This is more of a hypothesis than a well-rounded and evaluated theory. I could argue that rather than the viscosity of seawater the isolation of hydrothermal vents during the Snowball Earth period was the primary driver of eukaryote evolution and diversity. Isolation makes evolution speed up, and homogeneity slows it down in general. Fascinating to think about it though.
This hypothesis may happen to be correct regarding animals, who indeed have acquired their multicellularity either during Cryogenian or at its end, but it certainly is not applicable to algae or fungi, so the author should not have mentioned those.
Some of the algae, i.e. the red algae and the green algae, appear to have acquired multicellularity several hundred million years before the animals, i.e. around one billion years ago, if their fossils have been interpreted correctly.
The water viscosity hypothesis would not have been applicable to algae anyway, because their multicellularity has been developed to enable them to stay in a fixed position, attached to the ocean floor, not to move faster, like in the case of animals.
Similarly, the fungi have acquired their multicellularity later than the animals, after more than one hundred million years and their multicellularity has been developed during their adaptation to a terrestrial life, where fungi have lost their mobility due to acquiring a cell wall that prevents drying in air, but also movement.
For the animals, this hypothesis could be true, at least as a partial explanation of the advantages of multicellularity at that time.
In any case, besides the inappropriate mentioning of algae and fungi, the article has also other places with sloppy editing. It should have been revised more carefully.
For instance it says "In eukaryotes, adding cilia or flagella does not increase motility, nor does increasing cell size lead to a major increase in velocity", which is false and the article itself contradicts immediately this sentence by writing "Ciliates are much faster than flagellate eukaryotes".
Indeed, ciliates are much faster than flagellates, because they are bigger and they add a lot of cilia, which results in a major increase in their velocity, as correctly said in the second sentence, in contradiction with the first sentence.
So one way to overcome viscosity is the way of the ciliates, which, instead of associating multiple cells into a bigger body, like animals, have developed a more complex cellular structure, which allows bigger cells with a great number of cilia. There are many kinds of very small animals, with sizes typically under one millimeter, like rotifers and gastrotrichs, which are almost indistinguishable in size, speed and behavior from ciliates.
So up to a certain size threshold the two ways, of ciliates and of animals, are equivalent, but the ciliates have a size limit that is easily overcome by the multicellular animals.
It is very likely that the ancestor of all animals looked very similar to a huge ciliate, i.e. like the ctenophores (comb jellies) still look today. For ctenophores and for several other groups of simple animals the main method of locomotion is by using cilia, not muscles, and this appears to be a primitive characteristic of animals. The replacement of cilia with muscles appears to be a later development, caused by the evolution towards greater sizes, where cilia were no longer efficient.
All the sedentary groups of animals, like sponges and cnidarians, are unlikely to be primitive in this respect, but they have evolved from mobile ancestors. There are many well known cases when some groups of animals have evolved from a mobile life style to a sedentary life style, and then a few of the sedentary have then evolved to be mobile again, but in such cases those that have evolved to be mobile from sedentary, like medusae or salpae, have developed more peculiar ways of locomotion, they have never regained the same method of locomotion of their mobile ancestors. Therefore the evolution of most mobile animals from sponges is far more unlikely than the evolution of sponges from ctenophore-like ciliated animals, which is a kind of evolution that has been seen in a very large number of examples in other animals, for instance in tunicates.
The endosymbiosis that resulted in the harnessing of mitochondrial energy production appears to have only happened once (see work by Nick Lane) and resulted in a ~20 fold increase in available energy to the cell was followed very closely by the Cambrian explosion. We could still all still be bacteria today without it.
If it happened by chance, then we are alone in the universe. If literally trillions of bacteria all living and dying (ie evolving) with generations measured in hours ... and it still took a billion years to happen, then there is no hope for multicellular life elsewhere. There would have been more bacteria in earth's ancient ocean than there are planets in the visible universe. Hope of ever contacting multicellular aliens is over.
OR ... the switch to multicellular life was triggered by some sort of event, an evolved response to an environmental change. In that case then it is reasonable to think that it would happen on other planets too. So I really do hope that it wasn't a matter of pure chance. I want us to meet or at lease hear someone else some day.
This is still a "by pure chance" event, yea? Otherwise we're just back to intelligent design. Surely the issue is that we have fundamentally few ways of analyzing how probable that single event is/was.
Mutations occur by chance. With trillions of bacteria, mutations are a constant. Each possibly mutation is happening somewhere. Natural selection then dictates which mutations thrive when the environment favors them to do so. Evolution is a system, not a lottery.
There is also a meta layer to evolution, that organisms can "evolve to evolve", that the rate of "random" mutations can itself be an evolved trait. An organism unable to evolve, say with locked-in genetic code, will not adapt. Sexual reproduction arguably is such a system: an evolved system that regulates future evolution. So even base mutations may not be totally random.
My first thought was "Hah! Water viscosity doesn't change that much with temperature!" But it actually does; it's twice as viscous at 0°C than at 30°C.
It seems to me that multicellularity and the corresponding rise of complexity would be very useful adaptations in ANY evolutionary environment. They would give the species more capabilities and help to "game the game" by creating more axis of future variation. How can a unicellular critter compete with all the varied descendant species of a multicellular one?
No doubt water temperature/viscocity - environmental factors - did provide an evolutionary pressure for some extant species, but it seems highly speculative to try to write an exact evolutionary history of what factors drove what changes.
Yes, I suppose, although of course at one time it had been 100%. Maybe only the strongest lineages survived ?
What I really meant, rather than multicellularity being invincible, was that it's an obviously beneficial direction, and doesn't seem like it would need any extreme environmental stress to prove beneficial.
Such a mechanism would make complex life even more rare in the universe. Better this solution to the Fermi paradox than that sentient species tend to destroy themselves with technology.
I've always seen a problem with the idea of the great filter: why do we think the filter is one thing or one discrete barrier?
Why couldn't there be a very large number of low-probability filters and/or a continuous probability of failure spread across billions of years? Add them all up and the cumulative effect would be the same as a single great filter, but there would not be one single primary hurdle that needs to be cleared.
This and the Fermi paradox itself are really just speculations. We have only one data point, which makes it impossible to say anything definitive.
If we find abundant ET life, it's evidence we're fucked, since at least one filter is in front of us. Only by having little or no ET within cosmological distances would the possibility remain open there's no GF ahead of us.
Please explain because i would intuit the opposite. The less life's we see, the less likely that life survives to/through technological ages that would allow them to talk to us. If we see abundant life the it must be very easy to survive, thus less filters
If life is abundant, there must have been something to prevent it from spreading, now or in the distant past. The fact that we evolved in an apparently empty universe, rather than in one that's been totally colonized, implies something stops this from happening. This thing is called the Great Filter, and it's apparently extremely effective.
If the GF is in front of us, in a universe with lots of ETI, our chances of evading it are extremely small. After all, no one else has; why would we be so special?
Nick Bostrum has called detection of ETI, or even complex ET life, the worst news mankind would ever receive. It would be a death sentence for our future.
Space is called space for a reason. I feel like even scientifically knowledgeable people have a hard time wrapping their heads around just how much space is out there.
Signals diminish with the square of distance. SETI would never hear anything from other stars unless it was broadcast with insanely high transmit power or was directed right at us (and still very powerful). Nobody would do that unless they knew someone was here actively listening.
So for example, to reach the Centauri system (the nearest star) with a signal as powerful as the signal Voyager sends to Earth would require a transmit power of over 100 megawatts at a minimum. That's just the nearest star. It gets exponentially harder as you go further out. For more distant stars you're talking gigawatts or terawatts or beyond.
Even barring concerns about attracting attention, why dedicate that kind of resources to operate an interstellar border blaster if you don't even have a reason to think anyone's listening, and if you won't get a reply for hundreds or thousands of years?
This is not making a point you think it's making. If SETI is problematic, success becomes even more depressing, since it implies even more non-communicating civilizations out there (and, presumably, existing densely in the past as well). So why wasn't Earth colonized eons ago?
Why would it be? It’s full of corrosive oxygen and biomass that would be a biohazard to anything carbon based and organic. It’s habitable to us because we evolved here but it might be toxic hell to something else.
If you are going to sterilize Earth you might as well just terraform something dead. This isn’t even getting into the ethical questions, just looking at practicality.
If you’ve got the technology to come here, well, you might as well terraform something dead. We almost have the technology to terraform Mars (albeit over thousands of years) and it seems easier to do and possibly faster than interstellar flight.
Building giant space station habitats is also easier than interstellar flight. Uploading brains to machines, which would allow settlement of almost anywhere, is probably easier than interstellar flight. AGI is probably easier than interstellar flight.
In fact interstellar flight is so hard it might rarely be attempted anywhere by anyone unless there’s a forcing function like one’s star becoming unstable or some other cosmic scale calamity. Solar systems are huge and contain vast amounts of resources.
Why would a colonizing civilization have to choose between these options? If it experiences relentless growth, it would occupy everything. If it has grown to occupy some locations around a large number of stars, that's enormous growth from its initial system. Why would this growth stop? Instead, I'd expect it to fully exploit all the resources in all the systems reached. The solar system would look very different from what it looks like. There would be no asteroid belt; there would be a space colony belt.
They could be galaxies that have been fully colonized, but searches for them have turned up nothing. Specifically: if by "fully colonized" we mean fully exploiting the energy resources of the galaxy, this would show up in the spectrum of emitted radiation, with energy being emitted as waste heat instead of starlight. Infrared surveys put the fraction of such fully colonized galaxies at less than 1 in 100,000, and consistent with none at all.
Expanding is a significant use of resources. It requires consistent dedication over very long timeframes to succeed. Just look at how long it's taking to even have a permanent presence (not even close to a self-sustaining colony) on the Moon, which is one light-second away.
That’s one reason I’m skeptical of ultra-grabby hyper-expansionist aliens. Human cultures have become less obsessively expansionist as they’ve become more educated and advanced, with rare and usually transient exceptions.
Imperialism usually burns out after a while as the cost begins to exceed the benefit.
I could see a universe of mostly steady state civilizations that experience occasional bouts of expansionism or the sending of things like the Nauvoo from The Expanse. But those are transient events. This would greatly increase the time required for the galaxy to become saturated with life as per Fermi paradox models. It might happen eventually, meaning if we wait long enough some alien Mormons might show up. (The Expanse reference.)
Then consider that intelligence is probably a recent (on cosmic time scales) phenomenon due to metallicity. Previous generations of stars and planets probably lack the mineral diversity to support complex intelligent life.
There will be strong selection for expansionist cultures and species. As long as the universe is mostly unoccupied, expansion means increasing your share of the total population. Relatively static cultures will simply become irrelevant compared to their expansionist competitors. The experience on Earth recently is in the context of a world where all places that can be occupied (at the current tech level) have been.
The selection effect will operate at the biological level as well. Currently on Earth birth rates are declining. Modern civilization is acting like a kind of pesticide. But like any pesticide, it's going to select for resistance. This will likely mean more people will have an increased desire to have children. Selecting for this may take a while, but on the time scales involved in this thread it would happen very quickly.
In the long term, all societies in which selection is allowed to operate will push up against limits and be in states of Malthusian equilibrium. To counter this would require an unreasonable uniformity of control across time and space. All ET species, at all points in their history once they can undertake colonization, will have to exert rigid control on their societies and biology. Any defections from this uniformity will explode exponentially and overwhelm those who are not controlled.
You can have abundant life that is functionally isolated by physics, or said differently, by the degree of technological and societal advancement required to traverse the space between.
Great distances and inherent speed limits seem like a decent way to accomplish that.
I think it's clear that interstellar travel, while not trivial, should be possible for a civilization just a bit more advanced than our own. The energy or material resources required for interstellar colonization are very small compared to the resources available. It doesn't even require relativistic speeds, although reaching (say) 0.01 c doesn't require outrageous engineering, just a large spacecraft and lots of hydrogen bombs.
Now, perhaps it's universally true that civilizations don't advance much beyond where we currently are, or don't persist for sufficiently long for interstellar colonization to be successful. These would also be very depressing outcomes if they are imposed on us by the GF.
Interstellar colonization is more difficult than it's commonly imagined. To build a colony ship you need the same resources you'd need to build a large habitat. There would be economic pressure to build habitats that'd compete with the building of colony ships.
When a colony ship arrives to a new planet, it'll need to bootstrap a full industrial chain before it can build its own colony ships to expand to other systems. It's reasonable to assume there would be little incentive for that before the colony starts to require off-planet resources on their own system, and, therefore, has a compelling reason to build a space economy. For us, this would easily be in the thousands of years even if no terraforming is needed, and let's not even consider we'd need a consistent multi-century planning to get there. Precursor missions with automated in-situ resource collection and storage would allow colonists to arrive to a fully built and stocked colony, which would save some time. Existing biospheres might present issues of toxicity and biological risks: you really don't want to grow up on a (Dune's) Rossak (even if it grants you superpowers).
So, let's say Earth manages to establish 10 successful extra-solar colonies over 5 thousand years. It'll be at least a good couple thousand years until those successful colonies can each establish 10 others and raise the number to 100 worlds. If we manage to diligently expand at that exponential ration, we'll soon bump into other intelligent space-faring species that also share this commitment to reproduce.
And what motivates the continued expansion? Exhaustion of resources, if that is it, means that decay immediately eats into your reproduction and it isn’t a proliferation, but at best a continuous parasitic migration.
I don’t think that is clear at all. I think it reflects a massive gap in understanding of how difficult it is to do anything with humans for more than a generation.
I think the rare Earth hypotheses get it backward, it all looks unlikely when you only have one data point. I think that complex life finds a way and it all looks like a six cushion bank shot in retrospect.
