Evolution of Life

To suppose that the eye with all of its inimitable contrivances for adjusting the focus to different

distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration,

could have been formed by natural selection, seems, I confess, absurd in the highest degree.

- Charles Darwin

Water on Earth’s surface

Earth without free oxygen

Banded iron formations commonly occur in sedimentary rocks 2-3 Gyr old. They consist of alternating dark bands (containing FeO) and light bands of chert (silica and Fe2O3).

They occur from the deposition of alternately dissolved FeO & chert.

Todays oceans contain oxygen, which reacts with ferrous oxide (FeO) to form ferric iron (Fe2O3). This precipitates out of ocean. The formation of FeO sediment is not thus likely.

BIFs set an upper limit for the atmospheric oxygen of less than a few percent the present value.

The banded structure is thought to occur from fluctuating densities of bacteria in an ocean. When bacteria blossoms, it creates oxygen and thus chert, which falls to the ocean floor. An oxygen depletion allows for FeO.

Red beds

Earlier than 2 Gyrs ago red beds occur. These form when iron is weathered out of rock in the presence of oxygen. The oxygen needed is less than the present inventory. For several million years BIF and red beds overlap, indicating the presence of low levels of atmospheric oxygen.

Fossils of layers of calcium carbon sheets with concentric spherical shapes. These limestones date back 3.5 Gyr, with less definitive outcrops dating 3.9 Gyr. They are formed by colonies of cyanobacteria.

Chloroplasts are actually cyanobacteria living in plant cells

The Earliest Life

Precambrian (0.5-3.5 Gyr)

Shark Bay, Australia

Stromatolites, colonies of cyanobacteria, are alive in Australia today.

History of Atmospheric Oxygen

From Lunine 1999 based on Kastings (1991)

The presence of FeS2 and UO2 set upper limits because oxygen would have produced other oxides from U and Fe

Cambrian Period (505-570 Myr)

Rocks ½ billion years old differ from early outcrops because they contain multitudes of fossils of various forms of vertebrate marine life like the trilobite.

Ordovician Period (438-505 myr)

During the ordovician period, creatures resembling today’s hagfish emerge in the fossils. Rocks formed from fossils of these worm-like creatures can be seen in the Bright Angel formation in the Grand Canyon.

A hagfish

Silurian Period

The emergence of land-based life – both plants and animals. Fossils of scorpions and millipedes are common in these rocks.

Devonian Period (360-408 Myr)

Early sharks and hinge-jawed fish can be found in these rocks.

Carboniferous period

(286-360 Myr)

Much of today’s coal deposits in North America, Europe and China were formed from the decomposition of flora from this period.

Permian Period (245-286 Myr)

Rocks deposited in the Permian period indicate the first presence of reptiles.

These include those that are credited as being the progenitors of mammals.

Mesozoic Era

Rocks deposited 245-66 million yrs ago contain dinosaur bones as well as the first mammal bones. During this time the giant continent Pangaea is breaking up and the continents are beginning to form.

After the K-T BoundaryRocks deposited later than 66 million years ago indicate a lack of dinosaur bones. The abrupt end to massive dinosaurs is attributed to the impact of a 10 km meteor.

This event was perhaps a lucky one from our point of view, since it was followed by the proliferation of mammals. The profile of a ~100 km crater of an

impact that occurred 66 million yrs ago.

Cenozoic Era

From Gibbons, Science, 295, 1214, (2002)

Most skeletons of the earliest hominids (6-3 million years old) come from Africa.

This led to the idea that hominids evolved in Africa and progressed with time to Europe.

The oldest hominid bones from Europe are 800,000 yrs old.

From Gibbons (2002)

Early Hominids (3-6 million yrs ago)

Upright gate: after 4 million years ago

Small stature: 4 feet and a bit, 70-110 lbs

Small cranial capacity: ~400-500 cc

Tools: none discovered

Finds: Lucy

Lucy, 3.5 million years old

Michel Brunet digging in 6 million yr old sediment in Chad.

Homo Erectus (2 – 0.2 million yrs ago)

Taller stature: 5-6 feet and a bit, 120-150 lbs

Larger cranial capacity: ~ 850-1000 cc

Tools: tear drop hand axes

Finds: Java man, Peking man

Neanderthal (190-27 thousand yrs ago)

Stocky stature: 5 – 5.5 feet

Large size to the muscle attachments

Large cranial capacity: ~ 1500 cc (exceeds human)

DNA analysis: distinct from humans. 600,000 yrs ago - a common ancestor with humans.

Finds: La Chapelle-aux-Saints

The marks on Neanderthal bones indicate that the muscle was separated from the bone with stone tools.

Cannibalism?

Images from Science (2002)

Neanderthals (190,000 – 27,000 yrs ago)

Flaked stones that fit in wood handles.

Buried their dead with spices and bedding.

Built sturdy huts.

Made flutes!

A flute formed from a femur & 4 remaining holes.

The shapes & sizes of hominid heads can be seen to evolve with time.

Australopithecus vs. Modern

Australopithecus, 4-3 myrs ago Modern human

Chimpanzee

Cranial Comparisons

Neanderthal Homo Erectus Homo Sapiens

Homo Sapiens130,000 yrs

agoTall statureCranial capacity: ~1350 ccArt: Cave paintings, Venuses

Lascaux (17,000 yrs old)

Oldest paintings: Chauvet (32,000 yrs old)

Peche Merle (15,000 yrs old)

77,000 yr old artifact

From Blomlos, South Africa

House made of mammoth bones, Mezin Ukraine

A 20,000 year abode

Living quarters

Evidence exists for tents that date back ½ million years. One example is the remnants of animal hide draped over wooden pilings found in cave near Nice France.

First evidence of agriculture occurs in the Fertile Crescent, and date back 10,000 years.

From Science

Homo Sapiens have been around for a short time (130,000 yrs), just 0.003% the age of Earth.

No HS skulls occur in earlier rocks. DNA variation among humans indicate an age of 130,000 years.

In comparison, dinosaurs & stramatilites lasted 2% and 78% the lifetime of Earth.

SummaryThere is evidence for life on Earth for 3.5 billion years.

Rock formations indicate that little free oxygen existed in Earth’s early atmosphere.

Fossil records combined with radiometric dating of sediments indicates that life evolved slowly (over billions of years) along with the composition of the atmosphere.

Human evolution can be traced back ~6-4 million years.

Modern humans date back to less than 200,000 years. Lascaux, France