by Peter Ward
Benner also looked to extant life for clues. He analyzed the stability of various bacteria and found that the most ancient lineage may have formed at 65°C. This is hotter than any “warm little pond,” but much cooler than a hydrothermal vent, which typically has temperatures in the many hundreds of degrees. In fact, there are very few places on the surface of the Earth now, or even 3.7 billion years ago, that would have such temperatures—except for deserts.
Desert-like conditions, where the overall environment is alkaline and has calcium borate in abundance, is the only environment where the formation of ribose from borate minerals might be favored. Clay minerals of various kinds are also common in such settings, and increasingly it looks as if templates formed from clay would help bring about the synthesis of the complex organic compounds necessary for life.
To form the borate minerals needed to stabilize RNA, there has to be a liquid system that repeatedly decants and distills the liquids in a series of interconnected steps. Kirschvink, in collaboration with MIT professor Dr. Ben Weiss, has hypothesized a natural setting that could lead to the formation of RNA from borate in the rough fashion that Steve Benner has suggested. A good example is in California, where boron leached from the igneous rocks in the Sierra Nevada mountain ranges passes through a chain of transient lakes, including Mono Lake, Owens Lake, China Lake, Searles Lake, Panamint Lake, and finally into the bottom of Death Valley. Massive borate deposits form in the last few of these reservoirs. The most obvious candidate for such a system, at least on the early Earth, and especially between 4.2 and 3.8 billion years ago, the time during which life may have first formed, would be a series of impact craters linked in a desert setting, with communicating water systems among craters of higher to lower elevation. In this way the same series of distillations and decanting could be accomplished. But such a site would have been unlikely on Earth 4 billion years ago, when all of this early chemistry was taking place. Earth was also strongly reducing then, precluding the presence of the oxidized molybdenum for the final rearrangement of ribose synthesis.
All of the earliest Earth rocks appear to have been produced in a water setting. In fact, there is no good evidence of extensive, subaerial continents on Earth until less than 3 billion years ago—on a planet 4.6 billion years in age—and the oldest detrital zircons suggest oceans going back at least to 4.4 GA. Our best evidence is that the Earth, at the time when life would have first formed, had nearly global oceans, with at most strings of islands. But Earth was not the only inner terrestrial planet. Venus is the same approximate size as our Earth, but is so close to the sun that it is highly unlikely that life could have ever formed there. Yet we know that there is another possibility, one beloved by science fiction: Mars.
There has been great progress in understanding the ancient geological history of Mars during our new century. Mars never had planet-covering oceans; we are quite sure because the older rocks are still there, exposed at the surface. But the immense amount of new data from the various Mars rovers has told us that the so-called Red Planet had large lakes, maybe small seas, and possibly an ancient ocean in the north polar basin. There is also evidence that Mars had larger oxidation-reduction gradients than Earth, which are the important means used by life to gain energy. The deep mantle of Mars is so reducing that methane, H2, and the other gases needed for prebiotic syntheses of the carbon-rich chemicals needed for life should have been present, thus providing needed raw materials. There are some, coauthor Joe Kirschvink among the truest believers, who support the radical notion that life not only formed on Mars more than 4 billion years ago, but that it came to Earth on meteorites—and it is us. The question is if early Mars life could get to Earth at all.
PANSPERMIA AND THE CASE FOR MARS
Today, the surface of Earth is divided roughly into the larger ocean basins, which cover about 75 percent of the surface, and continental masses, which stand up above the mean sea level. We know from the simple age dating of the continents and a variety of other geochemical proxies that the continents have been slowly growing through time. New granitic basement rocks are added along the margins of the continents at subduction zones, where moist, sediment-laden rocks are carried down several hundred kilometers and are melted partially to form granites. Thus, as we go back deeper into geological time there is a good expectation that we would have less land area versus ocean area.
But there are even more constraints. We know from geophysical models that immediately after the moon-forming giant impact event at 4.5 billion years ago that the entire Earth was molten. A gigantic magma ocean existed, the result of the intense heat of the collision as well as the segregation of nickel-iron metal down into the Earth’s core. The first half billion years or more after this event was a time of intense heat flow coupled with the gradual solidification of the surface crust in the uppermost layers of Earth’s lithosphere. This increased heat flow limits the elevation that any landmass can reach above mean sea level. A continent stands high above the seafloor simply because it is underlain by less dense material that causes it to “float” upward. If the heat flow is high, the root underneath the continent will melt. That prevents high mountain ranges from forming.
Finally, geochemists suspect that the volume of Earth’s oceans may be slowly decreasing with time. After the giant Earth-forming event, it is likely that a lot of the water vapor present in the system condensed out as steam on the surface of the young Earth, and has been gradually worked back into the mantle through the process of plate tectonics. This reworking is certainly seen in the chemical fingerprint of the 4.4-billion-year-old zircons mentioned earlier. Estimates on the size of this initial ocean vary from a minimum of about equal to what we have today to three or four times more than presently exist. Given all of these constraints, it is extraordinarily unlikely that anything other than the tippy-top peak of some volcano ever stuck itself above sea level before about 3.5 billion years ago.
A water world is not a very good place to form ribose. It is also a terrible place to form large molecules like proteins and nucleic acids, which release a little bit of water each time they add a new subunit. For these reasons, Earth was probably not a very good place anywhere for the origin of life until about 3.5 billion years ago. And even then it was unlikely to have had a series of lakes like those in Death Valley capable of enriching calcium borate minerals to the levels needed to stabilize ribose and other carbohydrates that early life absolutely needed until much later. It certainly did not have large chemical characteristics producing enough energy to have fueled sloppy, early metabolism.
Extensive experiments conducted during the past decade have unequivocally showed that meteorites can go from the surface of Mars to the surface of Earth without being heat sterilized—and thus they could carry life from Mars to Earth.25 Over 1 billion tons of Martian rock has made this transition to Earth over the last 4.5 billion years. It is therefore important for the origin of life to consider the possibility that it arose first on Mars and was carried here by meteorites.
Mars is only about half the diameter of Earth, and about 10 percent of our mass. As a smaller planet, it has a smaller gravitational field. It is therefore easier for something like a meteorite or a molecule of gas to escape completely. For this reason when a small asteroid impacts into the Martian surface (traveling at 15 to 20 km/second) it can eject a lot of surface material into orbit around the sun, and the Mars rocks thrown off their planet would not suffer sufficient heating or “shock” to sterilize them. On Earth, the stronger gravity means that a lot more energy is required to launch material into deep space, making it very probable that material launched in this fashion will be melted. There is no record of unsterilized materials ever having been launched from Earth by natural processes.
Life, if it ever evolved on Mars, would thus escape easily. On the other hand, the stronger gravitational field of Earth means that it is much better than Mars at keeping its hydrosphere and atmosphere intact over geological time. The atmospheric pressure o
n Mars is so low that liquid water will simply boil away at room temperature. Data from the most recent Mars rover, the 2012-landed Curiosity, make it clear that there were bubbling streams merrily percolating down alluvial flans toward a large lake or perhaps an ocean at Gale crater, where Curiosity landed. A world with volcanic rocks, replete with bubbling streams and oceans and an active hydrological cycle, ought to have had life. Or it certainly could have had life. We argue that it was possibly the place where life, the life now of Earth, in fact first evolved.
If we go further back to the Hadean record on Earth it is clear that oceans did exist as far back as 4.4 billion years ago. A Martian setting for life’s first formation, using the borate pathway hypothesized by Benner, but then passing through linked craters in a desert setting, is this new possibility advocated by Kirschvink and Weiss26 earlier in this century. A number of experiments now confirm that complex organic molecules, and even the resting stages of microbes, could be transported from Mars to Earth through a process known as interplanetary panspermia—where a large impact on the Martian surface, say 3.6 billion years ago, hurls a great number of Mars meteorites onto the Earth—and in so doing seeded our planet with Mars life.
There is one more bit of evidence to support a Martian origin, based on new research by David Deamer of the University of California at Santa Cruz.27 One of the great problems in arriving at an RNA strand long enough to do anything is getting it to link to other of the component pieces of RNA to form a “polymer,” a long strand of RNA made up of many of the subunits, called RNA nucleotides. Deamer showed that freezing a dilute solution of single nucleotides forces many together along the edges of ice crystals. There was no ice on Earth back then. But Mars would have had plenty of polar ice, especially early in its history when the sun was dimmer, just as it does now.
FORMING LIFE—A 2014 SUMMARY
Advancing our understanding of how life first formed from nonlife on early Earth to some extent has depended on how close are we to producing life in a test tube. Even five years ago the answer would have been not very close at all. But thanks to a group at Harvard, led by biochemist and 2012 Nobel laureate Jack Szostak, we are closer than is perceived by most of the public.28 Szostak and his colleagues have for nearly two decades been experimenting with the chemistry of RNA. The earliest information molecule was either RNA or something much like it that later evolved into RNA as we know it. And it is in the study of RNA that the Szostak group has made great strides in this century.
The trick has been trying to get nucleotides in solution to link one to another into short lengths of RNA. Getting them to link into a chain is easier than getting them to reproduce, once formed. Yet they will do this if around thirty of the nucleotides are linked, because with such a length and longer, the RNA molecule attains an entirely new property: it becomes a chemical known as a catalyst, which is a molecule that helps speed up chemical reactions. In this case the reaction to be sped up is nothing less than the reproduction of the RNA molecule into two identical copies.
Getting RNA strands at least thirty nucleotides long somewhere on (or in) the early Earth perhaps required clay to serve as a template. The clay mineral montmorillonite seems the most favorable. According to this hypothesis, single nucleotides, floating in liquid, bumped into the clay. They became weakly bonded to the clay and held in place. On some parts of the clay mineral, chains of thirty nucleotides or more were produced. As they were only weakly bonded, they were easily detached, and if there were some sort of concentration of these strands that then became engulfed in a small bubble of lipid-rich liquid, much like a soap bubble, there would have been the makings of a first protocell.
The two major components necessary for life are a cell that can reproduce itself and some sort of molecule that can carry information, as well as performing chemical catalysis (changing conditions so that a chemical reaction that would not otherwise occur does take place because of the action of the catalyst being present). If enough new components of RNA can be brought into the cell, the catalyzing action of the RNA will make more RNA as appropriate new chemicals are brought into the cell itself. The old idea was that cells and the small information-carrying molecules formed separately somewhere and then later merged. Now it seems that they evolved in tandem.
Many biologists have argued that the first life was just that: a “naked” RNA molecule, floating around in a soup of nucleotides and reproducing itself, over and over. But a more favored view is that cells and RNA evolved as a single unit—double-walled cells of fat with small RNA nucleotides within them grew by obtaining more fat and more nucleotides, which could have passed through gaps in the fat of the cell wall, whereas the larger linked nucleotides of the interior would be too large to pass out of the walls. The material available on the early Earth necessary to make the protocells were chemicals that would have combined to form fatty (lipid) molecules, which themselves would readily link together to form sheets and then spheres.
The newly discovered Lost City mid-ocean ridge vents in the north central Atlantic Ocean, discovered by University of Washington oceanographers, are composed of lime-rich rocks and hence are whiter in color than the more common black smokers of the Pacific Ocean. These sites are considered prime possible places where life was first assembled on Earth. (Image from University of Washington, with permission)
Because of its chemical properties, accumulations of sufficient fatty molecules will easily form hollow spheres when agitated, just as water will form tiny drops on its surface for a short period. As these spheres form, they will be filled with the molecules that can form RNA if these molecules (the nucleotides) are present in the liquid. Again, this is where concentration is crucial, and why the analogy of a “prebiotic soup” is used so incessantly. There would have to be a great number of nucleotides caught up in a suddenly forming protocell sphere if there were to be any chance for RNA to form within. Unless, of course, some property of the new protocell either actively or passively moves nucleotides that are outside of the cell through its walls into the interior.
The cell wall would not only be “feeding” on nucleotides. It would also be accumulating more of the fatty molecules, and in so doing elongating into a sausage shape. Eventually it would split and two spheres would be present, with each now carrying around half of the RNA—and a lot more than just RNA, of course. To function for any length of time, the cell would then have to obtain energy, and that requires chemical machinery—made of proteins. So the interior would have to have a lot of chemicals within it, functioning in some orderly fashion so that needed chemicals can be brought in, unneeded chemicals tossed out, and there would have to be plenty of spare parts (molecules of various kinds) readily available.
This is the stage where evolution begins. Some of the cells might reproduce faster based on the nature of the molecules within the new cell. Natural selection thus kicks in, and the engine of life as we know it has been turned on, cells that are autonomous, metabolize, reproduce, and evolve. The rest, as the great Francis Crick so famously said, long ago, was history.
THE DARWINIAN THRESHOLD
Early Earth-life cells might have been like modular homes, with each part installed as a separate component in a different place and then transported to a single place. The transport system could have been through water or air. The latter case is receiving strong support from new work, begun in 2010, looking at the amount of life and life material found in the upper atmosphere.
The earliest life might have been composed of cells with very porous cell walls, allowing the swapping of whole genomes, a process known as horizontal gene transfer. But there came a time when the cell systems went from ephemeral to permanent. This is the point that biologist Carl Woese called the “Darwinian threshold.” It is the point where species, in something approaching the modern sense, can be recognized, and when natural selection—evolution in other words—takes over. Natural selection favored more functionally complex, integrated cells than simpler precursors, and they fl
ourished at the expense of the simpler modular varieties.
Modern Earth life was born when the radical changing of genes stopped. Some who study the evolution of the first life, such as Carl Woese, believe that arriving at this grade of organization is the most important event in all of evolutionary history. Yet those first cells were surely not alone, for there were probably ecosystems packed with all manners of complex chemical assemblages that had at least some life aspects. We can think of a giant zoo of the living, the near living, and the evolving toward living. What would that zoo contain? Lots of nucleic acid creatures of many kinds, things no longer existing and having no name because of this. We can imagine complicated chemical amalgamations that have been roughly defined as RNA-protein organisms, RNA-DNA organisms, DNA-RNA-protein creatures, RNA viruses, DNA viruses, lipid protocells, protein protocells. And all these huge menageries of the living and near living would have existed in one thriving, messy, competing ecosystem—the time of life’s greatest diversity on Earth, perhaps 3.9 to 4.0 GA (billion years ago), but with our new view being that it was later rather than sooner. Natural selection whittled what might have been a thousand really different kinds of life down to one.
Nobel laureate Christian de Duve stated that once the ingredients were in place with the right amount of energy present in the early Earth stove, life would have emerged from nonlife very quickly. Perhaps in minutes.
CHAPTER V
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From Origin to Oxygenation: 3.5–2.0 GA
