THE ORIGIN OF LIFE

http://www.youtube.com/watch?v=XvMgoelauLQ

At one time, every single living organism that lives on earth today, did not exist.

The life that existed in the past no longer exists today. In fact, 99.9 % of all organisms that ever existed are now extinct.

So….. where did life come from?

The History of Life Paleontologists “read” the history of life through

stories written in ancient rocks by studying fossils

They collect fossils, group similar organisms together and arrange them in the order in which they lived from oldest to most recent in what is know as the fossil record

The fossil record provides evidence about the history of life on Earth. It also shows how different groups of organisms, including species, have changed over time

Fossils occur in a particular order, certain fossils appear only in older rocks, others appear only in more recent rock

Fossils show that life on Earth has changed over time

More than 99% of all species that ever lived on Earth are extinct, species have died out

How Fossils Form Most fossils form in sedimentary rock, formed when

existing rocks is exposed to rain, heat, wind and cold, breaking down into smaller particles of sand, silt and clay which get carried away into lakes and seas, settling on the bottom

The weight of layers gradually compress the lower layers and turn them into rock, trapping remains of an organism and preserving soft and hard parts of the organism

http://phschool.com/atschool/phsciexp/active_art/fossil_formation/index.html

Interpreting Fossils As wind, rain and water erode the rock,

bit by bit the upper younger layers begin to wear away, exposing older fossil-bearing layers beneath

Paleontologists determine the age of fossils using two techniques: relative dating and radioactive dating

Relative Dating Relative dating

allows paleontologists to estimate a fossil’s age compared with that of other fossils based on placement in the rock layers

Radioactive dating In radioactive dating, scientists calculate

the age of a sample based on the amount of remaining radioactive isotopes it contains

Different radioactive elements have different half-lives (length of time required for half the radioactive atoms in a sample to decay)

Geological Time Scale Paleontologists use divisions of the

geological time scale to represent evolutionary time based on evidence from Earth’s rocks and fossils

After Precambrian Time, the basic divisions of the geological time scale are eras and periods

Eras and Periods The time between the Precambrian and

the present is divided into three erasPaleozoic EraMesozoic EraCenozoic Era

Eras are subdivided into periods

Formation of Earth Geological evidence shows that Earth,

which is about 4.6 billion years old, was not “born” in a single event

Pieces of cosmic debris were probably attracted to each other over the course of 100 million years with a large collision producing enough heat to melt Earth, rearranging elements according to density to help create today’s Earth

Earth’s early atmosphere probably contained hydrogen cyanide, carbon dioxide, carbon monoxide, nitrogen, hydrogen sulfide, and water

http://www.youtube.com/watch?v=M4pt0fFn_a4&feature=related

The Puzzle of Life’s Origins: The Early Ideas In the past, ideas that decaying meat

produced maggots, mud produced fish and grain produced mice were reasonable explanations for what people observed in their environment

Such observations led people to believe in spontaneous generation—the idea that nonliving material can produce life

Spontaneous Generation Disproved In 1668, an Italian physician, Francesco

Redi, disproved a commonly held belief at the time—the idea that decaying meat produced maggots, which are immature flies

Control Group

Experimental Group

Redi’s well-designed, controlled experiment successfully convinced many scientists that maggots, and probably most large organisms, did not arise by spontaneous generation.

Although Redi had disproved the spontaneous generation of large organisms, many scientists thought that microorganisms were so numerous and widespread that they must arise spontaneously-probably from a vital force in the air.

Pasteur’s Experiment In the mid-1800s, Louis Pasteur designed

an experiment that disproved the spontaneous generation of microorganisms.

Pasteur set up an experiment in which air, but no microorganisms, was allowed to contact a broth that contained nutrients.

Each of Pasteur’s broth-filled flasks was boiled to kill all microorganisms.

The flask’s S-shaped neck allowed air to enter, but prevented microorganisms from entering the flask.

Pasteur tilted a flask, allowing the microorganisms to enter the broth.

Microorganisms soon grew in the broth, showing that they come from other microorganisms.

Pasteur’s experiment showed that microorganisms do not simply arise in broth, even in the presence of air.

From that time on, biogenesis, the idea that living organisms come only from other living organisms, became a cornerstone of biology

Origins of Life: The Modern Ideas No one has yet proven scientifically how

life on Earth began. However, scientists have developed

theories about the origin of life on Earth from testing scientific hypotheses about conditions on early Earth.

Simple Organic Molecules formed (Primordial Soup) In the 1930s, a Russian scientist, Alexander

Oparin, hypothesized that life began in the oceans that formed on early Earth.

He suggested that energy from the sun, lightning, and Earth’s heat triggered chemical reactions to produce small organic molecules such as amino acids from the substances present in the atmosphere (nitrogen, methane, an ammonia).

Then, rain probably washed the molecules into the oceans to form what is often called a primordial soup.

In this soup, proteins, lipids, and other complex molecules found in present day cell formed

The formation of protocells The next step in the origin of life, as

proposed by some scientists, was the formation of complex organic compounds.

In the 1950s, various experiments were performed and showed that if the amino acids are heated without oxygen, they link and form complex molecules called proteins.

A similar process produces ATP and nucleic acids from small molecules.

By heating solutions of amino acids, protocells (large, ordered structure, enclosed by a membrane, that carries out some life activities, such as growth and division) arose

The Evolution of Cells Fossils indicate that by about 3.4 billion

years ago, photosynthetic prokaryotic cells existed on Earth.

But these were probably not the earliest cells.

The first “true” cells The first forms of life may have been prokaryotic

(lacking membrane bound organelles) forms that evolved from a protocell.

Because Earth’s atmosphere lacked oxygen, scientists have proposed that these organisms were most likely anaerobic (don’t require oxygen)

For food, the first prokaryotes probably used some of the organic molecules that were abundant in Earth’s early oceans.

Over time, these heterotrophs (unable to make own food) would have used up the food supply.

However, organisms that could make food (autotrophs) had probably evolved by the time the food was gone.

These first autotrophs were probably similar to present-day archaebacteria

Archaebacteria are prokaryotic and live in harsh environments, such as deep-sea vents and hot springs

Photosynthesizing prokaryotes Photosynthesizing prokaryotes might have

been the next type of organism to evolve. As the first photosynthetic organisms

increased in number, the concentration of oxygen in Earth’s atmosphere began to increase.

Organisms that could respire aerobically would have evolved and thrived.

The presence of oxygen in Earth’s atmosphere probably affected life on Earth in another important way.

The sun’s rays would have converted much of the oxygen into ozone molecules that would then have formed a layer that contained more ozone than the rest of the atmosphere.

Origin of Eukaryotic Cells: The Endosymbiotic theory Complex eukaryotic cells probably

evolved from prokaryotic cells. The Endosymbiotic Theory proposes

that eukaryotic cells arose from living communities formed by prokaryotic organisms

A prokaryote ingested some aerobic bacteria. The aerobes were protected and produced energy for the prokaryote.

Over a long time, the aerobes become mitochondria, no longer able to live on their own.

Some primitive prokaryotes also ingested cyanobacteria, which contain photosynthetic pigments.

The cyanobacteria become chloroplasts, no longer able to live on their own.

New evidence from scientific research supports this theory and has shown that chloroplasts and mitochondria have their own ribosomes that are similar to the ribosomes in prokaryotes.

In addition, both chloroplasts and mitochondria reproduce independently of the cells that contain them.

Other ideas: Meteorite Theory Some scientist believe that the

molecules necessary for life arrived here on meteorites, rocks from other space that collide with Earth’s surface

Many meteorites contain some organic molecules, necessary for cell formation, might have arrived on Earth and entered its oceans

Other ideas: Devine origins Many of the worlds major religions

believe that life did not arise spontaneously, rather life was created on Earth by supreme beings

http://www.youtube.com/watch?v=968ou_LcXLQ&feature=relmfu

THE EVOLUTION OF MULTICELLULAR

LIFE

Precambrian Time

90 % of Earth’s history occurred during this time

Simple anaerobic forms of life appeared, followed by photosynthetic forms, aerobic forms evolved, and eukaryotes appeared

Paleozoic Era Rich fossil evidence shows that early in the Paleozoic Era, there

was a diversity of marine life Cambrian Period: Cambrian Explosion” many organisms

develop hard parts (shells outer skeletons), invertebrates (jellyfish, worms, sponges), arthropods, segmented bodies, jointed limbs

Ordovician and Silurian Periods: first vertebrates, jawless fish, land plants

Devonian Period: vertebrates being to invade land Carboniferous and Permian Periods: life expands over Earth’s

continents, reptiles, winged insects, ferns and other plants

The mass extinction at the end of the Paleozoic affected both plants and animals on land and in the seas. As much as 95% of the complex life in the ocean disappeared

Mesozoic Era Events during the Mesozoic include the

increasing dominance of dinosaurs. The Mesozoic is marked by the appearance of

flowering plants Jurassic Period: Dinosaurs become dominant

animals on land, first bird Cretaceous Period: Dinosaurs still dominant,

flying reptiles and birds in sky, leafy trees, shrubs, small flowering plants,

At end of Cretaceous, mass extinction, killing half of all plants and dinosaurs gone

Cenozoic Era During the Cenozoic Era, mammals

evolved adaptations that allowed them to live in various environments-on land, in water and even in the airTertiary Period: Earth mild and warm, whales,

dolphins, flowing plants and insects, grasses, ancestors of today's cattle, sheep, deer

Quaternary Period: Earth cools, ice ages, then a warming, ocean life thrives, land mammals become common, Homo sapiens (200,000 y.a.)

In summary

http://www.youtube.com/watch?v=1I7Lyg7qD2A

In Summary

Fossils provide a record of life on Earth. Fossils come from many forms, such as leaf imprint, a worm burrow, or a bone

By studying fossils, scientists learn about the diversity of life and about the behavior of ancient organisms

Fossils can provide information on ancient environments. Ex. Predict river environments, terrestrial environments etc

Earths history is divided into geological time scale, based on evidence in rock fossils

4 major division: Precambrian Era, Paleozoic Era, Mesozoic Era, and Cenozoic Era. The eras are further divided into periods

Controlled experiments disproved spontaneous generation

Small organic molecules might have formed from substances present in Earths early atmosphere and oceans

The earliest organisms were probably aerobic, heterotrophic prokaryotes. Over time, photosynthetic prokaryotes that produced oxygen evolved, changing the atmosphere and triggering the evolution of aerobic cells and eukaryotes