The Pleistocene Epoch was characterized by multiple glaciation events. Ice advanced and retreated approximately every 100,000 years (glacial-interglacial cycles) beginning 2 m.y. ago.

(a) wooly mammoths

(b) mastodons

(c) wooly rhinos

(d) giant bison

(e) ground sloths

(f) saber toothed tigers

1. The earliest known hominid was Ardipithecus ramidus that appeared 4.4 m.y. ago.

2. Australopithecus afarensis appeared in E. Africa 4 m.y. ago

3. There were multiple lineages of hominids over next two million years

4. Genus Homo first appeared 1.8 m.y. ago

5. Homo sapiens appeared 90,000 years ago

6. Homo sapiens displaced Neanderthals in Europe 40,000 years ago.

7. Advent of modern man

1. Figure 15.37: From the middle Eocene to the middle Oligocene, the world underwent climatic deterioration (general global cooling).

2. Glaciation was initiated on Antarctica during the Oligocene due to establishment of the circum-Antarctic Current.

3. Arctic ice caps began to form about 3 m.y. ago during the late Pliocene. During this same time, glaciers began to advance in the northern continents.

4. Figure 16.3: Major continental glaciation was in progress by 2 m.y. ago and over the next 2 million years, ice advanced and retreated at approximately 100,000 year cycles.

5. The mid-latitudes emerged from the last ice age between 10,000 and 11,000 years ago. We are presently within an interglacial period.

1. Figure 16.2: Erratic boulders have been recognized in Europe since biblical times. Ancient mariners had seen boulders trapped in icebergs and explained the presence of erratic boulders in unglaciated regions as having been carried in icebergs that drifted across Europe during the biblical flood. These deposits today are called till.

2. James Hutton (1795) observed in Europe that till occurred far from modern glaciers and proposed that Alpine glaciers must have been more extensive in the past.

3. J. Esmark in Norway (1824) was the first to recognize that vast ice sheets once covered most of Europe.

4. Ignace Venetz-Sitten (1821) and Jean de Charpentier (1830) confirmed that alpine glaciers once extended beyond the mountains onto the Swiss Plain, leaving behind moraines and erratic boulders far north of present glaciers.

5. Louis Agassiz (1836-1840), a Swiss paleontologist, gathered evidence of a previous widespread ice cover that encompassed nearly all of northern Europe. He later also found evidence of former glaciers in New England, United States.

6. T.C. Chamberlin (1886) recognized striations in bedrock scoured by rocks in moving glaciers.

7. Geologists in the 1800's also recognized multiple deposits of glacial till layers separated by buried soils containing plant and animal remains. They correctly reasoned that these deposits represented multiple glaciation events.

8. By 1842, it became apparent to geologists that a worldwide lowering of sea level accompanied major glacial events.

1. Figure 16.3: The last glacial maximum occurred 18,000 years ago, during which time nearly one-third of the present global land area was covered by ice. This changed the oceanic circulation patterns and caused temperate climatic belts to shift to lower latitudes, causing the tropical belt to shrink.

2. During this period, regions like the Sahara and western United States, which are today deserts, were wet and fertile.

3. Glaciers also locked up much seawater on the continents, causing sea level to be lowered. The lowered sea level, in turn, exposed continental shelves in many regions, creating land bridges and aiding in the migration of humans throughout the world.

4. Figure 16.4: During the maximum ice advance, North America was largely covered with ice.

1. Figure 16.6: Following retreat of glaciers, isostasy (glacial rebound) occurred around the Great Lakes and northern Baltic Sea over the last 10,000 years.

2. Total rebound since last ice age range from tens to hundreds of meters total. Today, rebound rates in the northern Baltic Sea are several mm/yr.

1. Figure 16.6: Modern lakes formed by past glaciers include Finger Lakes of New York and the Great Lakes.

2. Lake Agassiz, which was 4 times the size of modern Lake Superior, formed temporarily in southern Canadian Plains when the ice began to retreat.

3. Immense terraces of coarse gravel and sand in the upper Columbia River Valley, western Canada, record the tremendous volumes of water which flowed through river valleys and into the Pacific following retreat of the last glaciers.

4. Figure 16.7: Ancient Lake Missoula, western Montana, was dammed by a huge ice-moraine in northern Idaho, causing immense volumes of melt water as much as 1,000 feet deep to collect behind it. Around 18,000 years ago, the dam broke, creating a gigantic flood that scoured large portions of the Columbia Plateau, known today as the scablands.

5. Lake Bonneville, Utah, was much larger in the past and covered up to 50,000 square kilometers and was as much as 300 meters deep.

6. Enormous amounts of sediment were dumped in front of receding glaciers. Wind winnowed out fine sand and silt and re-deposited these sediments as loess over large areas such as the mid-west.

7. Figure 16.11: Glacial low-stands were characterized by river terraces, land bridges and submarine canyons. Major drops in sea level occurred at least four times during last 2 m.y. with the largest drop 40,000 years ago.

8. Figure 16.10. Worldwide fluctuations in sea level were in response to glacial oscillations. The last rise in sea level began around 18,000 years ago and sea-level neared its present level by 7,000 years ago.

1. Table 16.1: During the early 20th Century, four major glacial advances were recognized and mapped in North America, while 3 to 5 were recognized in Europe. Sequences of strata from glaciated and un-glaciated regions on land and from deep sea cores were correlated in order to recognize these different glacial episodes.

2. Figure 16.12: At higher latitudes, intervals of glacial till were separated by soil horizons containing plant and animal fossils. At lower latitudes, interglacial strata were recognized by deposits related to higher sea levels such as cross stratification, marine fossils, etc. as well as interglacial lake deposits. In deep-sea sediments, glacial periods are marked by a greater proportion of coarse sediments while interglacial deposits contain abundant shale reflecting deeper water.

3. Table 16.1 gives Pleistocene Stratigraphic Classification for North America. Pre-Wisconsinan events are questionable since older deposits are more weathered, more difficult to date and in many places have been eroded.

4. The advent of Carbon 14 dating allowed scientists to date wood, charcoal, bone and calcareous shells back to 80,000 years, thus placing age constraints on the stratigraphic record.

5. Dating of fairly recent deposits can also be performed by fission-track methods.

6. K-Ar dating becomes applicable for ages older than 100,000 years. This method is particularly useful for lavas interstratified with glacial till.

7. Figure 16.14. Dating of glacial events was also possible using Pleistocene volcanic ash layers. Most ash layers originated from Yellowstone or Cascade volcanic centers. The various sources of ash layers can be determined by looking at the mineralogy and geochemistry of the ash, while mineral fragments and obsidian in the ash can be dated isotopically.

1. Figure 16.16: Protactinium 231-thorium 230 can be used for deep-sea clays dating back to 300,000 years ago.

2. Temperature fluctuation curves for deep-sea cores can be constructed using reversal of coiling direction of foram shells and oxygen isotope analysis, both of which can determine changing seawater temperatures. Using these methods, glacial-interglacial periods can be recognized within the deep-sea cores. Shells can also be dated by carbon 14 in order to place ages on the different temperature cycles.

3. Figure 16.16. Polarity reversals can be determined on magnetically susceptible minerals within deep-sea cores and the magnetic polarity reversal scale can be tied to the temperature curves determined from foram fossils.

4. Deep Sea cores suggest that there were significantly more than the 4 or 5 glacial episodes recognized on land. The deep sea sedimentary record is more complete and is in the process of being correlated with glacial deposits on land in order to better constrain past glacial episodes.

1. Continental glaciations have been relatively rare events in earth history

2. The "normal" climate during the past 1 billion years was milder than that of the last 20 m.y.

3. Past climatic conditions typically were not only milder but also more uniform over the earth.

(a) Temperature and moisture gradients less abrupt during much of the past.

(b) Polar areas coldest when continents layed at or near them

(c) Polar areas relatively milder when covered with open, well-mixed seas.

4. The magnitude of overall average yearly temperature differential between a normal and a glacial climate is less than 10o C.

(a) A drop in global temerature on the order of only 4o to 5o C from present mean annual temperature could cause renewed continental glaciation.

(b) The average sea-surface temperature near the peak of the last glacial advance (around 18,000 years ago) was only 2-3o C lower than now.

1. Changes in solar radiation. This is difficult to test due to lack of adequate measurements.

2. Astronomical or orbital effects involving changes in earth-sun geometry (Milankovitch cycle).

3. Terrestrial changes affecting net heat budget.

(a) Changes in atmospheric transparency

(b) Changes in reflectivity of the earth's surface

4. Variations in the heat-exchange rate from equatorial to polar regions due to paleogeographic changes (Plate Tectonics).

1. Climatic variations may be due to three cyclic properties of the earth's orbit.

(a) Figure 16.19a: Eccentricity of the orbit (period of 100,000 years). A more elliptical orbit puts the earth further away from the sun's heat during the summer, so not all the previous winter ice can melt.

(b) Figure 16.19b: Tilt of the axis (period of 41,000 years). Tilt only varies by a few degrees from the present 23.5o. Steeper tilt causes more polar snow to melt in summer while a shallower tilt allows more snow to accumulate in polar regions.

(c) Figure 16.19c: Precession (wobble) of the axis due to changing gravitational interactions with moon and sun (period of 23,000 years). The precession cycle can cause the earth to receive either more or less sunlight on its ice caps.

2. Only when the earth was made susceptible to glaciation by other factors could the Milankovitch effect cause expansion and contraction of glaciers.

3. Figure 16.16: Compilation of primarily Pleistocene oceanographic data was undertaken during the 1970's under the massive cooperative research program CLIMAP. Mathematical analysis of the frequencies of several parameters observed in deep-sea cores determined that the principle period of variation for the past 600,000 years had a frequency of about 100,000 years (orbital eccentricity). Less pronounced periodic features were also observed having periods of 23,000 and 42,000 years each.

4. The precession of the earth's rotational axis causes the date of nearest approach to the sun (perihelion) to change by about 1 day per 60 years. Today perihelion occurs in January (northern winters warmer today than they were 10,000 years ago). 10,000 years ago, perihelion occurred in July, which made northern summers warmer and winters colder than now.

Factors that may change balance between incoming and outgoing radiation include the following:

(a) Transparency of the atmosphere. More volcanic eruptions and greater ash content in the atmosphere would cause climatic cooling. CO2, O3 and water vapor inhibit escape of long wavelength radiation from the earth's surface, causing warming by the greenhouse effect. Atmospheric CO2 is consumed by plant photosynthesis and deposition of carbonates. Addition of CO2 to the atmosphere comes from igneous and hot spring activity, animal respiration, decay and combustion of organic matter. Atmospheric CO2 was generally lower during the ice age, although the reasons for lower CO2 are not obvious.

(b) Reflectivity of the earth's surface (albedo). Larger continental exposure during regressions produces higher global albedo where more solar energy is reflected back into space, causing global cooling. In contrast, larger sea-surface area reduces albedo, causing climatic warming. The growth of continents during the Cenozoic, coupled with the Milankovitch effect, may have triggered the ice age. In addition, formation of ice caps further increases albedo and contributes to expansion of glaciers.

1. Increased mountain building and enlargement of continents during the Cenozoic may have altered atmospheric circulation enough to bring on glaciation. Worldwide uplift of continents was particularly active during Miocene and Pliocene time due to continental collisions following the closing of the Tethys Sea.

2. An increase in elevation (altitude) of continents promotes alpine glaciation. Mountains and plateaus also deflect moist air upward to colder levels and induce considerable atmospheric turbulence, resulting in greater cloud formation and local precipitation. Greater cloud cover reduces the amount of solar energy reaching the earth and precipitation causes atmospheric cooling.

3. Large-scale uplift also causes retreat of seas and exposure of more continental area. Greater continental area increases the albedo that contributes to climatic cooling.

4. The relative positions of continents influence global climate. During the Cenozoic, the Arctic Ocean became almost completely surrounded by land masses that prevented warm equatorial waters from reaching the North Pole region, resulting in thermal isolation of the Arctic Ocean. Temperatures in the Arctic declined, ice formed, and the resulting increase in albedo perpetuated the ice age.

5. The positioning of Antarctica over the South Pole may have also contributed to global cooling. The increase in albedo, coupled with establishment of the circum-Antarctic current, promoted glacier formation and growth.

6. Following glaciation, an interglacial climate may soon develop as a consequence of widespread ice sheets. During the early stages of continental glaciation, moisture for snow and ice formation is largely derived from adjacent bodies of water such as the Arctic Ocean in the case of the North Pole region. When glaciation expands to the point that the surrounding ocean water is covered by ice, then the source of moisture is gradually cut off, resulting in dryer conditions and less snowfall. Eventually, ice caps cease to advance for lack of nourishment and soon begin to retreat. As ice melts, total climate becomes warmer, more ice melts, and so on leading to an interglacial period. The cycle is eventually repeated and another ice age occurs.

1. We still do not have a fully acceptable theory of glaciation, but the general consensus is that several factors combined to cause the Pleistocene ice age.

2. Movement of continents during the Cenozoic thermally isolated the poles. The Cenozoic was also a period when total land area was enlarged, thus producing a higher global albedo. The result was long-term global cooling.

3. Once global cooling was initiated by the positioning of continents, the Milankovitch cycle (which alone could not produce glaciation) may have been critical in enhancing the temperature oscillations between glacial and interglacial conditions.

4. Finally, reduction in atmospheric CO2 during the Pleistocene (the reason for which is unknown) aided in global cooling.

1. Pleistocene glaciation in the Northern Hemisphere caused the temperate zone to shift southward, significantly reducing the zone of tropical climate. These climatic shifts affected the distribution of life forms. For example, species living today along northern California only ranged as far north as San Diego during glacial periods.

2. Worldwide changes in sea level during Pleistocene time greatly affected coral reef growth. Fossil coral reefs that grew during interglacial periods stand higher than present low-tide level, suggesting that either sea level stood higher than present during past interglacial periods or that the islands containing the reefs had risen. Past sea-level fluctuations, caused by changing glacial-interglacial conditions, left scars on fossil reefs that can be studied today.

3. In many parts of the world, fossil plants and animals fossils indicate climatic conditions different from those of today (e.g. wetter conditions off western Mexico and the northern Sahara, semi-desert conditions prevailed along central North America adjacent to glaciers, etc. see Figure 16.3).

4. Faunal and floral changes due to glaciation were most extreme near the southern limit of the ice were alternating glacial-interglacial conditions prevailed.

5. Figures 16.22 & 16.24 shows the effects of climate change on plant communities of eastern North America and western Europe, respectively, during the last maximum ice advance (Woodfordian) compared with today.

6. Land bridges (e.g. Bering Bridge between Alaska and Siberia and the North Sea Bridge between Europe and England), were exposed during sea level lowstands, allowed plants and organisms to migrate between continents. The Bering Bridge was drowned 13,000 years ago.

7. Over 200 genera of mammals became extinct between late Pliocene and recent time, due either to climatic change and/or the advent of humans. Since the late Miocene, we lost the mammoth, mastodon, saber-toothed tiger, giant ground sloth, dire wolves, giant vultures and many other species.

Primates retain the primitive number of five digits, have teeth that are not specialized for dealing with either grain or flesh, and have never developed hoofs, horns, trunks or antlers. Hominidae is the human family of the order primates.

1. The early primates evolved from mammals that probably resembled shrews. The earliest primate was a creature named Purgatorius, the fossils of which were found in late Cretaceous strata in Montana.

2. Figure 16.28: From the coarse of evolution from shrew-like insectivores, the principal changes in primates involved progressive enlargement of the brain and modifications of the hand, foot and thorax (part of the body between neck and abdomen).

3. The earliest primates are represented in the Paleocene of Europe and North America as "squirrel-like nut eaters". The Paleocene procimian Plesiadapis was a tree-dweller living in several now widely separated continents. This animal had rodent-like characteristics and habits and its fingers and toes terminated in claws rather than nails. Plesiadapis became extinct by late Eocene time when true rodents were becoming abundant.

4. With the development of grasping, mobile hands, the eyes of primates became positioned towards the front of the face so that there was overlap of both fields of vision. This visual adaptation allowed early primates to gauge accurately the distance to insect prey without movement of the head.

5. During the Eocene, the Paleocene "nut eaters" were replaced by the ancestors of modern lemurs in Asia, Africa and North America. This evolution involved a reduction in the size of the muzzle, increase in brain size, shifting of the eyes to a more forward position, and development of a grasping big toe. Prosimian populations during the late Paleocene and Eocene also included tarsiers.

1. As primate evolution continued, the forelimbs and hind limbs of some species diverged in form and function and an inadvertent predisposition towards an upright posture developed.

2. Fossils of the earliest true apes were found in Oligocene deposits in Egypt, reflecting a region consisting of tropical forests and vast swamps. The fossils included skull fragments and teeth that clearly were not from prosimians, although subtle vestiges of prosimian ancestry were evident. They were fossils of primates that had developed monkeylike limbs and tail, a larger brain and eyes rotated to the front, indicating that the prosimian-anthropod transition had taken place by Oligocene time.

3. Fossils of late Oligocene-early Miocene monkeys were found in South America that differed enough from African counterparts to indicate that they evolved independently from prosimian ancestors.

4. During the Miocene, the evolution of primates was strongly influenced by plate tectonics as well as the replacing of dense forests by grass-covered savannas.

5. During the Miocene, a new group called the dryomorphs emerged in Europe, Asia and Africa. The skull, jaws and teeth of the early dryomorphs were more apelike rather than monkeylike.

1. The main radiation of hominid primates occurred during the Miocene when many of the characteristics of modern apes and humans developed, including a broad jawbone, low-crowned molars and reduced canine tooth size.

2. Early apes of the Miocene probably lived in forested areas near open grasslands and ate seeds, roots and possibly even meat. By the end of the Miocene, many apes have moved from the forests into open country.

3. Bipedality, the definitive characteristic of the earliest hominids, has been regarded as an adaptive response to a transition from forested to more-open habitats in East Africa sometime between 12 and 5 million years ago.

4. The divergence of hominids from the other African apes may have been associated with terrestrial adaptations to exploit resources in open habitats or in bridging forested patches. One of the greatest advantages of bipedality is the ability to carry large loads while walking or running.

5. The early hominids diverged from the African apes sometime near the end of the Miocene, 5 - 6 m.y. ago.

1. Australopithecines are the primitive hominids that represent the evolutionary link between apes and early man.

2. In 1924, Raymond Dart discovered the fossil remains of an immature primate in a limestone quarry in South Africa. He named the primate Australopithecus africanus. In the following years, many additional skeletal fragments of species of Australopithecus have been found in upper Pliocene and lower Pleistocene deposits of Africa, Java and China. Scientists eventually concluded that these fossils represented the ancestors of humans.

3. Figure 16.32: Many of the remains of early, undisputed Hominids are found in Tanzania and Ethiopia. These remains include fragments of jaws and teeth dated at 3.6 - 3.8 million years. Two sets of human footprints near one site reveal that the early hominids were indeed bipedal.

4. Figure 16.27: Then in 1974, Donald C. Johanson discovered "Lucy", a female Australopithecus afarensis, in Ethiopia. The bones of Australopithecus afarensis (including "Lucy") were found in sediments dated at 3.0 - 3.4 m.y. old. These bones revealed that A. afarensis was bipedal but not fully erect, had large canine and incisor teeth, and a large overhung jaw. Lucy herself was barely 3.5 feet tall, had a head about the size of a soft ball and probably had a brain comparable in size to that of a chimpanzee.

5. More fossils of A. afarensis that are 400,000 years older than Lucy were found in Ethiopia. The remains occur in tuff deposits dated by K-Ar methods as 4.0 m.y. old. Until recently, Australopithecus afarensis represented the earliest-known hominid.

6. Then in 1994, Ardipithecus ramidus was discovered in Ethiopia and dated at 4.4 m.y. old, becoming the earliest-known hominid. Many paleoanthropologists think that A. ramidus is the so-called "missing link" and fills the morphological gap between apes and known hominids, although other scientists dispute that theory.

7. Recent investigations by anthropologists working in Kenya and Ethiopia provide evidence that several humanlike lineages eventually evolved from A. afarensis and coexisted in Africa between 1 and 3 m.y. ago.

8. Figure 16.30a: Australopithecus africanus was found in fossil sites dated at 3.0 - 1.5 m.y. It was characterized as a gracile lineage with a relatively dainty jaw and small cheek teeth, no skull crest and walked in a vertical position. It generally had a lower forehead and a more projecting face than later humans.

9. Figure 16.30b: Australopithecus robustus was found in beds dated between 1.6 and 1.9 m.y. old. A. robustus is characterized by a massive jaw, large molars and a skull crest. It probably had the same-sized brain as A. afraicanus.

10. Most specimens of Australopithecus boisei was found in beds between 1.2 and 2.2 m.y. old and is characterized by massive robust jaws, widely flaring cheekbones, strong crest on top of the head and massive molars.

11. Figure 16.30c: The "Black Skull" is about 2.5 m.y. old, has a robust skull like A. boisei but also has many primitive features like A. afarensis. "Black Skull" may be a primitive link between A. afarensis and A. boisei (hypothesis 1 of Figure 16.32) or is a primitive relative of both A. robustus and A. boisei. (hypothesis 2 of Figure 16.32).

12. By 2 m.y. ago, four species of hominids lived side by side: A. boisei, A. robustus, A. africanus and the earliest species of our own genus, Homo.

13. The Australopithecus lineage eventually became extinct whereas the earliest species of Homo evolved towards modern humans.

1. Figure 16.32: Homo habilis appears to be one of the earliest ancestors of the genus Homo. H. habilis was discovered by Louis Leakey at Olduvai, eastern Africa. Dated at 1.75 m.y., Homo habilis has a less projecting face, larger brain and more rounded forehead than earlier hominids.

2. Another H. habilis skull was found at Lake Turkana, East Africa, and date at just under 2.m.y.

3. Figure 16.31: Homo erectus is the likely ancestor of Homo sapiens.. Specimens of H. erectus (also known as Peking Man) were found in Java and China during the 1920's and 1930's and dated around 1.0 - 0.25 m.y old. H. erectus is characterized as tall, large-brained and small-faced human forms. In 1984, a skeleton of H. erectus was found in East Africa and dated at 1.6 m.y. Additional fossils were subsequently found and dated at around 1.3 m.y. H. erectus dispersed from their African homeland around 1.0 m.y. ago and spread over most of Eurasia.

4. The earliest H. sapien bones were discovered in Africa and Europe and dated at about 500,000 to 300,000 years.

5. Figure 16.31b: Neanderthals were European and western Asian H. sapiens with heavy eyebrow ridges, pronounced chin, stocky build and the same or larger brain size than ourselves. Neanderthals first appeared about 170,000 years ago and were widespread throughout the glaciated regions of Europe and western Asia until their mysterious disappearance about 40,000 years ago.

6. Figure 16.31c: Modern humans (Homo sapienes sapiens) first appeared over 90,000 years ago in southern Africa and Middle East and 45,000 years ago in Europe. They are called "Cro-Magnon men" although they are anatomically identical to us.

7. By 10,000 years ago, all major races of modern humans had appeared.

1. Humans crossed the Bering land bridge from Siberia into Alaska 20,000 to 20,000 years ago during the time of lower sea level at the peak of Wisconsinan glaciation.

2. By 10,000 years ago, humans occupied both North and South America following the onset of the present interglacial conditions.

3. Figure 16.34: The climate continued to warm until 7,000 to 6,000 years ago when a Climatic Optimum was reached. During this climatic optimum, the ancient civilizations of Egypt and Mesopotamia experienced wetter and more favorable climates than are found in these regions today.

4. About 3,000 years ago, the climate again changed and the Middle East began experiencing droughts punctuated by annual flooding of rivers.

5. Around 2,500 years ago, the Mediterranean became considerably colder and wetter.

6. A warming period in southern Europe soon followed, allowing Romans to invade northern Europe.

7. Climatic fluctuations since that time contributed to the evolution and demise of civilizations.