Life on a Young Planet (28 page)

The transition to a fully oxic world appears to have been protracted. As discussed more fully in
chapter 11
, oceans rich in oxygen from top to bottom may not have developed until the Proterozoic Eon was almost over. When they came, however, the culmination of Earth’s environmental transformation paved the way for one last revolution in biology—the rise of animals.

__________

¹
My friend Dick Bambach objects to the word “only” in this sentence, reminding me that 50 million years is a very long time. Indeed it is. In 50 million years we could run the history of Egypt, from the pyramids to modern Cairo, more than 10,000 times. Two million human generations could come and go, and so could more than a billion generations of amoebas. My sentence is meant to convey the fact that
relative
to the enormous span of time that separates Great Wall and Doushantuo deposits, 50 million years is pretty short. But Dick’s point is a good one. Every now and again, we should sit back and contemplate the immensity of the canvas on which life’s early history is painted.

²
A nanometer is 10
^-9
meter, or one-thousandth of a micron. A 300-nanometer cell would thus be less than a third of a micron long.

³
The largest bacteria currently known are sulfur-oxidizing cells found in sediments off the coast of South Africa. These giants reach diameters of 500 microns or more, although, in a way, they cheat—the cells are hollow.

⁴
Discovered by Charles Doolittle Walcott in 1909, the Burgess Shale is renowned for its compressions of Cambrian animal remains. See
chapter 11
for discussion.

⁵
Recall from
chapter 2
that nitrogen gas (N
₂
) permeates air and ocean waters, but cannot be used directly by most organisms. Many prokaryotic microorganisms, including cyanobacteria, can “fix” nitrogen, converting gaseous N
₂
into ammonium ion (NH
₄
−) that can be incorporated into biological molecules.

In latest Proterozoic rocks, we find at last what Charles Darwin predicted long ago—the fossilized impressions of early animals. But the fossils are not at all what Darwin expected. The ancestors of modern animals undoubtedly lived in latest Proterozoic seaways, but most end-Proterozoic fossils have unusual forms that separate them from, rather than link them to, Cambrian and younger faunas.

F
OR PALEONTOLOGISTS
weaned on arctic research, the sun sets early in Namibia. By 6:00 P.M., packs and hammers must be stowed, and if we’re not to stumble around by flashlight, firewood must be gathered. Temperatures may have reached 100ºF in the afternoon, but by morning our sleeping bags will be rimmed by frost. As daylight wanes, the stark hills and fantastic shrubs of southwestern Africa fade from view. But even as they disappear, a new wonder takes shape in the evening sky—the Milky Way seen through clear desert air. Untold millions of stars form a broad arc across the southern sky, stars so densely packed that aboriginal Australians found their constellations in patches of emptiness amid the glimmer, rather than by connecting sparse points of light. Every few minutes, a meteor streaks across our celestial canopy.

The stars make good companions as we drift toward sleep, tightly wrapped against the cooling night air. But sleep can be fitful in the desert. The stars may disappear behind a curtain of clouds, prompting the worry that we’ll soon be wet as well as cold. Or, zebras may amble past camp, their quiet hoof-falls nudging weary geologists awake. Other animals are less respectful. More than once, I’ve been awakened by piercing shrieks just beyond our fading campfire—a troop of baboons,
irritated by human interlopers. Eventually, however, sleep returns until an amber rim on the eastern horizon heralds the end of stars and cold, and the start of a new working day.

The fossils of Gunflint and Spitsbergen offer few clues to the origins of creatures like zebras and baboons. Even Doushantuo phosphorites, formed just 50 million years before the Cambrian cliffs in Siberia, contain only microscopic hints of gathering animal evolution. But here in Namibian rocks deposited at the very end of the Proterozoic Eon (
figure 10.1
), and in beds of comparable age around the world, we see at last the palpable fuse of Cambrian Explosion—large animals that plausibly include the ancestors of our familiar biota. In some ways, it marks the realization of Darwin’s dream—animal life before the Cambrian. But Namibian fossils also deepen Darwin’s dilemma, because their unusual shapes challenge our efforts to locate them on the Tree of Life. Do these remains really trace a path to modern animals, or are they a dead-end fork on the evolutionary road?

Figure 10.1.
Sedimentary rocks of the Nama Group rise out of the Namibian desert. The large gray mounds to the left of the mesa and near its top are microbial reefs that contain calcified animal fossils. Ediacaran impressions occur in sandstones that form the conspicuous ledge high on the hill.

I first visited Namibia more than twenty years ago, in the company of South African geologist Gerard Germs. A gentle man of philosophical bent, Germs emigrated from the Netherlands as a graduate student and took up the challenge of bringing geologic order to a poorly known region the size of Texas. His success laid the foundation for continuing research that has changed the way we think about early animal evolution. Much of that research has been directed by MIT’s John Grotzinger, architect of our modern understanding of these rocks and their paleontological riches. John’s driving desire is to understand how sedimentary rocks accumulate and, especially, to know how they did so on the early Earth, when life and environments differed from today. To address these issues, he must find places where thick successions of ancient rocks are exceptionally well preserved and unusually well exposed. Southern Namibia fits the bill perfectly: its Proterozoic sediments remain almost untouched by tectonics or metamorphism, but they are dissected by canyons that allow geologists to map stratigraphic relationships in three-dimensional detail. With his students, John has taken the latest Proterozoic rocks of Namibia apart and put them back together, in the process learning how tectonics, sea level, climate, and biology shaped the sedimentary record seen today. Along the way, he has discovered a host of new fossils, including large reefs, built by microorganisms but bristling with the skeletons of early animals. It was the opportunity to study those skeletons that lured me back to Namibia.

Sedimentary rocks of the Nama Group accumulated in a broad basin formed in response to continental collisions that forged the supercontinent Gondwana. In its lowermost part, the Nama succession consists of pebbly and conglomeratic sandstones formed on an ancient coastal plain. Above these are finer-grained sandstones deposited along an ancient coastline, followed by siltstones and shales deposited farther offshore. In the basin center, beyond the reach of silt and mud, limestones precipitated from clear waters. Light green ash beds, introduced by nearby volcanoes, preserve a record of time: deposition began about 550 million years ago and continued until the very end of the Proterozoic (543 million years ago), when uplift and erosion carved deep canyons into underlying rocks. These paleocanyons are filled by more sandstones and shales; diverse trace fossils and a 539 ± 1–million-year-old
ash bed indicate that when Nama sedimentation resumed, the Cambrian Period was already under way.

The conventional hallmarks of Proterozoic biology, seen before from Spitsbergen to Siberia, appear once more in these Namibian rocks. Stromatolites are conspicuous if not abundant features of Nama limestones, and in Nama shales, cyanobacterial filaments lie buried with simple algal microfossils. Here, then, just below the Cambrian boundary, the paleontology still looks … well, Proterozoic. But, there is a difference. If we examine Nama sandstones carefully—preferably in late afternoon when the sun, set low in the sky, throws surface features into high relief—we see fossils that are almost shockingly different from anything found in older rocks (
plate 7
). We see the impressions of large, complicated organisms, as well as simple tracks and trails unambiguously made by animals. In truth, the Nama fossils are shocking whether we approach them from above or below, for if they have no counterparts in older beds, Nama impressions bear equally little resemblance to most fossils found in Cambrian or younger rocks. Thus, the debate: do the remarkable fossils in Nama and other latest Proterozoic rocks record the ancestors of modern animals or a failed evolutionary experiment at the dawn of animal evolution?

Fossils were discovered in Nama rocks as early as 1908, and between 1929 and 1933 the German paleontologist Gürich provided detailed descriptions of several species. Not much was made of this discovery, however, perhaps because the biological and geological frameworks needed to understand its importance were not yet in place. Scientists didn’t fully appreciate either the genealogical relationships we know from the Tree of Life or the time relationships among ancient rocks. By 1946, however, when Reg Sprigg began to uncover similar assemblages in the remote Ediacara Hills of South Australia, the necessary frameworks had begun to take shape. Moreover, the Australian fossils found a worthy champion in the great paleontologist Martin Glaessner (along with Barghoorn, Cloud, and Timofeev, the fourth patriarch of Precambrian paleobiology). “Ediacaran” fossils, as they came to be known, were interpreted by Glaessner—and many who followed—as the exposed roots of the metazoan tree: the earliest representatives of animal phyla that blossomed into diversity in the ensuing Cambrian. Glaessner took an active interest in the Ediacaran fossils of Namibia, but it was discoveries
in the 1970s by Gerard Germs and Hans Pflug, of Giessen University in Germany, that rekindled paleontological interest in Nama rocks. Another German scientist stirred the pot, as well. In 1984, Adolf Seilacher, one of the world’s most distinguished paleontologists, announced that Glaessner had gotten it all wrong.

One more time we must ask how paleontologists interpret fossils. How, in this particular case, do they coax biology from Ediacaran impressions, casts, and molds? Evidence of anatomy or physiology was stripped away shortly after the carcasses were impressed into surrounding beds, leaving only form to guide our interpretations. Of course, morphology is
usually
all that remains for paleontologists to ponder. Dinosaurs left only their bones (and, rarely, traces of skin), but that’s enough to reveal volumes about their biology, because dinosaur backbones, ribs, and teeth display morphological landmarks that relate them unambiguously to vertebrate animals alive today. The same is true for trilobites; they may be long extinct, but their segmented bodies and jointed legs tie them to living horseshoe crabs, shrimp, and other arthropods. Therein lies the problem. The impressions in Namibian sandstones have unfamiliar shapes, making it difficult if not impossible to map their features onto the forms of living animals.

The simplest impressions in Nama rocks are shallow disks up to a few inches across—the kind of fossils one might expect to be formed by jellyfish buried in a storm (
plate 7b
). Indeed, for many years jellyfish were considered likely counterparts of these fossils, but this interpretation has a serious flaw. Disklike fossils are common in Ediacaran-age sandstones, and nearly all occur as casts that bulge downward from the
bottoms
of sandstone beds. In other words, they are casts of organisms that formed
depressions
in the shallow seafloor. For jellyfish to form such fossils, they would have to land upsidedown (and with considerable impact) when cast onto sediment surfaces. We need only stroll along a beach in the wake of a storm to convince ourselves that this is not how jellyfish land. More likely, the disklike fossils lived on the seafloor, nestled into sediments much like modern sea anemones, bottom-dwelling cousins of the jellyfish. Other discoidal fossils represent originally bulbous or conical holdfasts, or anchors, that tethered more complicated constructions to the seafloor. And still others may not be animals at all—common
ball-shaped fossils called
Beltanelliformis
appear to have been fluid-filled seaweeds (
plate 7d
) comparable in size and structure to the living green alga
Derbesia
.

Relatively few disklike fossils have been found in Namibia, but elsewhere—in Ediacaran rocks from Australia and spectacularly fossiliferous beds from the White Sea region, Russia—these rounded fossils are far and away the most common components of Ediacaran faunas. There is
Cyclomedusa
, up to five inches across and marked by concentric folds and radial grooves—like a striated cone collapsed along its axis. Then there is
Mawsonites
(
plate 7b
), similar in size but ornamented by concentrically arranged lobes or bosses.
Medusinites
is smaller (less than two inches across) and smoother, but has a sharply defined circular groove in its center. In contrast,
Ovatoscutum
sports closely spaced, parallel grooves, as though it were fashionably decked out in corduroy. Thin tentacle-like projections ornament the disk of
Hiemalora
. And the lobed
Inaria
resembles a head of garlic flattened against the seafloor.