MediumTex wrote:
Bigamish,
You seem like someone who might have thought about this a lot, so how do you think that first cluster of proteins (and whatever else cells are made of) got together to create a self-replicating organism?
Going from nothing to something spontaneously is a pretty amazing step.
Now that my morning coffee is brewed and I find myself with some me-time, let me take a stab at your question. I'll even go one step further for you...I'll address where proteins, as well as other critical biological macromolecules like nucleic acids, complex carbohydrates, and lipids, may have originated from as this is just as important an issue as the question you pose.
First, some clarification.
(1) No scientist knows for sure how life originated. The Earth was a very different place back in the day, and the early geologic conditions were not conducive for the formation of much "hard" (fossil) evidence that could give us greater clarity. There are many competing views on the subject, but I will focus on the ideas that are most plausible and/or have the greatest amount of evidence supporting them. But in the end, we are just making a series of educated guesses with some evidence to back up their plausibility. I/we could be completely wrong. Scientists acknowledge this, so research continues.
(2) We are not dealing with Darwinian evolution here, regardless what you hear people spouting off about on the TV. Darwinian (i.e., biological) evolution assumes that life is already present, and then attempts to explain how population gene pools change over time. It makes no inference as to where that initial life came from. In fact, what we are dealing with here is entrenched in the fields of astronomy, geology, and chemistry/biochemistry. The goal of my manifesto is to attempt to explain how observable non-biological chemical reactions that widely occur in the natural world might produce the first cells.
(3) We are dealing with MASSIVE amounts of time during which these events take place. BILLIONS of years, in fact. As much as we say to the contrary, no-one can really fathom these kinds of time spans, but needless to say...these events did not occur overnight.
So here we go. I'll attempt to explain "cookbook style", since we are supposedly dealing with primordial "soup" here:
STEP 1: Establishing the chemical environment of the early Earth
Astronomers and geologists suggest that the Earth formed from the accretion of clouds of space dust (of composition similar to what you might find in the Eagle Nebula) around 4.6 BILLION years ago. This gravitational accretion created massive compression, generating temperatures several thousand degrees Celsius. The heat and subsequent cooling & condensation of materials present, resulted Earth crust formation, as well as a primitive atmosphere. This took place about 4 BILLION years ago. The composition of this atmosphere is thought (again based on the starting elements/molecules common in space dust) to consist primarily of gaseous hydrogen, nitrogen, carbon monoxide, and water vapor. Not much gaseous oxygen was present though, and this is important as I will later explain. What is the evidence for this? When free oxygen is present, some binds to the iron in rocks. However, the "rust" that forms is not present in rocks until relatively recently in Earth's history.
Note that when nitrogen, hydrogen, and carbon-based substances are in proximity, they spontaneously react (this is readily observable) and form substances like ammonia and methane, which are also considered to have been abundant in the early atmosphere.
When the Earth cooled to the point that liquid water (rain) would not vaporize when it came in contact with rock, rainfall then accumulated and eroded mineral salts from rock, creating a salty mineral-laden soup over a period of MILLIONS of years. We witness this kind of thing today, although the elements mineral/salt composition is different now (4 billion years later).
Also, this cooling period (as well as for a time afterwards) was a time of great volcanic activity. Volcanoes likely released tremendous amounts of hydrogen sulfide, carbon dioxide, and carbon monoxide into the atmosphere, as well as other inorganic compounds. This is based on analysis of modern volcanoes.
STEP 2: Forming simple organic molecules from non-living reactions
The fossil record indicates that cells appeared about 200 MILLION years after the crust solidified, so there must have been lipids, proteins, carbohydrates, and nucleic acids present by that time. All cells require these biological macromolecules (as they are called)...but where could they have come from?
Synthesis of organic molecules requires energy. On early Earth, chemists and geologists suggest that lightning, sunlight, and/or heat from hydrothermal vents could have fueled these reactions from the substances present in the early Earth's environment. How do we know that this isn't crazy-talk? The seminal experiment supporting this idea was performed by Stanley Miller in the 1950s. He put water vapor, methane, hydrogen, and ammonia (emulating the early atmosphere...note the lack of O2) in a reaction chamber and introduced electric sparks to the gaseous mixture. In less than a week, amino acids and other organic molecules formed. Interestingly, when hydrogen cyanide (HCN) and formaldehyde (CH2O) were added (both are considered to have been present in the early atmosphere), *all* of the organic building blocks for each of the biological macromolecules types were formed. An decent summary of the experiment can be found here:
http://en.wikipedia.org/wiki/Miller%E2% ... experiment
Other interesting and plausible explanations have been proposed (such as the observation that there are interstellar clouds and meteorites containing organic molecules), but the Miller experiment still seems to get the most traction among scientists.
One thing to note here is that the Miller's experiment only works in the *absence* of oxygen. Molecular oxygen (O2) is a reducing agent, meaning that it tends to promote molecular decay. Were O2 present in the early atmosphere, organic molecules would be broken down as rapidly as they were formed, thus leaving us without the complex organic molecules required to form life as we know it.
STEP 3: Forming complex organic macromolecules from non-living reactions
Assuming the building blocks of biological macromolecules are now present, how did they arrange themselves to form the large, complex proteins, lipids, carbohydrates, and nucleic acids we see in life today? Good question. Again, there are competing views on the subject, but experiments by Lahav & Chang have been performed where if water containing the building block products of STEP 2 above are dripped over clays, the molecular layering patterns of the clay forces the building block molecules to aggregate and encourages bonding. Thus bigger organic molecules (i.e., macromolecules) are formed from smaller ones. Clays are important to the equation here because they were an abundant component of the early Earth's geology, particularly in mud along shorelines.
So now we have a plausible situation where we have *all* of the required major categories of biological macromolecules present to sustain life. Keep in mind that the reactions taking place in STEP 2 & 3 are taking place over a 200 MILLION year time-span as stated previously, and concentrations are the aforementioned molecules are building up in the primordial soup of the ocean over this time.
OK MediumTex, now we have enough background information to directly address your question. However, before I begin I need to clarify what the essential parts of a basic cell are.
Without dwelling on the many nuances here, all living cells have an outer, semi-permeable, wrapping called a CELL MEMBRANE (aka PLASMA MEMBRANE). This membrane is important because it effectively creates a self-contained internal environment within which reactions important for life can take place. Bad substances can be kept (or moved) out, and desirable ones can be kept in. All cells also have some form of SELF-REPLICATING system based on nucleic acids. This is a "genetic code" that contains the protein-building instructions to build another cell of that type. Modern cells use DNA for this purpose, but it is unlikely that DNA was the original molecule used for this purpose. More on this later.
STEP 4a: From biological macromolecules to cell - the CELL MEMBRANE
Anyone interested in the basic biology of cell membranes can find a decent summary here:
http://en.wikipedia.org/wiki/Cell_membranes
So for a cell to form, a cell membrane is required. Cell membranes can easily form when other living cells are present. But how might a cell membrane form in the absence of living cells, but in the presence of biological macromolecules?
Good question, and there is no definitive answer on the subject. However, one of the better hypotheses describes the formation of what are called "protocells", which are no more than membrane-bound sacs filled with enzymes (which are proteins, by the way) and other organic molecules required for basic cell metabolism & replication. Interestingly, experiments have shown that membrane sacs can form spontaneously, as the result of naturally occurring reactions that occur among fatty acids (a lipid building block) and alcohols (a carbohydrate) in the presence of mineral-rich clays. Also, membranous sacs such as this occur in mineral-rich rocks at existing hydrothermal vents.
STEP 4b: From biological macromolecules to cell - SELF-REPLICATING SYSTEMS
OK, so we have the potential for cell membrane formation. But what about the basic molecules inside required for metabolism and replication? As stated previously, nucleic acids contain information for protein synthesis and new cell formation. Note that nucleic acids can't replicate without the help of protein enzymes. Proteins, however, require nucleic acids for their synthesis. This makes the evolution of self-replicating systems a circular problem. So what came first in self-replicating systems, proteins or nucleic acids, and how did they start copying each other?
Again there is no universal consensus here, but many biologists are convinced that the first system of self-replicating molecules involved the nucleic acid RNA. Experiments have shown that certain types of RNA can act as a catalyst for chemical reactions (as an enzyme can), and these types of RNA are called "ribozymes". These ribozymes can cut themselves in two and join themselves to other RNA molecules, or other nucleic acid fragments, thus catalyzing the replication of RNA molecules. Other experiments have shown that RNA in living cells today can catalyze protein fragments. So here we have an information-containing molecule that can replicate itself, as well as produce proteins according to its nucleic acid makeup.
Now, where did these ribozymes come from? Good question. No-one knows for sure (that i am aware of), but they may have formed during the chance assembly of nucleic acids back in STEP 2 & 3.
Last, it is also suggested that RNA may have resulted in the eventual development of DNA, which is a much larger and more stable molecule. Although the building blocks of RNA & DNA are similar, they are not identical. It is plausible that RNA could have catalyzed the formation of the formation of DNA through introduction of those differing components into the reaction process. In effect, it is thought that there may have been a form of chemical "natural selection" that resulted in the formation of DNA over RNA as a molecule for information storage. DNA is more chemically stable (RNA is made of one exposed nucleotide strand, where DNA is composed of two, coiled up as a double-helix), and it's size allows for the storage of more information. Some support for this train of thought is also found in computer modeling exercises where simple precursor molecules always "evolved" (in the chemical reaction sense) into larger, more complex ones, that then began to interact with each other as complex systems.
My overly simplistic "RNA first, DNA later" description is the basis for the "RNA world" hypothesis that stone brought up earlier.
stone wrote:
Medium Tex, they think it was first an RNA world. RNA can be both a genetic coding material and also fold up to form enzymes. We share relics of that RNA world in our ribosomes (the things that translate the genetic code into proteins). Ribosomal RNA shows amazing sequence conservation across all life. The key enzymatic parts of the ribosome are made from the ribosomal RNA not from proteins.
http://en.wikipedia.org/wiki/RNA_world_hypothesis
OK, so there is at least a plausible explanation for the formation of a primitive, self-sustaining cell. Cell membrane? Check. Self-replicating information system? Check. Explanation with informational & direct evidence gaps? Check. But, it at least provides a plausible scenario that is based on some evidence, reproducible experimentation, and observation. Research continues.
STEP 5: The first cells
So remember, STEPS 2 - 4 would be occurring in an atmosphere with little/no O2 for reasons described above, all over a 200 MILLION year period. So where did all of the O2 in the modern atmosphere come from? Well, the earliest fossil evidence suggests that the first cells were like modern prokaryotes (have DNA, but lack a nucleus that contains it), such as bacteria. The lack of O2 also indicates that their metabolism must be based on anaerobic (non-O2 requiring) mechanisms found in modern cells (such as lactic acid fermentation and alcoholic fermentation).
Microscopic evidence from fossils 3.5 BILLION years old clearly show that the earliest cells were abundant around hydrothermal vents, and were of a type closely resembling modern forms that make their cellular energy by harnessing sulfur-based compounds around these vents (this style of energy production is called chemoautotrophism).
By around 3 BILLION years ago, fossil evidence indicates that prokaryotes evolved into forms closely resembling modern cyanobacteria (blue-green algae), which is significant because they are the first photosynthetic organisms. This is notable because they use sunlight to make energy and give off O2 as a byproduct. These bacteria formed domed shaped fossils called "stromatolites", and can be observed in living versions today. See the link below for cool details.
http://en.wikipedia.org/wiki/Stromatolite
By around 2.8 BILLION years ago, the abundance of photosynthetic bacteria and the resulting 0.2 BILLION years-worth of O2-producing activity profoundly changed both atmospheric and oceanic composition because now O2 was abundant. This is significant, because the presence of so much O2 would have stopped the chemical reactions that produced life in most environments (remember, O2 breaks down organic molecules). However, by this time cells were reproducing on their own, so these reactions were no longer needed. Also at this time aerobic respiration (= O2 utilizing) evolved (again, based on the forms found as fossils during this time) which is significant because it effectively neutralizes (consumes) the O2 that would normally interfere with the ability of a cell to make it's own organic molecules.
I could go further with the development of cells, but I think I covered the big points and my fingers are bleeding from all of the typing.
What do I think? I compare the explanations that have the best logical and/or evidenciary-basis, and then go for the one with the best support. Do I think that the above scenario is plausible? Absolutely. Do I think that it is the only possible explanation? Absolutely not. Do I think that there may be other , more compelling explanations, offered in the future? Quite possibly. Could everything I just explained be found to be completely wrong? Possibly, but some parts of the above explanation are pretty thoroughly established. Other parts less so.
Anyhow, thanks for tolerating the long-winded response to MediumTex. Apologies to any folks who may take issue with some of the generalities I made in the interest of space and comprehension. This topic spans many areas of science, and I was forced to go beyond my area of expertise on several occasions. Be gentle with me.
Cheers! ;D
