Like I said; this is a general question, not a debate, and it’s about biogenesis, not evolution.
Given our current understanding of how life on earth may have originated…
[li]Could it be happening again right now in a puddle somewhere, or do the prevailing conditions prevent it?(i.e. there’s an ozone layer, there are all these organisms about to feed on any spare chemicals)[/li]
[li]Could it have happened more than once independently on earth? (in which case, any surviving descendants of the two instances would be totally unrelated in the biological sense)[/li]
Since we’re talking about a (admittedly very complex) chemical process, is there any reason why it should have taken a long time to happen? (seems to me that it must have happened quite rapidly, since we’re talking about it passing through transitional stages that would have been quite fragile - or are we?).
As usual, I don’t know any real good answers, but this is too good of a question to let drop off the first page unnoticed.
One thing I have heard is that most theories of early life could not work in an oxygen atmosphere. So if biogenesis were to occur now, it would have to be fundamentally different from the process that started all of us or would have to occur somewhere sheltered from the ubiquitous byproducts of the life already on this planet.
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*Originally posted by Mangetout * [li]Could it have happened more than once independently on earth? (in which case, any surviving descendants of the two instances would be totally unrelated in the biological sense)[/li][/QUOTE]
Why should they be totally unrelated? 2 or 3 billion years is a long time, and it’s difficult to imagine two compatible lines not interbreeding at some point.
Some popular models have it that originally there was an “RNA World” that evolved somewhat distinctly from the micro-spheres of lipids and amino acids that would eventually form cells and enzymes. These processes are very complicated and, obviously, have never been completely recreated in a lab. However, in lab experiments that have mimicked biogenesis a key ingredient has always been a large percentage of carbon-containing methane in the atmosphere and a much lower concentration of atmospheric oxygen. (The oxygen in the atmosphere today is believed to be a by-product of worldwide photosynthesis early in the history of life.)
Many of what are believed to be ancient organisms (members of domain Archea) now live in marginal environments that other more prevalent organisms cannot survive, for example in very salty ponds or extremely hot natural spring water. The hot, anaeorobic environments that many biologists believe were the original habitats of life are now few and far between. Biogenesis is presumably a very rare and complicated process that only occured once, or possibly a few times, either independently or in a limited set of locales. One supposes that it happened only due to the vast area of sea that was available for these original anabolic processes.
However, other theorists have conjectured that the first life was not at the top of the sea, but rather at geothermic vents on the sea floor. They argue that the first processes of life involved chemosynthesis (releasing chemical rather than light energy to synthesize food). These areas are still fairly abundant, but then again I’m no biologist and haven’t studied these environments. One presumes that with a large number of organisms living in these areas already, though, that there wouldn’t be much opportunity for spontaneously generated organic molecules to float around and assimilate into more complex structures, you’d think that filter feeders like giant tube worms would get them.
So, although I guess I’m obviously no expert, I’d say that most things that most biologists believe probably indicate that spontaneous biogenesis hasn’t happened for sometime and isn’t happening anymore.
Um, I suppose… if they were compatible, or so primitive that it didn’t matter much, or maybe they got into some sort of symbiotic or parasitic relationship that resulted in them becoming a single organism.
The reason I was assuming that they wouldn’t interbreed was compatibility; if the two instances arose in different places, they may not have been based on the same processes and may have been quite different.
Hydrothermal vents are one, but not the only, of the possible places life may have first developed. Unfortunately, they are relatively difficult to study and so far life there has turned up many surprises, but has not indicated any descent independent of the other life on Earth.
Until we find some organisms that appear to not share some of the common chemcial ‘choices’ made by the first life, such as the genetic code common to all known organisms, the same 4 nucleotide bases in DNA and RNA present in all known organisms, the same 20 amino acids used to build proteins in all known organisms, and the same stereoisomers of (hmmm… memory fails me, but I think all known life uses the L-isomers of all amino acids and D-isomers for all sugars).
If abiogenesis occurred in modern times, the most likely evidence for it would be locating organisms that used different nucleotide bases in their DNA or RNA, different amino acids to build proteins, or different isomers of all amino acids and all sugars.
To date, no one has found such organisms, so the hypothesis of continuing abiogenesis is not supported.
If we did find such organisms, we would have to try to distinguish whether they were the products of abiogenesis on Earth or extreterrestrial life.
As has been pointed out, I don’t think there could be interbreeding between the different lines as they would presumably use different structures for building blocks such as amino acids and nucleotides which would make them totally incompatible for “interbreeding”
As far as continued biogenesis… It seems like the success of a biogenetic process that takes a really long time depends on a very marked lack of competition… A bunch of amino acids form by a process of chemical evolution and since nothing is around to disturb this time consuming process, it can continue, but when the earth is suddenly covered in biotic organisms breaking down whatever substances they can find into simple “edible” molecules, this new process doesn’t stand much of a chance of gaining a foothold… So while its possible that relatively simple molecules are being formed (on the level of amino acids, for instance) that could eventually lead to complex life-forms if left alone, chances are they aren’t getting the chance.
My good sir, you are forgetting everything Star Trek has taught us about widely dispersed humanoid species being able to “get it on”, succesfully, with no problems at all! Human /Vulcan hybrids, Human/Klingon, Vulcan/Romulan…
If you can’t believe Star Trek, what can you believe?
Seriously, one of the best arguments I have ever heard against recurring abiogenesis is that any simple organic molecules that could conceivably cross the threshold from non living to living, in today’s world, would get eaten by something pretty quickly. As Woody Allen said, the world is like one big buffet. And my belief (I’m not an expert in this subject but I am a molecular biologist) is that any two lineages that became biotic at different times would most likely be so different that it would be obvious, like wevets said, and they would indeed be incompatible, Mr. Spock notwithstanding.
Most hypotheses about the origin of life depend on a reducing or nearly neutral atmosphere. The atmosphere now is very oxidizing. Biomolecules likely would not survive long enough in this environment to reach any level of self replication.
It could have happened more than once, but I think it is safe to say that all organisms living today are decendent from one lineage. I think the main reason for this is the fact that all organisms use three base codons for each amino acid and the codons stand for the same amino acids in all organisms (with a few minor exceptions). You might argue that independently arising organisms both must use DNA, RNA, and proteins, but there is no reason why they would use the same decoding chart. There is no special compatablity between the base sequence adenine, uracil, guanine and the amino acid methionine.
It is difficult to answer this, because it is hard to say when you start the clock and say, “Abiogenesis is starting now!” I would say that it would take millions of years to get from complex macromolecules to living cells simply because the number of intermediate stages was likely quite large. Self-replication, protein formation, DNA formation, enclosure in a lipid bilayer, these all happened independently and took some time to occur.
Yes, there is: We’re here, along with countless other living things. At some time, something must have happened. The only debate (admittedly a pretty Great one) is just what it was that happened.
As has already been pointed out, we’re here, ain’t we? (And we know neither Earth nor the Universe have always been around, so “there’s always been life” is not viable.) And, “Then a miracle occurs…” is not a scientific hypothesis.
MEBuckner- So are you saying that biogenisis occured, but not through a Creator, but rather through something natural?
I’d just like to say that yes, we are here. Yes, there is a theory of evolution and biogenis. However, it’s only a theory, and a weak one at that. What it comes down to is if one believes a theory which has never been proved correct, or believing in a Supreme being who created life in this world.
These are very interesting theories. Mine is that the Earth is but one of the forms god has manifested into, and all the different lifeforms on it play a part in the interesting creativity god has in acting out all the different senarios here, good ones and very bad ones that make us all cry.
No, he is saying that since life has not always existed, then, logically, life must have come from non-life. Biogenesis follows from the premise that life has not always existed. It must have occurred.
You know, it really bugs me when people who don’t know a thing about science start spouting off about it. Three quick things.
[ol]
[li]It is not a question of belief.[/li][li]Proof is an impossible standard.[/li][li]Theories explain observations, nothing more and nothing less. If you don’t like a theory find one that explains the observations better or go make some new observations. Just stop with the “it’s only a theory” nonsense.[/li][/ol]