daivid wrote:I now understand why you are a lot more sceptical than I about finding life on other planets of this solar system than I. It's hard to see how life could be sustainable without membranes - but then again, as we have not seen such life in action, how can we be sure.
It is certainly true that bacteria and archaea may be primitive compared to eukaryotes but they are also clearly the product a lot of evolution and are sophisticated compared with what must have gone before.
But the very fact we have two bacteria like forms on our planet (ie bacteria themselves and archaea) suggests that that was not where the most significant bottle neck lay - and it is their cell walls that one of the key differences lie.
I must say I'm not an expert on molecular biology, I just know what I remember from what learnt at medical school about 12 years ago. So take this with a grain of salt... But yes, I guess the bottle neck was probably before bacteria and archae separated. But like you say, we'd really need to have more examples to know for sure... Keep in mind though there's a big difference between cell membrane and cell wall - a cell membrane is a vital, thin lipid layer all
cells have, while a cell wall is a sort of extra support and is (I'm pretty sure) a relatively late development only found in some types of cell (some bacteria and plant cells at least).
Eukaryotes are one-offs and the fact they seem to have arisen from a fusion of an archaea and a bacteria suggests that arose from something very unlikely - certainly not text book small changes channelled by natural selection.
I have a faint recollection that the particularities of bacteria - like the fact they have little non-coding DNA, unlike eukaryotes - is rather an adaption than just a sign of their primitivity. But I'm not sure that the apparent similarities between eukaryotes and archae relative to bacteria is necessarily due to the eukariotic genome being derived from archae (though of course all known life here has ultimately a common origin), it could be just that they have been more "conservative" in some respects than bacteria. Also, the symbiosis of eukaryotes and the bacteria that would then become mitocondria didn't happen just once - chloroplasts also probably have a bacterial origin.
By contrast is far harder to even guess how hard it was to produce archaea and bacteria as the forms of life they grew our of are (probably??) now extinct.
No, they're not extinct - they have a lot of descendants, at least archae and bacteria...
In billions of years there's bound to be change.
But your point makes me aware that I don't know far too little about lipid membranes and would worth while for me to do at least a reading in this area.
The book I was reading long ago was Alberts et al.'s Molecular Biology of the the Cell. It was the 3rd edition, now the book is at its fifth
, apparently completely rewritten since then. The one I read was very good at least. It's a huge volume though, I never read it entirely, and there must be more concise books. The parts I preferred where short digressions where the authors speculated what this or that fact might indicate as to the origin of life...