Early Earth and the Origin of Life
The organisms that live on Earth today represent the evolution of a diverse assemblage of organisms. This diversity resulted from geologic and biologic changes that altered the environment since the origin of life some 3.5 to 4.0 billion years ago.
How life actually began is speculative, but clues for a number of hypotheses are present in the molecules, metabolic capabilities and the anatomical development of each species. These clues coupled with the fossil record have produced several theories about how life evolved on Earth.
Life on Earth began between 3.5 to 4.0 billion years ago.
Life appeared relatively early in the Earth's history. Fossils similar to spherical and filamentous prokaryotes have been recovered from stromatolites 3.5 billion years old in western Australia and southern Africa. Stromatolites are banded domes of sediment similar to the layered mats constructed by colonies of bacteria and cyanobacteria currently living in salty marshes. The fossils from western Australia appear to be of photosynthetic organisms that would indicate life evolved prior to the time of the fossils. Other fossils similar to the prokaryotes have been recovered from the Fig Tree Chert rock formation in southern Africa, which date to 3.4 billion years. Those early prokaryotic organisms probably originated within a few hundred million years after the cooling of the Earth's crust. These early prokaryotes appeared at least 2 billion years before the oldest eukaryotic fossils.
Here you can see layers of mud and living bluegreen algae. This pattern of layering resembles the structures found in fossil stromatolites.
Above are stromatolites and a fossil bluegreen algae.
The Origin of Life: Chemical Evolution Hypothesis
Life originated between 3.5 and 4.1 billion years ago. This is the time span during which the Earth's crust began to solidify (4.1 bya) and bacteria advanced enough to build stromatolites were present (3.5 bya). It is hypothesised that the earth's environment during this time was such that life could arise by spontaneous generation.
One hypothesis on the appearance of the first living organisms is that these organisms were the products of a chemical evolution that occurred in four stages:
1. Abiotic synthesis and accumulation of small organic molecules (monomers = amino acids and nucleotides).
2. Joining of monomers into polymers (proteins and nucleic acids).
3. Formation of protobionts due to the aggregation of abiotically produced molecules which differed chemically from their surroundings.
4. Origin of heredity during or before protobiont appearance.
Abiotic Synthesis of Organic Monomers
A. I. Oparin and J. B. S. Haldane (1920's) postulated that the reducing atmosphere and a greater UV radiation reaching the surface of the primitive earth, enhanced reactions that would have joined simple molecules to produce the first organic molecules. This would not possible today, since O2 in today's atmosphere attacks chemical bonds, removing electrons, and the ozone layer of the earth's atmosphere screens out most UV radiation.
In order to test this "chemical evolution" hypothesis, Stanley Miller and Harold Urey built an apparatus containing H2O, H2, CH4 and NH3, and used an electrical discharge to create organic molecules. Now we know the atmosphere of the early Earth probably included CO, CO2, and N2, and was less oxidising than the Miller-Urey model and thus, less favourable to formation of organic compounds.
However various experiments using the Miller-Urey apparatus, and different mixtures of gasses, have produced all 20 amino acids, ATP, some sugars, lipids and purine and pyrimidine bases of RNA and DNA.
Polymers = chains of similar building blocks or monomers.
After the synthesis of the organic building blocks the next step in the chemical evolution of life would have been the linking of these building blocks into polymers. Polymers are synthesised by dehydrating reactions.
Proteinoids are abiotically synthesised polypeptides. The first abiotically produced proteinoids were created in the lab by Sidney Fox, of the University of Miami. These amino acid polymers can be made by dripping organic monomers (amino acids) onto hot sand, rock or clay. When the water of these dilute solutions of organic monomers evaporated, the monomers could have concentrated to a point were polymerisation could have occurred.
Clay may have been an important substrate for abiotic synthesis of polymers since:
1. Monomers bind to charged sites in clay.
2. Metal ions could act as catalysts of dehydration reactions in clay.
3. The many binding sites on clay could have brought many monomers close together and assisted in forming polymers.
Pyrite may also have been important as it provides a charged surface and electrons freed during its formation could support bonding between molecules
The Formation of Protobionts
Protobionts are aggregates of abiotically produced molecules able to maintain an internal environment different from their surroundings and exhibiting some life properties such as metabolism, excitability and self-replication (yet still not able to precisely reproduce). These protobionts were probably antecedents of first true cells.
Evidence to support this hypothesis:
1. When mixed with cool water, proteinoids self-assemble into microspheres surrounded by a selectively permeable membrane.
Microspheres
2. Liposomes can form spontaneously when phospholipids form a bilayered membrane similar to those of living cells..
3. Coacervates (colloidal drops of polypeptides, nucleic acids and polysaccharides) self-assemble.
The Origin of Genetic Information
Today's cells transcribe DNA into RNA, which is then translated into proteins. This chain of command must have evolved from a simpler mechanism of heritable control. One hypothesis proposes that before DNA, there existed a primitive mechanism for aligning amino acids along RNA molecules, which were the first genes.
Evidence to support this hypothesis:
1. Short polymers of ribonucleotides that can base pair (5-10 bases without enzyme, up to 40 bases with zinc added as catalyst) have been produced abiotically in test tubes.
2. RNA is autocatalytic, as indicated by ribozymes (RNA that acts as a catalyst to remove introns, or catalyse synthesis of mRNA, tRNA or rRNA).
3. RNA folds uniquely depending on sequence (unlike DNA), thereby providing a mechanism for natural selection to act on different molecular shapes varying in stability and catalytic properties.
The next step may have been the formation of a membrane that concentrated favourable reactions benefiting unique molecular aggregates. Following this DNA may have replace RNA as the genetic material since DNA is more stable.
Some Alternative Views
No one knows how life actually began on Earth. The chemical evolution described above and supporting lab simulations indicate that the key steps that would have been needed could have occurred.
Several alternatives to the "Chemical evolution" hypothesis have been proposed.
1. Panspermia -- some organic compounds may have reached Earth by way of meteorites and comets. Organic compounds (e.g., amino acids) have been recovered from modern meteorites. These extraterrestrial organic compounds may have contributed to the pool of molecules that formed early life.
2. Most researchers believe life first appeared in shallow water or moist sediments. However some now feel the first organisms may have developed on the sea floor due to the harsh conditions on the surface during that time. This position was strengthened in the 1970's by the discovery of the deep-sea vent communities. Hot water and minerals emitted from such deep-sea volcanic vents may have provided the energy and chemicals necessary for development of the early protobionts.
Deep-sea vent communities
3. Simpler hereditary systems may have preceded nucleic acid genes. Julius Rebek synthesised a simple organic molecule in 1991. The importance of this molecule was that it served as a template for self-replication. This discovery supported the idea held by some biologists that RNA strands are too complicated to be the first self-replicating molecules.
