Life on a Young Planet (4 page)

The sedimentary beds in the Kotuikan cliffs aren’t quite flat-lying; tectonic movements over millions of years have tilted them slightly downward to the west. Because of this, a hike toward the east, upstream along the river, reveals layers that sit ever farther below the level of the Cambrian fossils. About fifteen miles up river—some 200 feet lower in the sedimentary rock column—we encounter a sharp stratigraphic break, the base of the sedimentary package that includes the latest Precambrian carbonate rocks and basal Cambrian animals (
figure 1.4
). Is that the end of the sedimentary trail?

Not at all. What lies beneath these rocks is another, older succession of
sandstones, shales and carbonates. Set at an acute angle to the younger beds, this older package is itself more than 3,500 feet thick. The base of the Cambrian System is not the bottom of the stratigraphic record—not in northern Siberia, and not in many other places where tectonic circumstance has preserved sedimentary rocks deposited one, two, or even 3 billion years before Cambrian beds began to accumulate.

We can put Darwin’s conjecture to the test. Is the Cambrian Explosion the beginning of biological history? Or is it the culmination of evolutionary events that extend much deeper into our planet’s past?

__________

¹
By convention, geologic time is divided into four eons: the Phanerozoic (0–543 million years ago), the Proterozoic (543–million 2.5 billion years ago), the Archean (2.5–ca. 4 billion years ago), and the Hadean (the time interval from the accretion of the Earth to the beginning of the preserved record, ca. 4–4.55 billion years ago). The Cambrian is the initial period of the Phanerozoic Eon; thus, all earlier time is commonly, if informally, referred to as “Precambrian.” See geologic timescale on page 2.

²
In the mid–nineteenth century, debate about how to define and differentiate the Cambrian and Silurian Systems remained unresolved. Darwin employed Londoner Roderick Murchison’s term Silurian for the oldest fossiliferous beds, despite the fact that his Cambridge mentor Adam Sedgwick had coined the name “Cambrian.” Not until 1879 did Charles Lapworth cut the Gordian knot, retaining Cambrian for the lower part of the disputed system, Silurian for its upper portion, and Ordovician (after the Ordovici, an ancient and putatively obstreperous Welsh tribe) for the contested interval of overlap.

In the Tree of Life, built from comparisons of nucleotide sequence in genes from diverse organisms, plants and animals form only small twigs near the top of one branch. Life’s greater diversity, and, by implication, its deeper history, is microbial. If we wish to explore Precambrian rocks for evidence of early life, we must first learn about Bacteria and Archaea, the tiny architects of terrestrial ecosystems.

M
OST OF US
learn about Richard III through Shakespeare’s eponymous drama, but as history, this account is suspect—after all, Shakespeare’s patrons
won
the War of the Roses. Biased, selective, incomplete, and even incomprehensible documents are the daily bread of historians. Despite the shortcomings of individual accounts, however, scholars can arrive at a balanced understanding of the past by sifting through a number of different records for points of agreement and complementary perspectives.

The study of biological history works much the same way. The fossiliferous cliffs along the Kotuikan River served to introduce one great library of Earth’s evolutionary past—the geological record. Sedimentary rocks preserve a remarkable record of life and environments through time, but as we’ve already observed, this accounting is episodic, not continuous. It is also highly selective, brightly illuminating some groups of organisms while leaving others in darkness. For example, we know a great deal about the paleontology of horses, but little about the earthworms beneath their feet.

Fortunately, we can consult a second library—the biological diversity that surrounds us today. Comparative biology offers rich resources for
evolutionary analysis, providing genealogy to complement paleontology’s record of time, and physiology to match geology’s chronicle of environmental change. The great cell biologist Christian de Duve has gone so far as to suggest that the genes of living organisms contain a
full
accounting of evolutionary history. If so, however, it is—like Shakespeare’s histories—limited to an account of life’s winners. Only paleontology can tell us about trilobites, dinosaurs, and other biological wonders that no longer grace the Earth. If we wish to understand life’s history, then, we must weave together insights drawn from geology
and
comparative biology, using living organisms to reanimate fossils and fossils to learn how the diversity of our own moment came to be.

Despite an almost bewildering diversity of form and function, all cells share a common core of molecular features, including ATP (life’s principal energy currency), DNA, RNA, a common (with a few minor exceptions) genetic code, molecular machinery for transcribing genetic information from DNA into RNA, and more machinery to translate RNA messages into proteins that provide structure and regulate cell function. The reciprocal observation is equally striking. In spite of their fundamental unity of molecular structure, organisms display extraordinary variation in size, shape, physiology, and behavior. Life’s unity and diversity are both remarkable in their own ways; together they comprise the two great themes of comparative biology.

Even a casual observer will notice the pattern of nested similarity displayed by Earth’s biological diversity. Humans and chimpanzees are clearly distinct, but they share many features of anatomy and physiology, resembling each other far more than either does, say, a horse. Humans, chimps, and horses, in turn, share features such as hair, lungs, and limbs that separate them from catfish. Yet, all animals with bony skeletons share a basic pattern of anatomical organization that unites them as a group and differentiates them from other sets of species built on different design principles—insects, for example, or spiders.

The nested similarity of species was well known to early naturalists. Linnaeus codified it in the 1730s, proposing a hierarchical system of taxonomic classification that is still in use today. It was Charles Darwin, however, who explicitly recognized the genealogical nature of this pattern. Biological differences have arisen through time, he wrote, because
of “descent, with modification,” that is, by evolutionary divergence from common ancestors under the influence of natural selection:

The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species.… As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications.

We can explain the similarities between humans and chimps by descent from a common ancestor that possessed the various features the two groups share. Their differences have arisen since they diverged. This makes the paleontological prediction that the oldest fossils of humanlike primates should resemble the last common ancestor of chimps and humans more closely than modern people do; the features that make us distinctly human should appear only in younger fossils of our lineage. The fossil record of human ancestry is notoriously sketchy, but skeletal remains unearthed in Africa and Asia confirm this prediction. (Note that there is no expectation that successively older members of our lineage should close in on chimp morphology. Humans didn’t descend from chimpanzees; humans and chimps
both
diverged from a common ancestor that was neither
Homo
nor
Pan
.)

Not all shared features are equally helpful in determining “propinquity of descent” (another delightful Darwinism). For example, birds, bats, and the extinct pterosaurs all sport wings, but their wings have different skeletal structures, and many other features show that these airborne animals are not closely related. Wings evolved independently in each group as an adaptation for flight; in the parlance of systematic biology, these features are
convergent
. Only features that are shared because of common ancestry (
homologies
, in evolution-speak) can be used to assess evolutionary relationships. In practice, we don’t always know whether similar features are convergent or homologous and so rely on sophisticated computer algorithms to sort out large sets of comparative biological data.

It is relatively easy to see how morphological characteristics might be used to articulate a hypothesis of evolutionary relatedness, or
phylogeny
, for all primates, all mammals, or even all vertebrate animals. We can also grant that an expert, at least, could do the same for mollusks or arthropods. But how can we place mollusks, arthropods,
and
vertebrates within a greater evolutionary tree of all animals? And, much harder, how can we reconstruct the whole of Darwin’s great Tree of Life, a phylogeny that encompasses all living things?

Wandering through an alpine forest or snorkeling above a coral reef, we observe an ecology shaped by plants (or seaweeds) and animals, with large vertebrates at the top of the food chain and other creatures below. Ecosystems also contain many organisms that we can’t see, but concern for their contributions is generally fleeting—surely bacteria and other microorganisms, tiny and simple, eke out their living in a world of our making?

As large animals, we can be forgiven for holding a worldview that celebrates ourselves, but, in truth, this outlook is dead wrong. We have evolved to fit into a bacterial world, and not the reverse. Why this should be is, in part, a question of history, but it is also an issue of diversity and ecosystem function. Animals may be evolution’s icing, but bacteria are the cake.

Plants, animals, fungi, algae, and protozoa are
eukaryotic
organisms, genealogically linked by a pattern of cell organization in which genetic material occurs within a membrane-bounded structure called the nucleus. Bacteria and other
prokaryotes
are different—their cells lack nuclei. In terms of biological importance, eukaryotes would seem to have a decisive edge; eukaryotic organisms display a variety of form that ranges from scorpions, elephants, and toadstools to dandelions, kelps, and amoebas. In contrast, prokaryotes are mostly minute spheres, rods, or corkscrews. Some bacteria form simple filaments of cells joined end to end, but very few are able to build more complicated multicellular structures.

Size and shape surely favor eukaryotes, but morphology provides only one of several yardsticks for measuring ecological significance. Metabolism—how an organism obtains materials and energy—is another, and by this criterion, it is the prokaryotes that dazzle with their
diversity. Eukaryotic organisms basically make a living in one of three ways. Organisms like ourselves are
heterotrophs
; we gain both the carbon and energy needed for growth by ingesting organic molecules made by other organisms. To obtain energy, our cells use oxygen to break down sugars to carbon dioxide and water, a process called
aerobic
(oxygen-using)
respiration
. In a pinch, we can gain a bit of energy from a second metabolism called
fermentation
, an
anaerobic
(no-oxygen) process in which one organic molecule is broken into two others—only brewer’s yeast and a few other eukaryotes make much of a living this way. The third principal energy metabolism found in eukaryotes is the
photosynthesis
performed by plants and algae: chlorophyll and associated pigments harvest energy from sunlight, enabling plants to fix carbon dioxide into organic matter. In order to convert light into biochemical energy, plants need an electron—water supplies the needed charge, producing oxygen as a byproduct.

A
Christmas Carol
, Charles Dickens’s classic tale of redemption, opens with an admonition for readers to pay close attention to a particular fact: “Old Marley was as dead as a door-nail.… This must be distinctly understood, or nothing wonderful can come of the story I am going to relate.” The early history of life has its own “Jacob Marley” facts that, like the old miser’s death in Dickens’s story, need to be understood if the narrative is to make sense. First of these is the metabolic diversity of prokaryotic microorganisms, key to any exploration of early biological history. We must come to grips with the many ways that prokaryotes make a living and how these tiny organisms fit onto the Tree of Life before we put our boots back on and return to the field as paleontologists.

Like eukaryotes, many bacteria respire using oxygen. But other bacteria can respire using dissolved nitrate (NO
₃
-) instead, and still others use sulfate (SO
₄
^2-
) ions or the metallic oxides of iron and manganese. A few prokaryotes can even use CO
₂
to react with acetic acid, generating natural gas, which is methane (CH
₄
). Prokaryotic organisms have also evolved a galaxy of fermentation reactions.

Bacteria ring changes on the theme of photosynthesis, as well. The cyanobacteria, a group of photosynthetic bacteria tinted blue-green by chlorophyll and other pigments, harvest sunlight and fix CO
₂
much like eukaryotic algae and land plants. However, when hydrogen sulfide
(H
₂
S, well known for its “rotten egg” smell) is present, many cyanobacteria use this gas rather than water to supply the electrons needed for photosynthesis. Sulfur and sulfate are formed as by-products, but oxygen is not.