Actually, I would expect the opposite. The environments on Earth with the highest diversity are those with the most congenial conditions for life, tropical forests and coral reefs, while deserts and arctic regions and other harsh environments have only a relatively small number of species. The greatest stimulus for the evolution of diversity seems to be interspecific competition, which is greatest where the greatest number of species can survive.
Bilateral symmetry is correlated with an active life style. If you are moving about actively, it helps to have a “head” at the front where sensory organs are concentrated. So I would expect it to be dominant in “animals” even in aquatic worlds.
From this and the wikipedia articles, it’s starting to make more sense to me now. I was thinking that DNA evolved in a more linear fashion like A->B->C->D. But instead it’s like a lot of these different compounds were sloshing around at the same time and some combined together, some became food, and some went away all together. So it’s not like there’s one line of evolution which produced all the genes and DNA we have. Rather, it seems like there was a lot of different lines of evolution and their interactions produced the DNA we see today.
So even if the single magic life molecule originated in one place, the primordial gloop around the world all got incorporated at some point. Even there where multiple magic life molecules which simultaneously and independently developed, they may have combined together at some point.
Yes. Except that ALL life forms today descend from a very SPECIFIC complex system based on DNA–>RNA–>Protein. This system requires a largish number of specific enzymes for transcription, assembly and disassembly, replication. All(?) life also uses a specialized system of metabolic reactions centered around ATP.
Just as the birds are the only survivors from a large radiation of dinosaurs, so all life on Earth passed through a bottle-neck: a very detailed biochemistry, with whatever competitors that then existed all disappearing.
Indeed, I took comedic license inferring dinosaurs were lumbering idiots. Dinosaurs reigned supreme for a long time and were at least as smart as chickens and most certainly more intelligent than my ex.
But, my point was that a major event took place at a particular point in the history of our biosphere, causing a paradigm shift in species dominance—favoring the smaller mammals. Mammals took the baton and ran with it.
The K-T extinction wasn’t even the biggest mass extinction on our planet, nor should we expect mass extinctions to be uncommon on other viable planets.
But, the K-T extinction combined with our planet’s other mass extinctions, along with other conditions and events unique to our planet, resulted (I believe) in a biosphere blueprint unique to Earth. Enough so, that with detailed, scientific analysis we could identify *our *blueprint as separate from *other *planet blueprints.
I’m not saying mammals are better/more successful than dinosaurs, or amphibians, or arthropods or whatnot. In fact, if your definition of success is sheer numbers, mammals float toward the bottom of the barrel. Compare the world population of humans (~7.7-billion) to that of the SAR11 bacteria (~24 billion billion billion). On a one-to-one hand-to-flagella knife fight to the death, it’s pretty clear who the winner would be.
Sure, but chaos theory tells us that when you have a complex system-- and I think “the ecology of planet Earth” qualifies – you DON’T see small changes get buffered out by selection pressure over time. Rather, tiny differences compound, over and over, and a small change in initial condition can lead to enormous differences in eventual outcome.
You are suggesting going back to the very initial condition – when life first arose – and tweaking it a bit, with the first living creature arising somewhere a little different. I propose that this tweak in initial conditions, 2-3 billion years later, would lead to radically different outcomes.
All life on Earth is based on left-handed protein. A fox can eat a rabbit and turn rabbit protein into fox protein, because they are basically compatible.
What if right-handed life had evolved too? Could a left-handed fox eat a right-handed rabbit?
I don’t pretend to understand the chemistry involved, but I always heard not. It’s a thing that has come up in several science fiction stories I’ve read, for instance.
I agree, if the criteria for planetary dominance is intelligence and/or technology-building, bilateral symmetry is probably the best route. Heck, even octopuses and squid are bilaterally symmetrical.
But, if the criteria for dominance is sheer numbers/ubiquitous of a species, radial symmetry may take the lead on a water world (e.g. jellyfish/corals/sea anemones).
Some of the work I did in high school organic chemistry class involved converting right-handed stuff to left handed (or vice versa). It often included the use of bacterial enzymes.
Bacteria can do some amazing things! And, yeah, they can totally eat cold raw poison!
Of course, finding any life at all is going to be a “holy crap” moment. It may be based on different chemistry, it may have 3 or 5 bases of encoding instead of 4, it may produce weird unfamiliar tissues. But (IMO) it will be really shocking if its DNA equivalent doesn’t at least vaguely resemble a computer-tape-storage thingy.
If we ever find alien life, I will be moderately surprised if it does not store genetic information in a split-ladder molecule of some sort, moderately surprised if their ladder molecule is of a structure we anticipated (i.e., one of those million calculated in the paper Colibri linked), and extremely surprised if it’s the same thing as our DNA.
The eating protein is chiral, and the food is chiral, but the eating chemical reaction is the same for both left and right handed food molecule.
So what this means is that a right handed eating protein can in principle eat either handedness of food molecule, but the rate of reaction will be very different, likely many orders of magnitude different. The correct handedness of food molecule will fit perfectly into the eating protein active site, but the wrong handedness will fit badly or not at all - wouldn’t be recognised.
So for this reason if a higher organism like a human tried to eat D-protein it wouldn’t be a success - it would likely be denatured and broken down a bit by non-selective oxidative metabolism but the cellular enaymatic machinery wouldn’t recognise any of the amino acid sequences to hydrolyse.
I wonder if anyone has ever eaten a D-meat pie? Perhaps it would cause a fatal blockage.
What all of this means for prebiotic chemistry and metabolism is a less well-defined question - obv there’s a lot of speculation as to the molecules that might have been involved.
Here is a point that I haven’t seen mentioned anywhere. As far as is known it took about 1.8 billion years to go from prokaryotes to eukaryotes. Although we can guess the mechanism (a prokaryote eating another and incorporating it instead of digesting it) we do not (AFAIK) know how unlikely the event was, but it seems to have been very unlikely. And without that, I guess multicellularity would likely be impossible. My question is, given a planet like earth, inhabited by prokaryotes, what is the average time for the development of eukaryotes? I assume no one could give any kind of estimate of that. It could of course be 1.8 gy but it could be .1 gy and earth was extremely unlucky. Or it could be 10 gy and earth was extremely lucky since the planet will be uninhabitable in another 2 or 3.
I think that, even more so than the KT extinction, is the sort of question SJ Gould had in mind by contingency. The evolution (or creation) of cells with organelles may–probably did–happen only once and we have no idea how likely it was to happen even once. Gould thought that if you started over, life on earth, if it even existed would be totally unrecognizable. British evolutionary biologist Simon Conway Morris disagreed and thought it would look about like it does today. It is known that the dinos were in serious decline even before the KT extinction. Still I think Gould had the better argument. Conway Morris is seriously religious and I suspect that this warped his thinking. Surely God would keep us on the right path. (I made that up, I have no idea whether he actually thinks that.)
Well, it sort of happened more than once, since chloroplasts in plants have the same origin as mitochondria in eukaryotes. But while it was a different organism that got assimilated, it was the same organism doing the assimilating as had already done it before. So the question then is, to what extent did organelleization depend on an adaptation in the assimilating cell, versus to what extent it depended on an adaptation in the cell being assimilated?
But that presupposes that on every planet the earliest lifeforms would be nearly equivalent to bacteria/archaea. What if they used some other compounds in making their cell walls, or had a different arrangement of their genetic material, or had other differences that made multicellularity possible. We don’t know exactly why bacteria never became multicellular, or that the symbiotic route taken by life on Earth was the only possible one.
IIUC, multicellularity arose independently several times among Earth’s life forms:
Animals, plants, fungi, separately in two different seaweed phyla, as well as, arguably, coral and slime molds.
I think sexual reproduction (meiosis) was a special singular breakthrough. It coincided closely with the arrival of eukaryotes, I think, and facilitated much faster evolution.
