Abbreviations used in this article:
By: billion (1,000 million) years
Bya: billion years ago
CC: climate change
ME: mass extinction(s)
My: million years
Mya: million years ago
The six chapters of Extinctions, each authored by a major researcher on mass extinctions (ME), are the fruit of a symposium convened by the Center for the Study of Evolution and the Origin of Life. The aim of both the symposium and the text "has been to make accessible - at undergraduate level - key findings and current debates concerning extinctions in the history of life". Hence: everything you wanted to know about ME but were afraid to ask..
Extinctions is a wonderful book, rather well written (or edited) and intellectually stimulating to no end - for dinosaur geeks and budding paleontologists..
However, it could be a valuable text for some other classes of readers:
- Anti-Climate Change (CC) activists who want to assure themselves of a really scientific base for their argumentation. In the mind of many earth scientists, CC is the major driver of biological evolution on earth.
- Green Energy advocates wishing to present the risks posed by CC as arguments in favor of non-carbon based energy sources: solar, wind, tidal, geothermal..
- Anyone interested in "environmental issues" such as species conservation and habitat preservation. Knowing more about extinction mechanisms contributes to the development of better programs for preserving biodiversity.
- General - but informed - readers of ecology and earth science. Readers should be minimally "informed" as Extinctions targets an undergraduate (college / university) audience. You might find the text a tad difficult if you think that "Paleozoic" refers to the antics of Sarah Palin.
As a kid, I was a dinosaur geek and could rattle off a bunch of species names and their descriptions. But I was puzzled by their disappearance. What would cause these great beasts to go extinct? Not obvious, that one..
to kill these boogers off..
As a kid, I was a dinosaur geek (still am) and could rattle off a bunch of species names and their descriptions. But I was puzzled by their disappearance. What would cause these great beasts to go extinct? Not obvious, that one..
No one, back then - the 1960s - had a clue it seemed. One theory even claimed that the herbivores dies of constipation - from changes in vegetation - and the carnivores followed them into oblivion, a domino effect.. Dinos died of constipation, OK..
I also learned about other mass extinctions, equally puzzling. In the Great Dying of the Late Permian, 250 Mya, most of the species capable of leaving fossils, on land and sea, went extinct. Why? No one knew..
Today, extinction researchers can piece together the Big Picture of Evolution and ME patterns from the accumulated wealth of several centuries of classification of fossils. New scientific and statistical methodologies permit deeper understanding of the dynamics of extinction and recovery. Thanks to stable isotope partition measures, a deeper understanding of ancient climatic conditions and extinction / recovery mechanisms is emerging. And the Big Pic is truly fascinating!
Earth scientist are more like detectives than physicists. Like forensic investigators they work backward from evidence - sometimes disturbed - left at a "crime scene" (ME as depicted in the fossil record) to establish the events, the actors and their interactions. Who did what to whom, when and how.. Unlike the physicist, earth scientists cannot simply translate a hypothesis into an experiment and test it in the lab. Evolution's time scale is in My and its scope, the entire earth!
Earth scientists - paleontologists, geologists, paleoclimatologists, paleo-ecologists,.. - work mostly with the clues Mother Nature has left behind. Like the forensic investigator, the earth scientist relies more on inference than experimentation (as physicists or chemists would do). Each branch of science eventually ends up developing its own particular style or blend of scientific methodology.
Mass extinction (ME): a geologically"short" period of time during which the earth's biota is severely depleted. Species numbers - biodiversity - drop as extinction rates rise above normal or "background extinction rates".
ME are, most realistically, a constructive factor in evolution, "removing incumbents and allowing other groups.. to prosper and diversify". In the long run this led to diversification and evolutionary "progress" (? bigger brains?). Although the anthropocentrism is obvious, we humans have "benefited" from increasing brain to body weight ratio among the "higher" vertebrates. This "cephalisation" appears to have been stimulated by past ME. We literally owe our human consciousness and intelligence to ME. If we believe that our consciousness and intelligence are gifts then we have ME to thank in large part.
Contrary to popular opinion, ME are not responsible for the majority of species extinctions. ME occur at intervals of tens of missions of years. Probably more than 95% of species extinctions are simple background extinction events. The average life expectancy for mammalian species is about 1 million year - much, much, shorter than the average lapse of time between ME.
ME are complex, multicausal events (the detective at the crime scene again! Who did what to whom, when and how..) Plants and animals tend to respond differently to ME. Genetic lines - clades - of plants generally include a broad range of morphologies: weeds, creepers, vines, little and big shrubs; little, medium sized and big trees... Thus, if ME selectively eliminates trees and shrubs, the clade will survive as weeds. Overtime, some of the weeds will re-evolve into new shrub and tree species. Thus plant clades tend to be more resilient than animal clades, although extinction levels at the lowest -species - level of classification may, in fact, be comparable between plants and animals.
One hypothesis I initially rejected but now lean toward: ME are not really "abnormal" events at all, rather they represent the random coming together of severa; factors (no one of which would be lethal) into a Perfect Storm - or sucker punch. Thus the accepted view that the dinos were killed off by an asteroid strike (Alvarez 1980). is today seen as an oversimplification. True, the strike was a big one and, chemically speaking, occurred in a bad type of rock (carbonates, sulfur). But evidence increasingly suggests that the earth has experienced numerous large bolide strikes without ME. One ME researcher, Tony Hallam, has labelled the asteroid strike as the "coup-de-grâce" which finished off an ailing Gaian ecosystem already fragilized by CC.
Among the potential causes of ME:
- volcanism (ejection of CC-producing gases like sulfur oxides, carbon dioxide and methane)
- bolide impacts. Only one ME, the K/T extinction that killed the dinos, has been rigorously linked to massive impactors.
- sea level rise or fall
- ocean anoxia ("dead zones" lacking oxygen)
- CC: global warming or cooling
- changing continental configuration which modifies important atmospheric and oceanic currents which, in turn, regulate climate. In addition, large continental masses experience greater climatic extremes in central regions than smaller continental masses.
Thanks to technical advances, all of these may now be detected and, to some degree, be quantified in the fossil record.
One of the "constants" that has consistently emerged over the last few decades is the important role that CC plays in ME. Some form of CC is found in most ME even if, as stated earlier, it is "only" part of a Perfect Storm that kills massively.
We need to pay heed! Today humankind is massively injecting gases into the atmosphere that, when massively injected into the atmosphere in the past (volcanism), are highly correlated with ME. Are we playing Russian Roulette with the air we breathe and the seas which feed us? The fossil evidence appears affirmative.
And so, what to do? (addressed to the reader..)
To repeat myself, this is a really great book for dino geeks and those with strong interests in the earth sciences, intellectual or practical. It summarizes, for an undergrad university readership, what is known today about ME and their impact on biological evolution.
Extinctions raises interesting philosophical questions for those so inclined. There is, for example, the question of the significance of human "control over" nature as we (potentially) enter a new geo-biological age, the Anthropocene (footnote 1). In contemplating the fossil record of ME, I conclude that any possible viable human regulation of natural processes would be founded, not on domination or "power over" nature, but on balance and equilibrium, on justice and wisdom - a rather tall order, given the state of the world today!
In reality, human have "benefited" from past changes, both "normal background" extinctions and the dramatic large scale transformations we call ME. Without these changes we would simply not exist! Today, however, we find ourselves in the curious position of provoking changes which - if past records (fossils) are reliable - will inevitably led either to the extinction of our species or, at the least, to the destruction of our "advanced" technical civilization. Ultimately one is led to a reflection on the very nature of human nature itself: given what we are doing to our planet, are we collectively MAD?
notes:
1- Anthropocene: to the traditional 5 bio-geological "eras" - the Archean, Proterozoic, Paleozoic, Mesozoic and Cenozoic - some contemporary thinkers have suggested that we are living the transition to a sixth era, one in which (human) consciousness becomes a major factor in biological evolution. James Lovelock, creator of the Gaia Hypothesis (the earth as a living "macro-organism"), refers to this new epoch as the Awakening of Gaia in which Nature becomes fully conscious in wo/man. One could cite the French theologian Teilhard de Chardin as an intuitive precursor, who saw a "planetary consciousness" emerging as the unfolding of God's Plan. More recently the French philosopher / sociologist, Edgar Morin, has written of consciousness now becoming an active player in the evolutionary process. For Morin, this has become possible because of the recent rapid evolution of science and technology and the power they confer to manipulate genes, mind, matter and energy on increasingly larger and more integrated scales.
The bio-geological eras of earth
Archean, 4.5 Bya to 2.5 Bya: appearance of life, early primitive microbes
Proterozoic, 2.5 Bya to 550 Mya: appearance of stable continental masses, more advanced microbes (eurkaryotes)
Paleozoic, 550 Mya to 250 Mya: appearance of "modern" - multicellular, oxygen breathing - animals and plants
Mesozoic, 250 Mya to 65 Mya: age of dinosaurs, first appearance of mammals and birds
Cenozoic, 65 Mya to 18th century CE: age of mammals and birds, appearance of humans
Anthropocene, 18th century to ??
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