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Islands in the Cosmos

The Evolution of Life on Land

Indiana University Press

TIME TRAVEL

FIRST STEPS

If the dimension of time is difficult to comprehend, introducing the history of life with the ordering of time may seem excessively burdensome. Yet time is intimately embedded in the nature of matter, with vastly differing manifestations on quantum and cosmic scales. The evolution of scientific thought has long been analytic in nature and tends to be increasingly focused on ever more minute scales. However, attention is also being directed toward broad syntheses of physical–biological theory, addressing even the possible influence of complex structures existing today on simpler structures in the remote past through quantum effects (chapter 15; Davies 2007). If one cannot fully comprehend the development of scientific thought, one can at least admire the courage and nobility of spirit that animate it.

The progression of events outlined in this work will be sketched at levels somewhere between the quantum and the cosmic. This is of course in deference to the author's limitations, whose imagination is sufficiently taxed by the more ordinary dimensions of space and time. Consideration will be given to geography, climates, and landscapes of the past. Living things that inhabited those landscapes will be perceived as whole organisms, interacting with their habitats and each other as parts of living communities, much in the way that a modern ecologist regards the denizens of a game reserve. How they changed in size, appearance, and general behavior will be traced through geologic time. For example, dinosaurs belonged to the era of middle life. To understand them as living creatures, they must be viewed within their times, and they must be distinguished from the life that preceded and followed them. A story thereby emerges that, one may dare to suggest, is more fascinating than even dinosaurs themselves. The tale is perhaps a rather simple one, but as will be seen, it is drawn from the considerations of many fine minds whose works are cited in the text.

We remember the past and ponder the future. However, time is encountered as an ever-present stream of consciousness in which are embedded fleeting memories of the past and an indistinct web of future possibilities. Future events are difficult to arrange in sequences; elapsed time is conventionally but paradoxically measured from the elapsing present, and furthermore is reversed. Thus, it is understood at this writing that humans first walked on the Moon about 40 years ago. By extension, it also seems intuitively reasonable to tabulate time as reversed in formal geologic timescales, wherein dinosaurs are described as becoming extinct 65 million years ago. Evolution is possibly best considered according to the sequence in which it occurred: from the past toward the present (cf. Witting 2008). Here an additional conceptual difficulty arises from the vastness of both past and future time. A year, a generation, and a lifetime are durations that are broadly understandable, but longer intervals are far beyond human comprehension. It is not easy to grasp the notion that our species may have been present on Earth for at least 6,500 generations (at 20 years per generation) or 2,600 life spans (at 50 years per life span).

In an example of a calendar of events in human history and prehistory (see the Microsoft Encarta Encyclopedia, 2002), dates are conventionally rearranged in readily understandable units (centuries) in order of decreasing antiquity from the present. There, the domestication of dogs is listed as 14,000 years, or 140 centuries, ago. Perhaps more realistically, dogs could be said to have been domesticated 116,000 years (1,160 centuries) after the appearance of their human domesticators. This of course is the natural direction in which all living things experience time.

In subsequent chapters, lists of events are ordered according to two timescales, each calibrated in millions of years. A natural timescale, representing elapsing time, is presented in italics and increases from top to bottom. It has the virtue of clearly portraying the duration of intervals of geologic time and facilitating comparisons with the pace of events during other intervals of geologic time. A reversed timescale, representing geologic age, is presented in normal type and increases from bottom to top. It facilitates comparisons with standard geologic timescales.

The compilation cited above can be arranged in such a fashion, supplanted with citations for a few additional events:

• o centuries (1,300 centuries, or 130,000 years ago): appearance of skeletal remains of Homo sapiens in the geologic record (or 195,000 years ago; McDougall et al. 2005)

• 100 centuries (1,200 centuries ago): burial of the dead documented

• 550 centuries (750 centuries ago): beginning of last glacial interval

• 870 centuries (430 centuries ago): iron extracted for dye

• 980 centuries (320 centuries ago): sophisticated cave paintings

• 1,000 centuries (300 centuries ago): appearance of counting notation

• 1,020 centuries (280 centuries ago): oldest known dwellings

• 1,030 centuries (270 centuries ago): oldest pottery

• 1,160 centuries (140 centuries ago): dogs domesticated

• 1,187 centuries (113 centuries ago): domestication of figs (Kislev et al. 2006)

• 1,197 centuries (103 centuries ago): domestication of cereals, invention of agriculture (Kislev et al. 2006)

• 1,190 centuries (110 centuries ago): end of ice age, sea levels rise, extinction of large ice-age mammals, sheep domesticated

• 1,235 centuries (65 centuries ago): beginning of Bronze Age, invention of plowing

• 1,240 centuries (60 centuries ago): horses domesticated

• 1,247 centuries (53 centuries ago): invention of writing

• 1,250 centuries (50 centuries ago): germination of the oldest living trees (Lewington and Parker 1999)

• 1,268 centuries (32 centuries ago): sacking of Troy (radiocarbon dated)

• 1,275 centuries (25 centuries ago): beginning of recorded history (Herodotus, fifth century bc)

• 1,280 centuries (20 centuries ago): birth of Christ, the conventional beginning of the modern Western calendar

Several inferences may be drawn from this compilation, some of which are particularly striking. If dinosaurs disappeared 655,100 centuries ago (Hicks et al. 2002), our species did not appear until 653,800 centuries later, a staggering span of time. The invention of agriculture approximately coincides with a global warming and a major extinction of large ice-age mammals. Did the domestication of plants and animals provide a food base from which humans could hunt large mammals to extinction without running the risk of starving to death themselves? Written history (or writing) spans only 4 percent of the total interval of our already brief existence as a species! The earliest narratives are characterized not so much by their technological content (e.g., plans for the construction of pyramids) as by a search for wisdom and an explanation for the origin of the world. The compilation also shows a relatively large number of significant events in the more recent centuries, suggesting that rates of cultural change may have become more rapid with time.

At best, oral traditions preserve an exceedingly hazy memory of large ice-age animals that became extinct about 110 centuries ago, although long after their extinction, large bones buried in the earth were recognized as remains of once-living, giant creatures (Mayor 2000, 2005). Malagasy people entered Madagascar about 24 centuries ago, precipitating the onset of megafaunal extinctions that lasted nearly a millennium. Reports of late-surviving large mammals and giant ground birds are vague, if only a few hundred years old (Burney et al. 2004). Maori legends refer to huge birds, moa, that inhabited New Zealand before their extermination by Maori colonizers seven centuries previously (Holdaway and Jacomb 2000; Wilmshurst et al. 2008). Evidently oral traditions of important, sudden events (e.g., those that took place within a human life span) might endure for several hundred years. That many oral epics must have been spun from interactions between ice-age animals and humans is suggested by a mythic quality imparted to European cave paintings of 320 centuries ago. These sagas are now forgotten. Writing and a culture that is aware of the importance of history are necessary to preserve human memories in detail for longer intervals. Thousands of years in the future, scholars may be very interested in commonplace diaries being written today.

There is no evidence that changes in shape and behavior accompanying the domestication of plants and animals were originally understood as an indication that plant and animal species could be modified artificially. Aristotle (384–322 BC) observed that mammals reproduced and matured according to a pattern. However, the species to which they belonged did not change significantly between generations. Since then, it has been discovered that mammalian species typically endure for much longer than the few tens of centuries of human history. Aristotle cannot be faulted for concluding that species were essentially changeless; the brevity of the time span over which his observations extended gave him little reason to think otherwise.

Ancient scholars tended to measure long intervals of time in the duration of the reigns of rulers and ruling families, calibrated in years. Examples include dynasties of Egypt and China, genealogies in the Bible, and sequences of Roman emperors. On the basis of biblical genealogies, James Ussher, in his Annals of the World, published between 1650 and 1654, calculated that the first humans appeared 60 centuries ago (4004 BC). This date approximately coincides with the invention of writing and the recording of preexisting oral traditions (Plaut et al. 1981). Large bones were known long before Aristotle's time, giving rise to heroic myths of former giants (Mayor 2000). In 1669 Nicholas Steno recognized that accumulations of sediment containing fossils represented very long intervals of past time (Cutler 2003).

By two centuries ago it was clearly apparent from sequences of fossils preserved in sedimentary rocks that life must have been present on Earth long before the appearance of humans. The human proclivity to invent names for extinct, newly discovered, or mythical animals, as well as for periods of ancient time, was reinvigorated during the nineteenth century: entering the English lexicon were mastodon in 1811, pterodactyl in 1830, Jurassic in 1831, Devonian in 1837, Pleistocene in 1839, dinosaur in 1841, prehistoric in 1851, and brontosaur in 1892 (see the Oxford English Dictionary, 1989). Expanding lexicons have kept pace with increasingly detailed scrutiny of the record of life and the exploration of deep space through the twentieth century. Yet the staggering extent of geologic time became appreciated at a slower pace. About a century ago it was discovered that radioactive elements gradually fractionate into simpler but stable daughter elements. The ratio of radioactive to daughter nuclei of a given element, when locked within a crystal, indicates how much time elapsed since the crystal was formed. Such evidence revealed that the Earth is thousands of millions of years old. Ratios preserved in crystals formed in rocks at different times in the past provide a framework for prehistory in which events in the planetary past can be situated.

As an example, what is popularly known as the age of reptiles or the dinosaurian era, and what is technically known as the Mesozoic Era, took place between approximately 251.4 and 65.51 million years ago, over an interval of about 186 million years (Smith and Ward 2001; Hicks et al. 2002). ("Million years ago" and "million years" are henceforth abbreviated throughout the text as "myr" and a thousand years as "kyr"; an approximate number is preceded by "~.") Artists have attempted to reconstruct the appearance of strange creatures that lived during the Mesozoic Era by combining image fragments of modern organisms (eyes, skin patterns, claws) with fossil skeletons recovered from sediments deposited during that long-lost time. Over the past century and a half, restorations have become increasingly realistic (Lanzendorf et al. 2000; Johnson and Raynolds 2002; Rudwick 2002). Similarly, by blending fictional accounts of the experiences of modern explorers with conceptions of dinosaur-dominated ecosystems, writers have acculturated prehistory in increasingly vivid time-adventure classics (Verne 1864; Doyle 1912; Crichton 1990).

The body of available information about the history of the Earth and the cosmos continues to expand. High-technology instruments, such as scanning electron microscopes, radio telescopes, mass spectrometers, and x-ray interferometers, have enormously extended our powers of observation. Explanations of the functions of these marvelous instruments could fill encyclopedias, and a rapid increase in knowledge is nearly certain to continue into the future.

CONTROVERSY, THE DIALOGUE OF SCIENCE

Over the last millennium, scientific methodology has increasingly replaced other routes to knowledge. In our day, evidence of the success of Science is everywhere. Public health and standards of living are continuously improved through applications of scientific methodology. Scientific research has probed the limits of the universe. It informs us that many stars are either too short-lived or too unstable for life to evolve in planetary systems orbiting them. Even within the solar system, most planetary surfaces are almost certainly inhospitable to anything but the simplest forms of microbial life. On the relatively idyllic surface of our own planet, evolution is often perceived as lacking orientation and dominated by erratic changes in physical parameters such as temperature, rainfall, and seasonality that are difficult to predict in detail. The evolution of humanlike organisms is considered to have been far from inevitable on Earth, and even less so in most environments elsewhere in the universe. In practice, the scientific method restricts itself to the material universe, eschewing speculation on nonmaterial questions. Justly impressed by the spectacular and continuing progress of Science, some or many have come to accept a material view of life as realistic and complete. Although Science rests on solid foundations, a perspective that is limited to the material world is considered by some or many to be incomplete.

The present essay resulted from an interest in life as life is successfully lived. The interest has been accompanied by a parallel sampling of various areas of knowledge through a lifetime that has now entered its seventh decade. Necessarily shallow and idiosyncratic, the resulting worldview was greatly shaped by controversies in the domain of the geosciences. The debates have often been passionate, but over time, they have usually given way to a larger, more comprehensive consensus. Many authors have recounted the fascinating flow of concepts that have emerged from scientific disputes. Excellent source references include Adams (1954), Eiseley (1958), Gould (2002) and Rudwick (2005). A few of the more interesting questions debated during the latter half of the twentieth century include:

• Are the positions of the continents fixed or do they change (drift) through time?

• Were dinosaurs cold- or warm-blooded?

• Did dinosaurs become extinct gradually from earth-limited causes, or catastrophically from an extraterrestrial agent?

• Will the search for extraterrestrial intelligence (SETI) succeed, or is it destined to be futile?

• Did birds descend from primitive reptiles or from dinosaurs?

• Is classification better served by gross anatomical comparisons (Linnaean) or analyses of distribution patterns of discrete characters (phylogenetic)?

• Is evolution random (contingent) or directional (teleological)?

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Excerpted from Islands in the Cosmos by Dale A. Russell. Copyright © 2009 Dale A. Russell. Excerpted by permission of Indiana University Press.
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