MarkE

This has already been mentioned by @Strange, but thanks anyway, it was an interesting article.

Is it fair to compare the metabolism of a prokaryotic cell with a eukaryotic cell, and conclude from it that the prokaryotic cell is metabolically more diverse? A eukaryotic cell is part of a multicellular system, in which parts of the metabolism have been taken over, have shifted. An analogy: would you claim that a plant cell's membrane is more robust than an animal cell's membrane because the plant has a cell wall? Well, an animal has a skin that protects all the cells inside the body, which is why it didn't need a cell wall anymore.

Eukaryotic animals within that niche have to, because they are in competition with one another. Yes, prokaryotes can't adapt to a different environments the way sexual reproductive eukaryotes can, because prokaryotic asexual reproduction creates identical copies, whereas eukaryotic sexual reproduction leads to more diversity in their genome.

Pretty interesting! Less (genome) is more it seems. The downsizing of genome of the angiosperms allowed greater CO2 uptake from photosynthesis, maximising their productivity. Just like C4 plants (all grasses, excluding rice) who are higher in efficiency (six times more efficient) than C3 plants. I guess that "efficiency" is the same reason why C3 grasses are so successful. Questioning why ferns and conifers still exist is perhaps in some way like questioning why C4 plants still exist. Isn't nature always looking for higher efficiency and productivity?

In the book 'Our inner fish' I've read that it's just this one gene (the SHH gene) that is responsible for our limbs, the fins of dolphins, and the fins of fish. So yes, the environment does influence the shape of an organism, but in the end it can be narrowed down to only one gene to rule them all.

What about the evolution of plants? Not only did chloroplasts develop only once by endosymbiosis of cyanobacteria, but all the flowering plants have an outer ring of male parts with an inner ring of female parts, which supports the idea that the flower arose only once as well.

What about two ape chromosomes fusing together to create, once again, just like 7 million years ago, a new/higher hominin lineage?

I wasn't talking about endosymbionts in general, even our own human stomach has endosymbionts, rather I was referring to the origin of eukaryotic cells (fungi and protists are also eukaryotes). My question was why a prokaryote never seems to invade/engulf another prokaryote anymore. For instance, only one cyanobacterium lead to the development of all plants. Why only one? Cyanobacteria still exist to this day.

Interesting. Could you please clarify this a bit? I'd like to understand the process.

@CharonY You're absolutely right, the animal lineage (Rickettsiales) as well as the green plant lineage (Cyanobacteria) have had their own endosymbiotic event back in the days. So that makes two, not one. Still, for 3,5 billion years, that's not so much. But isn't that the same as stating that it didn't happen? As an anology: I can try to score 1000 times, but if the goal keeper manages to stop the ball 999 times, I did NOT score 999 times, right? I failed, 999 times, therefore it didn't happen 999 times, even though I've tried. I'd like to know why it didn't happen, and if it could have happened, what went wrong. I don't know this of course, but I'm wondering why it has never been observed. Why should it happen all the time?

I've just showed you proof from genetic research, didn’t I? Moreover, professor M. Hazen made the same statement about self-replicating ribozymes, I didn't make it up myself. Occam’s razor suggests that, until proven differently, we ought to stay with the most probable explanation, because the statement with the fewest assumptions should be selected when you have two competing theories. You’re absolutely right to state that we know absolutely nothing about any life on the earlier tectonic configurations of the Earth, but I’m not convinced to accept that these three events could theoretically happen once again, it is therefore equally probable. So I'm not convinced of the alternative until there is any assumption to suggest that it could happens once again. But I'm always open for any suggestions.

@studiot Because evolution is an unbroken lineage of genetic transmission. The Y-chromosomal Adam is the most recent common ancestor from whom all currently living humans are descended patrilineally. We're all descended from one man. Mitochondrial Eve is the matrilineal most recent common ancestor of all currently living humans. This has been proven in genetical research. Whether an event like this won't ever happen again in the future however, I cannot provide any proof. Assuming that it could, then of course my question would by: Why didn't it happen yet? But if homo sapiens theoretically could evolve once again, then our notion of what evolution is and how it works has to be revised completely. Therefore it seems unlikely to me for this to even happen again in the future. But I'd like to know for sure of course, that's why I came to this forum with my question.

- Ribozyme self-replication happened only once in history, and it’s the ancestor of all eukaryotic life on Earth - Endosymbiosis that lead to eukaryotic cells happened only once in history, and it’s the ancestor of all multicellular life on Earth - Two ape chromosomes that fused together into one human chromosome (chromosome 2) happened only once in history, and it’s the ancestor of all human life on Earth What is the reason that these events could only have happened once in history, and will never happen again?

There are more confirmed exoplanets than I'd expected, thanks @Strange! How will we ever develop the technology to get more information about these exoplanets, such as their composition, their heat emission, and so on? What kind of technological innovations would that involve?

@Airbrush I've learned this a while ago from professor Laird Close. I don't know with certainty whether this percentage accounts for ALL stars in the Milky Way, because that would imply that all 100 billion stars, all of them, have already been studied, which seems rather unlikely to me. Moreover, the search for exoplanets would be over, which leads me to answer your last question about what percentage of stars have a habitable-mass planet: these small planets are very hard to detect, they're never detected directly but indirectly, because the slight wobble of a star indicates indirectly a massive Jupiter-like planet orbiting it, but an Earth-like planet doesn't effect its star that much. Maybe in the future we're able to use a different technique to detect Earth-like planets with Earth-like masses. @Strange I've followed your recommendation to visit the Cosmoquest forum. My question however somehow didn’t get through . I have no idea why, but I decided to move to another astronomy forum with the same question. There I've received a lot of response. First off, I have to admit that I was wrong! Well, I blindly quoted a professor who turned out to be wrong. The lecture I referred to was released in 2006. Dr. Filippenko was probably relying on old data that does not reflect the current best values. The metallicity of the Sun and population I stars has been revised downward in the past 10 years, and it could be that the mix of values was not properly vetted. About the changing of Sun’s magnetic orientation: it is expected that all stars do this. It's just hard to observe, but there is no proof whatsoever that our Sun is unique in this behaviour. About helioseismology, our sun that occilates every so much minutes: this also appears to be universal, that’s why it’s called ‘asteroseismology’. Unfortunately, I didn’t receive any examples of other stars who show this type of behaviour, but a little bit of research revealed Solar-like occillators, Cepheid variables, RR Lyrae variables and so on, all types of star occilation. About the unusualness of the heavy elements in our Sun and in the Solar System in general: our Sun does not have an unusually high abundance of heavy elements. It has an average composition for Population I stars. Older stars formed long before the Sun have fewer heavy metals, and stars younger than the Sun have more heavy metals than the Sun. The metallicity of the Sun has decreased over the last few decades, but our Sun has an average composition of heavy metals for stars that were formed near the same time. So, thanks for your comments, it lead me to find out more about the true nature of our universe!

@Strange I recently heard this from Alex Filippenko, but unfortunately he didn't say anything about the amounts. Our sun is quite unique for multiple reasons: there are only 5% Sun-like stars in the Milky Way (30% of which have Earth-mass planets in the habitable zone) and less than 3% are orbited by hot Jupiter-like planets. Only 1% of these run in a circular orbit (which is necessary for terrestrial planets to orbit). This is why I'm interested in how our Sun differs from other stars. For instance: do other stars also change their magnetic orientation every once in a while? (Our Sun changes its magnetic orientation every 11 year).

Not exactly, our sun has an unusually high proportion of heavy elements. Even our solar system contains much more heavy elements than is typically the case. That's why it's interesting to me. It's the unusualness that I'd like to examine, that's why I asked the question about the amount of heavy metals present on earth. If the accretion theory explains why Earth has this much heavy metals, why Venus has probably a little bit more of it, and Mercury even more (if I understand @beecee correctly) than there's no unusualness, and I withdraw my question. But not all (heavy) elements and molecules on earth can be explained by accretion only, take for instance water. It's though that water has been delivered to Earth by meteorites.

As a result of a supernova in the neighbourhood a few million years before?

@swansont The article states that the merger of two neutron stars could be an alternative source. A neutron star by itself is a potential result of a supernova, which makes it even more plausible that this is another source for the creation of heavy elements. I'm still interested when/how we, Earth, got them.

Interesting! When/how did we get these heavy elements?

Do supernovae scatter their heavy elements (after iron-56) far away in space, which, as a result of this event, could be absorbed by exoplanets in another galaxy? And why does Earth seem to have so many heavy elements? Shouldn't there be more heavy elements on Venus and Mercury instead?

Thanks @swansont and @Strange, I will look into those articles.

Does the law of conservation of energy mean that all photons and matter particles that exist in the universe right now have, in one form or another, always been around from the Big Bang on (only closer), and there will never be an increase in the amount of energy/mass? Doesn’t dark energy contradict the law of conservation of energy?

@@Airbrush Has it been measured that superclusters, who will become more massive in the future, won't ever be massive enough to have any gravitational impact on other superclusters?

@Airbrush Will this increase of mass (multiple black holes merging into one big super-supermassive black hole) have influence on the 'heat death of the universe' hypothesis?