A New History of Life (8 page)

Soon after this description of Earth’s earliest life originating in an anoxic, sulfur-rich environment appeared, NASA’s
Curiosity
rover
¹²
landed on the surface of Mars. Soon after his own discovery about earliest life on Earth, Martin Brasier was asked if the sulfur microbes whose fossils he had just found could have lived on Mars, or might still live there. His answer, after a brief rumination, was yes.
¹³

If the 3.4-billion-year-old life turns out to be the oldest on Earth, it puts into question many of the currently favored “crèches” where life might have first begun on Earth. Our planet at that time was already ancient in its own right—for our Earth coalesced, as we have seen, 4.567 billion years ago. If this was indeed the first life, it suggests to some that life must have been relatively easy to form in the first place.

But how easy? And in what order? Let us look at what it took to get life on Earth. In order for life to come into being there had to be four steps:

  1.   The synthesis and accumulation of small organic molecules, such as amino acids and molecules called nucleotides. The accumulation of chemicals called phosphates (one of the common ingredients in plant fertilizer) would have been an important requirement, since these are the backbone of DNA and RNA.

  2.   The joining of these small molecules into larger molecules such as proteins and nucleic acids.

  3.   The aggregation of the proteins and nucleic acids into droplets, which take on chemical characteristics different from their surrounding environment: the formation of cells.

  4.   The ability to replicate the larger complex molecules and establish heredity.

While some of the steps leading to the synthesis of RNA—and the even more difficult feat of producing DNA—can be duplicated in the laboratory, others could not. There is no problem in creating amino acids—life’s most basic building block—in test tubes, as shown by the Miller-Urey experiment of the 1950s. But it has turned out that making amino acids in the lab is trivial compared to the far more difficult proposition of creating DNA artificially. The problem is that complex molecules such as DNA (or RNA) cannot simply be assembled in a glass jar by combining various chemicals. Such organic molecules also tend to break down when heated, which suggests that their first formation must have taken place in an environment with cold to moderate rather than hot temperatures. Life on Earth has the nucleic acids RNA and DNA. Once RNA has been synthesized, the path toward life is open—because RNA can eventually produce DNA. But how the first RNA came into existence—under what conditions and in what environments—became the central problem facing those trying to work out the where and how of life’s origin. There is no shortage of hypothesized sites where life may have begun.

The first, most famous, and longest-accepted model for life’s first appearance on Earth was proposed by Charles Darwin, who in a letter to a friend suggested that life began in some sort of “shallow, sun-warmed pond.” To this day, this type of environment, be it of freshwater or perhaps in a tide pool at the edge of the sea, still remains a viable candidate in some circles and in textbooks. Other scientists early in the twentieth century, such as J. Haldane and A. Oparin, agreed with Darwin and expanded on this idea.
¹⁴
They independently hypothesized that the early Earth had a “reducing” atmosphere (one that produces chemical reactions the opposite of oxidation; in such an environment iron would never rust). The atmosphere at that time may have been filled with methane and ammonia, forming an ideal “primordial soup,” from which the first life appeared in some shallow body of water.

Until the 1950s and into the 1960s, it was thus believed that the early Earth’s atmosphere, thought to have consisted of methane and ammonia, would have allowed commonplace inorganic synthesis of the organic building blocks called amino acids by the simple addition of water and energy.
¹⁵
All that was needed was a convenient place to accumulate all the various chemicals. Seemingly the best place to do this was in a shallow, fetid pond, or a wave-washed tide pool at the edge of a shallow, warm sea. And, the idea goes, in such a place some kind of primordial soup filled with organic molecules sat around just waiting for Dr. Frankenstein.

Many scientists now looking at environments on the early Earth doubt this scenario. The organic compounds necessary to form life are complex and easily fall apart in heated solutions. Furthermore, an enormous amount of energy would be required to keep this soup out of equilibrium, which is necessary. What Darwin could not appreciate in his time was that the mechanisms leading to accretion of the Earth (and other terrestrial planets) produced a world that early in its history was harsh and poisonous, a place very far removed from the idyllic tide pool or pond envisioned in the nineteenth and early twentieth century.

Yet the
Alvin
dives to the deep, oceanic volcanic rifts described earlier in this chapter offered a new possibility in the early 1980s, championed by John Baross, now at the University of Washington: life on Earth began in the newly discovered deep-sea vents.
¹⁶
Soon new molecular techniques used to classify the vent microbes added confirmatory information to this idea. DNA tells us either that life spent its first eons in hot water, in fact
very
hot water, or that after forming in a cool place it was somehow scalded nearly to death in some ancient, highly energetic process.

Most of the microbes from the vents were eventually found to belong to the domain Archaea. The archaeans belong to the most ancient lineage of organisms known on Earth, and the oldest are thermophilic, or heat loving, to the point that they thrive in near-boiling water, something not found in ponds. This discovery suggested that the microbes from the vents were of a great antiquity.
¹⁷

During the period of heavy impact, between 4.4 and 3.8 billion years ago, the time of the heavy bombardment described in the last chapter, each successive impact event (caused by comets as large as 500 km in diameter) would have partially or even completely vaporized the oceans. Huge regions of the Earth’s rocky surface also vaporized, creating a cloud of superheated rock-gas, or vapors, several thousand degrees in temperature. It is this vapor, in the atmosphere, which caused the entire ocean to evaporate into steam, and thus could sterilize the surface, killing all nascent life. Cooling by radiation into space would follow, but a new ocean would not rain out for at least several thousand years after the event, and it is difficult to conceive that life would survive anywhere on the planet’s surface.

Large-body impact on the Earth had never previously entered into ideas about life’s origin. But now we know that during the period when life must have been first coming into existence on Earth, the only places that would have been insulated from the titanic energies of the heavy bombardment impacts would have been either in the deep oceans and/or in the Earth’s crust itself. Perhaps only depth in the sea or in rock provided the bomb shelters necessary for earliest life’s survival.

Even around 4 billion years ago there was little land area. Volcanism and the eruption of lava from the interior of the Earth were far more common and energetic than they are today. Thus the deep-sea ridges and vent systems being explored by the few small submarines of the mid-1970s were, in that long-ago time, much longer and more active than they are today. All of this translates to a very energy-rich volcanic world, with huge amounts of deep-earth chemicals and compounds spewing forth into the oceanic environment. Seawater chemistry would have been enormously different than it is than now. The ocean was what we would call reducing (as opposed to the present-day oxidizing oceans), since there was no free oxygen dissolved in the seawater. The temperatures of the oceans would have been scalding.

There may have been a hundred to a thousand times as much carbon dioxide in the atmosphere as there is today. There was also a constant bath of lethal levels of ultraviolet radiation on the surface. You need land to have a pond, and when life first formed on Earth, there may not have been any land at all. Perhaps there was only a hot, toxic ocean from pole to pole.

The hydrothermal vents and their biota of extremophilic microbes, including abundant and heat-loving archaeans, are still a favored site for the origin of life, and unlike the oceans and atmosphere of the early Earth, it is an environment that is indeed strongly reducing. The vents emit chemicals that are appropriate for the evolution of life, such as hydrogen sulfide, methane, and ammonia amid lots of hot water. The vent chemistry would be largely decoupled from the atmosphere, and thus the evolution of life could have taken place independent of the atmosphere. This removed the problem that the Earth’s atmosphere at the time was not chemically correct to form life. But the so-called vent origin had its own problems. How could RNA, that highly unstable molecule, have formed in the vents, with their high temperatures and pressures?
¹⁸

Early life may have formed on the surface of iron sulfide minerals, at least according to respected early-life theorist Günter Wächtershäuser. He named this idea the “iron-sulfur world theory.”
¹⁹
The hypothesis is that the earliest life, which Wächtershäuser termed the “pioneer organism,” was assembled within the high-pressure, high-temperature confines of an underwater hydrothermal vent, one created by undersea volcanism causing hot, mineral-rich fluids to bubble upward through the rock-lined vents along the many thousands of miles of these deep sea fissures. Life would have begun in temperatures that at the surface would boil water (100°C). Under pressure, however, water does not boil as it does on the surface, and the water coming out of the vents was a veritable chemistry set of elements and compounds. But for any sort of organic buildup to occur, the fluids coming from the vents had to have sufficient volumes of carbon monoxide, carbon dioxide, and hydrogen sulfide dissolved in them to provide the carbon and sulfur needed for the construction of amino acids and, eventually, nucleic acids, proteins, and lipids.

Eventually there were buildups of minerals containing iron, sulfur, and nickel as the hot, mineral-rich fluids jetted out of the volcanically heated vents. This allowed the formation of small regions that could capture carbon-carrying molecules, and then chemically change them to first free up carbon atoms and then to link these newly isolated carbon atoms together into ever more complex, carbon-rich molecules. When the poisonous gas hydrogen sulfide came in contact with iron atoms found in various minerals in the same region, the mineral pyrite (fool’s gold) was produced. This reaction produces energy-containing molecules, thus uniting two important aspects of life— the correct elements to produce life and sources of energy to fuel the necessary chemistry. But the energy produced by reactions with pyrite is very small and not enough on its own to fuel any sort of primitive life form. Wächtershäuser realized that it would take the reaction of a second gas, carbon monoxide, to serve as a fuel. This energy was the all-important driver for all that followed: the slow accumulation of Lego-like molecules, piecing slowly together to form a final product completely different from the sum of the various chemical parts.

The idea that mineral surfaces could act as templates for life’s formation is not new. The faces of these flat minerals such as clay and crystals of silicate minerals or pyrite could have been microscopic regions where early organic molecules could have accumulated. As envisioned by geologist A. G. Cairns-Smith some decades ago, the earliest life would have had several characteristics: it could evolve, it was “low -tech,” with few genes (sites on the DNA molecule that code for the formation of specific proteins) and little specialization, and it was made of geochemicals, arising from condensation reaction on solid surfaces, either from pyrite or iron sulfide membranes. Yet many students of early life remain skeptical of this scenario, particularly as the organic takeover lacks a process involving natural selection through which it might evolve.

Carbon monoxide and hydrogen sulfide are animal killers, with the former having taken untold numbers of human lives in poisonings both intentional and not. Yet if this idea is correct, the combination of two killer gases and fool’s gold was the pathway to life. This view was stated by Nick Lane: “The Last Common Ancestor of life … was not a free living cell, but a rocky labyrinth of mineral cells, lined with catalytic walls composed of iron, sulfur and nickel and energized by a natural proton gradients. The first life was (thus) porous rock that generated molecules and energy, right up to the formation of proteins and DNA itself.”
²⁰
William Martin and Michael Russell published a variant on this idea in 2003 and again in 2007.
²¹
They took the idea of hydrothermal vent origin a step further, arguing that such an environment could provide not only all raw materials and energy necessary, but one of the key aspects of life: a cell. Their view is that life began in highly organized minerals called iron monosulfide. The place where life would have formed would have been between the devil (too hot) and the deep blue sea (too cold)—in this case, somewhere geographically between a sulfide-rich (and hot!) fluid jetting out of the hydrothermal (volcanically produced) vents or seeps and an ancient seawater that would have been full of iron. But this is more than theoretical. There are indeed three-dimensional frameworks near fossil vents and seeps observable today—and these could have
been the precursors to cell walls. The “prebiotic synthesis” of organic molecules would have occurred on the inner surfaces of the microscopic compartments in the minerals forming near the vents or seeps. The chemistry of the ensuing “RNA-World” would have taken place within these mineralized cell walls.