The Origin of life has been one of the most puzzling questions scientists have ever faced. However, we now know much about the origin of life, much more than we did in the 1950's when Stanley Miller performed his famous experiment. This article is about what we know, and (some of) what we do not know is discussed under Addendum D.
1. The first step in the origin of life is producing the basic ingredients of life. Stanley Miller mixed cyanide and ammonia in a test tube and kept it frozen for 25 years. When he analyzed the mixture, he discovered that it had not only produced amino acids, but Adenine and Guanine (both RNA nucleotides) as well! Another experiment which Miller performed yielded the other two nucleotides used by RNA. All four nucleotides used by RNA could be created on the early earth.
2. The second step in the origin of life is getting these nucleotides to chain up to form a ribozyme, which is a strand of RNA that hooks other nucleotides together. Discover Magazine reports that, "[scientists] introduced random mutations into the hairpin RNA, shortened it from its normal length of 58 bases, and even cut it into pieces—all in an effort to produce RNA enzymes that were as dodgy and imperfect as early Earth’s first enzymes likely were. These pseudoprimitive RNA enzymes do nothing at room temperature. But freeze them and they become active, joining other RNA molecules at a slow but measurable rate."
These short, primitve enzymes could be formed by chance. Ice concentrates these molecules and "encourages" them to join together.
3. Another experiment showed that, (once again) under freezing conditions, RNA molecules up to 400 bases long formed with the help of an enzyme (Without an enzyme, chains of RNA formed which were 30 bases in ice and 40 bases in clay; Long enough to form something like the hairpin enzyme described above). The point of forming these long chains of RNA is to produce a chain of RNA which can replicate itself. In Richard Dawkins' evolutionary epic, The Ancestor's Tale (pg. 475-479), he describes a very interesting experiment in which a protein enzyme was added to a test tube along with the building blocks of RNA and, amazingly, the enzyme spontaneously assembled an RNA molecule which could reproduce! As signifigant as this is, the experiment is not quite a realistic scenario for the origin of life because of the presence of the (protein) Replicase Enzyme. What is needed is an RNA enzyme which can assemble an RNA replicator and catalyze its reproduction. There are high hopes that something like this will be found one day, as RNA has a lot of important catalytic properties, as are described in this article.
4. A membrane entrapped the genetic material. Membranes formed from lipids have been observed by experiment to form under prebiotic conditions. Even better, scientists are creating "protocells" and studying them for clues about life's origin. For more information about Abiogenesis research, please see this write up from Nick Matzke.
Once these four steps have occured, you are left with a simple cell. It is nowhere near as complicated as today's cells, obviously. This protocell reproduced offspring, some of which were imperfect copies. These imperfect copies may have survived longer or not as well because of their genetic changes. The cells that did reproduce most efficiently, however, became most common. This patter of trial and error occured again and again, gradually giving rise to today's complexity. For information reguarding the evolution of the genetic code and cellular complexity, please watch this video. Although I don't consider youtube a credible source, the video creator offers references and even gives a link to download the peer reviewed papers he took his information from. You can also read "Addendum B" below.
Addendum A: The Origin of Homochirality
In "The Case for a Creator", Jonathan Wells notes how all the amino acids of living things are "left handed" and how improbable it would be for only left handed amino acids to link up in an environment of both left and right handed molecules. Several factors have been discovered that account for this fact:
1) The Amino Acid Serine forms stable clusters of a single handedness which select other amino acids of like handedness by subtituting them for serine.
2) Calcite tends to adsorb just a single handedness in an environment of right and left handed amino acids.
3) Ultraviolet Circularly Polarized Light may have contributed to homochirality as well. (Meteorites often have more left handed amino acids because of U.C.P. Light)
Addendum B: A Summary of the Evolution of the Genetic Code
Components of the Genetic Code:
DNA, mRNA, tRNA, Ribosome.
DNA is the "storage medium" which holds all the information for making a living creature. Messenger RNA, or mRNA, "reads" and copies the information of DNA. Transfer RNA molecules, or tRNA, brings specific amino acids to the mRNA chain. Ribosomes are in charge of matching tRNAs with the mRNA code.The amino acids form proteins which make up all aspects of living things, from skin to internal organs and muscles.
So, as you can see from my simplified explanation of the genetic code, it is fairly complex and may seem (at first) to be impossible to explain by natural means. Yet it is not. Consider this: Scientist have discovered RNA which can act as both mRNA and tRNA (1). This in and of itself is a drastic simplification of the genetic code. But we can go even further: we can postulate that all of the components of the genetic code originally came from chains of RNA called Ribozymes. We now know that the Ribosome is a Ribozyme (2). Scientists have even successfully derived DNA from an RNA ribozyme through a process designed to simulate evolution (3). Essentially, all of the genetic code seems to be derived from chains of RNA.
Components of the Genetic Code and where they came from:
DNA - Ribozyme
mRNA, tRNA - Common ancestor ribozyme which performed both functions.
Ribosome - Ribozyme
1. Di Giulio M., The early phases of genetic code origin: conjectures on the evolution of coded catalysis. Orig Life Evol Biosph. 2003 Oct;33(4-5):479-89. (page 7)
2. Cech, T., Structural Biology: The Ribosome is a Ribozyme. Science 2000 Aug 11;289(5481):878-9.
3. http://www.sciencedaily.com/releases/2006/03/060327083737.htm
Addendum C: Creationist Arguments about the Origin of Life
I highly recommend all three of these essays which dismantle creationist arguments against abiogenesis quite thoroughly:
Lies, Damned Lies, Statistics and Probability of Abiogenesis Calculations by Ian Musgrave
Are the Odds Against the Origin of Life Too Great to Accept? By Richard Carrier
Dismantling Jonathan Wells' arguments against the Origin of Life
Addendum D: Unsolved Problems with the Origin of Life
1. The formation of Ribose. This is the "backbone" of RNA, and researchers have not yet managed to find a way to synthesize it under plausibly prebiotic conditions. Some scientists think that another nucleic acid, such as PNA, came first, but to my knowledge no one knows if a strand of PNA could replicate itself or not.
2. This information was provided to me by Alex over at "The Daily Transcript":
To be able to generate an RNA polymer, you either need the monomers in a "high energy state" or another source of energy. Since RNA polymerase uses NTPs (Nucleotide Triphosphates, or 'active monomers'), the energy that drives the polymerase reaction is derived from the hydrolysis of the extra two phosphates from each monomer. It would be very hard to imagine how such a high energy molecule could be created from non-biotic processes.
3. The protocells created by Dr. Jack Szostak, contrary to the claims of CDK's video on the origin of life, do not qualify as primitive life (CDK states in his followup video, about the origin of the genetic code, that it has been "proven" that life can arise spontaneously by Szostak's experiments). I emailed Dr. Jack Szostak and here is what he wrote in response to my email, asking him if his protocells qualified as life:
Dear Ryan,
The systems we have put together are not yet 'alive', because they cannot replicate their genetic material, and therefore cannot inherit information and evolve. Even we if incorporated a useful ribozyme, for example, it couldn't be passed on to future generations, because we don't yet have an efficient system for RNA replication. This is one of the main things we are working on.
So, as you can see, we are still a ways away from fully grasping the origin of life.
However, this does not mean we should give up: Many other problems for the origin of life, some far worse than this, were solved years ago. For example, DNA cannot form without proteins, but proteins cannot form without DNA. This seemed like an insurmountable problem just a few decades ago, and yet now we know that RNA disposes of this problem. We simply need to avoid having our curiosity satisfied prematurely about life's origins.
