|
|
|
|
Evolutionary Theory The Origin of Life
Three forms of matter are linked to the origin of life. The first is the matter composing stars, in which nuclear fusion reactions produce all the elements heavier than hydrogen, including the biogenic elements carbon, oxygen, and nitrogen. The second form of matter is the remains of exploded stars, a kind of dark matter that drifts through interstellar space,. Given the universe's age (12 to 15 billion years), our stars and our sun are probably the 2nd or 3rd generation of recycled stellar processes of death and re-birth. The elements synthesized in stars are ejected into the interstellar medium during novas and supernovas, then accumulate under the force of gravity into vast molecular clouds with dimensions measured in light years. In wide angle views of the Milky Way, such clouds of dust and gas can be as shadowy masses that obscure the background light produced by the hundred billion stars of our galaxy. Our solar system, and others that have been imaged by the Hubble telescope, arise within such molecular clouds. The third kind of matter is the material that accumulates into planets during the formation of a solar system like ours. All the matter of the Earth and its inhabitants was once a kind of stardust, and the detailed understanding of this revelation is at the core of astrobiology. Next Deamer considered how organic compounds are synthesized from stardust, then delivered to the Earth to produce the complex structures of the first forms of life. It is generally accepted that all planets are produced when the dust and gas of a molecular cloud forms a disk around a new star, then accretes first into planetesimals, which collide to produce ever larger objects that finally reach the size of planets. All the evidence suggests that a Mars-sized object collided with the early Earth about 4.4 billion years ago, and that the debris from the collision in turn formed the moon. The energy released by the collision heated the surface of the Earth and moon to the temperature of volcanic lava, too hot for any organic compounds to survive. After the Earth had cooled sufficiently for liquid water to produce the first oceans, organic compounds were stable and the processes leading to life could begin. Some organic compounds were likely produced at the Earth's surface by reactions first described in the 1950s by Stanley Miller and Harold Urey, while others were delivered to the Earth by extraterrestrial infall, a process still going on today. How could life begin on a sterile planetary surface, given only a thin soup of organic compounds as an energy source? The first isotopic evidence of microbial life on the Earth is dated at 3.8 billion years, and the first convincing microfossils of early bacteria are dated to about 3.5 billion years ago. It follows that life somehow got started only a few hundred million years after the last of the giant impact events that formed the Earth's crust. One of Deamer's key ideas is self-assembly, meaning the process by which certain organic compounds can spontaneously organize into larger aggregate structures related to life. A common example is the self-assembly of soap molecules into bubbles. All life today is cellular, meaning that life processes are enclosed in a kind of microscopic bubble defined by soap-like molecules called lipids. Because there were no genes and enzymes to initiate the life process, the origin of cellular life must have involved self-assembly of cell-like membranous structures on the prebiotic Earth, which then trapped larger polymeric molecules that had catalytic and self-replicative properties. At some point, one or more of the self-assembled protocellular systems happened to be able to capture energy and nutrients from the environment and began to grow and reproduce. This was the beginning of all life, in the form of relatively simple systems of self-assembled molecules, and at that point true evolution could begin. We don't know yet how this happened, but we do understand some of the steps involved, and particularly the self-assembly of cellular compartments. Deamer passed around a sample of the Murchison meteorite while he discussed molecular self-assembly processes. One of the main contributions his laboratory has made to this field was to show for the first time that organic compounds in the Murchison meteorite have the ability to assemble into membranous vesicles. These are about the size of small bacteria, and could represent the protocellular compartments available to initiate the first forms of cellular life. The last part of Deamer's presentation described a laboratory version of synthetic life that is being used as a model to test ideas of how life began. It is generally believed that the first forms of cellular life were simpler than the RNA-DNA-protein world we live in today. Because RNA can have catalytic activity in the form of ribozymes, it has been proposed that the first life used RNA both as a carrier of genetic information and as a catalyst. In earlier research, Deamer's lab showed that an enzyme that synthesizes RNA can be captured in lipid vesicles (liposomes) and produces RNA inside, using substrate and energy provided on the outside. This was the first demonstration of a working protocell capable of RNA synthesis. The next step is to establish a more complex system in which information contained in DNA can be transferred to RNA. This also has been achieved: an enzyme called T7 RNA polymerase was recently shown to transcribe a genetic message from a DNA molecule also inside the liposome. Neither of the two systems has the capacity to evolve, but the stage is now set for assembling a system capable of true evolution. Deamer's research goal is to include a second enzyme called reverse transcriptase, which takes the genetic message transcribed in the RNA and converts it back into DNA. The fact that this is a cyclic process permits the system to evolve. That is, an error made during DNA synthesis is in fact a kind of mutation which stays within the boundaries of the liposome. If the RNA is a ribozyme, it will now be possible to select a specific ribozyme catalytic action and begin to evolve toward that goal. This has already been achieved by other researchers in test tube reactions, but Deamer's group is the first to attempt such an evolutionary step within the confines of a lipid vesicle protocell. A question that came in response to Deamer's presentation concerns whether there could have been other routes to the origin of life on Earth. Are there multiple models for life's origin? The answer is clearly yes. A number of models are being tested and many questions remain to be answered. For instance, one possibility is that life began on Mars, since it is smaller and farther from the sun and would have cooled before the Earth did. There is clear evidence that Mars once had seas up to about 3.5 billion years ago. Furthermore, pieces of the martian surface are known to be ejected into space by giant impacts and then reach the Earth in the form of meteorites. It is a distinct possibility that life actually began on Mars and was then delivered to the early Earth. Could we all be martians?
|
|
|