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Islands in the Cosmos: The Evolution of Life on Land

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How is it that we came to be here? The search for answers to that question has preoccupied humans for millennia. Scientists have sought clues in the genes of living things, in the physical environments of Earth from mountaintops to the depths of the ocean, in the chemistry of this world and those nearby, in the tiniest particles of matter, and in the deepest reaches of space. In Islands of the Cosmos, Dale A. Russell traces a path from the dawn of the universe to speculations about our future on this planet. He centers his story on the physical and biological processes in evolution, which interact to favor more successful, and eliminate less successful, forms of life. Marvelously, these processes reveal latent possibilities in life's basic structure, and propel a major evolutionary theme: the increasing proficiency of biological function. It remains to be seen whether the human form can survive the dynamic processes that brought it into existence. Yet the emergence of the ability to acquire knowledge from experience, to optimize behavior, to conceptualize, to distinguish "good" from "bad" behavior all hint at an evolutionary outcome that science is only beginning to understand.

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1 Time Travel

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Fig 1.1. A group of muskoxen (Ovibos moschatus) on Ellesmere Island, province of Nunavut, arctic Canada, inhabits a simple biotic environment that is seasonally subjected to frigid temperatures. The megaherbivores tower above the stunted herbaceous vegetation on which they feed. For further discussion, see p. 18 and compare with fig. 1.2 on p. 20.

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.

 

2 The Extraterrestrial Pre-Hadean

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Fig 2.1. The field encompassed by the photograph is located in the constellation Fornax in the southern hemisphere and is one-tenth the diameter of a full moon. The field contains nearly 10,000 galaxies and penetrates nearly 13 billion light-years of space-time. Tiny red galaxies may be the most distant and oldest galaxies known, whereas the larger spiral and elliptical shapes represent galaxies typically ~1 billion years old. The few bright stars are nearby in the Milky Way galaxy. Photograph courtesy of NASA, the European Space Agency, and Steven Beckwith of the Space Telescope Institute and the Hubble Ultra Deep Field Team.

The origin and evolution of life on Earth took place within a universe that was unimaginably old and inconceivably vast. The structure of life is rooted within this universe. Life as we know it can flourish only under a limited range of conditions that include the availability of liquid water, suitable sources of chemical or radiant energy, and generally stable physical conditions that endure for thousands of millions of years. Such combinations may not be common within the visible universe, which is typically a cold, dark, near vacuum sparsely populated by brilliant nuclear furnaces (stars) emitting lethal high-energy radiation. Environments favorable to life may occur in scattered planetary oases, of which the surface of the Earth is an obvious, if uncommon, example.

 

3 The Hadean Eon

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Fig 3.1. The Island of Moorea, French Polynesia, southwestern Pacific, is the eroded summit of a submarine volcano, suggesting a common Hadean or early Archean landform. Soft volcanic ash has eroded from around solidified lava in volcanic throats. Similarly formed Hadean–Archean islands would have appeared dark as a result of the virtual absence of oxygen in the atmosphere.

Precambrian time includes the Hadean, Archean, and Proterozoic eras, encompassing nearly all (88 percent) of Earth history. Until recently, evidence for the evolution of Precambrian life was exceedingly meager. However, the search for life within and beyond the solar system has lately stimulated thoughtful investigations of the primitive Earth, in the expectation of better understanding the conditions necessary for the origin of life elsewhere. Rapid progress is now being made in elucidating the history of the Precambrian Earth.

The Hadean Era is here taken to include the interval spanning the appearance of a gravitational field around a minuscule protoearth strong enough to attract adjacent particles of matter and, several hundred million years later, the termination of an intense barrage of interplanetary asteroids, known as the late heavy bombardment. No surface rocks are known from this interval, for they have long since been destroyed as a result of dynamic processes within the interior of the Earth (Kamber et al. 2003; Zahnle et al. 2007). Hadean history is presently inferred from many sources. These include observations of young planetary systems beyond the solar system, elemental abundances (isotopes) in the inner solar system, crystals formed on the ancient Earth and later recycled into younger rocks, and in the genetic code preserved in surviving lineages of ancient bacteria (Schoenberg et al. 2002; Valley et al. 2002; Gaucher et al. 2003; Kamber et al. 2003; Kramers 2003; Menneken et al. 2007; Zahnle et al. 2007).

 

4 The Archean Eon

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Fig 4.1. Rotorua hot springs mud volcano, North Island, New Zealand, a terrestrial environment representative of those colonized by early Archean bacteria.

Not much land was exposed at the beginning of Archean time. Bacteria, although probably present, had little effect on the appearance of rugged volcanic terrains that periodically emerged from the oceans. By the eon’s end, over a billion years later, it looked much the same. No multicellular organisms were present, although the Earth was already over 2 billion years old. Under such circumstances, a similar planet, orbiting another star, might not be viewed as an auspicious cradle for the future evolution of complex life. Yet the planet was infected with life. Bacteria flourished within the upper limits of the Earth’s crust and in shallow seas, and had spilled onto lowlands of nascent continents. Change was literally in the wind; bacterial activity was polluting the atmosphere and threatening the primeval balance of greenhouse gases.

 

5 The Proterozoic Eon

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Fig 5.1. The Hoggar Mountains, near the hermitage of Charles de Foucault in southern Algeria, are representative of late Archean and early Proterozoic continental landscapes that hosted only microbial life. The local environment is too dry to support complex Phanerozoic-like ecosystems, such as the rain forests to the south in the Congo basin. Photograph courtesy of Harold Heatwole.

Spanning nearly two billion years, the Proterozoic is the longest division of geologic time. All of later time is less than the average span of its three subdivisions, the Paleo-, Meso- and Neoproterozoic (Amthor et al. 2003; Knoll 2003). The eon may be characterized as an age of microbial evolution. Most organisms, as individuals, remained microscopic during all but the last 3 percent of the eon. It has been necessary to rely on radiometric dates to calibrate its physical history, rather than fossils as is typical of post-Proterozoic divisions of geologic time. Severe glaciations and stepped increases in the oxygen content of the atmosphere occurred near the beginning and end of the eon. As presently understood, various interpretations of the physical and biological history of the Proterozoic are not entirely consistent with each other. The narrative presented here represents an approximation that in the future will be rendered more precise with the rapid accumulation of knowledge.

 

6 Phanerozoic Marine Life

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Fig 6.1. Patterns in the diversity of multicellular organisms during Phanerozoic time (after Benton and Emerson 2007; courtesy of Michael Benton). For further explanation, see text.

With a duration less than a third (28 percent) of that of the Proterozoic, the Phanerozoic is the shortest of the four eons of Earth history. In contrast to nearly all the preceding history of life, fossils visible to the unassisted eye can be found, often in great abundance, within Phanerozoic strata. The eon is characterized by a great diversification of multicellular organisms that gradually merged into the biosphere we know today. The fossil record of marine life is outlined here because it provides the geologic timescale with subdivisions (periods, epochs, ages) familiar to all students of Earth history. These subdivisions are well calibrated by numerical (radiometric) dates. Warm and cold intervals, changing configurations of continents and oceans, and episodes of mass extinction that influenced the evolution of terrestrial life have all been placed within the marine timescale. The marine record thus provides the temporal framework for the evolution of life on land.

 

7 Origin of Complex Terrestrial Ecosystems

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Fig 7.1. Geography of the world ~480 myr ago, showing a major global expansion of shallow seas over equatorial continental shelves. Reconstruction courtesy of Chris Scotese.

By Early Paleozoic (Cambrian through early Devonian) time, multicellular terrestrial organisms had begun to diversify rapidly, and some were large enough to be seen by the unaided eye. The record of life on land is very meager during the earlier part of the interval. It is likely that mats of lichens and liverworts associated with supporting (symbiotic) or degrading (saprophytic) fungi differed but little from their Proterozoic predecessors. The same was probably true of a host of tiny soil animals bearing jointed, chitinous body walls (exoskeletons), such as nematodes, tardigrades, mites, millipedes, and perhaps the progenitors of insects (see chapter 5). These organisms inhabit weathered rock surfaces and modern soils in astronomical numbers, and their combined weight (biomass) distributed globally across continental surfaces may have equaled or exceeded that of life in the oceans before the Paleozoic began (McMenamin and McMenamin 1994; LePage and Pfefferkorn 2000; McLeod and Braddy 2002; Prave 2002; Retallack 2004).

 

8 Toward the Coal Age

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Fig 8.1. A giant lycopod (Huperzia deuterodensa) from the Plateau de Dogny, Grande Terre, New Caledonia, is a small surviving member of a group of plants (lycopsids) that dominated swamp forests in middle Paleozoic time. Photograph courtesy of Jérôme Munzinger.

The Middle Paleozoic (middle Devonian through all but the latest Carboniferous) culminated simultaneously in a coal age and an ice age. Planetwide deposition of coal and forest fires indicated that accumulating plant materials had far outstripped the recycling abilities of microorganisms, and fungal and animal life. An age of plants had begun. The overwhelming success of plants was preceded by the differentiation of function-specific organs, such as roots, woody stems, branches, leaves, and seeds, a process analogous to the differentiation of tool kits in eurypterids. The complexity of terrestrial ecosystems increased as plants diversified, spread across the continents, and became vertically multilayered with root systems below and branch systems above. An even more rapid diversification of arthropods began as insects flew into forest canopies to harvest spores, pollen, and seeds. In response, plants began to develop chemical defenses. Dynamic links between vascular plants and insects were established that would broaden in the future. As global climates deteriorated and marine biodiversity stabilized, terrestrial biodiversity burgeoned.

 

9 Ascendancy of Life on Land

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Fig 9.1. Arid vegetation, Northern Territory, Australia, is perhaps illustrative of middle to late Paleozoic vegetation but is now restricted to hot, arid environments hostile to the growth of dense forests.

About 50 myr (latest Carboniferous through to end of Permian) separated the draining of coal swamps in the tropics and the beginning of the era of “middle animals” (Mesozoic). The interval witnessed the greatest planetary proliferation of terrestrial life known up to that time. Glaciers retreated and increasingly complex terrestrial ecosystems spread toward the poles as life in the tropics underwent a simplification linked to increasing aridity and irregular, monsoonal rainfall. Seed plants rose to dominance, and lizardlike herbivores radiated into a variety of clumsy reptilian “cows.” The appearance of large land-dwelling herbivores signaled the appearance of terrestrial ecosystems that were more than a simple extension of freshwater aquatic ecosystems—they contained large animals (megaherbivores) with bacterial intestinal symbionts enabling them to prosper on coarse, poor-quality plant food. Rodentlike burrowing vertebrates proliferated in seasonally cool subantarctic floodplains. Toward the end of the interval, vertebrates that had evolved in the southern temperate zone crossed ecological obstacles in the tropics to colonize the northern hemisphere. And the interval ended in great global catastrophes.

 

10 Bridging the Eras

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Fig 10.1. Geography of the world ~240 myr ago, when continental plates were united into the supercontinent of Pangea. Reconstruction courtesy of Chris Scotese.

The Triassic was named after a threefold division of strata of early Mesozoic age in northern Europe. A tripartite succession of terrestrial ecosystems also characterized the evolution of Triassic vertebrate life. The oldest interval is typically represented by aquatic environments dominated by large amphibians, an intermediate interval by floodplains dominated by crocodile-like ruling reptiles (archosaurs), and the youngest by fully terrestrial, primitive dinosaurs. Emergent regions of the globe had largely gathered into a single supercontinent under a greenhouse climatic regime.

A brief chronology of Triassic inter-era time follows, measured from the final Paleozoic extinction, in italics, and time measured before the present, in parentheses. For a recent and detailed review of Triassic biota and environments, see Fraser (2006).

 

11 The Natural History of Natural Selection

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Fig 11.1. An accelerating curve with number of species shown in millions (no. spp./millions), vertical axis, plotted against elapsing geologic time in billions of years (B.yr.), horizontal axis. One species is taken to have been present at 1 billion years, corresponding to the oldest well-documented presence of bacterial life, and 10 million species are presumed to exist today, plotted at 4.6 billion years. The age of dinosaurs, represented by cross-hatching, brackets the midpoint of the species diversity axis (after Russell, 1996). See also Fig. 5.5 and “Ascending the Curve of Fitness” in chapter 15.

With the previous chapter, a geometric (logarithmic) midpoint in the diversification of terrestrial life had been approached (Benton 1995, figure 1B) and, by implication, in the accelerating fitness of terrestrial organisms (chapter 1). Nearshore Archean bacterial mats had been largely replaced by araucarian forests, and prey-engulfing single-celled organisms (stem eukaryotes) by bulky carnivores and herbivores (rauisuchians and dicynodonts). This progressive polarity toward the appearance of higher levels of fitness had been maintained from generation to generation for several billion years. Much would yet be accomplished by the end of the most recent ice age, then ~230 myr into the future. The marvel of continuity of form between generations has already been alluded to in regard to air-breathing eggs (chapter 8). It may be useful to review briefly current understanding of the mechanism of inheritance that has been conserved throughout the history of life, from bacteria to dinosaurs and beyond.

 

12 An Age of Giants

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Fig 12.1. A tree fern growing in dense vegetation on the North Island, New Zealand, is closely related to similar and common constituents of Mesozoic vegetation.

Early in the nineteenth century evidence of an astonishing antediluvian age of reptiles was discovered in Jurassic strata of England and France. Now, nearly two centuries later, popular expectations require that Jurassic rain forests remain veiled in mystery, broken by vaguely defined giant silhouettes slowly gliding between massive trees. From high overhead, the canopy is broken by stabbing screams that slowly fade into soundless vaults of vegetation. The humid forest floor is briefly illuminated by flashes of lightning, revealing scarlet pools of blood. Similarly jarring the foundations of reality are immense shudders in the Earth’s crust as a world-continent ponderously ruptures into precursors of modern continents. Volcanoes erupt, streams change their course, the already dim light fades, and terrified Lilliputians are lost in an age of giants.

 

13 One Earth, Two Worlds

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Fig 13.1. Geography of the world ~80 myr ago, as the fractionation of terrestrial environments approached a maximum. Reconstruction courtesy of Chris Scotese.

Cretaceous means many things to students of Earth history. Its name stems from the chalk that was so widely deposited in shallow European seas during the latter part of the period. A giant marine lizard (Mosasaurus maximus) was the first huge vertebrate specimen to be collected from Cretaceous strata. The interval is regarded as a time when sea levels were high and global climates were warm. Yet during the Cretaceous, the major clades (families) of multicellular terrestrial organisms surpassed those of marine organisms in number (Benton 2001). Dinosaurs, such as the familiar Tyrannosaurus and Triceratops, symbolize the climax of dinosaurian evolution, and their disappearance marked the end of the Mesozoic Era. More recently, it was discovered that giant predatory dinosaurs unrelated to Tyrannosaurus once existed in the Cretaceous of the southern hemisphere, and that the northern hemisphere record was not necessarily representative of Cretaceous life on land everywhere. Indeed, late Cretaceous forests in the interior of North America more closely resembled modern forests in the southeastern United States than those now growing in New Zealand. Similarly, reptiles and amphibians in the western interior of the North America near the end of the Cretaceous more closely resembled those now living in the southeastern United States than those in Africa. Recently the genetic record preserved in living plants and animals has been applied with notable success in clarifying the history of Cretaceous life, which formerly was interpreted only from fossil residues of Cretaceous organisms. Thus, drawing from available osteological and genetic evidence, it may be inferred that modern terrestrial biogeography and modern terrestrial life are seamlessly linked to the Cretaceous.

 

14 The Modern Earth

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Fig 14.1. The smoothly undulating surface of a tropical rainforest in Bellenden Ker National Park, Queensland, Australia, is characteristic of a closed-canopy plant biome that appeared during mid-Cenozoic time. Photograph courtesy of David Jarzen.

The duration of the Cenozoic Era (Paleocene through Pleistocene), which separates the extinction of the dinosaurs from the present, was less than that of the Cretaceous, the last period of the Mesozoic Era. Midway through Cenozoic time, global climates abruptly changed from greenhouse to icehouse conditions. World geography moved toward the coastlines and ice caps portrayed in modern atlases. The diversity of terrestrial species increased as animal forms merged into the graceful shapes illustrated in textbooks of natural history. Familiar as the biological world beyond our doors is, its finesse far surpasses the finesse embodied in vanished ecosystems of the remote and seemingly mysterious past. Ours is a truly special time, and one that encourages admiration and optimism for the processes that mold the history of life.

 

15 Synthesis

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Fig 15.1. Earth and Moon photographed by Voyager 1 at a distance of 11.6 million kilometers (7.25 million miles) from Earth. Because it is not nearly as bright as Earth, the image of the Moon was artificially brightened by three times in the photo. Average temperatures on its dull gray, airless, and waterless surface rise to 107°C by day and fall to −153°C at night. Mantled by an oxygen-rich atmosphere, blue water oceans, white ice and water clouds, green vegetation, and bright yellow deserts, the maximum recorded temperature on earth is 57.8°C (Sahara) and the minimum is −89.5°C (Antarctica). Photograph courtesy of NASA.

The object of this essay has been to portray the evolution of terrestrial ecosystems as a process that contains regularities meriting further investigation. There is of course more to evolution than the sum of the differences between a bacterium and a human, and the process cannot be characterized as fundamentally random. Further, the history of science itself suggests that perceptions of evolution change, in no small measure because the domain of science itself increases with time. Two thousand years ago, time, gravity, light, and air were nebulous concepts. Today the list includes subatomic strings, dark matter, multiverses, and mind. Much information has been gleaned from the geologic record during the past two centuries. How is this detail to be understood?

 

Epilogue: The Way of Life

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Although questions concerning the meaning of life are beyond the domain of science, they are almost without exception of interest to everyone. The rapidly growing volume of information on the history of life has also stimulated much reflection. What, then, might be the existential implications of the present, all-too-brief survey of the history of life on land? When I was small, my worldview was that of a child, and fond memories remain of a time when the world of giant, simple-minded dinosaurs seemed fresh and young. That worldview has given way to one of a grandfather, who profits from reading thoughtful discourses on life penned more than 23 centuries ago by Aristotle, the genial genius of classical Greece. A reinspection of the night sky reveals no evidence of change since childhood. However, its seeming immutability is better understood as a consequence of the brevity of a human life. Measured in units of thousands of millions of years, the dynamism of the universe is obvious.

 

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