A New History of Life (10 page)

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.
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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.

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.
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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.

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 flourished 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.

One of the least visited (and populated) corners of the Earth is the northern half of Western Australia. Covering a land area close to that of the western United States from the Rocky Mountains to the Pacific Coast, this gigantic region is arid, mainly red in color—and contains some of the most important sites for understanding the history of life on Earth. The most important of these are the sites where the earliest known life on Earth (to date) has been found. In a desolate region known as the Pilbara, ancient hills rich in oxidized iron lend a burnt umber canvas to the remains of life’s first chapters—on our planet, at least. The red hills of the Pilbara are created by massive amounts of iron ore—and because of this, the region is the site of massive open-pit mining of its ancient iron-bearing strata, most of it being shipped to China as fast as it can be loaded onto the endless succession of freighters.

Yet there is more than iron ore to be found in the Pilbara’s ancient hills. In the treeless landscape are rocky outcrops that have long been thought to contain the Earth’s oldest fossils—including the Apex shale described in a previous chapter, as well as the newest entrant in the “oldest life on Earth” derby, the Strelley Pool site, less than twenty miles from the Apex Chert locality in the Pilbara.

The Apex Chert and Strelley Pool sites do not trumpet that they bear fossils (or not, in the case of the Apex Chert). Yet in the surrounding countryside there is unmistakable evidence of early life, for the landscape is rich in stromatolites, the layered, humped deposits created by shallow water and intertidal bacterial slicks that proclaim the presence of life—and indeed were the most common kind of life on Earth from some time after its origin until about a half billion years ago. It is ironic, and entirely coincidental, that Western Australia, at the end of a long estuary known as Shark Bay, is also the place where
one of the last oceanic remnants of a far more ancient world, one without any atmospheric or dissolved oxygen at all, still lives today.

The presence of both the oldest fossil life known and the coincidental existence of the best examples of what that oldest life may have looked like have left an indelible impression of Western Australia as the world’s most important “museum” of early Earth life. From the first time life arose, until the first snowball Earth episode essentially ended the Archean era, the fossil record of this long time interval, more than 1 billion years, is known mainly from the presence of stromatolites, as well as rare, exceptional fossils found within agate-like rocks called chert. The stromatolites present in the two regions that yielded the most information about the nature of the oldest life on Earth—the North Pole region of Western Australia, and an area in South Africa called the Barberton Greenstone Belt, located near the famous Kruger National Park in South Africa—both show the presence of very ancient forms of stromatolites.

Through most of the 1900s, we all thought that these structures were formed as a by-product of algal mats, which can induce carbonate precipitation as a result of photosynthesis. But over the past two decades, many Earth scientists have concluded that some (but not all!) of the finely laminated structures can also form from direct chemical precipitation from salty brines. To distinguish those that did come from the processes of life, it is necessary to study the modern representatives, which, in fact, are few indeed.

The best place to observe living stromatolites today is in the World Heritage site, again in Western Australia, called Shark Bay, mentioned above. There, large, sometimes meter-wide mounds of interbedded sediment (mainly sand and mud) are found atop and below communities of photosynthetic bacteria. If one of these stromatolites is sectioned in half with a rock saw, the two halves show finely layered intervals—layers that show some very characteristic undulations. Stromatolites are generally round at the top, but the cuts show a wonderful diversity of shapes and structures.

The stromatolites of Shark Bay have been long lauded as one of the best ways to understand the Archean. Once again, we see
uniformitarianism at work in this: the structure, chemistry, and biology of these structures living in the broiling corner of Australia are unquestioned as windows into the deep past, and their presence there is of inestimable value in interpreting the fossil stromatolites. But there are things about Shark Bay not mentioned or described in the endless television specials and written and photographic treatments of this site that are decidedly
not
a model for Archean oceans. Principal among these is the identity of other organisms inhabiting the most important of stromatolite-bearing regions in Shark Bay (the bay is huge, and covered more than 2.2 million square acres). They are also reminders
of what life on Earth would have been like for at least the first billion years of its existence.