Nick Lane’s “Power, Sex, and Suicide” is about just this question. Lane makes a strong case that for muticellular life, the individual cell has to subvert its goal to survive to the organism. That is, a successful multicellular organism needs cells to implement apoptosis (cellular suicide).
As we know, it’s very hard to evolve in a way that benefits a group at the expense of the individual, requiring specific circumstances.
Lane also explains why eukaryotes are required for MC life, and argues that the development of the first eukaryote was extremely unlikely … even more so than the 2B year gap would imply.
A fascinating book, also showing how a number of apparently unrelated features (mitochondria, sexual reproduction, apoptosis) are intricately involved in permitted MC life.
One way to guess which parts of evolution were difficult is to consider how often they occurred on Earth.
Engulfing of organelles to create new kingdoms of eukaryotes? This seems to have happened multiple times.
Multicellular organisms? My understanding is that multicellular plants, animals, seaweeds, and fungus were all independent developments. (I think I’m missing some kingdoms from that list.)
Ocean life becoming land life? Multiple occurrences.
This is why I think the “invention” of sex was a very key development. It seems to have happened just once. I think it may have been prerequisite to rapid evolution.
True; somewhere there are aliens chatting on their own version of the Internet about how methane volcanoes, or tidal atmospheric swapping with a partner world, or extreme short-period coronal mass ejections, were clearly essential to the development of their life and intelligence.
septimus, I’ve considered that same question. The vast majority of “key developments” have, in fact, happened multiple times (I’ve even seen some circumstantial evidence that abiogenesis happened multiple times, perhaps as many as thousands). If meiosis in fact only evolved once, that might be a key filter. The only other one I’m not sure of is photosynthesis (which wasn’t a part of our lineage but was still essential for it): I know that there are multiple photosynthetic pigments in use by living things, of which chlorophyll is merely the most efficient, but I don’t know if any of them developed independently, or how big the gap is between chlorophyll and the second-best.
The big step is that the different cells have to evolve differential functions to serve he whole collective. The catch is - then that has to be hereditary, so the cellular changes have to be “generic” so any single cell can split to create all the different players in the collective.
In Blake’s counterexample, the ant queen does not lay aphid eggs. Each anthill, once established, has to go out and find aphids to enslave.
That’s probably the “gotcha” - the organism needs to evolve differentiation and then a mechanism to reproduce that from a single cell, and then the sexual mechanism to exchange/improve DNA - meosis would be another huge and unlikely major change in function.
I don’t quite understand. You initial objection was that an organism simply attaching to another orgnanism in order survive won’t produce multicelular life 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.
Then when it’s pointed out that many colonial organisms do indeed incorporate unrelated organisms in exactly that way and it works just fine, you retort once again that the organism needs to evolve differentiation and then a mechanism to reproduce that from a single cell.
As already noted, it doesn’t. An organism just has to evolve some humoral mechanism to control all the different cells.
Mammals don’t come from a single cell, we derive from two cells that combine, and then the mitochondria and other organelles. All this works just fine because the mother organism is able to humorally control the replication of all the component cells in order to reproduce her own DNA.
There’s no absolute need for genetic relatedness in producing a colonial organism.
Beekeepers do this all the time. If the queen dies the hive may start to create a new queen. But an apiarist may note the loss of the queen and simply replace her with a new one. Or simply take a portion of a working hive and create a new hive with the new queen. There is a trade in lone queens for this purpose. Eventually the new queen’s offspring will dominate the hive, but until then the existing unrelated bees will do her bidding and support her.
Not sure why you find this confusing. If the object is to find an aggregation of completely unrelated organisms that cooperate, yes, that can and has happened.
The problem with the evolution multicellular, complex organisms is the step of reproduction. This is simple for single cells - they just split, now there are two distinct organisms each complete in itself.
If the cells start to aggregate but evolve different functions - how does the creature reproduce? And, how does it go on to become more complex?
One can imagine several scenarios - say, a simple collective where cells differentiate to A and B types.
-the grouping grows to be a larger and larger collection until it splits, and now we have two collectives. Hopefully in the split, the result was both pieces have some A and B cells. However, the long-term result is likely a divergence, where A and B become separate “species” of cells with less and less genetic material in common.
-If an A and a B swap some genetic material during some stage, the next step is for the resulting cell to figure out how (and why) to become A or B… let’s say, a concave/hollowed collective where the outer layer becomes hard and crusty (A) and the inner layers are soft digesters (B) that in turn feed (share with) the outer A cells.
Along with this sort of specialization comes the question - how and when does a new organism form? The ones that can simply push off one or two cells (over and over) and let them grow into a full organism will be less vulnerable than a grouping that has to split down the middle and become two half organisms/collectives every time it needs to reproduce.
…and that’s my point. A mixed organism that does this limits itself. An organism that can grow from a single cell is much more flexible, than something that needs to hive off a large collection of specific different cells in order to reproduce.
I suspect you are right, that differentiated collections were easy - hence a dead-end step for a billion years until something evolved DNA-driven differentiation of a single cell into a specialized collective - grow a complete team instead of trying to collect one. .
The evolution of sex would have been a major step - by having a complete duplicate set of DNA, the cell avoided the possibility it was trading away its trump card when exchanging some DNA material with another cell. It perhaps starts with the mechanism of complete duplicate DNA (give or take an X or Y) and swapping one of two sets - eventually moving to the process of specialized cells that fuse to make a full set.
I presume the steps from primordial soup to cellular to nucleus cells to multicellular to multicellular differentiated to full sexual reproduction took so long because the steps involved were that much more complex than the last step - the evolution of size and shape in multicellular organisms.
There’s no mystery here at all. Evolution is just a hill climbing algorithm. An organism is somewhere on a hill, a graph of fitness. Some changes raise the organism’s position on that hill, other changes lower it. Over time, the organisms that get mutations that raise their position are more likely to succeed that ones that lower it.
Eventually, organisms reach a local maxima - a peak on the hill. Any change will push them lower on it. They now are stuck here and will remain so forever unless something changes the landscape. You can imagine a hill as just a peak on a 2d graph, though in reality it is in fact probably hundreds of dimensions and can’t be visualized by human minds.
Rare random changes can sometimes happen that make such drastic changes to an organism in a single generation that they end up on another hill, and the algorithm can go to work again. But most such changes result in death - the organism with the mutation just dies. These “hills” are surrounded by vast probability spaces of nonviable patterns.
It took a lot of jumps, and a lot of randomness to end up with what we see. Quite possibly, had events not gone like they did, life on earth might still be stuck in the unicellular stage, and it might remain so until the sun grows to burn off the biosphere. This, I think, is the answer to the SETI paradox that fits the available evidence the best. I think if we ever manage to explore other star systems with probes of some sort, we’ll find lots of cases where life never passes the unicellular stage.