Fathoming Geologic Time
The very earliest lifeforms of which we have record date back to nearly 3.8 billion years ago. These were procaryotes (a group that includes bacteria) consisting of little more than tiny bags of self-replicating chemicals, without so much as a localized nucleus. The oldest known eucaryotic cells (those with high-tech advances, including - most importantly - a membrane-bound nucleus) are single-celled organisms called protoctists (a group which includes amoebas, dinoflagellates and slime molds) dating back some 2.2 to 1.8 billion years. The Ediacaran assemblage in southern Australia is about 580 million years old and provides us with a look at the earliest-known multicellular life, a collection of bizarre invertebrates unlike anything alive today. Armored jawless fishes known as ostracoderms were the first true vertebrates, dating back to about 510 million years ago. Sharks appeared on the fossil scene about 455 to 425 million years ago. Those embodiments of prehistoric life, the dinosaurs, were relative late-comers - one of the earliest known genera (Herrerasaurus) dates back only about 230 million years.
The hominid fossil record goes back only about 4.5 million years, represented by a species called Ardipithicus ramidus from early Pliocene deposits in Ethiopia. These ape-like creatures could only be considered 'human' in the broadest, most generous sense of the word. Modern humans date back perhaps 60,000 years. Written language was developed only about 5,000 years ago. Therefore all of recorded human history - from the ancient Sumerians to you reading this sentence right now - represents a ridiculously tiny fraction (0.00013%) of the history of life on Earth. Days, weeks, months, years, decades - indeed, entire human lifetimes - are infinitesimal against the backdrop of geologic ages. By comparison, we seem to measure out our lives in coffeespoons of time. The timescale on which evolution occurs is so utterly without connection to the scale of our lives as to be virtually unfathomable. Yet that is precisely the timescale with which one must come to terms when tracing the history of life on Earth.
Geologic time is divided into units based on major geological and biological events, such as mountain building phases ('orogeny' in Geo-speak) or mass extinctions (relatively brief periods during which several - sometimes many - lineages die out). The largest unit of geologic time is the eon, of which only the most recent - the Phanerozoic ("abundant life") - is of concern to us here. Spanning from about 570 million years ago to the present, the Phanerozoic includes all known strata which bear fossils of multi-cellular life. The next largest unit is the era; the Phanerozoic is divided into three eras: the Paleozoic ("ancient life"), Mesozoic ("middle life"), and Cenozoic ("recent life"). Each of these eras is divided into periods, and each of these periods is divided into epochs. In practice, however, usually only the Cenozoic Era is so finely divided. The Cenozoic is popularly known as The Age of Mammals*, as this is the period during which we fabulous fur-balls appeared and thus in which we have the greatest self-interest.
The accompanying chart presents geologic time as blocks of history. Many of these blocks of time have strange, exotic-sounding names that are generally derived from the region where they were first clearly differentiated from other such blocks. Familiarity with the sequence of the major units of geologic time will greatly enhance your understanding of the evolution of life in general and that of sharks in particular. A handy mnemonic for recalling the sequence of geologic periods and epochs - from oldest to youngest - is, "Camels often sit down carefully. Perhaps their joints creak. Possibly, early oiling might prevent painful rheumatism". The first letter of each word in the mnemonic is the same as the first letter of the corresponding geologic block of time. As a good rule-of-thumb approximation, periods lasted about 50 million years and epochs about 10 million. With a little practice, you'll soon get the hang of it. But remember: Rome wasn't built in a day. In terms of geologic time, it just seems that way.
Determining the Ages of Rocks
The history of life is recorded as fossils embedded in strata of sedimentary rock. Each of these strata is stacked in chronological order, like stony pages in an immense book. Stratigraphy is the science of determining the relative age of rock strata by their position, with older strata generally occurring deeper than younger ones. Similarly, biostratigraphy is (in part) a technique for determining the relative age of fossils by the depth of the strata in which they are buried: fossils in deeper strata are generally older than those nearer the surface. However, geological events - such as fault slippages, overthrustings, subductions, and even inversions of large chunks of the Earth's crust - can change the relative depth of older and younger strata, frustrating biostratigraphic analyses. If one has only a small number of specimens, as in the case with Tyrannosaurus rex (17 skeletons to date), tectonic activity can really muck up paleontologists' attempts to affix relative ages to them. But since each shark produces and sheds teeth by the thousands during its lifetime, biostratigraphy of fossilized shark teeth becomes highly reliable. Fossil shark teeth are so super-abundant that stratigraphic anomalies become statistically insignificant glitches in an otherwise straight-forward sequence from oldest to most recent. But to determine absolute ages of rock strata, radiometric dating must be employed.
Radiometric dating depends on the fact that rocks often contain certain chemical elements - such as uranium, argon, potassium, and carbon - that occur in two or more forms, called 'isotopes'. Isotopes of a given element all have the same electron configuration and thus behave basically the same way in chemical reactions. Isotopes differ only in the number of neutrons tucked away in the nucleus. But some nuclei have more neutrons than they can hold onto indefinitely, making some isotopes inherently unstable. Unstable 'parent' isotopes break down into stable 'daughter' forms (either a lighter form of the same element or a lighter element altogether) through the processes of radioactive decay. Radioactive decay follows strict pathways governed by the weak nuclear force and occurs following well-understood laws of probability.
The rate of decay is remarkably uniform over time, usually expressed in terms of a half-life. The half-life of a radioactive substance is defined as the time it takes for half of a sample to decay into a stable form. Each radioactive isotope thus has a characteristic decay pathway and half-life. For example, Carbon 14 decays to Carbon 12 fairly directly and has a half-life of 5,730 years, while Uranium 238 decays to Lead 206 through a complex cascade of nuclear events and has a half-life of 4.5 billion years. By measuring the ratio of a radioactive isotope to its stable break-down products, one can back-calculate to estimate the time when the decay process began. Such an estimate produces not a single, definitive age, but instead a range of ages clustered around a central value ('normally distributed', in statistical parlance). The longer an isotope's half-life the longer the span of measurable time, but the less precise the estimate. As a result, the farther back we peer into the history of life, the fuzzier our sense of time becomes.