Download Chapter 8 - Cynthia Clarke

Chapter 8
The Fossil Record
Overview
• Not all information on human evolution comes from what can be dug out of the ground. Studies of the genetics of
living humans and living nonhuman primates can provide clues. Studies of the behaviors of living humans and living
nonhuman primates also gives us clues.
• Our past can be revealed from clues accumulated over time. The fossil record provides us with bits and pieces of a
small proportion of living organisms that have lied and died in the past.
• Paleoanthropology is a specialty of paleontology. All of these avenues of research and others make up the field of
paleoanthropology.
•
•
The name paleo- (ancient) and –logos (study of) means ‘study of ancient humanity’
A multidisciplinary pursuit seeking to reconstruct every possible bit of information concerning the dating,
structure, behavior, and ecology of our hominin ancestors.
Dating the Past 1
•
•
Perhaps one of the most common questions asked of archaeologists is, ‘How old is it?”
The answer is that they have a wide variety of dating methods that fall into two major types: relative dating methods
and chronometric dating methods
•
•
Relative dating methods establish the order from oldest to youngest, but not the time that separates them from
each other
Chronometric dating methods also provides relative order, but also the date (this is way these methods are often
called absolute dating methods.
Dating the Past 2
• Relative dating methods
• Relative dating methods determine the sequence in time by showing which fossils are older but not their exact
date.
• Stratigraphy
•
•
•
Stratigraphy makes use of the geological process of superposition, the cumulative buildup over time of the
earth’s surface.
In a stratigraphic sequence, the lower layers were deposited before the upper layers.
•
•
A layer is called a stratum is a single layer of soil or rock; sometimes called a level (plural: strata)
At times stratigraphy is difficult to use. In particular, the geologic record contains evidence of many
disturbances that the record is often jumbled
Other relative dating methods
•
Biostratigraphy (also called faunal correlation) involves comparison of animal remains found at different sites
to determine similarity in time levels.
•
•
It uses index fossils to date a locality, based on regular changes seen in evolving groups of animals as
well in the presence or absence of particular species.
Using data from another site as to age of the species, one can make estimates in the new site
Dating the Past 3
•
Relative dating methods (continued)
• Fluorine dating measures the accumulation of fluorine levels in bone
• This type of relative dating depends on the presence of living tissues such as teeth, antler, or bone in which
protein and fat were present at death.
• When an organism dies, it accumulates more fluorine and less nitrogen
• The rate varies from locale to locale so there can be no cross-site comparisons.
• If you are curious visit this site for a demonstration of how this process works
•
•
Paleomagnetic reversals
• Paleomagnetic reversals is a method using sedimentary rock to indicate shifts in the earth’s magnetic field
over time.
• The shift is between ‘north’ and ‘south’
• Last documented reversal was about 780,000 years ago (called the Brunhes-Matayama Reversal)
Chronometric dating methods
• Uses constant physical and chemical processes to determine exact dates, subject to statistical variation.
• Many are based on the average rate of radioactive decay of radioactive materials
• Carbon-14 dating
• What materials are dated?
• Bone, with organic, is a poor source for this dating technique
• Charcoal is the best.
Dating the Past 4
• Chronometric dating methods (continued)
• Carbon-14 dating
• The process
•
Ordinary carbon (C-12) is absorbed during the life of an organism, as well as the radioactive form (C14). The ratio is constant during the life of the organism
•
•
•
After death, no more C-14 is absorbed and new ratios are created as the C-14 decays
The cycle of decay is from C-14  N-14 (nitrogen)
The rate at which a radioactive isotope decays is called it’s half-life. Carbon-14 dating is only useful for
sites as old as 50,000 years as its half-life is 5,730 years.
• Radioactive isotopes are unstable and over time will decay to form an isotopic variation of another
element
•
•
•
The time period in which one-half of the amount of a radioactive isotope is converted
chemically (into a daughter product).
For example, if the half life of a radioactive material is 100 years:
•
•
•
1 half-life ((100 years) 50% of the original material remains (½)
2 half-lives (200 years) 25% of the original material remains (½ of ½)
3 half-lives (300 years) 12.5% of the original material remains(½ of ½ of ½)
• Recalibration of the C-14 values has been performed to correct for fluctuations in C-14 values over
time.
• Confused? This site is great: Radioactive Dating Game
One more point, some researchers use B.P. (before present) as a way to denote the date. It is a hassle in that the
present is 1950 and one is always doing math to figure out the exact date.
Dating the Past 5
• Chronometric dating methods (continued)
• Argon dating
• Two related chronometric dating methods make use of radioactive decay of isotopes into argon gas using
samples of volcanic rocks
• One method is potassium-argon dating
• Abbreviated as 40K/40Ar and has a half-life of 1.25 billion years
• It is best used on samples older than 100,000 years.
• The sample needs to have been one without any argon at the start and this means it is best for
use with volcanic rocks
Relethford Chapter 8 Page 2
•
•
We date fossils by dating the layer above and below
• Argon-argon dating is abbreviated as 39Ar/40Ar
• This technique is much more commonly used today as it is considered more accurate.
• The sample is irradiated to convert 39K isotope of potassium to 39Ar
• This is then a ratio of the two argon forms
• Less sample is needed
Other chronometric dating methods: dendrochronology, fission-track dating, thermoluminescence, and electron
spin resonance
• Dendrochronology
• This is tree ring dating
•
The differences in annual rainfall mean that the growth patterns of trees reflect these differences.
•
Bristlecone pines are ideal for dendrochronology as they are long-lived
• One problem is that the date of the tree ring may not match the date it was used (or reused).
• Other areas lack trees or long-lived trees.
Dating the Past 6
• Chronometric dating methods (continued)
• Fission-track dating
• Uranium decays into lead inside of volcanic glass (obsidian) and other igneous rocks
• During the decay of 238U to 206Pb-206
• The alpha particles ‘burn’ tracks in the crystal structure of the rock.
• The greater the number of tracks the older the material because they are proportionate to the remaining
uranium.
• Not often used for artifact aging as it does not tell when it was made only when the rock was formed.
• Thermoluminescence
• Heated objects accumulate trapped electrons over time.
• For some objects, if reheated, these electrons can be released and counted
• Materials most likely to exhibit this trait are pottery, bronze and burned flints.
• Can date back as far as 1 million years.
• Electron spin resonance (ESR)
• Measures radioactive atoms trapped in calcite crystals, such as bones and shells
• Best within 300,000 years, but can be used up to 1 million years
Reconstructing the Past 1
• Interpreting fossils
• Identifying species
• Using the biological species concept to determine species is problematic for fossil specimens
• Identifying species mostly relies on an examination of the morphology or structure of skeletal remains to
infer evolutionary relationships with other specimens.
• Often use the term paleospecies to refer to fossil species
• The term paleospecies indicates that the identification of fossil remains is based on physical
similarities and differences
• Which traits are most indicative can be the basis of debate
• Variation of species
• Must keep in mind the variation found within a paleospecies.
• Variation within species should provide lists of traits common to all members of that species, but different
from other species.
• Sources of variation
Relethford Chapter 8 Page 3
•
•
Sex is one source of variation (sexual dimorphism)
Age of the specimen is another issue
• Teeth are an excellent source of information on age
• Presence of epiphyses (fused cartilage) indicates greater age as another example
Reconstructing the Past 2
• Interpreting behaviors
• Paleoecology
• The study of ancient environments.
• One example is palynology, the study of fossil pollen.
• Diet can be studied using stable isotope analysis of fossil remains
• Stable isotopes are non-radioactive isotopes and so remain stable over time
• An example is 13C to 12C ratio in animal bones as this ratio depends on what plants were eaten.
• Paleoclimate studies are particularly important to human evolution in the last 2 million years
• Past temperatures can be measured with 16O and 18O found in skeletal materials.
• 16O evaporates more quickly; the ratio helps plot global temperature over time.
• Taphonomy
• The study of what happens to plants and animals after they die.
• Need to take into account:
• The effects of geological processes such as water movements, wind erosion and other forms of
weathering
• The effects of scavengers and predators on the scattering of bones.
• The original pattern of disposal: were they buried or left on the surface and discarded.
• The nature of the material that was discarded.
Cycle of Taphonomy
The Making of a Fossil
• Fossilization
• Fossilization of bones is a continuous process that occurs as the organic portion of the bone (about 1/3 of the
weight of living bone) is replaced by minerals from the soil.
• Fossilization also involves a slower replacement of the bone’s inorganic minerals--primarily calcium phosphate-with minerals from the soil.
• Effects of the environment
• Great environments for making fossils:
•
•
•
•
•
Hot, dry environment (dehydrates)
Cold and dry (dehydrates)
No oxygen (no or low bacterial growth)
Peat bogs (chemical preservation)
Poor environments for making fossils: Acidic soils, gravelly soils, too wet
Relethford Chapter 8 Page 4
•
•
Worst possible is hot and wet (tropics)
Think about where primates tend to live!!
Reconstructing the Past 3
• Interpreting behaviors
• Experimental archaeology
• Your book only addresses tool making but this is actually a number of types of experiments performed to test
hypotheses in a hand-on style
• Involves learning how to make and use tools in the present in order to shed some light on tool making and
tool use in the ancient past.
• One classic example was the sailing of Kon Tiki by Thor Heyrdahl in the 1950s from South America in
the Pacific Ocean
• Nonhuman primate models for behavior
• Provides insights into behavior during human evolution using comparative data from studies on nonhuman
primates.
• There have been many different approaches
• One early approach looked at society and behavior such as dominance hierarchies as models for hominin
behavior by looking at baboon troops.
• We now know that the earliest hominins did not develop on the savannah, so that some researchers argue
that looking closer to home in the apes is more useful.
• The discussion of chimps and bonobos is a recent example
• Wrangham suggests comparing hominins with the 3 African apes
Box 8.1: Deep Time/Recent Human Evolution
• Geological time is immense
• One characteristic that separates geology from other scientific disciplines is the attention paid to time
• Geological time is much longer than the measures of time that we encounter in our daily lives.
• This makes it harder to imagine the great depth of geological time.
• Perspective on geologic time
• The universe is about 14 billion years old and the earth about 4.6 billion years old
• When we compare this to human evolution (200,000 years ago) the immensity of geologic time is apparent.
• Carl Sagan created an analogy to help us grasp geological time: The Cosmic Calendar.
• Using a single year as a collapsed version of geologic time, he was able to bring it into terms most can interpret
• See the calendar on the next page
• An alternative version is the length of a football field (100 yards)
• About 25 yards for the first single-celled organisms; 87 yards for the first single-celled organisms.
• 89 yards for the first vertebrates (500 mya)
• All hominin activity began after 99 yards, 2 feet, 7 inches, the first tools at 10 inches.
Life Before the Primates 1
•
All events we address occur in a relatively short segment of geological time.
•
Primates and humans did not appear out of nowhere, but instead evolved from earlier animals.
• The first primates appear about 50+ million years ago.
• The first hominins only 6 million years ago.
The origin of life
• The history of the earth is divided into 4 eons which are further broken down into eras, and then into periods.
• The basis of the geological time scale is that these units of time are denoted based on major geological and
biological events.
•
Relethford Chapter 8 Page 5
•
The first three eons encompass 90% of the existence of the earth in a time called the Precambrian: (4.6 bya to
5.42 mya) which encompasses the first 3 eons:
• Hadean eon (4.6-3.8 billion years ago) covers the time from origin of the earth prior to any fossil evidence
for life.
• Archean eon (3.8- 2.5 billion years ago) includes fossil evidence for the first forms of life.
• Proterozoic eon (2.5 bya-542 million years ago) is the transitional period to an oxygen atmosphere and
the first multicelled organisms.
Life Before the Primates 2
• Vertebrate Evolution
• The Phanerozoic eon covers the last 542 million years of the earth’s history The Phanerozoic eon is divided into 3
eras:
• The Paleozoic era (542 to 251 mya) begin with the Cambrian period which was a period of rapid
diversification of species and continued evolution of more complex life occurred (called the Cambrian
explosion).
• Origin of first vertebrates; jawless fishes
• Evolution and diversification of fishes, amphibians, and reptiles.
• Tiktaalik is the first indication of early fish evolving into tetrapods (vertebrates with limbs)
• First complete adaptation to land was seen in the first primitive reptiles
• Mammals and birds eventually evolved from primitive reptiles and into two major evolutionary lines
• One line became the dinosaurs and birds
• The other lead to therapsids (‘mammal-like reptiles)
• The Mesozoic era of this eon (to 251 million to 65 Ma) is called the “Age of Dinosaurs.”
• Many were bipedal.
• Before the therapsids died out, some evolved to become the “true mammals.”
• The Cenozoic era of this eon comprises the last 65 million years of the earth’s history.
• It is called the “Age of Mammals.”
• It is during this time that all modern groups of mammals evolved, including the primates.
• The first appeared 50 million years ago.
• Early primitive apes appear by 20 million years ago, and the first hominins by 6 million years ago.
Relethford Chapter 8 Page 6