|Chapter 89||Table of Contents||Chapter 91|
Ecology 330 — Extinction, July 7, 2060
I only had time for a few weeks of classes while I was back. The students were eager to hear of my exploits in the North and what I had learned about EF1. I related several tales and then brought them back to course content.
“The subject of this lecture can be disturbing. When I first started to study the patterns of life on Earth, I several times found myself unnerved at contemplating extinction — the deaths of so many life forms.
“It is important to realize that the major pattern of life on Earth has been change. We humans get stuck in our ways and imagine that our cherished dreams will last forever. ‘Look on my works, ye mighty, and despair,’ the poet, Shelley said. But I say to you, ‘Look on these species and do not despair, for all of life is a miracle of change.’
“The extinction event I am sure you’ve all heard about is the demise of the dinosaurs. It makes a good starting point.
“What do we know for certain? We are talking about events that transpired millions of years ago. How can we know what happened that far in the past?
“The evidence we have is of two sorts. First, we have the information buried in rocks and fossils. In many regions of the world, in quarries, in mountain outcroppings, we see layers of sedimentary rock that have built up over millions of years. In these layers, we note the presence and the absence of various life forms.
“The evidence results from many hours of painstaking work by paleontologists and geologists digging out fossils, dating layers by stratigraphy and radioisotopes, testing the concentrations of elements or the isotopes of elements.
“Secondly, we have the information in the DNA we have inherited from the past. One of the unsung scientific achievements of the late twentieth century was demonstrating the unity of life. When scientists started unravelling the genomes of humans and other species, they discovered that many of our genes are shared. Humans have slightly different versions of many of the genes of other lifeforms. As recently as the nineteenth century many people, and indeed scientists, had imagined humans as apart, as somehow above the rest of the biosphere. Genetics showed us we are indissolubly linked.
“During reproduction, DNA mutates at a steady rate. By taking the DNA of different species we can calculate an approximate time those species diverged. We can also see when certain genes emerged to handle specific problems. For example, the presence of high concentrations of oxygen in the atmosphere approximately 2.5 billion years ago is marked in the genome.
“Now back to the original problem: the dinosaurs. For 100 million years, these gigantic reptilian creatures were the dominant form of life on Earth. Their fossils are found in many places. And then, something happened. What? In a relatively short time, they ceased to exist.
“What could cause the deaths of so many life forms? The list of candidate hypotheses for extinction is long, including sea level change, a disruption of oceanic chemistry, climate change, massive volcanoes, asteroid impacts and other increasingly unlikely events such as a supernova in our astronomical neighbourhood.
“The ocean calamity could be triggered by massive releases of methane, a depletion of oxygen in water called an anoxic event or by the generation of hydrogen sulfide from either biological or volcanic sources. Climate change could be triggered by many factors including changes in the Earth’s orbit around the sun, continental movement, volcanism, greenhouse gases and our old friend, the asteroid. Perhaps the most extreme climate hypothesis is the Snowball Earth, which theorizes a planet covered by snow and ice from the poles to the equator. And if you think volcanoes are an unlikely culprit, check out the size of the Deccan Traps in India and the Siberian Traps in Russia.
“These are the candidate hypotheses. There may be other possibilities as well, but how can we know what really happened?
“In the case of the dinosaurs, the first solid evidence found was the anomalous presence of the element iridium in a reddish brown stripe between the layers of the Cretaceous and the Tertiary, commonly called the K-T boundary. The ‘K’ is from the German Kreidezeit to avoid confusing a ‘C’ with the Cambrian.
The concentration of iridium in this stripe was 20 times higher than normally occuring in the layers above and below. The measurements were done by neutron activation analysis in which a mixed sample is briefly irradiated and the absorption of neutrons counted. It is very accurate. These measurments have been confirmed at many K-T sites around the world. Where did this extra iridium come from? It could have been carried by an asteroid. Later shocked quartz crystals typical of high velocity meteorite impacts were found at many of these same sites. It remained only to find the impact site, which, as I’m sure you know, turned out to be Chiczulub in the Yucutan peninsula.
“The dinosaurs disappeared a mere 65 million years ago. When it comes to looking hundreds of millions years ago, the investigation is that much more difficult.
“We need to put these queries in a proper context.”
I opened a wallscreen with my padd and put up a graphic of the solar system.
“The Sun is about 5 billion years old.
“The Earth is slightly younger, at perhaps 4.9 billion years.
“Life on Earth started about 4.5 billion years ago, give or take a few hundred million years.
“For the first 3.5 billion years or so, single celled animals ruled the roost. For the first 2.8 billion years, life was prokaryotic — no nucleus, no organelles, no complex cytoplasm — just cells walls around a loose agglomeration of DNA. Then for 700 million years, eukaryotic cells were the most complex form of life on earth. They were essentially the same all over the planet. By doing a genomic analysis of many different species of bacteria, we have discovered that as much as a quarter of their DNA had evolved 2.8 billion years ago. Remember that when you think about bacteria. They have had more than 4 billion years to figure out new tricks.
“And then something miraculous happened: the Cambrian Explosion. Multicellular life forms blossomed on the Earth. Weird and wonderful life forms with body arrangements that seem bizarre to our eyes — five eyes, dual spines, nose projections longer than the body — animals that drove paleontologists into paroxysms of descriptive extravaganza — Anomalocaris, Hallucinogenia, Marella Splendens — flourished in wild abandon. Life differentiated into myriad forms. Your key term of reference here is the Burgess Shale. Within the space of a few million years, the precursors of every form of life on Earth came into being.
“This is one of the great scientific mysteries of all time. Within these few millions of years, a biological engine of invention generated dozens of basic anatomical designs, dozens of phyla. Since that time, evolution has generated only variations on these existing themes. Why has there not been one new phylum since the Cambrian Explosion? Nobody knows, although there are a lot of theories.
“The pattern we see is massive production of many disparate life forms, followed by decimation in extinction events and then the evolving specialization of the survivors.
“It was also during this time that the patterns of macroecology first emerged. Specialization for geography began. Remember that prior to this time the planet was essentially one big petri dish covered by virtually identical single celled organisms. With differentiation, life began to complexify. Animals became larger. You had complex food webs forming. The succession of species began. And the extinction of species became a possibility.
“Look at this geological timeline.”
On the wallscreen I put up a diagram with the Eons delineated.
“The timeline is divided into Eons — Hadean, Archean, Proterozoic and Phanerozoic.
“The Eons are divided into Eras. For example the Phanerozoic is divided into the Paleozoic, the Mesozoic and the Cenozoic Eras.
“The Eras are divided into Periods. For example, the Cenozoic is divided into the Paleogene and the Neogene Periods.
“And the Periods are divided into Epochs. For example, the Neogene is divided into the Miocene, the Pliocene, the Pleistocene and the Holocene Epochs.
“Now just to keep things interesting, this schema has mutated over the last 200 hundred years. In the older literature you have to be careful just which time an author intends. There have been new discoveries, improvements in techniques of measurement and name changes. It is not unusual to hear of the Tertiary and Quaternary for example. You will also hear of Ages, SuperEons and other demarkers. It can be confusing to the uninitiated because a given time can be correctly denoted in multiple ways. For example, we are currenly living in the Phanerozoic Eon, the Cenozoic Era, the Neogene Period and the Holocene Epoch, although some say that should now be the Anthropocene Epoch.
“So why do I go over this stale outline?”
I opened up the timeline to show all the Eons, Eras, Periods and Epochs.
“Because the dividing lines between these times are cataclysmic changes, often extinctions. As you can see, there have been a lot of them. In the entire history of the Earth some 30 billion species have existed. The average age of a species works out to be about 4 million years. It is correct, although a little melodramatic to say, that 99.5% of all species that ever existed are extinct.”
I put up a diagram of the extinction rate in the Phanerozoic.
“There are five major extinction events recorded in the Phanerozoic, which you can see here as bumps in the extinction rate. I will just go over these quickly, as they are detailed in your readings.
“The first extinction event was the Ordovician-Silurian about 440 to 450 million years ago, which killed off 60% of marine invertebrates.
“The next extinction event was the Late Devonian about 360 to 375 million years ago, which killed off about 70% of all species
“Then we have the biggie. The Permian-Triassic extinction event 251 million years ago killed off about 96% of all marine species and about 70% of land species. This event has the unofficial moniker of the Great Dying.
“The fourth extinction event was the Triassic-Jurassic about 205 million years ago, which killed off about 50% of species.
“And then we have the demise of the dinosaurs in the Cretaceous-Tertiary extinction event about 70 to 65 million years ago. During this event about 75% of species went extinct.
“Finally we have what some are calling the Sixth Extinction. That is the current extinction event triggered by humanity. The International Union of Conservationists tells us that 35% of all mammals, 27% of all birds, 45% of all fishes and 57% of all reptiles are either endangered or critically endangered. Some survive only in zoos. You can see for yourself how the Sixth Extinction stacks up against the previous Five.
“These extinction events prune the tree of life and change the game for the survivors. And that is what I would like you to consider: What kind of a world will the Sixth Extinction leave for us to face?
“Okay, that is enough. I’d like 2500 words on the extinction event of your choice in two weeks time. See you next Monday.”
Excerpted from _The Bottleneck Years_ by H.E. Taylor
For further information, see
A Gentle Introduction.
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Last modified April 29, 2014