WHEN did “it” happen? Weʼre going to talk, in many instances, about “dates” - e.g., the amount of time before the current day when specific types of organisms were present on Earth Weʼre not going to look in detail at the methods, but it is essential that we understand something about how these dates are estimated Two components: “Stratigraphy” and various types of “radiometric dating”

The History of Life on Earth

Stratigraphy

Radiometric Dating

The idea is that layers of rock closer to the surface are younger, while those further below the surface are older This approach gives “relative” dates, i.e., species X was present on Earth before species Y

Fig 25.5 and handout These dates are measurements, based upon well-established properties of natural radioactive materials - they are not “guesses” This approach provides “absolute” dates, i.e., how many years ago was species X present on Earth

(continued)

What does “theory” mean?

The theory used to establish the ages of rocks by radiometric dating is the very same theory used in construction of nuclear power plants and nuclear bombs I.e., this is a theory that is wellsupported by data (evidence) and that “works” (makes accurate predictions)

Before we discuss this, Iʼd like your thoughts… Write a paragraph on what the word “theory” means to you Put your name - first and last (LEDGIBLY!) on the paper and hand in Worth 2 points!

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Science With Practice A College of Agriculture and Life Sciences experiential learning and work program for undergraduate students in the College of Agriculture.

Students work with faculty and staff on specific projects or work assignments, earn money and earn academic credit while fulfilling work responsibilities!

NOW TAKING APPLICATIONS FOR FALL 2008 SEMESTER!

What do we, in science, mean by the word “theory”? Contacts: [email protected] Mr. Mike Retallick Program Coordinator [email protected] Ms. Amy Burmeister [email protected] Justine Hosch [email protected]

There’s a sign on the wall But she wants to be sure ’cause you know sometimes words have two meanings. In a tree by the brook There’s a songbird who sings, Sometimes all of our thoughts are misgiven. Ooh, it makes me wonder, Ooh, it really makes me wonder.

Merriam-Webster Definitions of “Theory”

1 : the analysis of a set of facts in their relation to one another 5 : a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena 6b : an unproved assumption

- Led Zeppelin, Stairway to Heaven

In the realm of science…

ANYBODY can have an “idea” and call it a “theory”using the “street definition” (~ definition #6b in the MerriamWebster) But for an idea to be a scientific theory, it has to be supported by EVIDENCE - LOTS of evidence You can read more about theories in science on page 23 of your text.

“Theory” means an explanation for a natural phenomenon that is wellsupported by data and incorporates relevant laws The word “theory” can have other meanings in other contexts

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Facts, Hypotheses, Laws and Theories

A fact is something that can be observed to occur (the observations do not have to be with the unassisted human eye) An hypothesis is really a prediction about what will happen in a certain set of circumstances Hypotheses can be based on considerable previous knowledge or relatively little previous knowledge (potentially, merely a “guess” that can be tested)

Newtonʼs Universal Law of Gravity:

where:

A law is a “universal relationship” describing how the natural world behaves under specific conditions Laws can often be expressed mathematically

- F is the magnitude of the gravitational force between the two point masses - G is the gravitational constant - m1 is the mass of the first point mass - m2 is the mass of the second point mass - r is the distance between the two point masses

A theory is an explanation that takes into account, and is consistent with, all known facts and all applicable natural laws E.g., the “theory of gravity” is our best scientific explanation for how and why objects fall to the ground when dropped Physicists continue to work on improving the theory of gravity - I.e., providing an ever more accurate and detailed explanation

Newtonʼs universal law of gravity does NOT fully explain the phenomenon of gravitational attraction E.g., How is the gravitational force transmitted? What the law does is define the magnitude of the force of gravitational attraction between any two “point masses”

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Theories NEVER become “facts” - they are based on the available facts Theories NEVER become “laws” - they include the applicable laws A well-accepted scientific theory will be supported by many facts, be in agreement with all known natural laws, and make accurate predictions that can be tested by observation and/or experiment

So, the next time youʼre wondering whether a well-accepted scientific theory is likely to be correct, remember that gravity is “only a theory”…

WebCT Question

(Radiometric Dating - continued)

While I understand what stratigraphy is. I don't understand how it is accurate. Snow falls, so the top layer is the last to have fallen, but rocks are just there, aren't they? How do the rocks on top become on top? In other words, how did "newer" rocks come to be on top of the "older" rocks?

By using various radioactive isotopes, times from 100 years to 4.6 billion years before the present can be measured Radiometric dating is typically carried out in igneous (e.g., granite or basalt) or metamorphic (e.g.,quartzite or gneiss) rocks

Carbon Dating

Radiometric dating is actually measuring the time since the rock cooled sufficiently to prevent loss of the “daughter isotope” e.g., K-40 ---> Ar-40 (daughter isotope) Ar-40 is a gas The “half-life” of K-40 is 1.3 billion years So - a rock with 25% K-40 and 75% Ar-40 is years old

A specific type of radiometric dating When employing radiocarbon dating (only usable to ~ 75,000 years before the present) the fossil (or artifact) ITSELF is dated - not the surrounding rocks

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The margin of error in radiometric dating is typically less than 10% - so a rock dated to 10 million years is between 9 and 11 million years old Radiometric dates are “facts” These dates tell us nothing about HOW life began, or about WHAT mechanism has led to the great diversity of life BUT they do set a time frame for the history of life of Earth and they tell us that the types of organisms present on Earth have changed (dramatically) through time

Putting stratigraphy and radiometric dating together Basalt - dated to ~65 MYA Fossil-bearing sedimentary rocks Quartzite - dated to ~ 70 MYA

How old are the fossils?

History of Life on Earth (Table 26.1)

Stromatolites Fig. 25.4

~ 4,600 million years ago (MYA) formation of the Earth ~ 4,000-3,500 MYA - origin of life ~ 3,500 MYA - oldest prokaryotic fossils Oldest prokaroytic fossils found in formations called stromatolites

History of Earthʼs Atmospheric Oxygen

~ 2,700 MYA - atmospheric oxygen concentration begins to increase:

How can we know ANYTHING about the atmosphere 2.7 billion years ago??

The types of rocks formed at various times (dates from radiometric dating) provide clues regarding the conditions under which the rocks were produced

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Banded Iron Formations

History of Life (cont.)

~ 2,100 MYA - oldest eukaryote fossils Reminder: Two basic types of cells found in all living organisms on Earth - Prokaryotic cells (no nucleus) - Eukaryotic cells (possess a nucleus)

~2 billion-year-old rocks with bands of iron oxide (“rust”) Fig. 25.8

History of Life (cont.)

Using your “clicker”

Push the “go” button Push 4 Push 1 Push “go” Indicate your response by pushing the appropriate number

~ 600 MYA - oldest animal fossils ~ 500 MYA - oldest plant fossils ~ 65 MYA - extinction of the dinosaurs ~ 0.2 MYA - origin of modern humans ~ 0.00002 MYA - your birth

Before today my understanding of what the word “theory” means in science was

1. Quite accurate 2. Somewhat accurate 3. Somewhat inaccurate 4. Quite inaccurate

Some Common Assumptions

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Species that directly impact humans are more important than species that donʼt

The world exists to serve human interests

1. Strongly Agree 2. Agree 3. Disagree 4. Strongly Disagree

1. Strongly Agree 2. Agree 3. Disagree 4. Strongly Disagree

Scientific knowledge is a fixed and unchanging system

These assumptions are typically NOT held by biologists (or by scientists in general) YOU donʼt (necessarily) have to change YOUR assumptions.... But when trying to learn biology, itʼs quite important to understand how biologists think

1. Strongly Agree 2. Agree 3. Disagree 4. Strongly Disagree

Iʼm most interested in…

1. 2. 3. 4. 5.

A little bit about you…

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Animals Fungi Protists Prokaryotes Plants

After graduating Iʼd like to pursue…

Learning Community Participation 1. BEST (Biology Majors) 2. LAS Connections (LAS 125X) 3. Another learning community 4. Not a member of a learning community

1. 2. 3. 4. 5. 6. 7. 8. 9.

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Grad school in ecology Grad school in biodiversity and evolution Grad school in cell and molecular biology Medical school Another human health profession Veterinary school Teaching high school biology Working as a technician in a biological/medical lab Working for a business or government agency focused on environmental issues/resources