How is the past dated
There are two primary techniques used to determine the age of fossils or related materials:
relative and absolute.
Relative dates are based on the arrangement of artifacts or events in a sequence. Dates
produced with this measurement are relative to one another but lack ties to calendrically
measured time. As a result, relative dates are expressed as a range of time (date and +/years). One example would be the arrangement of artifacts or fossils in a typological sequence
or seriation based on stratigraphic analysis. Age is inferred based on the evidence surrounding
the deposit/artifact/fossil. Here is an example of stratigraphic layers at an archaeological site in
Canterbury. The same method applies to paleoanthropological efforts. Unless intrusion has
occurred, generally, the deeper levels will be the oldest because they were deposited first.
By contrast, absolute (chronometric) dating methods rely on measurements derived from
processing chemical, organic, or radiometric material. These are accurate and precise but do
not always reflect when human interaction occurred. Some of these techniques can date rock
material but that does not differentiate when a rock might have been shaped into a tool. Carbon
dating is a well-known absolute method of dating based on the decay rate of the half-life of
carbon isotopes. It is important to remember that each technique has strengths and
weaknesses. When dating paleontological material, the greater the number of techniques and
dates that can be determined about a site or find, the stronger the evidence to support
conclusions based on those discoveries. Here is a hypothetical C14 dating sample.
The textbook lists many of the most common dating techniques and methods but there are tons
of others, some quite unusual, you are welcome to explore.
For example, let's consider that not all hominid characteristics evolved at the same time. The act
of becoming human was a process of adding different components such as bipedalism, the
manipulation of objects into tools, the physiological expansion of the brain, etc. These events
occurred at different times in the history of hominin development...so… trying to determine the
actual moment when our ancestors became recognizable as humans is complex. This is
a highly contentious argument - you'll find different answers dependent upon identifying criteria.
Hominid Origins
Definition of Hominid
Large brain size
Australopithecus
afarensis
Becoming Human
Learning Center,
Calculating Cousins
Definition of Hominid
Tool making behavior
Becoming Human
Culture,
No. 3 – Making Tools
Definition of Hominid
Bipedal locomotion
All the major structural
changes required for
bipedalism are seen in
early hominids from East
and South Africa.
Laetoli footprint
Walking Tall
Bipedalism
Patterns of Evolution
Mosaic evolution
Evolutionary pattern in which
physiological and behavioral
systems evolve at different
rates.
Biocultural evolution
Biology makes culture
possible and developing
culture further influences
biological evolution.
Evolving Ideas
How do we
know evolution
happens?
Paleoanthropology
Paleoanthropologists reconstruct
the anatomy, behavior, and
ecology of our ancestors:
Geologists work with
anthropologists to locate
potential early hominid sites.
Archeologists excavate the site
and search for hominid traces.
Becoming Human
Prologue
Australopithecus
africanus
Hominids appear
in the fossil
record following
the long
development of
primate adaptive
radiation.
Dating Methods
Paleoanthropologists use two types of dating
methods to tell us the age of sites and fossils:
Relative dating determines only whether an
object is older or younger than other objects.
Chronometric (absolute) dating provides an
estimate of age in years based on radioactive
decay.
Relative Dating Techniques
Stratigraphy - based on the
law of superposition, that a
lower stratum (layer) is older
than a higher stratum.
Fluorine analysis applies to
buried bones and
groundwater seepage bones incorporate fluorine
during fossilization.
Relative Dating Techniques
Biostratigraphy related to changes in
the dentition of
animals.
Paleomagnetism based on the shifting
of the geomagnetic
pole.
Chronometric Dating
Techniques
The age of an object can be determined by
measuring the rate of disintegration:
Potassium/argon (k/Ar) dating involves the
decay of potassium into argon gas. K/Ar has a
half-life of 1.25 billion years.
Carbon-14 is a radiometric method commonly
used by archeologists. Carbon 14 has a halflife of 5730 years.
Becoming Human
Evidence
Paranthropus
boisei
Primate Speciation
The first evidence of
primate speciation
occurs 65mya.
The first evidence for
the appearance of
early hominids is
much more recent.
Becoming Human
Anatomy
Paranthropus
Boisei
The Primate Descendency
Primates appear in the fossil record after the
beginning of the Cenozoic Era.
Prosimians thrive during the Paleocene & Eocene.
Monkeys become dominant by the Oligocene.
Apes evolve by the Miocene.
In the late Miocene, the hominid evolutionary line
finally becomes distinct – and includes our direct
ancestors.
Did Humans Evolve?
PBS
Evolution
The East African Rift Valley
Known for mountain
building, faulting and
volcanic activity over the last
several million years.
Early sediments were thrown
to the surface where they
were located by
paleoanthropologists.
Volcanic sediments make it
possible to chronometrically
date the sites.
Becoming a Fossil
“Lucy”
Earliest Traces of
East African Hominids
The oldest specimen that
is believed to be a
hominid comes from
Lothagam, northern
Kenya.
Several other fragmented
specimens have been
found around the same
area of east Africa
Origins of Humankind
Paranthropus
boisei
Aramis
Dated at 4.4 million years old,
this is the oldest collection of
hominids discovered.
The remains provide anatomical
evidence of bipedalism, the
criterion for hominid status.
The excavators suggested that
the Aramis hominids be assigned
to a new genus and species,
Ardipithecus ramidus.
Australopithecines
Laetoli
Dated between 3.5-3.7
mya.
Fossilized hominid
footprints found in an
ancient volcanic bed.
The individuals were
bipedal, but maybe not
in the same way as
modern humans.
Laetoli Footprints
Hadar (Afar Triangle)
Dating suggests a range from 3.9-2.3 mya
Recovered:
"Lucy" an Australopithecus afarensis female,
was recovered here.
Group of bones representing 13 individuals,
including 4 infants, suggest a social unit died at
the same time.
Stone tools that may be 2.5 million years old,
making them the oldest cultural evidence.
Finding Lucy
Bouri (Middle Awash)
Several fossils were discovered dating to 2.5 mya
These fossils are quite different from other PlioPleistocene hominids:
Projecting face
Very large back teeth
Long hind limbs
Animal bones found with these fossils show clear
signs of butchering.
Koobi Fora
(East Lake Turkana)
This site yielded the richest
assemblage of Plio-Pleistocene
hominids from the African
continent.
Most of the hominids date to 1.8
mya, others date back to 3.3
mya.
The 150 hominid specimens
recovered at Koobi Fora in
Kenya represent at least 100
individuals.
West Turkana
Two important discoveries:
The nearly complete 1.6 mya Homo
erectus adolescent.
The black skull,
a well-preserved
2.4 million year
old skull which
caused a major
re-evaluation of
Plio-Pleistocene
evolution.
Olduvai Gorge
Louis and Mary Leakey
conducted continuous
excavations from the
1930's to early 1980.
Paleontological evidence
includes more than 150
species of extinct
animals which can
provide clues to the
ecological conditions of
early hominid habitats.
Central Africa
A hominid mandible was discovered in Chad
dating from 3.5 to 3.0 mya.
Preliminary analysis suggests that this fossil's
closest affinity is to Australopithecus afarensis.
The fossil was found more
than 1,500 miles west of the
previously established range
of early hominids.
Australopithecus afarensis
South African Sites
The first australopithecine was
discovered at a quarry at Tuang.
As the number of discoveries
accumulated, it became clear that
the australopithecines were not
simply aberrant apes.
The acceptance of the
australopithecines as hominids
required revision of human
evolutionary theory.
Groups of
Plio-Pleistocene Hominids
Specimens represent 200 individuals from South
Africa and more than 300 from East Africa.
Divided into four broad groupings:
Set I Basal Hominids.
Set II Early Primitive Australopithecus.
Set III Later, more derived Australopithecus.
Set IV Early homo.
Becoming Human
Lineages,
No. 3 - The Human
Family Tree
Estimated Body Weights and Stature
in Plio-Pleistocene Hominids
Body Weight
Stature
Male
Female
Male
Female
A. afarensis
99 lb
64 lb
59 in.
41 in.
A. africanus
90 lb
65 lb
54 in.
45 in.
South African
“robust”
88 lb
70 lb
52 in.
43 in.
East African
“robust”
108 lb
75 lb
54 in.
49 in.
H. habilis
114 lb
70 lb
62 in.
49 in.
Set I.
Basal Hominid
(4.4 mya)
The earliest and most
primitive remains are
those from Aramis.
They have been
classified as
Ardipithecus ramidus,
a different genus
from all other PlioPleistocene forms.
Set II.
Early Primitive Australopithecus
(4.2-3.0 mya)
The hominids from Laetoli and Hadar are
assigned to Australopithecus afarensis.
A. afarensis is so primitive
in the majority of dental
and cranial features
that if it were not for
evidence of bipedalism,
this primate would not
be classified as a
hominid.
Set III.
Later, More Derived Australopithecus
(2.5-1.0 mya)
Robust Australopithecines
Larger body size
Small cranial capacities
Very large, broad faces
Massive back teeth and
lower jaws
Gracile Australopithecines
Different face dentition
Set IV.
Early Homo
(2.4-1.8 mya)
The earliest appearance of our genus, Homo
may be as ancient as the robust
Australopithecines.
Leakey named these specimens Homo habilis
("handy man").
H. habilis differs from
Australopithecus in
cranial cavity and dental
proportions.
Becoming Human
Lineages
What we know…
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