Figure 18.8 puts the Phanerozoic in perspective with the rest of the Earth’s history. The Cambrian revolution in the appearance of animals is clearly seen, along with the “boring billion” of years between the assured appearance of eukaryotes and the Cambrian revolution. What is striking about this figure cannot be directly inferred from its content: one has to go back to Chapter 15 to recall that, within a billion to two billion years, Earth will lose its surface hydrosphere and hence complex life, if not all life, will become extinct. If Earth’s situation is typical, and it takes billions of years after the formation of life for complex organisms to arise and become intelligent, then the probability of this happening on any given planet may be very small indeed. Is there time for another innovation akin to the Cambrian revolution to kick in before the brightening Sun closes the curtain on the history of life on Earth, or have we seen, in the record of fossils of trilobites and other wonderful creatures, all the innovation there really is in the genome?
Summary
The Phanerozoic eon began about 600 million years ago and is characterized by the diversification and global spread of mul- ticellular organisms. While such organisms may have existed well before the start of the Phanerozoic, it was not until then different types of multicelled organisms rapidly diversified. The remarkable appearance of a variety of animal forms at the start of the Phanerozoic, the so-called Cambrian explosion, is a dramatic example of the process of evolution in action. Evo- lution is made possibly by the mutability of the genome in all organisms, but the nature of the changes that survive and propagate is shaped by natural selection: the effect of the envi- ronment on the organism. Without the two acting in tandem, the appearance of different and more complex forms might not have occurred. Evolution is not a slow, gradual process; species may remain stable for long periods of time, and only in the face of an event that isolates a breeding population might one see the appearance of a new species. For this reason and for the reason that the fossil record is an imperfect one, there are few cases of species change that are well documented in the fossil record, but those that are provide strong arguments in favor of evolution as the process by which new species appear.
In the case of the Cambrian explosion, essentially all of the major animal branches or phyla appear at that time, along with some that did not survive to the present. Clues to the trigger
for such a dramatic flowering of species may be found in the Ediacaran period that immediately preceded the Cambrian; a minor flowering of very primitive animal species took place at that time. What remains perplexing is the long delay between the development of eukaryotes and that of complex plants and animals. The delay may have to do with slow lengthening of the genome to allow for multicellularity, a sulfur-rich ocean, and one or more near-global glaciations that greatly restricted suit- able habitats. Subsequent to the Cambrian revolution, much of the history of complex life has involved the interplay between ecosystem-emptying great extinctions, and the co-option of such ecosystems by new forms that diversified from classes of animals or plants that previously were unimportant. Thus the mammals were a relatively unimpressive class of animal until the dinosaurs, who occupied a much larger range of ecosystems, suffered extinction 65 million years ago. The cause of that great extinction remains controversial, but compelling evidence exists that a 10-km sized fragment of an asteroid struck the Earth, causing massive damage and climate change for a period of time. Unimportant in the overall history of the solar system as just another impact, the K/T boundary impactor paved the way for the diversification of mammals and hence, eventually, to ourselves.
Questions
1. Can you conceive of several alternative explanations for the lack of transitional forms in the fossil record? Explain why, logically, “absence of evidence” (of fossils) is not “evidence of absence” (of the evolutionary process).
2. Is the Ediacaran–Cambrian revolution an inevitable result of increasing genetic complexity? If so, what might you imag- ine could happen in a putative future revolution? Is such a revolution prohibited by external environmental conditions?
3. Using the formula for kinetic energy compare the amount of energy deposited by projectiles with radii of 1, 10, and 100 km, all moving at 10 km/sec. What happens to the energy if
the speed is doubled? Assume the projectiles are spherical and have densities around that of rock (3 grams per cubic centimeter).
4. The concept that genome size must increase for more com- plex animals to arise seems to be contradicted by the obser- vation that amphibians have a larger genome size than do all other types of animals. It is also contradicted by the fact that the human genome has half the number of genes that wheat does. Can you think of some other aspect of the genome that might determine the sophistication or complexity of an organism? (This may require a literature search.)
General reading
Gaidos, E. and Knoll, A. H. 2012. Our evolving planet: from the Dark Ages to an evolutionary renaissance. In Frontiers of Astrobiology(eds. C. Impey, J. Lunine and J. Funes). Cam- bridge University Press, Cambridge UK. In press.
Gale, J. 2009.Astrobiology of Earth: The Emergence, Evolution and Future of Life on a Planet in Turmoil.Oxford University Press, New York.
Margulis, L. and Sagan, D. 1986.Microcosmos. Summit Books, New York.
References
Cloud, P. 1988.Oasis in Space: Earth History from the Beginning.
W. W. Norton, New York.
Eldredge, N. and Gould, S. J. 1972. Punctuated equilibria: an alternative to phyletic gradualism. InModels in Paleontology (T. J. M. Schopf, ed.). W. H. Freeman and Company, San Francisco, pp. 82–115.
Gaidos, E., Dubuc, T., Dunford, M.et al.2007. The Precambrian emergence of animal life: a geobiological perspective.Geobi- ologyDOI: 10.1111/j.1472-4669.2007.00125.x.
Gould, S. J. 1969. An evolutionary microcosm: Pleistocene and recent history of the land snailP. (Poecilozonites) in Bermuda.
Bulletin of the Museum of Comparative Zoology138, 407–531.
Gould, S. J. 1985.The Flamingo’s Smile: Reflections in Natural History. W. W. Norton, New York.
Keller, G., Adatte, T., Stinnesback, W.et al.2004. Chicxulub impact predates the K-T boundary mass extinction.Proceedings of the National Academy of Sciences of the USA101, 3753–8.
Kring, D. A. 1993. The Chicxulub impact event and possible causes of K/T boundary extinctions. In Proceedings of the First Annual Symposium of Fossils of Arizona(D. Boaz and M.
Dornan, eds). Mesa Southwest Museum and Southwest Pale- ontological Society, Mesa, Arizona, pp. 63–79.
Lyson, T. R., Bercovici, A., Chester, S. G. B., Sargis, E. J., Pearson, D., and Joyce, W. G. 2011. Dinosaur extinction: closing the 3 m gap.Biology Letters7, 925–8.
Milne, D., Raup, D., Billingham, J., Niklaus, K., and Padian, K.
(eds) 1985.The Evolution of Complex and Higher Organisms.
NASA SP-478. U.S. Government Printing Office, Washington, DC.
Vickery, A. C., Kring, D. A., and Melosh, H. J. 1992. Ejecta associ- ated with large terrestrial impacts: implications for the Chicx- ulub impact and K/T boundary stratigraphy.Lunar and Plan- etary ScienceXXIII, 1473–4.
19
Climate change across the Phanerozoic
Introduction
The preceding chapter focused on singular events in the later history of the Earth – the flowering of multicellular com- plex organisms at the start of the Phanerozoic eon and the widespread extinction of species some 65 million years ago at the close of the Cretaceous period. Although these events stand out in their drama and the mystery of their causes, any under- standing of the interactive history of life and Earth’s environ- ment cannot rest on their study. Throughout the Phanerozoic, and before, the relatively steady rhythms of plate tectonics brought continental masses together and then moved them apart, creating new seafloor and destroying old. The process of great landmasses moving around the planet must have had profound effects on the environment, and indeed this is seen to be the case in the geologic record.
This chapter begins by reconsidering plate tectonics with an eye to understanding the apparently cyclical creation and break
up of multicontinent landmasses, orsupercontinents. We con- sider the effects of such supercontinent cycles on the amount of volcanic activity, and hence atmospheric chemistry, on the ocean circulation patterns, on mountain building, and hence on the available area for storage of continental snow and ice deposits. Such considerations touch on a major theme of the latter portion of Earth history, the comings and goings of great ice ages. Finally, we draw our attention in detail to a par- ticularly warm time in recent Earth history, the Cretaceous period. Ice free and showing much less drop in temperature from equator to pole than Earth experiences today, the Creta- ceous has become a proving ground for climate modelers who seek to predict the amount and nature of global warming in humankind’s future.