Life on a Young Planet (35 page)

Figure 11.9.
This sinuous tube, tapered at both ends, is a nematode—a tiny (less than a millimeter long) animal found almost ubiquitously in present-day environments. Nematodes have complex tissues and organs, but almost never fossilize. For comparison, the filaments in the lower part of the figure are sulfur-oxidizing bacteria! (Photo courtesy of Andreas Teske)

__________

¹
Some molecular phylogenies indicate that cnidarians and bilaterian animals are specific relatives of carbonate-precipitating sponges, with siliceous sponges alone on the first branch. Several features of cell ultrastructure support this view, although it remains a subject for debate. If correct, more complicated animals must have arisen from
within
early diverging sponges.

²
The number of animal species alive today remains unknown. About 1.5 million species have been described (more than half of them insects), but the actual number could be much higher. Ten million lies near the midpoint of current estimates.

³
Steve originally called his book
Homage to Opabinia
, but his editor, perhaps wisely, rejected this title in favor of
Wonderful Life
, an agreeable allusion to Jimmy Stewart and to all that is, well, wonderful about life.

⁴
This point of view, first articulated by British paleontologists Derek Briggs, Richard Fortey, and Matthew Wills, receives extended treatment in Simon Conway Morris’s book
Crucible of Creation: The Burgess Shale and the Rise of Animals.

⁵
Similar, and equally spectacular, fossils from Chenjiang, China, and Sirius Passet in northern Greenland are somewhat older. These deposits, which include the earliest fishlike animals, may have formed as early as 520 million years ago—still more than 20 million years after the Cambrian Period began.

⁶
Claims of older animal fossils surface every few years. The most celebrated candidates are sinuous impressions reported by Dolf Seilacher in rocks now known to be 1.6 billion years old. Dolf’s structures may be biological, but features such as branching pattern suggest to me that they are more likely to be algal than animal. More to the point, I see no evidence to connect these structures stratigraphically to the record of abundant trace fossils that begins 555 million years ago or phylogenetically to the animals that made those younger trackways.

Supercontinental breakup, globe-swaddling ice, rising oxygen levels, and short-lived environmental perturbation at the Proterozoic-Cambrian boundary—great events in Earth’s planetary history framed the early evolution of animals, generating successive waves of permissive ecology that fueled metazoan diversification.

F
OR MORE THAN
a century, paleontologists have scrambled across rock faces in search of early animals. Until recently, however, we’ve labored alone—other scientists had their own problems and their own agendas. Now, as already seen, paleontology has found an ally in molecular biology. And another partner, equally important, has emerged from our traditional home base of geology. Increasingly, Earth scientists are working to understand how rocks, life, air, and water interact to produce the environment around us. Much of this research, christened Earth system science, is motivated by concerns about our environmental future. But geochemists and climatologists have also begun to study Earth’s environmental
past
, allowing us, for the first time, to link early animal evolution to late Proterozoic and Cambrian environmental history. And what a history it is turning out to be.

In Spitsbergen, the fossil-packed cherts of the Akademikerbreen Group are separated from younger Cambrian rocks by thick beds of tillite, the coarse and poorly sorted sediment deposited by glaciers. As observed in
chapter 9
, tillites also occur just below the extraordinary Doushantuo fossils of southern China. And in Australia, glacial rocks lie just beneath the thick sedimentary pile that contains Ediacaran fossils near its top. The same stratigraphic pattern can be found in subhimalayan India; in European Russia; in Norway, Namibia, and Newfoundland; in the Rocky Mountains, from Death Valley to northern Canada; even in Boston Harbor (
figure 12.1
). Ice heralded the age of animals.

Figure 12.1.
Coarse and poorly sorted sedimentary rocks of the Numees Tillite, Namibia. Similar rocks seen around the world document widespread glaciation on the late Proterozoic Earth.

Brian Harland—the same Brian Harland who invited me to work in Spitsbergen—was the first to recognize the implications of these widespread tillites, proposing in 1964 that Earth experienced a global ice age near the end of the Proterozoic Eon. We tend to be impressed (and rightly so) by the fact that 18,000 years ago Boston and Chicago lay beneath glacial ice, but Pleistocene ice sheets never extended south of Long Island, even at the height of glaciation, leaving much of North America ice-free. In contrast, if Harland was correct,
most
landmasses on the late Proterozoic Earth must have been covered by ice.

Decades of careful research have confirmed Harland’s proposal. In fact, continental glaciers spread across the globe more than once, although the exact number of late Proterozoic ice ages remains contentious. Noting that only two tillite successions are found in most regions, some stratigraphers believe that the Earth froze twice. On the other hand, applied to Phanerozoic rocks, the same approach would
suggest that our planet endured two ice ages in the past 500 million years, when in fact there were three.
¹
Only precise radiometric dates can solve the problem. My own view is that Earth chilled at least four times during the late Proterozoic: an initial event a bit earlier than 765 million years ago and arguably confined to Africa, two truly global ice ages 710 ± 20 and a bit more than 600 million years ago, and at least one last (relatively small) hurrah before the Cambrian began.

In general, sedimentary geologists associate glacial rocks with cold climate and carbonate accumulation with warmth, but, as seen in Spitsbergen, late Proterozoic tillites commonly lie sandwiched between carbonate-rich successions. In particular, late Proterozoic glacial rocks are nearly always capped by distinctive carbonate beds that display unusual sedimentary features, including sheaves of slender crystals like those found in Archean and earliest Proterozoic limestones (
figure 12.2
). In many places, you can place a knife blade at the sharp contact between glacial rocks and the blanketing cap carbonates.

Our research in Spitsbergen turned up another unusual feature of these cap carbonates, in this case a chemical oddity. Recall from
chapters 3
and
6
that two different types of information can be gleaned from carbon isotopes in sedimentary rocks.
Differences
in the isotopic compositions of limestone and organic matter reflect the metabolisms of organisms in the local ecosystem. On the other hand, the
absolute
ratio of
¹³
C to
¹²
C in limestones and dolomites allows us to estimate the relative contributions of carbonate and organic matter to sedimentary carbon burial at the time the rocks formed—higher C-isotopic values imply higher rates of organic carbon burial (
figure 6.6
). Thick carbonate successions below late Proterozoic glacial rocks generally have unusually high C-isotopic ratios, nearly matching the extremes found in 2.4–2.2-billion-year-old rocks and exceeding anything else seen in the geologic record.

Figure 12.2.
A cap carbonate above the Numees Tillite, Namibia. Cap carbonates characteristically show unusual bedding features, including the thin and contorted laminations seen here and fans of crystals deposited directly on the seafloor.

In contrast, the C-isotopic compositions of cap carbonates fall to extremely low values. This chemical pattern occurs not only in Spitsbergen but worldwide (with an additional twist, discussed below). Moreover, it applies to each major ice age.

Late Proterozoic ice ages, thus, can be linked to unusual behavior of the global carbon cycle. How do we explain these chemical fluctuations, and what do they tell us about the late Proterozoic world?

Geology helps to explain our isotopic data. The onset of unusually high C-isotopic ratios coincides with the rifting and breakup of one or more late Proterozoic supercontinents. As great continents broke apart, narrow seas opened, perhaps facilitating the burial of organic matter in rapidly accumulating sediments. That is to say,
tectonic
changes may explain the
chemical
observation of high C-isotopic values. High rates of organic carbon burial may, in turn, have helped to keep atmospheric CO
₂
at relatively low levels, cooling global climate and leaving the Earth vulnerable to glaciation. The intricate linkage of Earth and environment implied by these relationships is what Earth system science is all about.

The low C-isotopic values in cap carbonates have been interpreted in several ways. One possibility is that algae and cyanobacteria were scarce in the postglacial ocean and the burial rate of organic matter correspondingly low. Another is that cap carbonates accumulated at extremely rapid rates, swamping any influence of organic carbon burial on the C-isotopic record. We might also conjecture that as glacial ice retreated, methane (which has very low C-isotopic values) was belched from the margins of warming continents.

There’s more. Conspicuous carbonate precipitates were not the only archaic sedimentary features reprised in association with late Proterozoic glaciation. Iron formations returned, as well. As documented by Grant Young, a geologist at the University of Western Ontario, iron formations occur among late Proterozoic tillites throughout the world, especially those formed during the great ice age circa 710 million years ago. We explained earlier iron formations as the products of dissolved iron transport in deep oceans that lacked oxygen or sulfide. How could such oceans return more than a billion years after they disappeared, seemingly for good?

We will return to the question of ice and iron, but there is one more aspect of late Proterozoic glaciation that requires attention, because it may provide the key to everything else: at least some tillites formed at sea level near the late Proterozoic equator. This was not your Cro-Magnon ancestors’ ice age.

How do we know this? Given that continents have migrated through time, rafted by tectonic plates, how can we tell that rocks formed deep in our planet’s history accumulated in the tropics? The answer lies in the magnetic properties of sedimentary and volcanic rocks. When rocks form, iron-bearing minerals crystallize in alignment with the Earth’s magnetic field. Frozen in place, this magnetic orientation can be preserved through geologic time, enabling geologists to determine the latitude—but not longitude—at which ancient rocks originated. (Magnetic orientation can also be reset by later events, so geologists must be exceedingly careful in interpreting paleomagnetic data.) Meticulous studies of glacial rocks in southern Australia and western North America show that these beds formed within 10º of the late Proterozoic equator. At circa 40º paleolatitude, the Nantuo tillite in China is among the most
poleward
of the admittedly few tillites whose magnetic signatures have been reliably measured.