A Brief History of Creation (36 page)

One issue was left unresolved: the microscopic fossils that McKay and others believed they had seen. Even discounting the possibility that the fossil-like structures in ALH84001 were the result of microscopic contamination on Earth, the issue of whether they were fossils at all was something that was bound to be wrapped up in subjectivity. In many ways, it was the same problem faced by those trying to establish the fossil history of ancient microbial life on Earth. Identifying such microscopic fossils has long been often a matter of perception. In the case of ALH84001, the problem was compounded by the fact that the fossils seen by McKay would have been made by microorganisms smaller than any microbe known on Earth. This raised questions about the requisite minimum size required for life.

Those same questions were raised by another discovery, which took place in 1996, just months after the details of ALH84001 were made public. Off the west coast of Australia, an oil-drilling rig had brought sample cores of ancient sandstones to the surface, some as old as 250 million years. They were shipped off to the University of Queensland for study. Four years
later, the Australian team led by scientist Philippa Uwins announced the discovery of tiny life-forms that were composed of the standard elements one would expect, including carbon, hydrogen, oxygen, and nitrogen, and that appeared to have cell membranes and stained positively under the microscope for DNA. Uwins called them “nanobes” and suggested they might be a biochemically run-of-the-mill but previously undescribed terrestrial life-form. Yet decades after the discovery of Australian nanobes, nobody could say conclusively whether they were organisms, fragments of organisms, or something else entirely. It is the same dilemma that still surrounds the microfossils that McKay and his team observed on ALH84001. It is hard enough for the scientific community to agree on the authenticity of ancient terrestrial microbial fossils. The burden of proof for extraterrestrial bacteria is naturally, and rightly so, much heavier.

But a growing number of scientists in the field of the origin of life were beginning to realize that fossils are not the only means of looking into the past history of life on the ancient Earth. Soon, one scientist in particular would realize that an imprint of that history was embedded in the genes found in every living cell, and that it could be used to paint some sort of portrait of the earliest living organisms.

*
“But why, some say, the moon? Why choose this as our goal? And they may well ask why climb the highest mountain? Why, 35 years ago, fly the Atlantic? Why does Rice play Texas? We choose to go to the moon. We choose to go to the moon in this decade and do the other things, not because they are easy, but because they are hard, because that goal will serve to organize and measure the best of our energies and skills, because that challenge is one that we are willing to accept, one we are unwilling to postpone, and one which we intend to win.”

†
The orbits and the relative strengths of the two planets' gravitational fields suggest that transport from Mars to Earth is much easier than the other way around, leading to the rather startling conclusion that we are all, in fact, Martians. Such an inference cannot be made lightly, but there is experimental evidence that modern terrestrial microbes could at least survive such a trip.

ONE PRIMORDIAL FORM

Nature, displayed in its full extent, presents us with an immense tableau, in which all the order of beings are each represented by a chain which sustains a continuous series of objects, so close and so similar that their difference would be difficult to define
.

—GEORGES-LOUIS LECLERC,
Comte de Buffon,
Histoire naturelle des oiseaux
, 1770

 

T
HE YEAR IS 3,500,000,000 BC
. The place is a rocky outcropping that juts out into a shallow, wave-lapped inlet on a landmass that will one day be called Australia. The seas are bright green and have the sulfurous stench of rotten eggs. The moon looms large in the sky, twice as large as the moon does today because it is only half the present distance from the Earth. The sun, however, is only about three-quarters as bright as it will one day be. Still, it bombards the Earth's surface with deadly ultraviolet radiation unchecked by a protective layer of ozone. The atmosphere is filled with toxic gases, and almost completely devoid of oxygen. That will come much later, the product of photosynthesis by tiny organisms that will one day churn away in the primitive oceans.

But the ancestor of those creatures is already here. It lives in the ocean near the shore, close to a hydrothermal vent that keeps the temperature of nearby water close to boiling. It is a tiny, single-celled organism, no more than a lipid membrane that encases an early but functioning genome composed of DNA, as well as proteins and the RNA with which these parts
communicate. Billions of years in the future, scientists will give it a name: LUCA, short for “last universal common ancestor.”

Eventually, LUCA will give birth. This will be a virgin birth, which it will accomplish through binary fission. When it is done, LUCA will have divided into two of what will be essentially clones, distinguishable only by the odd genetic mutation. Soon other such clones will appear. They will share their genetic information with each other in a haphazard fashion. Their genetic code will be pooled, their evolution shared. Collectively, they will exist less like a community of organisms and more like a communal organism.

In time, the shallow lagoon will be filled with such organisms, forming small domed masses that peek out from the surface of the water. They will appear to be made of mud. This will be an illusion. Inside, they will be composed of finely laminated layers of silt and biological material. These are microbial mats, not unlike the masses of scum that float to the surface of modern ponds, filled with complex symbiotic communities of microorganisms interspersed with fine particles of clay and other minerals that have adhered to their cell walls.

Under a microscope, these communities would appear as tiny oval cells juxtaposed with filamentous bacterial forms. They look a lot like stromatolites, a few versions of which still exist in Australia, as well as on the shores of such far-flung places as the Yucatán, British Columbia, and Turkey. Dynamic and resilient microbial ecosystems, they are capable of thriving in environments hostile to most modern forms of life. In the years to come, these early descendants of LUCA will grow and multiply, carried by winds and currents to distant refuges across the Earth, until one day they will collectively evolve into human beings and every other living thing on the planet today.

E
VER SINCE
the publication of
On the Origin of Species
, scientists have speculated about what Charles Darwin tantalizingly called “one primordial form.” Though based on reasonable evidence, the scenario just depicted is purely hypothetical. There are many such guesses about the
precise nature of LUCA. The reality is that nobody knows exactly what the environment was like when life arose some four billion years ago. Instead of the hot water of a hydrothermal vent, for instance, LUCA might have lived in a warm little pond not unlike the one once suggested by Charles Darwin. Nor does anyone know what LUCA's internal chemistry was like. It may have possessed unique features that have been lost to its ancestors over the course of several billion years of evolution and natural selection.

Stromatolites in Australia's Yalgorup National Park.

Until the late twentieth century, scientists had precious little to go on. The reason modern scientists can draw a reasonable picture of LUCA at all is due in large part to the evolutionary detective work of one man, the biophysicist Carl Woese. One of the most creative, revolutionary, and underappreciated biological thinkers of the twentieth century, Woese upended nearly everything that biologists had once assumed they knew about the earliest organisms on Earth, and laid the groundwork for a new understanding
of how those organisms existed and evolved. After his death in 2012, some of Woese's most ardent admirers would invoke comparisons to Einstein and Darwin.

A
LTHOUGH CARL WOESE
would go on to become one of the world's most important biological thinkers, he never much cared about the life sciences when he was young. Growing up in Depression-era Syracuse, New York, Woese was a painfully shy child who fixated on mathematics. Math offered a respite from the chaotic world around him. It was objective, consistent. In his later years, that shyness would keep him away from most academic meetings or conferences, and it probably contributed a great deal to the relative underappreciation of his work by the general public, as well as to the resistance that his most revolutionary ideas would one day meet in the field of biology.

Woese went on to study at Amherst College, where he did his undergraduate work in mathematics and physics. By the time he arrived at Yale to work toward a doctorate, however, he had turned to biophysics, steered, like so many would-be physicists of his generation, into the relatively new science that had been invigorated by Schrödinger's
What is Life?
Woese's work at Yale revolved around radiation and how it might be used to change the molecular structure of viruses, particularly the one that causes Newcastle disease, which afflicts poultry. After graduating, he spent two years in an unsatisfying and ultimately unsuccessful pursuit of a medical degree before securing a position as a biophysicist at General Electric's primary research laboratory in Schenectady.

Crick and Watson's discovery of the structure of DNA had by then begun to reshape the scientific world's understanding of genetics, and Woese spent the next five years at Schenectady trying to decipher the genetic code. The problem, as he saw it, was one of translation. There were just four “letters” of nucleic acid bases, arranged in three-letter “words.” These had to correspond to the twenty amino acids found in proteins, allowing them to be strung together in precise sequences. How this was accomplished was anyone's guess.

Woese turned his attention to the problem of understanding the cellular translators of the genetic code, ribosomes, the large molecular structures composed mainly of RNA that read out specific genetic instructions from DNA into proteins. The little-understood ribosomes had started to interest Woese as early as his Yale studies on bacteria, but at Schenectady he was able to focus on them exclusively.

Though Francis Crick and others would work out the problem of the genetic code before Woese, he would come to understand the code's potential in ways they did not. They treated it is a physics problem to be solved mathematically. Ironically, Woese, who had once rejected biology for mathematics, saw the genetic code as a distinctly biological phenomenon rooted in evolution. He saw that the code might be able to serve as a kind of evolutionary time machine, enabling scientists to peek back through successive generations all the way to the dimmest and most distant period in evolution. Instead of trying to gauge the changes between species by measuring the differences between physical manifestations—searching, as Geoffroy Saint-Hilaire had once done, for the similarities between a human hand and a whale fin—Woese believed that evolutionary links could be more conclusively elaborated by tracing the evolution of the cellular machinery that governed the translation of DNA into protein.

Woese would set himself to this task for the next decade, until he understood the history of life well enough that he could completely reshape what had long been thought to be one of the most unshakable foundations of biology, the tree of life.

T
HE FIRST SYSTEMATIC ATTEMPT
to classify every living thing in the natural world was made by the Swedish physician Carl Linnaeus, one of the eighteenth century's most influential naturalists. In his groundbreaking 1735 book
Systema naturae
, or
The Natural System
, he sorted organisms into three distinct “kingdoms,” consisting of the plants, the animals, and the minerals. By 1758, the tenth edition of
Systema naturae
had grown to include seventy-seven hundred species of plants and forty-four hundred species of animals, all systematically grouped and categorized. These numbers
seemed awfully large at the time, but in the two and a half centuries since, estimates of the varieties of species have grown exponentially. By the beginning of the twenty-first century, scientists would suspect that there may be as many as a billion distinct species of bacteria, about three hundred thousand species of plants, and perhaps ten to thirty million animal species, most of which are yet-to-be-discovered insects.