Looking at the history of life on Earth, one may think it is easier for the inorganic to turn into organic than it is for unicellular life to turn into multicellular life. Earth is 4.5 billion years old and life has been around for 4.1 billion years old. Multicellular life has been around for only 600 million years. Certain scientists think multicellular organisms might have appeared earlier but their claims are not widely recognized.
Multicellular life is said to have appeared due to (1) the tendency of natural systems to become complex and (2) the survival advantages of individual cells to form stable colonies known as multicellular organisms. It is strange though. If (1) and (2) are true, why did it take unicellular life 400 million years life to appear on Earth only to take it 3 billion years to become multicellular?
I don’t know the answer to this question, but I can see an important ramification. If it is so difficult for multicellular life to develop (as opposed to unicellular life) it seems to me that although life may have appeared on certain exoplanets the life that exists outside Earth is most likely unicellular-like: that is, primitive and non-intelligent.
We don’t consider it multicellular until there are several conditions. Specialization is a big one, and recognizing a cell that’s “different from me but still me” as ok is chemically more complicated than recognizing one that’s identical as being ok.
The conclusion was that oxygen, manufactured by one-celled creatures, was a prerequisite for higher life, and it took a long while to get sufficient oxygen. Dopers seemed satisfied … but 12 years later someone bumped that thread to ask if there were any other ideas. Being a narcissist, I’ll repeat my answer to the bump:
Simply repeating your answer to the bump doesn’t make you a narcissist. Even loving oneself doesn’t make one a narcisist. Thank you for your reply.
I didn’t know the question had been asked before but I expected it had already been discussed. It is true, however, that I hadn’t searched for it. Although I usually manage to google things satisfactorily, I can’t seem able to perform the same within this site.
My question has a tiny twist though. It implies complex life may be a rare occurrence - rarer than we think.
I know the process of unicellular life turning into multicellular life has occurred repeatedly, several times that is. If I understood well, plants and animals have become multicellular separately from each other. This means, I think, the ‘invention’ of sex has occurred more than once (that is, at least twice) because sexuate multiplication can be found in both kingdoms.
But if the evolution of life on Earth leads to the conclusion that oxygen is a necessary condition for complex life to appear, and taken the homogeneous nature of the universe, it means that (1) complex life whose metabolism is maintained with the help of other types of gas does not seem a feasible idea and (2) extraterrestrial intelligent life may be a really rare occurrence because it seems more difficult for unicellular life to turn into multicellular life than it is for the inorganic to turn into organic.
I miswrote in that long-ago post. It is the mechanism of Meiosis — key to sexual reproduction — which seems to have been invented a single time, in an early eukaryote.
(A variety of independent mechanisms have been invented for haploid gametes from different parents to make contact!)
And the real answer is that anything anybody–from arm-chair scientist to multiple-PhD–has come up with is at best a guess. Nobody knows, possibly nobody can know why something didn’t happen.
The Triassic Period with all of its large and very complex life forms was about 200 million years ago. It would take an absurdly huge jump in evolution for the first multicellular life forms to evolve that much in only 400 million years.
One thing that I don’t have enough of a grasp of, is the question of the relative complexity of the inside of a single cell that forms part of a multicellular organism versus the complexity of the systems that those cells form as the multicellular organism. From a naive point of view I have a feeling that the complexity of stuff inside a cell dwarfs that of the composite entity. Inside our constituent cells lurks an astonishing level of complexity of machinery. For that to take a few billion years to evolve doen’t surprise me. Just clumping together and differentiating into some sort of gross machine of large scale bits is much less surprising. We did need to evolve some interesting tricks - the ability to build different cell types at the right time and so on. But compared to the evolution of things like the molecular machinery that mediates the operation of a complex cellular form this is fiddlesticks.
Another, perhaps even more interesting question is the huge ~2Gyear delay between Abiogenesis (~4Gy ago) and the appearance of complex cells (Eukariotes) at around ~2Gy ago. This means that even when replicator cells (bacteria and archea) were present, and evolution was on, so to speak, it took so much time for a relatively much smaller increase in complexity (bacteria to eukariotes) than the development of the DNA or RNA life with all the absolutely needed mechanisms and enzymes. How the heck those developed without an evolutionary mechanism ?
This is something like the “half eye” argument of the creationists, but lacking evolution, a valid one. There are a number of conjectures to this question, such as a much simpler, mostly chemical replicator mechanism, but as much as I can judge, none more than speculations with many attack surfaces.
The most realistic speculation, in my view, absent God or little green men, is that the beginning of life was a freakishly low probability event. Our Anthropocentric slant tents to believe that, in similar conditions as those occurring on early earth, life was inevitable, or at least a high probability event. We have no evidence for that. It may very well be that out of billions of similar planets, very few, or maybe none have any life on them.
That is, oxygen is NOT required for multicelluar life. The anoxic multi-cells in the link do not have mitochondria and have no use for oxygen.
Now, they’re still very very tiny and they’re not intelligent, but then, the Earth doesn’t have a lot of non-oxygen environmental niches. Maybe on another planet multicelluar anaerobes might predominate. Or not. We don’t know. But it’s interesting that this has arisen on Earth. They do exist.
As for why it took so long for multicelluar life to really get going on Earth… the truth is no one really knows. Lots of theories, though.
Article says that the creature is still a metazoan, i.e., an animal. It’s only secondarily anaerobic, having evolved from ancestors that had mitochondria.
There is another mystifying question: on Earth, amino-acids have the L chirality, and sugars D-chirality. Now, there is nothing we know to prevent R-amino-acids and L-sugars. These would be incompatible with L-life (as we know it) and thus present no competition. Why, if life on Earth is relatively inevitable, don’t we see L-aminoacid life anywhere ?
This question is a bit similar to the question of why we have almost no anti-matter in the observable universe.
There is at least another prior thread on this topic aside from the one linked above: [THREAD= 472539]The rise of multicellular organisms[/THREAD].
One particular element of true multicellular organisms (rather than just homogeneous cellular colonies) is how they share capabilities, with some cells dedicated to metabolizing nutrients and producing energy and the rest to other functions such as structure, circulation, sensory activity, cognition, et cetera. This requires a degree of cooperation which is difficult to explain in purely selective terms without invoking extended phenotypes or other interdependencies. A partial answer which had previously occurred in prokaryotes is endosymbiotic evolution, where some functions of a cell are performed by other previously independent organisms which have been absorbed within the cell, either integrating into the nuclear genome or existing in free-floating fashion, such as mitochondria and plastids. Such internal specialization could also lead to the cell being ‘optimized’ for a particular function via modification during DNA-RNA transcription and control of translation via epigenetic (heritable) regulation, methylation, and histone acetylation during development and post-development translation. This may offset the additional risks and complexity of being multicellular, at least from a gene-centric point of view, but we clearly don’t have a comprehensive understanding of why it occurred.
We can’t really infer anything about the speed or likelihood of any particular evolutionary development from a single datum (that is, life on Earth). We may have evolved rapidly compared to other CHONPS-based chemical life, or we may be the ploddingly slow child in evolutionary development. Without any basis for comparison it is impossible to make any definitive claim or even particularly informed speculation. However, it should be understood that ‘oxygen’ is a proxy for any environmentally available oxidizer suitable to support respiration or some similar process; without a strong oxidizer, an organism is restricted to generating energy by much weaker and slower respiration processes or by fermentation, which we believe to be insufficient for multicellular life, or at least any that operate on the same timescale that we do. Another oxidizer, such as diatomic chlorine or a more complex compound could also be used.
Cyanobacteria for colonial “organisms” but are not true differentiated multicellular organisms and are not the direct precursors to complex plants and animals, which are eukaryotic organisms. Cyanobacteria have played a crucial role in the development of multicellular life in making oxygen available (and by the way, killing off most of the competitive anaerobic life to that point, which may actually be the largest mass extinction event in terms of biotic mass) but differentiated multicellular life arose much later. This is not “an absurdly huge jump in evolution”, for which a few million years is sufficient to produce widely differing species from a common origin, and once multicellular organisms with all of their advantages (protection and robustness from aggressive single-celled competitors, mobility, high energy gradient production, primitive nervous and sensory systems, et cetera) were established they rapidly flourished, which is quite evident in the fossil and genetic record.
There is no real answer to this beyond the speculation that chirality could have been influenced by circularly polarized radiation, which would tend to affect one chirality over another. However, it could be a purely random incidence that the first proteins were formed from levorotatory amino acids and their subsequent replication followed form. This isn’t a very satisfying conclusion but I have not seen a more definitive answer or better supported speculation.
Random is as good an answer as any, since the probability is half. But this is not the question. The question is why in the 4.5Gy since earth formation there has not been any new life creation with the opposite direction. Such life would not be competing with the established life since D-aminoacids and L-sugars are unusable by known form of current and past life, this protecting them from destruction.
Not all resources used by life are chiral. For example: sunlight, water, nitrates, salts, other minerals, etc. A retro-chiral lifeform must still outcompete pro-chiral lifeforms for those resources.
And of course, the biggest resource for life is other lifeforms. While a retro-chiral lifeform could not be metabolized by pro-chiral ones, neither could it metabolize them. Retro-chiral lifeforms would have to boot-strap their own ecosystem while outcompeting pro-chiral life for achiral resources. And pro-chiral life has a big headstart on utilizing those.
