Life on a Young Planet (38 page)

More broadly, Earth’s long Precambrian history provides illuminating perspective on the great idea of twenty-first-century Earth science—that biology is inexorably linked with tectonics and climate, atmosphere and oceans in a complex and interactive Earth surface system. The pageant of Cambrian evolution simply provides one last, and dramatic, confirmation that life did not evolve on a passive planetary platform. Rather life and environment evolved together, each influencing the other in building the biosphere we inhabit today.

The last word on this history will not be written for many years. At present, we don’t really know what processes drove the stepwise growth of oxygen in the atmosphere and oceans. Neither do we understand how and why climates shifted so violently at the beginning and again toward the end of the Proterozoic Eon. And we can only conjecture how these and other events shaped (and were shaped by) biology. We have some leads, some good ideas, and much more data than we had a decade ago—but no resolution. The absence of a definitive punch line may disappoint some readers, but as a paleontologist, it is why I get up in the morning. For scientists, unanswered questions are like Everests unclimbed, an irresistible lure for restless minds.

__________

¹
A relatively brief ice age centered on northern Africa took place near the end of the Ordovician Period, some 440 million years ago. Continental ice sheets returned late in the Paleozoic Era, covering much of Gondwana from about 355 to 280 million years ago. Glaciers began to expand for a third time 33 million years ago, in Antarctica, although ice sheets spread across northern continents only within the last 2 million years. No one region preserves a sedimentary record of all three events.

²
These data potentially undermine another argument advanced for the Snowball scenario. Alternative explanations for low C-isotopic values in cap carbonates include a postglacial methane burst and the upwelling of deep waters enriched in
¹²
C—mechanisms that can influence the composition of seawater for a few hundred thousand years, at most. If the C-isotopic value of seawater remained low for several million years, from the onset of global glaciation until its aftermath, hypotheses that call for upwelling or methane release must be rejected. If, on the other hand, low isotopic values began only with deglaciation and ended shortly thereafter, carbon chemistry does not necessarily favor any one explanation over the others.

³
Oxygen availability places biochemical as well as biophysical constraints on animals. For example, Kenneth Towe of the Smithsonian Institution noted years ago that the biosynthesis of collagen, the extracellular matrix protein par excellence, requires O
₂
in relatively high concentrations.

⁴
The idea of Ediacaran mass extinction didn’t originate with me. In 1984, Dolf Seilacher proposed that Ediacaran taxa disappeared many millions of years before the Cambrian began. As John Grotzinger showed, however, this isn’t the case. It is the geochemistry of Proterozoic-Cambrian boundary sediments that leads me to think that extinction paved the road to Cambrian diversification.

⁵
One or two Ediacaran taxa unambigously show up in earliest Cambrian sandstones, but this tells us no more about end-Proterozoic extinction than Triassic brachiopods tell about devastation at the end of the Permian.

Debate about a small meteorite from Mars has catalyzed scientific interest in one of humanity’s oldest questions: are we alone in the universe? What do we know at present; how, in the dawning age of astrobiological exploration, can we learn more; and are there effective limits to what we can learn?

L
IFE AROSE AS
a product of physical processes in play on our planet’s youthful surface, and it has been sustained for nearly 4 billion years by tectonic, oceanographic, and atmospheric processes that modulate climate and cycle biologically important materials. Perhaps most important, as it has increased and diversified through time, biology has come to provide a suite of planetary processes that is important in its own right. Accepting this planetary view of life on Earth, the next thought is large, if obvious. There are probably a lot of planets out there.

Are we alone in the otherwise sterile vastness of space? Or, might our particular corner of the cosmos be representative of the universe as a whole? To pose such questions is to be human; our grandparents asked them, and their grandparents before them. To find answers, however, may be the special privilege of our own generation.

Of course, the meaning of “we” is critical in this context. If by “we” we mean life, then “we” might well be distributed abundantly across the universe, mostly as simple bacteria-like microorganisms. If, on the other hand, “we” is more narrowly self-referential (life-forms capable of introspection and technology), then “we” might be rare, or even unique. Despite a raft of speculation, we really don’t know how to calculate the odds of extraterrestrial life—there is only one data point, and we are it. But speculation is beginning to yield to exploration. I fully expect that
one morning in the future, as I sip my coffee in preparation for another day of grateful retirement, I will open the newspaper to find the screaming headline: life found in space. Again.

On August 7, 1996, I woke up in a hotel room in Beijing, where I was taking part in the International Geological Congress. Still groggy after a fitful night’s sleep, I shuffled over to the television in hopes of catching some news before plunging into the day’s meetings. As the picture flickered and then flashed into focus, a CNN correspondent, reporting from Baghdad, told the camera (as best I can remember), “I thought I had a big story, but now that they’ve found life on Mars, my report doesn’t seem so important any more.” I woke up fast.

By the time I reached the breakfast room, opinions about the announcement were flying thick, fast, and mostly negative: “The fossils are too small.” “They got the history of the meteorite all wrong.” No one knew any details beyond the few gleaned from television, but everyone had an opinion. Of course, unknown to us, our breakfast in Beijing was duplicating in microcosm an intellectual firestorm that had begun to sweep across the scientific world.

The match that ignited this conflagration was struck by David McKay, a courtly geologist at NASA’s Johnson Space Center in Houston, Texas. Along with colleagues at JSC and Stanford University, McKay had reported the discovery of biological fingerprints in a small piece of rock called ALH-84001, a grapefruit-size fragment of Mars blasted into space by bolide impact and delivered to Earth as an unusual and distinctive meteorite (
figure 13.1
). As announced triumphantly to a credulous press corps, the answer to one of humanity’s oldest questions was not only at hand but had, in fact, been sitting quietly in antarctic ice for thousands of years. At least that was NASA’s version of the story.

The debate about ALH-84001 has been recounted in countless newspaper stories and a dozen or more books. It has been portrayed both as a courageous leap into the unknown and as a cautionary tale of judgment clouded by desire. The former is certainly true, the latter more arguable. But if David McKay’s team
was
misled by an exciting hypothesis, their story resonates because of its universality, not its uniqueness. What scientist has not marched purposefully down a blind alley, buoyed by a great idea that just might be true? Our wings usually get clipped in arcane journals and scholarly symposia. Because of its extraordinary public interest, however, the Mars meteorite paper was debated, sometimes acrimoniously, in
Newsweek
and the
New York Times
.

Figure 13.1.
ALH-84001, the grapefruit-size meteorite from Mars that touched off debate about Martian biology. (Photo courtesy of NASA/JPL/Caltech)

My own early response to the McKay report was neither dramatic nor incisive—I really didn’t know what to make of it and thanked the scheduling gods that my trip to China spared me from the frenzy of sound bites unleashed that August. In fact, the opening-night reviews and subsequent pop psychology contain little of lasting value. More impressive are the painstaking analyses that continue to this day, carefully applying color to a decidedly gray area of science. Answers remain elusive, but we now understand that on other planets the familiar questions of life detection must be asked in new ways. As our first, halting exercise in Mars paleontology, ALH-84001 has furnished a planetary extension of the Precambrian research I hold dear. As midwife to the dawning age of astrobiology, it has given us much more.

Before touring the front in the battle over ALH-84001, we should reflect at least briefly on issues that have
not
been contested. First of all, no one argues that ALH-84001 is misidentified as Martian. That, in and of itself, is remarkable. How do we establish the planetary pedigree of a small bit of
rock collected from an ice field? Like other meteorites, ALH-84001 has a surface rind of fused minerals that records its heated passage through the atmosphere. In the 1970s, two graduate students named Hap McSween and Ed Stolper proposed a Martian origin for some unusual meteorites called shergottites (after the village of Sherghati, India, where the first known example fell in 1865). To put it politely, their scientific elders were skeptical. More than two decades later, McSween and Stolper are both distinguished scientists, and at least eighteen meteorites are accepted as immigrants from Mars. The most convincing evidence comes from studies of glass in several of these rocks, formed by the impact that blew them into space. The glass contains small inclusions of gas—samples of the atmosphere on the meteorites’ parent body. The gas mixture does not resemble terrestrial air, either modern or ancient, but it closely matches the Martian atmosphere as measured by Viking spacecraft. ALH-84001 itself contains no glass that might have captured an atmospheric sample, but is interpreted as Martian on the basis of oxygen locked into its constituent minerals. This oxygen differs in isotopic composition from terrestrial rocks but fits the pattern of other Mars meteorites.

McKay’s hypothesis requires that we accept, in principle, two additional suppositions before we pick apart specific points of evidence. First, we must accept that ancient Martians lived in tiny cracks formed within the red planet’s crust. Second, we must be willing to believe that these tiny aliens left an interpretable biological record that has survived intact for nearly 4 billion years. This, of course, is where terrestrial experience begins to frame the debate. We find these claims unremarkable because both are met on Earth. Bacteria thrive today thousands of feet beneath the Earth’s surface, sustained by chemoautotrophic metabolism in networks of fractures flushed by groundwater. And, as previous chapters make clear, terrestrial rocks preserve an unambiguous signature of early life—admittedly smudged in our oldest examples, but by metamorphism, not age per se. If life ever arose on Mars, it should have left its mark in ancient Martian sediments.

ALH-84001 consists mostly of volcanic rock formed soon after Mars accreted some 4.5 billion years ago. Around 3.9 billion years ago—about the time that life first gained a foothold on Earth—tiny deposits of carbonate minerals formed within cracks that had developed in the rock.
Opinion remains divided on whether the carbonates formed at high temperatures associated with meteorite impact or precipitated from cooler groundwaters that percolated through the cracks in calmer times. Regardless of their origin, the minerals were subsequently modified by transiently high temperatures and pressure imparted by meteorites that continued to pummel the Martian surface. Much later—a mere 16 million years ago—ALH-84001 was ejected into space by one more impact. Captured by Earth’s gravitational field, it came to rest 13,000 years ago in the Allan Hills (hence the moniker ALH) of Antarctica.

David McKay’s team gleaned four lines of evidence from this meteorite, which collectively convinced them that Mars once supported microbial ecosystems. The carbonate minerals in ALH-84001 (1) resemble terrestrial deposits formed where bacteria are active, (2) contain distinctive grains of the iron oxide mineral magnetite that compare closely to magnetite crystals formed inside bacterial cells, (3) preserve complex organic molecules thought to be derived from biomolecules, and (4) harbor tiny round and rodlike structures interpreted as microfossils.

By themselves, the carbonate minerals tell us only that the cracks in the Allan Hills meteorite once served as a conduit for fluids charged with carbonate and other ions. Like the seafloor precipitates in Archean limestones, they provide unreliable guides to biological processes. The minerals may, however, furnish clues to physical conditions at the time they formed, and our terrestrial experience tells us that environmental setting is key to any paleobiological interpretation. Temperature is of particular interest—life as we know it can persist only where water remains in its liquid form, a few degrees below 0ºC in salty, ice-covered ponds of Antarctica to about 113ºC in hydrothermal rifts beneath a mile or more of ocean. The isotopic composition of oxygen in the carbonate crystals permits us to estimate temperature, but only if we know the conditions and processes in play when the minerals formed. Geochemists have carefully measured the isotopic composition of ALH-84001, but assumptions about early Mars vary from one computer model to the next, leaving the temperature of carbonate deposition unresolved. If the minerals formed well above the boiling point of water, then biology wasn’t present. But if—as many Marsophiles believe—the carbonates formed at cooler temperatures, then organisms could have
existed in the cracks of ALH-84001. In neither case, however, do the carbonate crystals
require
biology for their formation.