11
$\begingroup$

How did the earliest life forms exist without DNA? The most likely scenario I can think of for life happening from nothing is that, over billions of years, with trillions of water molecules and dust particles and chemicals moving around, eventually a small clump of this murky stuff formed a pattern. Or maybe, under some lucky conditions, a whole mud pit of such a inorganic system started forming into structures which had a pattern of motion, and one of those structures became the first sub-cell, like a primitive ribosome or something. I remember learning some parts of cells were once separate entities, but there are massive gaps in that idea.

The primary gap being: How did reproduction occur in that incredibly lucky, freak incident of a tiny dust clump or whatever? How did reproduction occur without DNA? Do scientists have a theory on how the first reproduction occurred?

$\endgroup$
  • 2
    $\begingroup$ I find it surprising that people usually think of incredibly lucky events. I mean, 3-4 billion years is an incredible amount of time. 100 years is already a lot of time. Adding a zero makes it 1000. What happend 1000 years ago... hum. Another 0 will bring us beyond the origin of cultural development I guess. Oh hey, there are another 5 zeros to add :) $\endgroup$ – cel Sep 28 '15 at 18:20
  • 2
    $\begingroup$ Jack Szostak's lab works on this kind of stuff. It really seems impossible the know how life first originated, but he and his colleagues have some hypotheses that can at least be shown to be plausible. Check out this video: m.youtube.com/watch?v=PqPGOhXoprU $\endgroup$ – canadianer Sep 28 '15 at 19:52
  • 2
    $\begingroup$ Before DNA must have come RNA, as DNA is only a way of storing information. And primeval soup experiments suggest that amino acids should have played a significant role in the early stages of reproductive life, because they are one of the first molecules that form. $\endgroup$ – AstronAUT Sep 28 '15 at 21:17
  • $\begingroup$ There are a lot of organic molecules formed naturally in the universe, I'd guess that those are a part of the pussle. $\endgroup$ – Alex Sep 28 '15 at 22:24
  • 2
    $\begingroup$ @canadianer I think Jack's claims (specially about lipid membranes) are controversial. $\endgroup$ – Dexter Sep 29 '15 at 4:07
14
+200
$\begingroup$

So earth is estimated to be around 4.5 billion years old, and life has existed for about 3.4-3.9 billion years of that, around >75% the time. For a little perspective, Homo sapiens have been around for up to ~250,000 years, just 0.00005% of the time earth has existed. In that time the earth has changed massively, early earth was pretty hostile, but that could have made it a good place to develop early life.

What was the pre-cursors to self-replicating DNA like?

Well one popular theory is the RNA world hypothesis, which suggests that precursors to life as we know it were RNA molecules that could self replicate (reproduce). These RNA molecules supposedly evolved in to life in the DNA/RNA/Protein world via an intermediate stage of ribonucleoproteins. The reason why RNA could have preceeded DNA based life is because it has the potential to carry information and to be the catalyst of its own synthesis.

"The unique potential of RNA molecules to act both as information carrier and as catalyst forms the basis of the RNA world hypothesis.

"RNA therefore has all the properties required of a molecule that could catalyze its own synthesis (Figure 6-92). Although self-replicating systems of RNA molecules have not been found in nature, scientists are hopeful that they can be constructed in the laboratory." in Molecular Biology of the Cell

However, the RNA world probably wasn't the first step on the road to self-replicating life, but other polymers with characteristics similar to RNA that could have acted as the template and catalyst required for RNA synthesis.

"Although RNA seems well suited to form the basis for a self-replicating set of biochemical catalysts, it is unlikely that RNA was the first kind of molecule to do so. From a purely chemical standpoint, it is difficult to imagine how long RNA molecules could be formed initially by purely nonenzymatic means. For one thing, the precursors of RNA, the ribonucleotides, are difficult to form nonenzymatically." in Molecular Biology of the Cell

-

"Shapiro doesn't think it's necessary to invoke multiple universes or life-laden comets crashing into ancient Earth. Instead, he thinks life started with molecules that were smaller and less complex than RNA, which performed simple chemical reactions that eventually led to a self-sustaining system involving the formation of more complex molecules." on LiveScience.com

This means that the first self-replicating RNA molecules were preceded by other molecules that were important in the progression from simple atoms, through non-biotic molecules, to large self replicating polymers which can be considered life. Copley et al suggest a progression in detail, with monomers being an early step in the progression (see figure 4 in the paper, and section 4 for extensive detail) where monomers precede multimers, which precede micro-RNA.

"Our model describes a continuous path for emergence of this sophisticated system from a simpler reaction network fueled by geochemical processes. The processes involved in metabolism and replication were intertwined from the very beginning, a concept that neatly eliminates the chicken/egg problem. Further, this model suggests that many features of the RNA World, and indeed modern life, arose long before the RNA World and were retained as pre-biotic systems became more sophisticated." - Copley et al


An alternative and slightly different theory suggests that viruses were the precursors to modern life, a so called Virus world. Similarly it supposes that early replicators were RNA based, but that viruses evolved in to the earliest forms of self replicating DNA carriers.

"The Virus World Theory is closely related to the RNA World Theory, which says life first evolved as small pieces of RNA that slowly developed into complex DNA-carrying organisms. The Virus World Theory agrees that life's genetic material began as RNA. But it differs by arguing that the ancestors of viruses evolved before cells." from NationalGeographic.com.


Further good reading here, here, and here. I also recommend the book The Story of Life as an interesting related read.

$\endgroup$
  • $\begingroup$ I just want to add a link to this answer. This video youtu.be/w2lqZL153JE illustrates how simple molecules can replicate without the need of any enzymatic reaction. $\endgroup$ – alec_djinn Apr 6 '16 at 14:32
8
$\begingroup$

Short answer
Before the RNA world, mineral surfaces may have facilitated the prebiotic containment and organization of biomolecules. Minerals are believed to have promoted the transition from a dilute chaotic prebiotic “soup” to highly ordered local domains rich in key biomolecules.

Background
As pointed out by others, the transition from a hereditary DNA world as we now know it, is believed to have evolved through an RNA dominated world. Life in this RNA world is believed to have deployed self-replicating ribonucleic acid (RNA) molecules and it is hypothesized it was the precursor to all current life on Earth. Because RNA can store hereditary information and can have enzymatic properties, it can potentially be self-replicating and catalyze critical biochemical processes. In contrast, DNA can only store information. Hence, RNAs could be used as information storage and catalysis (Alberts et al., 2002). One of the strong motivators for this hypothesis is that ribosomes contain catalytic RNA that is critically involved in protein synthesis (Orgel, 2004).

However, I think the question here is, what happened before the emergence of RNA, i.e., what led to the formation of RNA in a prebiotic world? The question of life’s origin is in essence a problem of the information storage and transfer from a prebiotic, geochemical environment to the first life forms, as aptly stated in the question. Crystalline surfaces of common rock-forming minerals are likely to have played an important roles in life’s prebiotic origins.

  • Metal sulfides (such as pyrite) and oxides promote a variety of organic reactions, including nitrogen reduction, hydroformylation and amination.
  • Fine-grained clay minerals and hydroxides facilitate lipid self-organization which could have mediated the formation of lipid membranes necessary for cell membrane formation. The cell is the building block of all life and critical to its existence is its integrity. A closed membrane allows to build up a membrane potential necessary to sequester biomolecules inside and keep toxins and waste products out.
  • Inorganic minerals have been shown to be able to mediate the polymerization of RNA monomers. Hence, minerals may have been the prebiotic template for RNA.
  • Surfaces of common rock-forming oxides, silicates, and carbonates selectively concentrate amino acids, sugars, and other molecular species essential to present-day life. These minerals are also capable of stabilizing these monomers. Cells are much like little containers with essential biomolecules. The capabilities of minerals to sequester them, accompanied by the intrinsic capability to form lipid structures could potentially make minerals hot spots for life formation.
  • Chiral mineral surfaces are actually able to separate left- and right-handed molecules. Chirality is central to current life. For example, in humans the major form of amino acids is the levo form, or left-hand variation. (Hazen & Sverjensky, 2010)

References
- Alberts et al., Molecular Biology of the Cell. 4th ed. New York: Garland Science (2002)
- Hazen & Sverjensky, Cold Spring Harb Perspect Biol (2010); 2(5): a002162
- Orgel, Crit Rev Biochem Mol Biol (2004);39(2):99-123

$\endgroup$
3
$\begingroup$

You can do without DNA, you need RNA to make proteins. Living organisms create and maintain a bubble that is very far removed from thermal equilibrium. So, you can speculate that at the origin of life, conditions existed where processes that are far from thermal equilibrium could still occur naturally without the support structures one finds inside living cells. This suggests conditions involving very low temperatures e.g. in comets.

If you consider complex molecules forming in comets then some unstable intermediary molecule can be metastable at the low temperatures there. Chemical compounds will only react with their immediate neighbor, not with molecules that are a bit farther away. This allows for intermediary unstable compounds to be created and to survive for long enough until it gets a chance to undergo further reactions making it more stable. Of course many more unstable molecules will be formed in such a process.

Imagine this happening in a comet kicked out from the Oort cloud that is periodically coming close to the Sun for a short while. When it is far away from the Sun, chemical reactions induced by cosmic rays happen in the interior which creates all sorts of strange chemical compounds. When it comes close to the Sun part of the comet melts and then some of the chemicals formed can disperse. The molecules that are more stable will have a better survival probability.

Darwinian selection of molecules may thus happen inside comets and that may have led to RNA molecules and many of the proteins and enzymes that we today find inside cells. All these enzymes and RNA together could then replicate by interacting indirectly with each other, but they are then not organized inside cells. You can speculate that cells could have evolved much later on Earth after the organic materials from comets arrived on Earth.

Alternatively, you can consider scenarios where during the Late Heavy Bombardment processes leading to life where going on both on Earth, Mars and Venus. It is known from studies that the impacts that happened during that time were powerful enough to eject materials from one planet and dump them on other planets and that molecules essential for life could survive the entire transfer process. This means that it's possible that one part of what is necessary for life was developed on, say, Mars, another part developed on Venus (which at that time had oceans) and yet another part developed here on Earth and that it all came together during the Late Heavy Bombardment.

The results of a recent experiment on the formation of organic compounds in the interstellar environment has appeared in this article.

$\endgroup$
  • $\begingroup$ The molecules that are more stable will have a better survival probability. Darwinian selection of molecules may thus happen [...] and [...] may have led to RNA molecules and many of the proteins [...]* - Of all the compounds I can think of, RNA and especially proteins are ranking amongst the least stable. I like your approach, but I don't get your reasoning. Further, references would be appropriate too. $\endgroup$ – AliceD Apr 5 '16 at 13:38
  • 2
    $\begingroup$ @Christiaan I'll try to dig up some refs, the basic scenario was mentioned in an old NGC documentary, they showed experiments being done involving freezing materials similar to what you find in comets and then bombarding that with radiation, partially thawing it, refreezing and then repeat this cycle. $\endgroup$ – Count Iblis Apr 5 '16 at 16:44
-3
$\begingroup$

Well another (and less popular) theory called Panspermia says that life, actually, came from space, it was not originated here in earth. So this completely change everything, because the conditions where life would have been formed could be very different from our's planet, so we can't make any predictions with this considerations in mind. But thinking in the DNA and our planet, the possibility that the DNA replicates with the help of other molecules could be possible. I don't think that it's necessary the steps of DNA to RNA and proteins or even DNA to proteins, to explain his reproduction, maybe some proteins with catalytic activity already exists and DNA with some good information in itself, it reproduce and continue evolving till have enough information to create other molecules with beneficial properties. We all know that proteins can actually be created by pure chemical reactions, i think that's the real secret of the life. We dont need a creator molecule to manufacture the entities entrusted with the job of create other molecules. The rest of the story is just the azar.

$\endgroup$

protected by Chris Apr 5 '16 at 6:56

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

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