Take the 2-minute tour ×
Biology Stack Exchange is a question and answer site for biology researchers, academics, and students. It's 100% free, no registration required.

If the "primordial soup" theory of abiogenesis is to be believed, self-reproducing organisms spontaneously arose on Earth at least 3.5 billion years ago, surprisingly soon after the Earth cooled down enough to potentially harbor life.

It may never have happened again, however, since all lifeforms on Earth today are similar on a molecular level (DNA), suggesting a common origin.

This appears to imply at least one of the following:

  • Immediately after the first spontaneous abiogenesis, environmental conditions on Earth changed dramatically, making a repeat impossible.
  • The first organisms that arose consumed any subsequent organisms that came into being. This was suggested by Alexander Oparin, but I find it very difficult to believe since it would have to happen everywhere on Earth. Also, most single-celled organisms found today do not consume other organisms, and even where they do that does not usually lead to their complete extinction.
  • The common origin theory is in fact false, despite the similarities between organisms, and abiogenesis did occur multiple times in the same way.
  • Abiogenesis happened more than once, but only the descendants of one occurrence survived till today.
  • Spontaneous abiogenesis never occurred on Earth after all because the conditions never allowed it; instead, a proto-organism arrived on Earth from a planet where the conditions do (Panspermia).
  • The standard theory of geological history is wrong (i.e. the Earth was in a "fertile" state for much longer before the eventual origin of life than commonly thought, due to some unknown mechanism causing radiometric dating to give wrong results).

None of these seem likely to me, yet even less likely I find the idea that life arose spontaneously almost immediately (within a few hundred million years, possibly even faster) after the Earth's crust solidified, and never again afterwards.

What are the currently held theories on that matter? Both random speculations and references to relevant publications are welcome.

share|improve this question

3 Answers 3

It may never have happened again, however, since all lifeforms on Earth today are similar on a molecular level (DNA), suggesting a common origin.

An important distinction to make here is that all extant life on Earth has a common origin.

It's completely possible that abiogenesis occurred many times, but whatever organisms emerged as a result became extinct early on or are not preserved in the fossil record.

Immediately after the first spontaneous abiogenesis, environmental conditions on Earth changed dramatically, making a repeat impossible.

Nope. The red bands and subsequent dating techniques suggest it took a very long time for a change to occur.

The first organisms that arose consumed any subsequent organisms...

Possible, but we have solid data suggesting the first functional organisms were autotrophs. That is, they made their own food from whatever they were exposed to, and the predator/prey relationship had yet to play a major role.

The common origin theory is in fact false, despite the similarities between organisms, and abiogenesis did occur multiple times in the same way.

The Common Origin Theory - as you've defined it - is almost certainly true. DNA is a constant, as is RNA, mitochondria, etc.

That, however, doesn't mean abiogenesis couldn't have happened more than once. As I said above, the Common Origin Theory applies to extant (living) species, not necessarily all organisms that have ever existed. Species go extinct all the time.

Spontaneous abiogenesis never occurred on Earth after all because the conditions never allowed it; instead, a proto-organism arrived on Earth from a planet where the conditions do (Panspermia).

That merely pushes the question of abiogensis back a bit. Why did it evolve on another world and not Earth, then? How did it survive in space? Why was it so well suited to Earth's environment if Earth-like planets are (relatively speaking) rare?

The standard theory of geological history is wrong (i.e. the Earth was in a "fertile" state for much longer before the eventual origin of life than commonly thought, due to some unknown mechanism causing radiometric dating to give wrong results).

Estimations and conclusions have come from more than radiometric dating. We have fossil records, geological records spanning continents, and even ice-core records from the poles that both confirm radiometric dating and establish themselves as independent evidence.

None of these seem likely to me, yet even less likely I find the idea that life arose spontaneously almost immediately (within a few hundred million years, possibly even faster) after the Earth's crust solidified, and never again afterwards.

Why?

What are the currently held theories on that matter? Both random speculations and references to relevant publications are welcome.

This SE is not the place for random speculations. I don't have publications handy, so I may return and edit my answer when I have the impetus.

The most widely held theory is that all extant life, with a few possible exceptions, has evolved from a single organism that existed billions of years ago. That organism may or may not have been the only one to arise. If it was one among many variations of progenitors, we have yet to find the progenitors in the fossil or biological records. Given the extremely long time span between now and the beginnings of life, we don't expect fossil records to exist of all the organisms that existed some 3 B.Y.A. so the best we can do is speculate on what might have existed to account for what we see today.

share|improve this answer
    
Great answer, but what "few possible exceptions" are you talking about? I'm genuinely interested. –  Chinmay Kanchi Dec 30 '13 at 2:22
    
@ChinmayKanchi - I remember a few years back that some researchers found some (assumed) Archae members that had some surprisingly different inheritable material. Not completely new genetic material, but odd nucleotides IIRC. I haven't heard about it since, so I don't know if it was ever verified. –  MCM Dec 31 '13 at 0:40
    
Right. I seem to remember that vaguely as well. Wasn't it something like a tetrose sugar instead of a pentose? –  Chinmay Kanchi Dec 31 '13 at 14:53

It may never have happened again, however, since all lifeforms on Earth today are similar on a molecular level

Actually, the similarity is at the chemical level where the chirality of all known-life's amino acids are "left-handed" (see http://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry#Alternative-chirality_biomolecules). Since there is no advantage to having left-handed proteins over right handed, we would expect the chemistry in a spontaneously organism to randomly chose one or the other. So, it's 99% likely that life didn't arise more then 7 times: (1/2)^7 < .01.

Or, all the descendants of the right-handed protein life could have died-off, but this seems unlikely if there is no inherent efficiency difference.

The first organisms that arose consumed any subsequent organisms that came into being. This was suggested by Alexander Oparin, but I find it very difficult to believe since it would have to happen everywhere on Earth

This is where your missing a big point:

  • It's not necessarily eating them, or even outcompeting them once they arise, it's about cutting the legs out of the environment - eating the primordial soup of any loose complex organic matter. The places where the minimal materials for self replication machinery would have been available (since crystal-based origin of life was rejected in the 60's) would be very rare, and only on very specfic parts of the Earth...
  • Two of the current theories for this location, were in seafloor thermal vents, or on beaches with radioactive sand. Once life arise once in one of these niches, it will gain a strong foothold in these exact locations, crowding out those places orginal "ideal conditions" that led to the generation of life in the first place.

Ultimately, we'll probably be better able to answer this question by looking out to other planets than by un-earthing any satisfying evidence that could be found intact from 3 billion years ago.

share|improve this answer

Some people think that abiogenesis did occur more than once, and that the results of those occurrences are still with us. Paul Davies, for instance, is a proponent of what he calls 'the shadow biosphere' (here's a link to a popular article on his views: http://www.theguardian.com/science/2013/apr/14/shadow-biosphere-alien-life-on-earth).

I've seen him talk about it, and while he has pretty much zero evidence for the idea, it's an intriguing thought. His argument is basically that we're not finding the organisms in the shadow biosphere because we're just not looking in the right places or in the right way; our discovery techniques are all biased towards our particular RNA/DNA-based life.

Note that this doesn't really put common origin in the ground, it would just mean that there's more than one common origin for each 'type' of life.

Also, it isn't necessary for early organisms to consume each other in order for there to have been multiple origins that subsequently went extinct. Our brand of life could simply have out-replicated the competition.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.