Special guest column by Reginald Selkirk:
Many Creationists will point to the complexity of modern biological cells and proclaim that something so complex could not possibly have assembled by pure chance, therefore God (or the unnamed-for-legal-reasons Intelligent Designer) must have done it. Sometimes the argument will be made more specific, such as that N proteins of M residues each must be necessary for a minimally functioning cell, and when you calculate the probability of those proteins appearing together simultaneously, with a 1/20 chance of a particular amino acid residue in each spot of each protein sequence, the odds are astronomical. Or they may point out that proteins are necessary for the replication, transcription and translation of genes, and genes are necessary for encoding proteins, so that there is a chicken or egg circularity to any origin schemes. Such arguments do not point out legitimate weaknesses in evolutionary theory, they only highlight the ignorance of the typical Creationist about matters of biology.
Before we get started on a discussion of the origin of life, let's get the timeframe straight. Current evidence tells us that the age of the universe since the Big Bang is about 13.7 billion years. The age of planet Earth and the solar system is about 4.5 billion years. These dates are supported by several independent fields of science and lines of evidence, including astronomy and radiochemistry. Anyone denying these figures can take their ball and go home right now, because the game they are playing is not science.
The main problems with the "cells are too complex" argument are 1) that it looks at a modern cell, and 2) that naturalistic evolutionary processes are mistakenly perceived as being entirely random.
To tackle the second misconception first, evolution is not entirely random. Variation in organisms due to various forms of mutation or recombination are indeed presumed to be random, but the natural selection which weeds out the failures and allows the successes to flourish is anything but random. To suggest that evolution is entirely random is akin to stating that since any foot race has an element of randomness which might influence the outcome, the Olympic 100 meter dash is equivalent to a lottery. Not so, the participants were selected at many different levels before they even made it to the Olympics. With evolution, the argument is even more absurd, since successful organisms replicate themselves more successfully, and thus have more chances for their offspring to participate in subsequent rounds of competition.
Modern cells are indeed complex. A great deal of the chemistry involved has been worked out in the last century or so. First of all, there is no evidence indicating that the chemistry that takes place within organisms is different than chemistry that takes place outside of cells. Biochemistry has identified most of the metabolites involved, and they conform to known physical and chemical laws. The same sort of reactions occur inside and outside cells. The same atomic theory holds, and the same electron orbital theory. There is no magic ingredient that makes a living thing alive. Chemicals can be synthesized from nonliving sources and introduced into living systems, where they will behave identically with chemically identical biogenic compounds. Much of modern molecular biology consists of using biologically derived catalysts (enzymes) to facilitate reactions outside of living things. Vitalism is dead, and biochemistry killed it.
The Creationist mistake is assuming that modern, complex cells were present at the origin of life. This is analogous to supposing that since cavemen could not construct a modern Mercedes Benz, therefore automobiles cannot have been invented by men, but must have been introduced into modern society by gods or space aliens. (This is an analogy. Analogies are made to illustrate arguments. They should not be stretched too far.) We know that early humans worked their way up through wheelbarrows, carts, chariots, animal-drawn carriages, and early engine-driven carriages before the current incarnations of autos "evolved." In a similar manner, biologists who accept and understand evolution (i.e. almost all of them) believe that early forms of life were much simpler than those we see today, which are the result of billions of years of evolution. Thus, the Creationist argument based on the complexity of modern cells is a straw man fallacy.
Modern cells mostly follow the Central Dogma of Molecular Biology, which proposes that genetic information is stored in DNA, that the information is transcribed into RNA, and then translated into proteins. At the time of the Central Dogma (late 1950s), DNA was viewed as the central repository of information, RNA was viewed as a relatively unimportant go-between. Most of the functional stuff in organisms is done by proteins. Proteins form most of the enzymes which catalyze metabolic reations. Proteins fill structural roles within the cells (microtubules and actin fibers) and outside cells (collagen in ligaments, keratin in skin, crystallin in the lens of your eye). Proteins transport things from here to there (hemoglobin in your red blood cells, ion pores in cell membranes). It is hard to imagine modern cells without proteins. But remember, we're not concerned with modern cells, but with the ancient precursors of modern life, which may even predate the development of cells.
One important exception to the Central Dogma which carries important insights into biological function and history (and shows the lack of respect scientists have for any "dogma") is reverse transcriptase, an enzyme which copies RNA into DNA. RNA (ribonucleic acid) and DNA(deoxyribonucleic acid) are closely related linear polymers which both fall into the general class of nucleic acids. DNA's ability to store and replicate genetic information is based on its ability to form double-stranded helices with base pairing. RNA shares this important property. DNA can form a double helix with DNA, RNA can form a double helix with RNA, and DNA and RNA can form a double helix together. This is how DNA is copied into RNA in cells in order that genes might be read and translated by the ribosomes. Reverse transcriptase is an enzyme which performs the opposite function, it starts with a strand of RNA, and using the double helical base-paring properties of nucleic acids, constructs a complementary DNA chain. In the modern world, reverse transcriptase is used by retroviruses, viruses which transmit their genes as RNA, but which can copy themselves into the DNA genome of the host. Prominent examples of retroviruses include HIV (human immunodeficiency virus) and chicken pox. The discovery of reverse transcriptase allows us to imagine a world without DNA, in which celular genetic information was carried in RNA, as it is in many viruses even today.
As already stated: consistent with evolutionary thinking, biologists propose that early forms of life were simpler than those available for study now. Once the main ingredients of cells were worked out in the middle of the 20th century, people started wondering about which of the major linear biopolymers, DNA, RNA or protein might have come first. RNA was a favorite candidate because not only can it carry and replicate information in the same manner as DNA, but it is more chemically reactive than DNA and could presumably catalyze chemical reactions. The term RNA World was first used by Walter Gilbert in 1986, although others had already made similar suggestions.
Although RNA's theoretical ability to catalyze reactions was known, no examples of RNA catalysts had been identified, and this was a sticking point for the RNA World hypothesis for quite a while. I will digress a bit here and mention that RNA is comparatively difficult to work with in the laboratory. DNA and protein can be extracted from cells without too much difficulty, but RNA is chemically less stable than DNA, and RNase, an enzyme which specifically breaks down RNA, is ubiquitous. It is all over your hands, it is pretty much everywhere. An obvious reason for this is that RNase helps protect you against attack by viruses, many of which carry their genome as RNA. To isolate RNA in the laboratory requires careful technique and the use of chemicals which inhibit the action of RNase.
Francis Crick, famous as one of the discoverers of the DNA double helix, wrote a book on directed panspermia in 1982, Life Itself. Directed panspermia is the idea that aliens deliberately seeded life on earth by sending starting organisms across interstellar space. This does not really answer the question of the origin of life, but it does allow one to push the answer back a few billion years, since the aliens would presumably have to evolve for a few billion years before they reached the technological sophistication necessary to seed other planetary systems. However, one cannot push the origin of life back too far, because astrophysics tells us that the heavy elements which make up much of life ("heavy" means anything heavier than hydrogen and helium to an astrophysicist) were created in early stars. Directed panspermia has a lot of difficulties of its own, and the only reason I mention it is because Crick stated as one motivation for writing his book the fact that no RNA catalysts had been identified at that time.
It wasn't too much longer before the first RNA catalysts were identified, and Thomas Cech and Sidney Altman shared the Nobel Prize in 1989 for those discoveries. RNase P is a piece of RNA which cleaves RNA. These discoveries renewed interest in the RNA World.
What other evidence might exist for the RNA World? First of all, we must adjust our expectations to the question. We are wondering about an environment that existed over 3 billion years ago (possibly closer to 4 billion) and left no macroscopic fossils lying around, as the dinosaurs did. A large portion of the Earth's surface has been recycled over the planet's history by the forces of plate tektonics. Any chemical evidence of the RNA World may have been scavenged by later, more successful life forms to which it gave birth. Modern cellular life has now been identified at deep ocean vents, in polar ice caps, in hot springs, and deep underground. Life is now almost everywhere on our planet, and is even believed to be responsible for creating the oxygen-containing atmosphere which allowed for the evolution of large oxygen-breathing life forms such as ourselves.
One way to deal with this challenge is to look for "molecular fossils" within the cells of modern organism. That is, evidence of our origins may be embedded within the make-up of our own bodies, and within the cells of all living organisms. To summarize briefly, additional RNA enzymes have been identified. DNA raw materials in cells are constructed from RNA raw materials, which points to RNA metabolism as being earlier and more central than DNA metabolism. This conversion is carried out by ribonucleotide reductase, and forms of that enzyme in all known branches of life appear to be homologous (descended from a common origin.) This supports the RNA World, and also suggests that gene-encoded proteins preceded the introduction of DNA to cellular metabolism. More recent speculation supposes that the shift from RNA to DNA as the primary genetic storehouse came about as a result of competition between viruses and their hosts.
The capstone of evidence for the RNA World has come within the last decade from researchers working on ribosomes, the protein-producing factories within cells. Ribosomes have been worked on for quite a while, but they are huge as enzymes go, consisting of dozens of subunits of both RNA and protein. Only around the turn of the century did these huge complexes succumb to the technique of X-ray crystallography, which uncovers the structure of molecules down to the atomic level. And the information uncovered was very exciting: the catalytic core of the ribosome is a ribozyme, i.e. an RNA enzyme. The implications of this for origin of life studies is clear: cellular proteins are manufactured by RNA (with a few odd exceptions), and they always have been. The Creationist charge of chicken:egg::DNA:protein circularity is based in ignorance of biological fact.
The evidence for the RNA World is not as extensive as the evidence for evolution through natural selection, or the evidence for quantum mechanics, but it is solid enough to convince most biologists that it is a sound theory of what an early stage of life on this planet looked like. This still leaves many questions: What was the nature of the RNA World: what, where and when, etc. What came before it? How did we get from there to where we are now? The existence of unanswered questions is not a reason to give up and praise God, it is a reason to do more science, and researchers are doing just exactly that.
If you would like to learn more about current theories on the origin of life, i recommend the book Genesis: The Scientific Quest for Life's Origins by Robert Hazen (ISBN-13: 978-0309103107)
A few web links on the origin of life:
- The two ages of the RNA world, and the transition to the DNA world: a story of viruses and cells
Patrick Forterre, 2005 Biochimie 87, 793-803.
This research paper does a nice job of summarizing the evidence for the RNA World, then goes on to propose a role for viruses in the transition from the RNA to the modern DNA world.
- MOLECULAR EVOLUTION: Do Proteins Predate DNA?
Stephen J. Freeland, Robin D. Knight, Laura F. Landweber, 22 Oct. 1999, Science 286, Number 5440 690-692.
This short commentary is a bit older, as it was written before the ribosome structure was completed. It discusses evidence for the RNA world and speculates about the order of development of protein and DNA in biological systems.
- Jump-starting a cellular world: investigating the origin of life, from soup to networks.
Richard Robinson, 2005 PLoS Biology 3(11) 1860-1863.
Another summary of current work.
- Emergence of a replicating species from an in vitro RNA evolution reaction. by R R Breaker and G F Joyce. Some RNA studies take place in the test tube, not in cells.
- Researchers Study Formation Of Chemical Precursors to Life. Precursors of biological molecules exist in interstellar clouds so huge they can be identified by spectroscopy from a distance of hundreds of light years.
- 'Lost' Miller-Urey Experiment Created More Of Life's Building Blocks
EVOLUTION WEB RESOURCES
- Science and Creationism A View from the National Academy of Sciences
- AAAS, American Association for the Advancement of Science
- Talk.Origins is the most complete repository of information on evolution on the web. Unfortunately it is unreachable as I write this, hopefully it will be back up soon. Featuring:
- Talk.Design Similar to Talk.Origins, but centered on Intelligent Design claims, rather than the more general Creationist claims.
- National Center for Science Education
- Statement on the Teaching of Evolution by 68 national academies of science.
- Press release describing the above statement.
- Understanding Evolution at Berkeley
- Tree of Life web project
- List of scientific organizations which support evolution and oppose creationism
- Panda's Thumb Group science blog with a decent caliber of scientific content.
- Pharyngula "Evolution, development, and random biological ejaculations from a godless liberal"
- Sandwalk "Strolling with a skeptical biochemist" by Larry Moran