Building a Habitable Earth

The Earth is the only world known, so far, to harbor life. There is nowhere else, at least in the near future, to which our species could migrate. Most of the Universe is too cold, too hot, too dense, too vacuous, too dark, too bright, or not composed of the right elements to support life. Only planets and moons with solid surface materials provide plausible oases for life as we know it. And even among planets with surfaces, most are highly undesirable. As we noted in the Introduction to this...

Spectrum of a Life Bearing Planet

Earth's atmosphere would actually be quite unnatural for a nonbiotic planet. It is clearly different from the nearly pure carbon dioxide atmospheres of its neighbors, Mars and Venus. The mix of nitrogen, oxygen, and water vapor is chemically unstable and would never arise on a dead planet. Without life, nitrogen and oxygen in the presence of water would combine to form nitric acid and become a dilute acidic component of the ocean. Earth's peculiar atmosphere is not in chemical equilibrium, and...

Chapter The Surprising Importance of Plate Tectonics

Radiogenic isotopes The case for crustal recycling on a near-steady-state no-continental-growth Earth. Philos. Trans. R. Soc. London Ser. A 301 443-472. Arrhenius, G. 1985. Constraints on early atmosphere from planetary accretion processes. Lunar and Planetary Sciences Institute Rep 85-01 4-7. Beck, M. E., Jr. 1980. Paleomagnetic record of plate-margin tectonic processes along the western edge of North America. J. Geophys. Res. 85 7115-7131. Broecker, W. 1985. How to...

Whatif Anything Triggered the Cambrian Explosion

Near the end of the last chapter, we pondered whether the initial diversification of the animal phyla was stimulated by evolutionary or environmental causes specifically, the Snowball Earth events of between 800 and 600 million years ago. This same question can be posed about the subsequent Cambrian Explosion Did it occur as late as it did in Earth history because it took that long for the establishment of an environment conducive to animals of large size, most with skeletons, or because it...

The Archaeans

Biologists have long recognized that species can be grouped into hierarchical assemblages. These units are linked by lines of descent that is, all species that make up a higher category share a common ancestor. species are grouped into genera. (Our species is grouped, along with the extinct human forms, into the genus Homo. This means that all species of Homo, including Homo sapiens, Homo erectus, and Homo habilis, among others, have a common ancestor.) Genera are grouped into families,...

The Nuclear Family

In summary, we can characterize eukaryotic cells as having seven major characteristics that distinguish them from prokaryotes. 1. In eukaryotes, DNA is contained within a membrane-bounded organelle, the nucleus. 2. Eukaryotes have other enclosed bodies within the cell the organelles such as mitochondria (which produce energy) and chloroplasts (tiny inclusions that allow photosynthesis). 3. Eukaryotes can perform sexual reproduction. 4. Eukaryotes have flexible cell walls that enable them to...

Environmental Conditions Leading to the Evolution of Eukaryotes

What environmental conditions led to the evolution of the forerunners of animal life New discoveries of the 1980s and 1990s have given us a much clearer view of the early Earth during the great evolutionary transitions we saw in the last chapter. The Earth's earliest life seems to have formed during or soon after cessation of the heavy comet bombardment. By about 3.8 billion years ago that heavy cosmic bombardment ended, and by 3.5 billion years ago we find the first fossilized evidence of...

Chapter Mass Extinctions and the Rare Earth Hypothesis

Asaro, F. and Michel, H. 1980. Extra-terrestrial cause for the Cretaceous-Tertiary extinction. Science 208 1094-1108. Alvarez, W. 1997. T. Rex and the Crater of Doom. Princeton, NJ Princeton University Press. Annis, J. 1999. Placing a limit on star-fed Kardashev type III civilisations. Journal of the British Interplanetary Society 52 33-36. Bourgeois, J. 1994. Tsunami deposits and the K T boundary A sedimentolo-gist's perspective. Lunar Planetary Institute Cont. 825 16....

Diversity and Disparity

One of the central (and controversial) aspects of the Cambrian Explosion especially with reference to the wondrous assemblage of fossils found in the Burgess Shale localities in Western Canada (where not only early animals with hard parts but also forms without skeletons are preserved as smears on the rocks) concerns what are called diversity and disparity. Diversity (or in this case biological diversity) is a term familiar to most of us. Overtly, it is usually understood as a measure of the...

What Animals Were Involved in the Cambrian Explosion

Ediacara Fauna

No one disputes that a huge diversity of large animals emerged with alacrity between 600 and 500 million years ago. The event itself took place in the sea, for the land areas of the time were largely barren except for lichens and perhaps a few low plants there were no trees, no shrubs, no stemmed plants at all. Because of the lack of rooted vegetation, little soil would cling to the land surfaces. In the shallow seas and waterways, however, life was plentiful (though clearly different from that...

The History of Mass Extinctions on Earth Ten Events

Paleontologists have discovered many mass extinction events that occurred since the Cambrian Explosion (that is, in the past 540 million years). Yet other mass extinction events of earlier times are largely unknown to us, because they occurred when organisms rarely made skeletal hard parts and thus rarely became fossils. Perhaps the long period of Earth history prior to the advent of skeletons was punctuated by enormous global catastrophes that decimated the biota of our planet mass extinctions...

Creation of the Elements

Although we understandably date Earth's history from the origin of the planet, a considerable prehistory preceded Earth's formation. one of the most important aspects of this period was the origin of the chemical elements. The elements are the building blocks of both planets and life. Consider that in a sort of cosmic reincarnation, every atom in our bodies resided inside several different stars before the formation of our sun and has been part of perhaps millions of different organisms since...

The Oxygen Revolution

It is probably impossible for us to conceive how entirely alien to ours this world truly was. Yet the strange microbial world of 2 billion years ago may be the norm in the Universe for those planets that harbor life. Traces of it exist still, here on Earth, in the bacterial froths and pond scum that persist across our planet, and perhaps nowhere more prolifically than in the rotting garbage dumps and landfills created by our own species places where huge, visible colonies of rapidly growing...

An Ancient Dichotomy

Microsporidia Not Ancient

The gulf between the complexity of a bacterium and the complexity of even the simplest multicellular animal, such as a flatworm like Planaria, is immense. The number of genes in a bacterium can be measured in the thousands, whereas the genes in a large animal number in the tens of thousands. To illustrate this, we can liken a bacterium to a simple toy wooden sailboat. With only three or four very tough parts, the toy boat is virtually indestructible, just as a bacterium is impervious to most...

Why Does Earth But Not Mars or Venus Have Plate tectonics

Why is there plate tectonics on Earth The recipe for plate tectonics seems simple enough at first glance. You need a planet differentiated into a thin, solid crust sitting atop an underlying region that is hot, fluid, and mobile. You need this underlying region to be undergoing convection, and for that you need heat emanating from even deeper in the planet. And you are likely to need water oceans of water Much new research suggests that without water you cannot have plate tectonics though...

Chapter Assessing the Odds

The life span of the biosphere revisted. Nature 360 721-723. Caldeira, K., and Kasting, J. F. 1992. Susceptibility of the early Earth to irreversible glaciation caused by carbon ice clouds. Nature 359 226-228. Dole, S. 1964. Habitable planets for man. Waltham, MA Blaisdell. Goldsmith, D., and Owen, T. C. 1992. The search for life in the universe. New York University Science Books. Gonzalez, G. 1999. Are stars with planets anomalous Monthly Notices of the Royal...

What Would Happen If Plate Tectonics Ceased

The fossil record suggests that there are more species of animals and plants alive on Earth today than at any time in the past estimates vary between about 3 and 30 million species. This great diversity has come about through many physical and evolutionary factors. We contend that the effects of plate tectonics are among the most important. But once created, does high biodiversity require the continued presence of plate tectonics We can examine this question with a thought experiment. Imagine...

What Is Plate Tectonics

Geologists of the eighteenth and nineteenth centuries had little difficulty understanding the origin of volcanoes Hot magma from deep within the planet rose to the surface regions and spewed forth lava, ash, and pumice to form a cone. Understanding how nonvolcanic mountains and mountain ranges could form, however, was more problematic. Countless hypotheses were proposed. These included buckling of the crust as a result of sediment loading where the weight of slowly accumulating sediment finally...

Surprising Importance of Plate Tectonics

Imagine that we have a spacecraft capable of swiftly taking us to each planet in the solar system. Our goal on this voyage is to try to determine what features of Earth are essential to animal life. On our voyage, therefore, we are looking for some clue to why animal life has been able to survive on Earth for a time period approaching a billion years. What are those factors that have fostered diversity on Earth We begin our celestial survey with Mercury, a cratered world of great heat on the...