A New History of Life (9 page)

By the turn of our new century, many clues were present, and many potential places for life’s first origin had been mooted. The oldest surviving life on Earth was certainly heat loving, of a type found still in the hydrothermal vents. All chemicals and energy necessary for life were found in the vents, although it did not necessarily evolve there. And finally, the vents offered a refuge from the rigors of the early Earth’s surface, most important as a bomb shelter from the murderous asteroid barrage of the Earth’s first billion years. But there was one great obstacle to uniform acceptance of this theory: RNA, and to a lesser extent DNA, are highly unstable at high temperatures such as those found in hydrothermal vents. Once RNA was created, the leap from RNA to DNA would have been more straightforward. RNA serves as a template for DNA. But getting from small molecules to something as complex as RNA, which even in its simplest forms is composed of many atoms in very precise positions, is still a mystery. Yet while a mystery, it is not an impossibility by any means, and rapid progress in the artificial formation in what are essentially test tubes shows us the overall pathway if not yet every detail.

Biologist Carl Woese theorized
²²
yet another possible origin of life pathway, with life starting even before the Earth was fully formed and differentiated into the core, mantle, and crust components that we see today. Thus, in these early times, there would have been large amounts of metallic iron present on the surface of the Earth in contact with steam and some liquid water, amid an atmosphere filled with carbon dioxide and hydrogen. It is the latter that is so interesting, since hydrogen is a potent driver of chemical reactions. But because of hydrogen’s light weight, it is easily lost to space on small-mass planets like the Earth, Mars, and Venus (the gas giants are so massive that they can hang on to their hydrogen). At this time the Earth was being barraged by space debris large and small, causing the planet to be
encircled by a haze of dust particles and water vapor. High clouds of water vapor would form, and these tiny droplets would have served as protocells—tiny cell-like objects. With sunlight serving as an energy source, and the dust thrown up from the surface carrying organic molecules among the many other molecules and elements blasted into the sky by the asteroid bombardment, there would have been plenty of raw materials to make life from. With lots of hydrogen present as well, the first primitive organisms to evolve could have produced methane after using carbon dioxide as a carbon source. Microbes using this pathway today—hydrogen for energy and CO
₂
for carbon—are called methanogens. As the Earth cooled, oceans formed, and life would have fallen from the sky to populate the oceans.

One of the newest suggestions comes from the University of Florida’s Steve Benner
²³
and a coauthor of this book, Joe Kirschvink. As mentioned earlier, the hardest step of all is making RNA. This is because RNA is a very fragile molecule, large and complicated, and thus very easily destroyed. Water attacks and breaks up the nucleic acid polymers (strings of smaller molecules) that make up RNA. In fact, it appears that there are many steps required in making RNA, and each step would require different conditions, or a different chemical environment. Biochemist Antonio Lazcano has described this problem as follows: “The RNA-world model confronts several serious challenges, including the lack of plausible primitive abiotic mechanisms to account for the formation and accumulation of ribose.”
²⁴
A possible way out is the hypothesis that ribose can be made under current temperatures from common desert minerals.

Benner noted that the major problem was not making carbohydrates (including ribose), the problem was
preventing
the reactions that created them from continuing madly, producing a sticky brown coal tar that gummed everything up. By looking at the synthesis pattern carefully, and after staring at a table of ionic radii, he realized that the pathway to coal tar could be blocked specifically by reaction with
calcium (Ca
⁺²
) and borate (BO
₃
^-3
) ions. These calcium-borate minerals (e.g., colemanite, ulexite) are often used in soap, and they form by evaporation of salty brines in dry, hot environments. One additional step, a subtle rearrangement catalyzed with oxidized molybdenum, is all that is needed to produce biologically active ribose.

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, H
₂
, 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.

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 on 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 Weiss
²⁶
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.