IN THE BEGINNING...
The Earth began as a twinkle in the Solar Nebula's eye some 4.5 billion years ago and it - along with the rest of the planets, asteroids, meteors, comets - formed, it is thought, through the tendency of matter to clump together, ever more until finally there were substantial bodies, the planets and their moons, sweeping up all left-overs in their orbits. During this era, approximately one billion years long, the newly-borning Earth was pummeled mercilessly by these left-overs. This was the so-called "Hadean Period" (and well named at that!), a "hell-ish" time indeed when the Earth's surface was periodically broiled, flash-fried so to speak. Incoming asteroids of sufficient size would actually vaporize, themselves and the part of the surface they impacted and this would turn into a seering plasma that would tsusami around the globe - not a pretty picture. Not to mention volcanic eruptions.
This made it a little hard, one would think, for life to get a toehold on the young planet's surface.
Abiotic Production of Organic Molecules
The classic experiment demonstrating the mechanisms by which inorganic elements could combine to form the precursors of organic chemicals was the 1950 experiment by Stanley Miller. He undertook experiments designed to find out how lightning--reproduced by repeated electric discharges--might have affected the primitive earth atmosphere. He discharged an electric spark into a mixture thought to resemble the primordial composition of the atmosphere. In a water receptacle, designed to model an ancient ocean, amino acids appeared. Amino acids are widely regarded as the building blocks of life.
Although the primitive atmosphere is no longer believed to be as rich in hydrogen as once thought, the discovery that the Murchison meteorite contains the same amino acids obtained by Miller, and even in the same relative proportions, suggests strongly that his results are relevant.The Beginnings of Life on Earth
Others have made similar experiments. A group at the Department of Chemistry and Biochemistry at the University of California, San Diego, exposed sulfur-bearing molecules like those thought to have been present before the Earth formed to low levels of light. The presence of the light was enough to generate organic compounds - molecules containing carbon, which form the chemical basis of life as we know it. Meteorite Reveals Life Not Difficult to Make
The new compounds had a distinct isotopic (atomic makeup) signature, not normally found on Earth. In fact, the peculiar part is that these isotopes have only been found one other time, in compounds removed from the Murchison meteorite.
Hypotheses on Origin of Macromolecules
1.RNA-First Hypothesis
RNA could carry out processes associated with Life
Nobel Prize 1989 (Cech, Un of Colorado & Altman, Yale)
RNA can act as a substrate and/or an enzyme
2.Protein-First Hypothesis Sidney Fox (above)
Proteinoids form from amino acids at 180o
Proteinoids can form Microspheres
3.Clay catalyzed RNA & Protein synthesis (Both First)
Graham Cairns-Smith (University of Glasgow)
Clay is helpful in polymerizing Proteins & Nucleic Acids
Attracts small organic molecules
Contains zinc & iron (metal catalysts)
Collects energy from radioactive decay and releases it when Temperature and/or Humidity change.
Macromolecules to Living Cells
Took half a billion years
Event Still a Mystery
1. Prebionts
Nonliving structures that evolved into the first living cells
2. Coacervates
Organic molecules surrounded by a film of water molecules
Selectively absorb materials from surrounding water
Incorporate them into their structure
Not a random arrangement of molecules
3. Microsphere
Organic molecules surrounded by a double membrane
Can be formed from Proteinoids, when placed in boiling water & cooled.
Shrink & swell depending on the osmolarity of the water.
Can absorb material from the environment & grow & form buds.
Internal streaming similar to cells
Have been shown to form nucleic acids & polypeptides (ATP present)
Microspheres = Protocells!
RNA Stuff
Why is modern metabolism dominated by protein enzymes? There is a "chicken-and-egg" paradox of how to initiate theflow of genetic information at life's origin, since proteins encoded by DNA are required for the replication of DNA. To circumvent this dilemma, Francis Crick and others proposed thirty years ago that primitive forms of life contained RNA as their sole genetic and catalytic molecules (the "RNA World"). The discovery of natural catalytic RNAs renewed vigorous speculation over the role of RNA in early evolution, most of it going well beyond the reach of supporting data. Through in vitro selection technologies, we can now test RNA World hypotheses through direct experimentation.
Based on Early Earth Environment
This organism is referred to as the Universal or Common Ancestor. It would have had the following characteristics because of the environment in which it evolved:
it would have been anaerobic
it would have been hyperthermophilic and halophilic
it would have been a chemolithoautotroph, obtaining both energy and carbon from inorganic sources, using H2 or reduced sulfur compounds as electron donors and CO2 or oxidized sulfur as electron acceptors to provide energy and fixing CO2 as their carbon source.
(Chemolithoheterotrophs would have evolved later in this scenario as "opportunistic" consumers of organic matter formed by autotrophic producers. There is also a hypothesis that the first living organism was heterotrophic but this could only have been true if the prebrotic broth contained significant concentrations of abiotically produced organic molecules, which is not likely, especially from the point of view of continuous supply.)
Modern chemolithoautotrophs
Organisms thought to be similar to these first chemolithoautotrophs have been isolated in the last few years from what we would call "extreme environments". These organisms are isolated from hot sulfur springs on the earth's surface or hydrothermal vents ("black smokers") on the ocean floor where these organisms form purely prokaryotic ecosystems.
Conditions in these environments are thought to mimic those present on the early earth, i.e. high temperature, high ulfur, anaerobic, high salt.
These organisms grow optimally under anaerobic conditions in high salt at 80-110 C, in fact they grow completely independent of oxygen and sunlight; they could even grow on another planet if water was available.
Cells existing prior to 1.5 billion years ago were all prokaryotes, such as bacteria. Eukaryotes appeared about 1.5 billion years ago.
Prokaryotic Cells
Cells of the most primitive organisms have cells that lack a nucleus.
Monerans (bacteria and related organisms) are prokaryotes.
Prokaryotes also lack other membrane-bound internal structures.
Eukaryotic Cells
All organisms, other than monerans, have cells with nuclei.
Cells with nuclei = eukaryotic cells, and the organisms made of such cells are eukaryotes.
Eukaryoyic cells have membrane-bound internal structures (organelles) in addition to the nucleus.
In whichever way life developed, we have fossil traces of it that have been reliably dated at 3.5 billion years old. And there are suggestions from carbon deposit analyses (looking at percentages of the kinds of isotopes that we know are produced by life) that hint at an even earlier animation. The fossil remains, of bacteria, look similar to a modern variety, the Cyanobacteria, which are actually thought of as the most advanced.
We are looking at some two to three hundred million years for the appearance of a form of life on Earth.
Stromatolites are the oldest known fossils, dating back more than 3 billion years. They are colonial structures formed by photosynthesizing cyannobacteria and other microbes. Stromatolites are prokaryotes(primitive organisms lacking a cellular nucleus) that thrived in warm aquatic environments and built reefs much the same way as coral does today. Cyannobacteria were likely responsible for the creation of earth's oxygen atmosphere. They were the dominant lifeform on Earth for over 2 billion years. Today they are nearly extinct, living a precarious existence in only a few localities worldwide.
Some have said that there was too short a time for life to have evolved from inorganic chemicals and go on to propose that life itself was seeded on Earth from extraterrestrial sources. Let's consider this.
One. We know that meteors infallen on Earth have come from other planets in the Solar System, in particular from Mars. How can we know this? We actually can - ever since the Viking Lander missions to Mars, we know precisely what the makeup of the Martian atmosphere is - it has a specific "fingerprint" that identifies these meteors beyond a trace of doubt. How could a meteor from Mars get here? Well, a pretty good-sized asteroid crashing into Mars at, say, an oblique angle, would hurl surface material at escape velocities off-planet. We can reasonably assume that the planets have traded rock. Could microbial life have hitch-hiked from one planet to another?
Two. Consider this. We can calculate that four billion years ago the sun was almost half as hot as it is today. It seems likely that Earth and, say, Mars, were frozen during this time. On the other hand, Venus may have been sitting pretty and perhaps had warm liquid oceans. This is pretty speculative but perhaps life evolved there and spread by the mechanism just detailed to Earth (and Mars?). Some suggest that perhaps life is endemic to the universe: note the presence of complex carbon compounds in interstellar space and on comets and meteors.
The Panspermia Hypothesis goes a step further and suggests that life is basically a property of the universe and it will be found in all possible habitats as a matter of course.ref
But - we are still left with the question: how and under what circumstances does life start?