Bio 5th

The History of Life

• Our planet formed 4.6 billion years ago.
• The chemical signatures of life date back to 3.8 billion years ago.
• 3.5 billion years ago, the earliest evidence of life came from fossils (The fossils of early prokaryotes).
• These early prokaryotes were Earth’s first organisms.
• Their prokaryotic descendants had the planet to themselves for about 1.5 billion years, until the first eukaryotes appeared.
• The first eukaryotes appeared about 1.8 billion years ago.
The Cambrian Explosion
▪ Many present-day animal phyla appear suddenly in fossils formed 535– 525 million years ago, early in the Cambrian period. This
phenomenon is referred to as the Cambrian explosion.
▪ Fossils of several animal groups—sponges, cnidarians (sea anemones and their relatives), and molluscs (snails, clams, and their
relatives)—appear in even older rocks dating from the late Proterozoic.
▪ Fossils and DNA analyses suggest that animals originated about 700 million years ago and then remained small for over 100 million years
—until they diversified explosively during the Cambrian period and beyond.
Prior to the Cambrian explosion, all large animals were soft-bodied.
▪ The fossils of large pre-Cambrian animals reveal little evidence of predation. Instead, these animals appear to have been grazers (feeding
on algae), filter feeders, or scavengers, not hunters.
➢ The Cambrian explosion changed all of that. In a relatively short period of time (10 million years), predators over 1 m in length emerged
that had claws and other features for capturing prey.
➢Simultaneously, new defensive adaptations, such as sharp spines and heavy body armour, appeared in their prey

The Colonization of Land

The colonization of land was another milestone in the history of life.
▪ There is fossil evidence that some prokaryotes lived on terrestrial surfaces as early as 3.2 billion years ago.
▪ However, larger forms of life, such as fungi, plants, and animals, did not begin to colonize land until about 500 million years ago.
▪ This gradual evolutionary venture out of aquatic environments was associated with adaptations that made it possible to reproduce on land
and that helped prevent dehydration.

Plate Tectonics
The continents we live on move over time.
• Over the past billion years, there have been three occasions, when most of the landmasses of Earth came together to form a
supercontinent, then later broke apart:
• 1 billion years ago
• 600 million years ago
• 250 million years ago
Each time, this breakup yielded a different configuration of continents.
➢Based on the directions in which the continents are moving today, some geologists have estimated that a new supercontinent will form
roughly 250 million years from now.

The Theory of Plate Tectonics

According to The Theory of Plate Tectonics, the continents are part of great plates of Earth’s crust that essentially float on the hot,
underlying portion of the mantle.
Movements in the mantle cause the plates to move over time (usually only a few centimetres per year) in a process called Continental Drift.

Many important geologic processes, including the formation of mountains and islands, occur at plate boundaries.
✓In some cases, two plates are moving away from each other, as are the North American and Eurasian plates, which are currently drifting
apart at a rate of about 2 cm per year.
✓In other cases, two plates are sliding past each other, forming regions where earthquakes are common (California’s San Andreas Fault is
part of a border where two plates slide past each other).
✓In still other cases, two plates collide, producing violent upheavals and forming new mountains along the plate boundaries.
▪ One spectacular example of this occurred 45 million years ago, when the Indian plate crashed into the Eurasian plate, starting the
formation of the Himalayan mountains.

Consequences of Continental Drift

Continental drift also promotes allopatric speciation on a grand scale. When supercontinents break apart, regions that once were
connected become isolated.
About 250 million years ago, plate movements brought previously separated landmasses together into a supercontinent named Pangaea.

About 250 million years ago, plate movements brought previously separated landmasses together into a supercontinent named Pangaea.
▪ Ocean basins became deeper, which drained shallow coastal seas.
▪ At that time, as now, most marine species inhabited shallow waters, and the formation of Pangaea destroyed much of that habitat.
▪ The formation of Pangaea greatly altered the physical environment and climate, which drove some species to extinction and provided
new opportunities for groups of organisms that survived the crisis.
▪ Organisms are also affected by the climate change that results when a continent shifts its location.
▪ The southern tip of Labrador, Canada, for example, once was located in the tropics but has moved 40° to the north over the last 200
million years.
▪ When faced with the changes in climate that such shifts in position entail, organisms adapt, move to a new location, or become extinct
(this last outcome occurred for many organisms stranded in Antarctica, which separated from Australia 40 million years ago).
▪ Continental drift also promotes allopatric speciation on a grand scale.
▪ When supercontinents break apart, regions that once were connected become isolated.
▪ As the continents drifted apart over the last 200 million years, each became a separate evolutionary arena, with lineages of plants and
animals that diverged from those on other continents.
▪ For example, marsupial mammals fill ecological roles in Australia analogous to those filled by eutherians (placental mammals) on other
continents
▪ Finally, continental drift can help explain puzzling geographic distributions of extinct organisms, such as fossils of the same species of
Permian freshwater reptiles found in Brazil and the West African nation of Ghana.
▪ These two parts of the world, now separated by 3,000 km of ocean, were joined together when these reptiles were living.

Mass Extinction
▪ The fossil record shows that the overwhelming majority of species that ever lived are now extinct.
▪ A species may become extinct for many reasons:
✓ Its habitat may have been destroyed, or
✓ Its environment may have changed in a manner unfavourable to the species.
➢For example, if ocean temperatures fall by even a few degrees, species that are otherwise well adapted may perish. Even if physical
factors in the environment remain stable, biological factors may change.
▪ Although extinction occurs regularly, at certain times disruptive changes to the global environment have caused the rate of extinction to
increase dramatically.
▪ The result is a mass extinction, in which large numbers of species become extinct worldwide.

The “Big Five” Mass Extinction Events

Five mass extinctions are documented in the fossil record over the past 500 million years.
Mass extinction events, indicated by red arrows, represent peaks in the extinction rate of marine animal families (red line and left vertical
axis).
These mass extinctions interrupted the overall increase, over time, in the number of extant families of marine animals (blue line and right
vertical axis).

➢In each mass extinction, 50% or more of marine species became extinct.
➢During the Permian mass extinction (252 million years ago) about 96% of marine animal species became extinct and drastically altered
life in the ocean.
➢ Terrestrial life was also affected. For example, 8 out of 27 known orders of insects were wiped out. This mass extinction occurred in less
than 500,000 years.
➢The Permian mass extinction occurred during the most extreme episode of volcanism in the past 500 million years.

▪ Geologic data indicate that 1.6 million Km (roughly half the size of Western Europe) in Siberia was covered with lava hundreds of meters
thick.
▪ The eruptions are thought to have produced enough carbon dioxide to warm the global climate by an estimated 6°C, harming many
temperature-sensitive species.
▪ The rise in atmospheric CO2 levels would also have led to ocean acidification,
thereby reducing the availability of calcium carbonate, which is required by reef- building corals and many shell-building species.
➢The volcanic eruptions would also have added nutrients such as phosphorus to marine ecosystems, stimulating the growth of
microorganisms.
➢Upon their deaths, these microorganisms would have provided food for bacterial decomposers.
➢Bacteria use oxygen as they decompose the bodies of dead organisms, thus causing oxygen concentrations to drop.
➢This would have harmed oxygen-breathers and promoted the growth of anaerobic bacteria that emit a poisonous metabolic by-product,
hydrogen sulfide (H2S) gas.

▪ The Cretaceous mass extinction occurred 66 million years ago.

▪ This event extinguished more than half of all marine species and eliminated many families of terrestrial plants and animals, including all
dinosaurs (except birds, which are members of the same group)
▪ One clue to a possible cause of the Cretaceous mass extinction is a thin layer of clay enriched in iridium.
▪ Iridium is an element that is very rare on Earth but common in many of the meteorites and other extraterrestrial objects that occasionally
fall to Earth.
▪ As a result, researchers proposed that this clay is fallout from a huge cloud of debris that billowed into the atmosphere when an asteroid
collided with Earth. This cloud would have blocked sunlight and caused a sudden drop in global temperatures lasting for several months to
years.

A 2018 study estimated that wildfires resulting from the impact contributed to a sharp rise in atmospheric CO2 levels, causing a period of
global warming that lasted 100,000 years. Critical evaluation of these and other hypotheses for mass extinctions continues.

Is a Sixth Mass Extinction Under Way?

▪ Human actions, such as habitat destruction, are modifying the global environment so that many species are threatened with extinction.
▪ More than 1,000 species have become extinct in the last 400 years.
▪ Scientists estimate that this rate is 100 to 1,000 times the typical background rate seen in the fossil record.

Consequences of Mass Extinctions:

▪ Mass extinctions have significant and long-term effects.
▪ By eliminating large numbers of species, a mass extinction can reduce a thriving and complex ecological community to a pale shadow of
its former self.
▪ And once an evolutionary lineage disappears, it cannot reappear.