The figure below shows a simplified geologic cross-section of an impact crater in Chicxulub, Mexico. This crater is thought by most scientists to be the impact crater resulting from the asteroid collision that caused the mass extinction event at the end of the Mesozoic era about 65 million years ago. In this lesson, we will discuss prevailing hypotheses for this and other mass extinction events during Earth's history. We will also discuss the effect on evolution/diversification of life following mass extinction events.
I think the subject matter of this lesson is an important educational topic for two reasons. The first is that any discussion of the pattern of evolution, diversification, and extinction of life on Earth over geologic time must necessarily bring up the subject of deep time and the age of our planet. The age of the Earth is not at all a controversial subject among scientists, but recently in the United States, public schools have been pressured to teach "alternative explanations" that have no scientific merit. The second reason is that the subject of mass extinction events ties together the disciplines of geology and biology; it is an important part of teaching and learning science to recognize that scientific disciplines are linked, even though they are usually taught in schools as completely separate fields.
By the end of Lesson 3, you should be able to:
Lesson 3 will take three weeks to complete. 18 Sep - 8 Oct 2019.
The chart below provides an overview of the assignments for Lesson 3. There are two problem sets and two discussions. One of the problem sets is broken into two parts (each part with a different due date).
Requirement | Submitted for grading? | Due date |
---|---|---|
Reading: An introduction to recent debates | No |
24 Sep |
Problem set: Antipodes and Geologic Time, part 1 |
Yes—Submitted to the "Antipode and timescale problem set part 1" assignment in Canvas |
24 Sep (end of week 1) |
Problem set: Antipodes and Geologic Time, part 2 | Yes—Submitted to the "Antipode and timescale problem set part 2" assignment in Canvas | 1 Oct (end of week 2) |
Reading: The K/T extinction event | Yes-Graded discussion in Canvas | participation spanning 25 Sep - 1 Oct (2nd week) |
Reading / Discussion: Exploring a controversial theory—the Permian/Triassic Extinction | Yes-Graded group discussion in Canvas | 1 Oct (end of week 2) |
Problem set: Correlating impacts and extinctions | Yes—Submitted to the "Impact craters problem set" assignment in Canvas | 8 Oct (end of week 3) |
Reading: Recovering from an extinction | No | 8 Oct |
Discussion: Teaching and learning about mass extinctions | Yes—Graded discussion in Canvas | participation spanning 2 - 8 Oct (3rd week) |
If you have any questions, please post them to our Questions? discussion forum (not e-mail). I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.
The pattern of evolution, diversification, and extinction of species on Earth is a topic worthy of an entire course (but so are the other topics in Earth 501, as you have no doubt realized at this point). Here we will focus on two major extinction events in geologic history. One of them happened approximately 250 million years ago. It marks the end of the Permian period of the Proterozoic Era and marks the beginning of the Triassic period of the Mesozoic Era. This extinction event was the most catastrophic in geologic history in terms of the number of species that disappeared at this time. The other event we'll study is better known. It happened 65 million years ago, ending the Mesozoic Era and the Cretaceous period, and beginning the Tertiary period of the Cenozoic Era. This is the event that killed the dinosaurs and is often called the "K/T" extinction (K is the abbreviation used for Cretaceous; T is the abbreviation for Tertiary).
These two extinction events have some characteristics in common: extraterrestrial impacts as kill mechanisms have been proposed for both, and in both cases, scientists continue to debate other possibilities as well. In this lesson we will examine different hypotheses proposed for each of the two events, we will examine a database of known impact sites on the planet, and we will try to wrap our minds around the concept of deep geologic time.
Read the following article, available through Canvas:
This article gives a brief overview of some of the most recent debates surrounding the causes of mass extinction events that happened during the Mesozoic Era. Read this now as an introduction to some of the topics we will pursue more deeply as this lesson progresses.
When you read this article, think about the following:
This article does not answer all the questions above, and I have italicized the questions that you may or may not know the answer to depending on your background knowledge of the geologic timescale.
Please post any comments or questions to the Questions? discussion forum, especially if you want more background pertaining to the italicized questions (or other questions). If you have the answers to help out a fellow student, please post!
Geologic mapping involves a certain amount of detective work. Layers of rock preserve a record of the planet's past climate and its biological population. Early versions of the geologic timescale were mostly based on index fossils—these are fossils commonly found in a particular sequence of rock around the world. Finding a certain index fossil meant being able to correlate that rock with other rocks containing that same fossil in other places on Earth. Divisions of time were generally assigned based on the disappearance of index fossils or on changes in rock type at certain points in geologic history. Relative ages of rocks around the world have been cataloged for over a hundred years. However, it has only been since the advent of radiometric dating and paleomagnetic measurements that we have also been able to assign numerical ages to the different divisions.
Here are two versions of the geologic timescale (both are in PDF format):
We do not have perfect preservation of the entire 4.5 billion year history of the planet. Preservation increases towards the present time, as you would expect. Therefore the geologic timescale contains more detailed divisions the closer we get to the present. The first abundant organisms with shells in the fossil record don't even appear until about 542 million years ago, which is a fifth of the way back from the present time. I annoted the second of the two time scales linked above to show where the two extinction events pertinent to this lesson appear in time, and to give you a warning about how the layout isn't to scale.
In the activity below, the whole class will work together to make a representation of the geologic timescale. The way we'll do this uses the concept of antipodes. The antipode is the opposite point of any point on the surface of the Earth so that if you connected the two points with a line through the center of the Earth, that line would be an exact diameter. Mathematically, the antipode of a point whose latitude and longitude are (A, B) equals (-A, B ± 180°).
As an example, the latitude and longitude for State College, PA, is
40.8° N, 77.9° W
Its antipode is 40.8° S, 102.1° E, as shown on a map of where this is located [5].
As you can see, when my children get in the sandbox, they are not digging a hole to China. They are in fact digging a hole to the Southern Indian Ocean off the west coast of Australia.
Here's an animation of an antipode using Eliza's Cherry Skewer Mental Model (Click to download) [6]
Calculate the antipode of your home town. Here is a good link for looking up the latitude and longitude of many locations in the world: Get Latitude and Longitude. [7]
Our planet is approximately 4.55 billion years old. It is a challenge for any human being to comprehend just how long a billion years is. This is one of the reasons that many people have trouble accepting the fact that biological processes such as natural selection can work. How can we try to represent the age of the Earth in a way that people will really understand what a billion years mean?
In this activity, we'll use distance as a proxy for time. Penn State's campus at University Park, PA (in State College) will represent the present time, and PSU's antipode will represent 4.55 billion years ago when the Earth was formed. The great circle distance between PSU and its antipode then represents 4.55 billion years.
Save the Antipode and Timescale Worksheet [8] to your computer. Use that word processing document to record your work.
Record your answers to the following questions on your worksheet:
First, add your data to our class table in Canvas. My data is given as an example. Next, save your worksheet as a Microsoft Word, Macintosh Pages, or PDF file in the following format: Antipode_AccessAccountID_LastName.doc (or .pages or .pdf). For example, Cardinal starting pitcher Adam Wainwright's file would be named Antipode_apw50_wainwright.doc
Upload your Antipode Worksheet to the Antipode and timescale problem set part 1 assignment in Canvas by the end of the first week of this lesson. I will check your work so that you can make any corrections to the class table if you need to before Part 2 is due. Part 2 will be due a week later (the end of the second week of this lesson).
There is a week in between the due dates of part 1 and part 2 of this assignment so I can check part 1 and have you make corrections if necessary. Meanwhile, go on and do the reading assignments/discussions on the following pages, then come back here to finish part 2.
I want you to make a geologic timescale in which the dimension of time is measured consistently at a single scale. The two pdf files of the timescale produced by GSA and the USGS linked from page 3 of this lesson are made the way they are because the authors wanted each named portion of time to be legible, and they wanted to produce something that more or less fit on a page. This is instructive, but it can be misleading because periods of time closer to the present time are given more space. For example, see in the USGS timescale how the Holocene epoch (in which we now live) is given approximately the same amount of space as the late Jurassic. The Holocene is about 11,500 years long and the late Jurassic lasted about 15 million years. So, obviously, we are distorting time in favor of the present. Let's have a little humility and make a model to scale.
It is not as easy to appreciate the vastness of time before anything cool (like life!) began on this planet unless you can see what the timescale looks like when it is drawn to scale, so that's what we'll do next.
Choose a scale that makes sense to you and allows you to fit your timescale on one 8.5" x 11" page. This means you must come up with a scale that will fit 4.55 billion years into something less than eleven inches. Keep in mind that you are not being asked to recreate the standard-issue geologic time scale as they are rarely created with a consistent time scale. HINT: The easiest way to do this will be to draw your timescale on graph paper. You can make your own graph paper online for free and print it out at Incompetech Inc. [9]
You will need to submit your timescale electronically. It is okay to draw your timescale by hand and then scan it. Or, draw your first draft of it by hand and then make an electronic version of it. If you scan something that you have drawn by hand, please check for legibility before submitting it.
Save an electronic version of your timescale as a Microsoft Word, Macintosh Pages or PDF file in the following format:
Timescale_AccessAccountID_LastName.doc (or .pdf or .pages).
For example, Cardinal outfielder Dexter Fowler's file would be named "Timescale_wdf25_fowler.doc"
Upload the electronic version of your timescale (from Part 2) to the Antipode and timescale problem set part 2 assignment in Canvas by the end of the second week of this lesson. NOTE: The timescale (part 2) is due a week later than the antipode worksheet calculations (part 1). This is because you will need the data your classmates provide in Part 1 to complete Part 2. You are probably wondering why I've repeated this fact about four times now. Just to see if you're still reading! No, but, really, I've had somebody get confused about this every time I've taught this class, I swear.
I'll use my general rubric for grading problem sets [10] to grade this activity. I made each half of this problem set worth 50 points so together they'd add up to the usual 100 points.
The mass extinction event that occurred about 65 million years ago brought about an end to the domination of the planet by reptiles and, in so doing, opened up ecological niches within which mammals flourished several million years later (including, happily, human beings!).
History of the impact theory as the cause of the Cretaceous/Tertiary extinction 65 million years ago begins in the Italian town of Gubbio (green arrow in the map below).
The rock sequence preserved in a gorge outside of Gubbio preserves the transition between the Cretaceous and Tertiary periods. The site was visited by geologists conducting paleomagnetic surveys used to make precise dates of geologic horizons. These workers had noticed that while the lower beds of the Cretaceous contained many fossils, the Tertiary beds above the boundary were surprisingly depleted in fossils. There was a thin layer of clay at the boundary, which turned out to have an extremely high concentration of iridium. Iridium does occur in the Earth's crust but the concentration of it in this layer was so high that either the layer must have been deposited over a very long period of time in a way that concentrated the iridium, or else this iridium must have been delivered to Earth all at once from an extraterrestrial source. Precise dates from paleomagnetic data on the beds above and below the clay layer eliminated the possibility that the clay was just a thin feature representing a very long time, so an extraterrestrial source was hypothesized by a team of scientists led by Luis Alvarez and his son Walter Alvarez. We'll read their 1980 paper in Science that outlines their theory. (Alvarez, L., et al., 1980, Extraterrestrial Cause for the Cretaceous-Tertiary Extinction, Science 208, p 1095-1108.) Since this discovery, iridium-rich clay layers have been found at the K/T boundary in rocks all over the world, making the hypothesis for a planet-wide ecological catastrophe caused by an asteroid or comet impact much stronger.
Other evidence of an extraterrestrial impact from the Cretaceous/Tertiary boundary rocks includes tektites, which are glassy spherules of melt ejected from the crater, and shocked quartz, a form of high-pressure quartz only found at other known meteorite impact craters on Earth. The photo below shows tektites from the K/T boundary in Haiti.
Tektites and shocked quartz are found in K/T boundary rocks all over the Earth, but the concentrations of them in rocks around the Gulf of Mexico narrowed the search for the crater to that area. Calculations based on the amount of iridium in the boundary clay gave scientists an estimate of the size of the crater they were looking for. About 15 years ago scientists "found" the Chicxulub crater in the Yucatan peninsula and began a drilling project there to date the crater. I wrote "found" in quotation marks because this crater's existence was known in the oil industry for some time, but the idea that it could be the K/T crater was not thought of until the early 1990s. The 1992 Swisher et al. paper in Science that we will read as part of the reading assignment for this section of this lesson details the age-dating of this crater and other evidence to support Chicxulub as the site of the K/T impact.
Well, what would the fun of that be? The majority of Earth scientists do agree with the Alvarez impact hypothesis because so many lines of evidence support it. However, there are some other fairly catastrophic events that happened on the planet at about 65 Ma, making for an odd coincidence at the very least. For example, the flood basalts of the Deccan Traps in India spewed out at about 65 million years ago. Such extensive volcanism would no doubt have altered the climate seriously. Would that have been enough to cause a mass extinction event? Plenty of geologists think so. It is safe to say a significant number of geologists who work in this field think that there was a sequence of events that led to the end-Cretaceous extinction event, with the asteroid impact being one of them.
My suggestion is to read through the press releases and summaries first because they are intended for a more general audience. Then read/skim the scientific papers, which have been written for experts. Note the different writing styles and differing amounts of technical jargon in the different papers! I have posted some discussion questions you might want to look at first help guide your thinking, so you are ready to discuss.
We will discuss these papers together via a discussion in Canvas. This discussion will take place over the second week of this lesson.
Grading rubric: Please see the rubric for discussions. [12]
Now we're going to begin thinking about a different, even more, catastrophic mass extinction event.
The Permian-Triassic extinction event marked the end of the Paleozoic era and the beginning of the Mesozoic era, which, in turn, was ended by the K/T mass extinction we just finished reading about.
Approximately 250 million years ago, the biggest extinction event in the history of the Earth (in terms of the number of species that disappeared) took place at the end of the Permian period. This event marks the end of the Paleozoic era and the beginning of the Mesozoic era. The rise of reptiles, such as the dinosaurs, is most probably a direct outcome of these species flourishing in the ecological niches left by the end-Permian extinction event. Several theories have been proposed to explain the "Great Dying," but many of these lack global evidence to prove or disprove them, or they do not provide a kill mechanism that is quick enough or extensive enough. One of the difficulties in pinning down a kill mechanism is the dearth of well-preserved outcrops that record this time period in geologic history.
As you read the papers in our next activity that deal with the Permian/Triassic extinction, I want you to keep in mind the major difference between what the scientific community thinks about this extinction, as opposed to the Cretaceous/Tertiary extinction that we read about previously. In the case of the Cretaceous/Tertiary extinction, the impact hypothesis is widely believed and supported and has been for at least a decade. I can verify this because when I teach large-enrollment residential courses for non-science majors here at Penn State, the great majority of the class knows that "a meteor killed the dinosaurs." Usually, none of these same students have even heard of the Permian/Triassic extinction event, despite the fact that whatever happened on Earth at the end of the Permian was quite a bit more catastrophic. At the end of this section of the lesson, we will have a summary discussion so we can compare the story of two different extinction events and discuss why the explanation for one of them is apparently much less controversial than the other.
To see some 250-million-year old rocks, check out this video clip from NOVA [13] in which Neil deGrasse Tyson interviews several geologists who are hunting for answers about the end-Permian extinction out in the field and through computer models.
Lee Kump, Dean of PSU's College of Earth and Mineral Sciences (we filmed this back when he was merely a professor in Penn State's Department of Geosciences), has been studying ancient climates based on the evidence left behind in marine sediments for most of his career. Watch this video below to see him explain some of his hypotheses about the end-Permian mass extinction and to hear more about his research.
In addition, here is a press release about Lee's hydrogen sulfide hypothesis:
Here's the citation for the scientific paper:
In this next activity, we'll concentrate on the relatively new and controversial theory that an asteroid impact caused the Permian/Triassic extinction event. By doing this, I admit that I'm deliberately not spending time on the other theories that have more adherents, such as the theories that Lee talked about in the video on the previous page. First, I want us to talk about impacts, in general, this week. Second, this gives us a way to compare what the state-of-the-art thinking is with respect to extraterrestrial impacts for the Permian/Triassic extinction while the evidence for an impact at the K/T boundary is still fresh in our minds.
Fall 2019: We will not discuss the papers below as part of a graded discussion. Please do skim them because some of the knowledge you get from them will help inform you when you work on the impact craters problem set. I alternate between discussing the K/T and the P/Tr papers each time I teach this class. So the assignment below is just for reading, this time, not discussing.
Then, engage in a class discussion of these hypotheses, which will take place in the "Lesson 3 - Permian/Triassic Extinction" class discussion forum. (See "Submitting your work," below.)
Special note to Group-Work-Haters: I am dividing you up in teams because I have assigned more reading than I think each of you can be expected to process on your own. So, each of you only has to read half of it. Therefore, the job of the group discussion is to clarify your thinking about the papers and also to provide an overview to the other team, through your discussion, of the content of the papers you read.
QUESTIONS:
QUESTIONS:
QUESTIONS:
You will need to participate multiple times during the discussions.
You will be graded on the quality of your participation, both in your team discussion and in the discussion involving the whole class. See the grading rubric [12] for specifics on how this assignment will be graded.
The articles below are press releases that accompanied scientific papers that detail other theories for the end-Permian extinction event. Check them out:
In this activity, you will explore the relationship between known impact events and sudden extinctions on Earth. Want to see a hand sample from an impact crater? Here is a piece of rock from the Vredefort structure in South Africa. The Vredefort is so big that you can't even see across it. The extreme heat of impact created glassy flow bands in the rocks of the crater ring. That's the black part in the photo. It's a little easier to see in the outcrop scale, but it's still cool!
The world map shown in this screencast comes from the impact crater database [21] maintained by the University of New Brunswick's Planetary and Space Science Centre. It shows the locations of known impact crater sites (noted as white dots on the map). In this screencast, I explain the map in more detail [22]. You can also read a transcript of my discussion of the map [23].
As discussed in the screencast, it is instructive to see where craters are and are not found on Earth. For example, craters are conspicuously absent from the oceans, Antarctica, Greenland, the Amazon river basin, central Africa, and Indonesia. Is this real (i.e., is there a scientific reason why asteroids or comets would fall to Earth in certain locations?) or is this an artifact? The reason why craters are not found uniformly over the globe is a result of preferential preservation in some places and also some places on the planet are not as easy to get to, thus potential impact sites have not been explored there.
In the case of the oceans, plate tectonic activity means that the oldest ocean crust dates from the Jurassic, so no impacts older than that could be found there. In addition, oceanic crust is quite thin and not conducive to preserving evidence of an impact.
Frame 1 - Man: I used to think correlation implied causation.
Frame 2 - Man: Then I took a statistics class. Now I don't.
Frame 3 - Woman: Sounds like the class helped. Man: Well, maybe.
For this problem set, create your own document instead of downloading a worksheet.
Your completed problem set should include your timescale, your scatter plot of crater diameter vs. age and answers to the follow-up questions. Save an electronic version of your work in the following format:
Impactcraters_AccessAccountID_LastName.doc (or .yourFileExtension).
For example, Cardinal relief pitcher Tyler Lyons' file would be named "Impactcraters_twl70_lyons.doc"
Upload your document to the Impact craters problem set assignment in Canvas by the end of week 3 of this lesson.
I'll use my to grade this assignment. [10]
How did the mass extinction events of the past change the trajectory of evolution on Earth? How much do human beings impact the future of biodiversity on the planet? In the following reading assignment, we'll explore these topics.
Read the following articles:
As you read these articles, think about these questions:
Feel like sharing your thoughts? Use the Questions? discussion forum to do so!
Let's take some time to reflect on what we've covered in this lesson!
For this activity, I want you to reflect on what we've covered in this lesson and to consider how you might adapt these materials to your own classroom. Since this is a discussion activity, you will need to enter the discussion forum more than once in order to read and respond to others' postings. This discussion is scheduled to run during the last week of this lesson.
You will be graded on the quality of your participation. Please see the rubric for teaching/learning discussions. [12]
Keeping an Eye on Space Rocks [33]: this is an interactive flash movie made by Caltech's JPL about monitoring asteroids and comets close to Earth.
Map Tunneling Tool [34]: this is an interactive web site that helps you locate the antipode of any point on Earth.
Check out these readings if you are interested in other extinction events and/or other impact events. Note: I didn't put any of these readings in our course reserves, but as a PSU student you have access to the PSU libraries, which contain all of these articles. To find out how to get access to them, see Online Students Use of the Library [35].
Have another reading or Web site on these topics that you have found useful? Share it in the Questions? discussion forum!
In this lesson, we briefly covered the construction of the geologic timescale and discussed two of the five great mass extinction events that happened since life began on the planet. These topics are controversial in two important ways. First, scientists still argue about the exact cause of mass extinction events on Earth. Impacts have been invoked for both of the two we discussed but it is interesting how much contention there is, especially for the Permian/Triassic extinction. Second, anytime biology and geology meet, the subject of the age of the Earth and the processes of evolution and natural selection naturally come up. These are not scientifically controversial topics but are often represented that way by nonscientists.
You have reached the end of Lesson 3! Double-check the list of requirements on the Lesson 3 Overview page to make sure you have completed all of the activities listed there.
Links
[1] http://http://www.webcitation.org/5VdFMykv2
[2] https://www.lpl.arizona.edu/sic
[3] https://www.e-education.psu.edu/earth501/sites/www.e-education.psu.edu.earth501/files/file/lesson 4/l4p3_fs2007-3015.pdf
[4] https://www.e-education.psu.edu/earth501/sites/www.e-education.psu.edu.earth501/files/file/lesson 4/l4p3_timescl.pdf
[5] http://maps.google.com/maps?f=q&hl=en&geocode=&q=40.8+S,+102.1+E&ie=UTF8&t=h&ll=-25.482951,110.742188&spn=53.886903,74.707031&z=3&source=embed
[6] https://www.e-education.psu.edu/earth501/sites/www.e-education.psu.edu.earth501/files/flash/MassExtinction%20lesson/cherrySkewerCC.mov
[7] http://www.latlong.net/
[8] https://www.e-education.psu.edu/earth501/sites/www.e-education.psu.edu.earth501/files/file/lesson%204/Antipode_worksheet.doc
[9] http://incompetech.com/graphpaper/
[10] https://www.e-education.psu.edu/earth501/node/1807
[11] http://www.lpl.arizona.edu/SIC/
[12] https://www.e-education.psu.edu/earth501/sites/www.e-education.psu.edu.earth501/files/file/rubrics/Online_Discussion_Forum_grading_rubric.pdf
[13] https://www.pbs.org/wgbh/nova/transcripts/3318_sciencen.html
[14] https://www.youtube.com/user/duttoninstitute
[15] http://www.sciencedaily.com/releases/2003/11/031104063957.htm
[16] http://www.sciencedaily.com/releases/2005/01/050121101514.htm
[17] http://www.sciencedaily.com/releases/2005/02/050223130549.htm
[18] http://www.sciencedaily.com/releases/2007/10/071025091047.htm
[19] http://www.sciencedaily.com/releases/2006/10/061021115722.htm
[20] http://www.sciencedaily.com/releases/2007/08/070831171556.htm
[21] http://www.passc.net/EarthImpactDatabase/New%20website_05-2018/Index.html
[22] https://www.e-education.psu.edu/earth501/sites/www.e-education.psu.edu.earth501/files/flash/MassExtinction%20lesson/impactmap.swf
[23] https://www.e-education.psu.edu/earth501/sites/www.e-education.psu.edu.earth501/files/file/massExtinctions/impactMapTranscript.txt
[24] http://imgs.xkcd.com/comics/correlation.png
[25] http://passc.net/EarthImpactDatabase/New%20website_05-2018/Index.html
[26] https://www.e-education.psu.edu/earth501/sites/www.e-education.psu.edu.earth501/files/file/massExtinctions/impactCraterDiameterAge.csv
[27] https://www.e-education.psu.edu/earth501/sites/www.e-education.psu.edu.earth501/files/file/massExtinctions/impactCraterDiameterAge.xlsx
[28] https://www.e-education.psu.edu/earth501/sites/www.e-education.psu.edu.earth501/files/file/massExtinctions/impactCraterDiameterAge.pdf
[29] https://www.e-education.psu.edu/earth501/sites/www.e-education.psu.edu.earth501/files/image/lesson4/kirchnerfig1_nature2000.jpg
[30] http://www.sciencedaily.com/releases/2000/03/000329080536.htm
[31] http://www.nature.com.ezaccess.libraries.psu.edu/nature/journal/v404/n6774/full/404129a0.html
[32] http://www.nature.com.ezaccess.libraries.psu.edu/nature/journal/v404/n6774/full/404177a0.html
[33] http://www.jpl.nasa.gov/multimedia/neo/index.cfm
[34] http://www.freemaptools.com/tunnel-to-other-side-of-the-earth.htm
[35] http://guides.libraries.psu.edu/onlinestudentlibraryguide
[36] http://www.sciencedaily.com/releases/2001/08/010828075843.htm