked would disagree. Sexual eukaryotes have a single ancestor; one clue is that the genes associated with meiosis are similar.
The current origin-of-life speculation is that the first pre-biotes that formed were RNA chains. The reason is that only RNA is known to have both catalytic and self-reproductive ability. RNA, however, is less stable than the closely related DNA. The next important stage was the endosymbiosis of mitochondria and, specially important here, chloroplasts. It was this that enabled (and still enables) our partly oxygen atmosphere. That alone may have taken a billion years. So evolution was taking place, just not obvious in the fossile record. Once there was oxygen available (that was the first great eco-catastrophe if you were an obligate anaerobe) then organisms evolved to take advantage of it. Then came the multi-celluar explosion.
This leads to an interesting speculation. Let us say that the development of endosymbiosis took a billion years. Presumably, it was a more-or-less random event. A predator cell swallows a prey and, instead of digesting it, allows it to live. The prey cell does something that the predator cannot do (say, photosynthesize) and the combination is successful and the compound cell thrives, duplicates and, once having the habit of successful endosymbiosis, absorbs the precursor of mitochondria and so on. As I said, I am assuming this was a random mutation that allowed it. suppose that the average time for this to have happened was actually 5 billion years and that that earth was lucky (or maybe unlucky) in having it happen after only a billion. Then multi-celluar life might not have formed at all or formed only in time to be destroyed by the red-gigantism of a dying sun. This sort of question is the biggest hole in the Drake equation.
Along these lines, both the earth’s tilt, that creates seasons, and the extremely large moon relative to the size of the earth are also possibly crucial. We may find many planets in the goldilocks zone, but how many of them will have large moons creating strong tides and a strong (but not too strong) axial tilt?
Without those factors introducing a hell of a lot more “churning” of the system, it might take on average many billions more years to go from single celled to multi celled, or from slug to mammal… so long on average in fact that that the sun dies before it happens, or a planet busting asteroid resets the clock.
Or it might not have. We know that tidal pools were involved in key steps of life’s evolution here on Earth, but we don’t know if less extreme tidal pools (like we’d still get just from the Sun’s tides) would have worked just as well, nor do we know what the next-best option after tidal pools would be, or how good it would be.
You’re skipping a whole bunch of steps here. In between the self-catalyzing RNA chains and endosymbiosis, you’ve got all of the steps needed to get to what we would now call prokaryotes: The transition from RNA to DNA, the development of the cell membrane and cell wall, and a bunch of other things that I don’t know about. Nor was endosymbiosis necessary to get an oxygen atmosphere: The blue-green algae were doing a fine job of producing oxygen, well before some of them happened to get assimilated into other cells and turned into chloroplasts.
If evolution had morality, this is quite interesting. Dog eat dog world, big fish eat little fish, I’ve thought of. But this seems sadder, all those unfit little guys.
Of course we don’t know but its fun to speculate. Also, if our huge moon and axial tilt are indeed one of the factors that caused life to evolve more quickly to intelligence on Earth, thats potentially a good thing as it may mean we are already past “The great filter”. I prefer to be optimistic and think that the jump to multi-cellular life and then the rise of intelligence are the main highly unlikely great filter events that explain the fermi paradox.
The big evolutionary leap would be going from single-cell to multi-cell. Logically, the first step is for split cells to “stick together”. For some of them, up to a certain size of colony, this may have advantages.
The next step (probably the one that took over a billion years) would be gradually changing from a collection of identical cells to cells that specialize - some cells will do one thing - move, digest - and then receive from others whatever benefits from other functions they gave up. I.e. I’ll help move the colony, but you guys have to feed me". In this case, the standard logic “I attached to another organism and it will do the work” - this won’t work because all cells have to come from the same DNA, it has to have a means of triggering what cells develop in many different forms.
Sex at that point seems like a trivial development- like swarms from beehives, a colony of specialized cells may have had ones that split off to exchange genetic material from a similar neighbour. This sort of behaviour can over time evolve to something more purposeful.
That’s not entirely necessary. If you look at social insects, the colony will quite happily absorb members of other colonies regardless of their DNA. Some ants and wasps actually specialise in capturing the workers of other species and enslaving them.
There’s no reason why a colonial organism couldn’t do the same. So long as the “gonad” organism has the ability to produce hormones to control the rest of the colony, then the degree of relatedness isn’t essential.
Siphonophores work that way, albeit at an already multicellular level each individual organism serves a specific function within the colony.
IIRC sponges are at the crossroads between unicellular and multicellular organisms, they function as a collection of individual, undifferentiated cells working together.
Oh you are absolutely right about that and I knew it. I just wanted to emphasize that the evolution into eukaryotes was a crucial step towards multi-celluarity that took place sometine during that 2 billion years. The real mystery is how life formed originally. I mean we kind of understand most of the steps in evolution but the original formation of life is still a mystery. And it happened, relatively speaking, so quickly.