Okay, I've been wanting to do this for a while but have never found the time. I'm sure many of you have a decent grasp on how evolution/natural selection works, but that doesn't explain where life came from. I'm hoping that this gives you a better understanding.
Let me start off reminding you that this all took a very, very long time. Hundreds of millions of years makes this theory very plausible. That's a hell of a long time. If you took a calender and mapped out the evolution of the entire earth in one year, humans would appear a couple of minutes before midnight on December 31st. We're so incredibly insignificant in the big picture, yet so amazing when you actually think of how we got to where we are.
Formation of the earth
So... in the beginning.. the planet looked nothing like it does now. The universe was much different and clouds of dust/gases and clumps of rock/ice started to gather around the sun. These clouds began to collide, compress.. gravity took effect and the earth was slowly formed. These "clouds" probably cooled around 4.5 billion years ago . Since the earth was in a mostly molten stage, density took effect and heavier metals (nickel, iron, etc) sank towards the core and lighter ones 'floated' to the surface and this process began to form the crust, the mantle and the core. (crust is the outer surface, basalt, granite, low density rocks.. The mantle wraps around the core and is very high density, molten nickel/iron). We can assume that about 4 billion years ago, the earth had a very thin, very hot crust, but withing 200 millions years, life originated.
Early earth
Of course the conditions were much different than they are today, the atmosphere was probably a mix of gaseous hydrogen, nitrogen, carbon mon/dioxide and probably little to no pure oxygen -- that likely built up as superheated rocks (volcanic activity) gave off oxygen. Also the oxygen probably would have reacted quite readily with the other free elements. There was also likely water vapor, as the surface was probably much too hot for water to pool on, but condensation did start to cool the earth down, and this is essential for the formation of the first (and really any) cell membranes which could not have formed without water. Water likely did collect after the first several million years along with eroded minerals in depressions that would become the first seas. So, this was huge no water = no cells. No cells = no life.
Organic compounds (this one's huge if you skipped the first two)
Keeping in mind that this process has taken millions and millions of years, we move onto the next big event, the formation of organic compounds (carbon containing molecules). Essentially, cells are made up of proteins, complex carbohydrates, lipids (fats), and nucleic acid (DNA). Cells assemble these molecules from smaller organic compounds (simple sugars, fatty acids, amino acids and nucleotides). Now these can be assembled spontaneously, and energy (heat, sunlight) drives these reactions. How you ask? I'll explain it in a pretty general descriptions, if you want more Stanley Miller and the Miller-Urey Experiment are the places to look.
Miller-Urey Experiment
Okay, so these two scientists had an idea that if they stuck a whole bunch of gases that were present in the atmosphere of early earth in a complex array of tubes, added water vapor and a spark (to simulate lightening), they could essentially recreate some of the first compounds that led to life. So, they mixed methane (CH4), ammonia (NH3), water vapor and gaseous hydrogen in a glass ball connected in an extensive array of tubes that included a chamber of boiling water, a condenser, a vacuum and electrodes (for the spark). The boiling water gave water vapor, which traveled up into the ball of gases/electrode, then down into the condenser where it dripped into a little collecting tank. Within a week, they had amino acids and other small organic compounds inside of that chamber.. the bases of life. In other similar experiments, scientists were able to get the synthesis of glucose, ribose, deoxyribose and other sugars, along with adenine using different gases that would have been present on early earth. Glucose, is essential for life (it's a basic carbohydrate/sugar that is pretty much pure energy), and ribose and adenine are found in ATP and NAD+ and nucleotides which are also essential.
Okay, so we have organic compounds.. what now?
There are plenty of hypothesis's that predict what happened next. I'll outline a couple. One says that when water drained, clay served as a template for the spontaneous assembly of proteins/other complex organic compounds. How? Clay is made of thin layers of aluminosilicates with metal ions on the surface, which attract amino acids. Amino acids + clay + warmth + rays from the sun = condensation reactions that yield proteins and other complex organic compounds. Another hypothesis states that complex organic compounds formed spontaneously near hydrothermal vents on the sea floor. Amino acids + heat + water = protein. Regardless of how proteins came to be, they had the structure to form weak enzymes, and would have had an advantage to compete for available amino acids. (First natural selection anyone? ) Proteins would also have had an advantage in that they would have had the capacity to bind metal ions against other agents of metabolism. Metabolism = life.
Metabolism
Metabolism is the capability of cells to harness energy by reactions in order to drive their activities. So during the first 600 million years, we already have enzymes, ATP and other organic compounds that probably assembled spontaneously (and I should probably say this now, spontaneously doesn't mean that they assembled by chance. But rather they assembled because of favorable conditions) and in the same location. This close association would have encouraged chemical interactions/the beginning of metabolism.
Self replicating systems
Another factor needed for life is the ability to replicate. How do we do this? DNA, a stable molecule that is easily replicated before each cell division. Of course, DNA was not spontaneously created in the primordial goo. However, in experiments containing the compounds previously talked about, RNA, enzymes and coenzymes (I won't go in depth with what these do because it's kind of confusing and quite frankly, not all that important at the moment) have been synthesized. Why does this matter? RNA would have originally served the basic process that DNA does now. RNA is limited in that it can't form long nucleotide chains as DNA can, but it would have aided the creation of early life and helped early life reproduce.
We have proteins, we have enzymes, we have RNA, what's next?
We now have many of the basic components needed for life, what else is there? Well we need something to encompass everything and separate it from the outside world. Plasma membranes serve this purpose. They are basically a "phospholipid bilayer" that surrounds cells (hydrophilic phosphate head with hydrophobic lipid tails). They use proteins to keep a balance between what the cell needs inside, and what it doesn't need outside. They probably initially were simple membrane sacs that surrounded and protected information-storing templates and metabolic agents from the environment. Again, there are experiments that show that they can form spontaneously, one guy heated amino acids until they formed protein-like chains and then put them in hot water. Once cooled, they formed small stable spheres, another experiment exposed them to lipids and they created the phospholipid bilayer. Now not all of the pieces of the puzzle are complete, and a lot of 'chance occurrences' would be needed for everything to work as needed... but we're talking hundreds of millions of years.
Finally, cells...
The first living cells originated in the eon known as the "Archean eon" (3.9-2.5 billion years ago). Chances are, they originated on tidal flats or in seafloor sediments. The fossils that we have (stromatolites) if you live in Australia, go check them out, while they're much more recent, they descended from the same thing you did..) indicate that early cells were like bacteria we see today. They're called 'prokaryotes' and they have no nucleus. In fact, the first living cells were probably nothing more than membrane-bound, self replicating sacs of DNA and complex organic molecules. They probably secured energy through anaerobic pathways (as oxygen was scarce if it existed at all in its natural state O2) like fermentation. Energy was plentiful, geologic cycles had already filled the seas with organic compounds and predators were nonexistent, obviously. From there, the rest is history. The prokaryotes evolved into more advanced eukaryotes (cells that have a nucleus and probably initially only an endoplasmic reticulum for protein transport), then cell organelles (mitochondrion, vacuoles.. etc..), protists, fungi, plants, animals.
Let me start off reminding you that this all took a very, very long time. Hundreds of millions of years makes this theory very plausible. That's a hell of a long time. If you took a calender and mapped out the evolution of the entire earth in one year, humans would appear a couple of minutes before midnight on December 31st. We're so incredibly insignificant in the big picture, yet so amazing when you actually think of how we got to where we are.
Formation of the earth
So... in the beginning.. the planet looked nothing like it does now. The universe was much different and clouds of dust/gases and clumps of rock/ice started to gather around the sun. These clouds began to collide, compress.. gravity took effect and the earth was slowly formed. These "clouds" probably cooled around 4.5 billion years ago . Since the earth was in a mostly molten stage, density took effect and heavier metals (nickel, iron, etc) sank towards the core and lighter ones 'floated' to the surface and this process began to form the crust, the mantle and the core. (crust is the outer surface, basalt, granite, low density rocks.. The mantle wraps around the core and is very high density, molten nickel/iron). We can assume that about 4 billion years ago, the earth had a very thin, very hot crust, but withing 200 millions years, life originated.
Early earth
Of course the conditions were much different than they are today, the atmosphere was probably a mix of gaseous hydrogen, nitrogen, carbon mon/dioxide and probably little to no pure oxygen -- that likely built up as superheated rocks (volcanic activity) gave off oxygen. Also the oxygen probably would have reacted quite readily with the other free elements. There was also likely water vapor, as the surface was probably much too hot for water to pool on, but condensation did start to cool the earth down, and this is essential for the formation of the first (and really any) cell membranes which could not have formed without water. Water likely did collect after the first several million years along with eroded minerals in depressions that would become the first seas. So, this was huge no water = no cells. No cells = no life.
Organic compounds (this one's huge if you skipped the first two)
Keeping in mind that this process has taken millions and millions of years, we move onto the next big event, the formation of organic compounds (carbon containing molecules). Essentially, cells are made up of proteins, complex carbohydrates, lipids (fats), and nucleic acid (DNA). Cells assemble these molecules from smaller organic compounds (simple sugars, fatty acids, amino acids and nucleotides). Now these can be assembled spontaneously, and energy (heat, sunlight) drives these reactions. How you ask? I'll explain it in a pretty general descriptions, if you want more Stanley Miller and the Miller-Urey Experiment are the places to look.
Miller-Urey Experiment
Okay, so these two scientists had an idea that if they stuck a whole bunch of gases that were present in the atmosphere of early earth in a complex array of tubes, added water vapor and a spark (to simulate lightening), they could essentially recreate some of the first compounds that led to life. So, they mixed methane (CH4), ammonia (NH3), water vapor and gaseous hydrogen in a glass ball connected in an extensive array of tubes that included a chamber of boiling water, a condenser, a vacuum and electrodes (for the spark). The boiling water gave water vapor, which traveled up into the ball of gases/electrode, then down into the condenser where it dripped into a little collecting tank. Within a week, they had amino acids and other small organic compounds inside of that chamber.. the bases of life. In other similar experiments, scientists were able to get the synthesis of glucose, ribose, deoxyribose and other sugars, along with adenine using different gases that would have been present on early earth. Glucose, is essential for life (it's a basic carbohydrate/sugar that is pretty much pure energy), and ribose and adenine are found in ATP and NAD+ and nucleotides which are also essential.
Okay, so we have organic compounds.. what now?
There are plenty of hypothesis's that predict what happened next. I'll outline a couple. One says that when water drained, clay served as a template for the spontaneous assembly of proteins/other complex organic compounds. How? Clay is made of thin layers of aluminosilicates with metal ions on the surface, which attract amino acids. Amino acids + clay + warmth + rays from the sun = condensation reactions that yield proteins and other complex organic compounds. Another hypothesis states that complex organic compounds formed spontaneously near hydrothermal vents on the sea floor. Amino acids + heat + water = protein. Regardless of how proteins came to be, they had the structure to form weak enzymes, and would have had an advantage to compete for available amino acids. (First natural selection anyone? ) Proteins would also have had an advantage in that they would have had the capacity to bind metal ions against other agents of metabolism. Metabolism = life.
Metabolism
Metabolism is the capability of cells to harness energy by reactions in order to drive their activities. So during the first 600 million years, we already have enzymes, ATP and other organic compounds that probably assembled spontaneously (and I should probably say this now, spontaneously doesn't mean that they assembled by chance. But rather they assembled because of favorable conditions) and in the same location. This close association would have encouraged chemical interactions/the beginning of metabolism.
Self replicating systems
Another factor needed for life is the ability to replicate. How do we do this? DNA, a stable molecule that is easily replicated before each cell division. Of course, DNA was not spontaneously created in the primordial goo. However, in experiments containing the compounds previously talked about, RNA, enzymes and coenzymes (I won't go in depth with what these do because it's kind of confusing and quite frankly, not all that important at the moment) have been synthesized. Why does this matter? RNA would have originally served the basic process that DNA does now. RNA is limited in that it can't form long nucleotide chains as DNA can, but it would have aided the creation of early life and helped early life reproduce.
We have proteins, we have enzymes, we have RNA, what's next?
We now have many of the basic components needed for life, what else is there? Well we need something to encompass everything and separate it from the outside world. Plasma membranes serve this purpose. They are basically a "phospholipid bilayer" that surrounds cells (hydrophilic phosphate head with hydrophobic lipid tails). They use proteins to keep a balance between what the cell needs inside, and what it doesn't need outside. They probably initially were simple membrane sacs that surrounded and protected information-storing templates and metabolic agents from the environment. Again, there are experiments that show that they can form spontaneously, one guy heated amino acids until they formed protein-like chains and then put them in hot water. Once cooled, they formed small stable spheres, another experiment exposed them to lipids and they created the phospholipid bilayer. Now not all of the pieces of the puzzle are complete, and a lot of 'chance occurrences' would be needed for everything to work as needed... but we're talking hundreds of millions of years.
Finally, cells...
The first living cells originated in the eon known as the "Archean eon" (3.9-2.5 billion years ago). Chances are, they originated on tidal flats or in seafloor sediments. The fossils that we have (stromatolites) if you live in Australia, go check them out, while they're much more recent, they descended from the same thing you did..) indicate that early cells were like bacteria we see today. They're called 'prokaryotes' and they have no nucleus. In fact, the first living cells were probably nothing more than membrane-bound, self replicating sacs of DNA and complex organic molecules. They probably secured energy through anaerobic pathways (as oxygen was scarce if it existed at all in its natural state O2) like fermentation. Energy was plentiful, geologic cycles had already filled the seas with organic compounds and predators were nonexistent, obviously. From there, the rest is history. The prokaryotes evolved into more advanced eukaryotes (cells that have a nucleus and probably initially only an endoplasmic reticulum for protein transport), then cell organelles (mitochondrion, vacuoles.. etc..), protists, fungi, plants, animals.
Last edited by CommieChipmunk (2008-03-12 21:11:22)
