Chapter 17 – The History of Life

Section 17-1: The Fossil Record

The fossil record shows how species have changed over time.

Paleontologists are scientists who study fossils

Fossils can provide insight into where an organism lived, food, predators, and physical structure all of this information = the fossil record

The fossil record shows that fossils occur in a certain order and that life has changed

More than 99% of all species that have lived on Earth are now extinct

Most fossils form in sedimentary rock

For a fossil to form, the remains of the organism or some trace has to be preserved

The quality of fossil preservation varies; sometimes there are just imprints of soft parts, sometimes hard parts get replaced with long-lasting mineral compounds, and sometimes organisms get buried so quickly they are almost perfectly preserved

Once a fossil is discovered, there are several ways a paleontologist can interpret them Many times, they must

reconstruct an extinct species so they have to look for similarities & differences between the fossil and living organisms

A fossil’s age is also important and can be determined using 2

techniques; relative dating and radioactive

dating

Relative dating = when the age of a fossil is determined by comparing its placement with that of fossils in other layers of rock

Index fossils can also be used to compare relative age they are easily recognized, only lived during a certain period of time, and were widely distributed

With sedimentary rock, older layers are on the bottom and newer layers are on the top

Some elements in rocks are radioactive and take a certain amount of time to break down

A half-life = the amount of time required for half of the radioactive atoms to decay

Radioactive dating = the age of the fossil is calculated based on the amount of remaining radioactive isotopes it contains

Carbon-14 is a common isotope used to date fossils

The scale was first developed by studying rock layers and index fossils

When scientists found major changes in fossil animals and plants they used those times as markers between segments of geologic time

Paleontologists use a geological time scale to represent evolutionary time

Later, radioactive dating was used to put specific ages to the segments of time and they found that there was not a standard length of time for the segmentsGeologic time begins

with Precambrian time, it is about 88% of Earth’s History

There are currently 3 eras after Precambrian time; Paleozoic Era, Mesozoic Era, and the Cenozoic Era

The eras are broken up into smaller time frames called periods

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

Section 17-2: Earth’s Early History

Geologic evidence shows that Earth is about 4.6 billion years old and was not created by one single event

Early Earth was much different than the current Earth we know

Earth’s early atmosphere probably contained at least 5 compounds that are no longer part of the atmosphere

Earth was also very hot, it was not until about 3.8 billion years ago that Earth was cool enough for oceans to form

Scientists now know that their experiments were not accurate, but similar experiments have produced organic compounds

1953 – Miller and Urey tried to figure out how the first beings formed

They used the suggested gasses of the early atmosphere and electric sparks for the lightning

They were able to produce organic compounds including amino acids

Under the right conditions, RNA can help DNA duplicate

Some RNA can even grow and duplicate itself

One hypothesis is that an RNA based form of life could have led to the DNA control we know now

Another unanswered question is which appeared first, RNA or DNA?

Earth’s first atmosphere contained very little oxygen

Over time, photosynthetic bacteria became common in shallow seas and started to produce oxygen

The oxygen combined with iron in the oceans and caused the oceans to rust

The iron fell out and formed bands of iron in Earth’s crust

The rise of oxygen in the atmosphere drove some life forms to extinction, while other life forms evolved new ways to use oxygen for respiration

The oxygen also started to accumulate in the atmosphere and replace several other compounds

The endosymbiotic theory says that eukaryotic cells formed from a symbiosis among several different prokaryotic organisms

About 2 billion years ago, prokaryotic cells began evolving into the ancestor of all eukaryotic cells

Prokaryotes that used ATP evolved into mitochondria

Other prokaryotes that carried out photosynthesis became chloroplasts

Basically, smaller prokaryotes lived inside larger prokaryotes and eventually formed organelles

This sped up the evolution process by diversifying organisms they no longer had the exact same DNA as the “parent”

Sometime after eukaryotic cells arose, they began to reproduce sexually

Section 17-3: Evolution of Multicellular Life

The fossil record indicates that major changes occurred in Earth’s climate, geography, and life forms

Photosynthetic forms then started to add oxygen to the atmosphere

Aerobic forms of life appeared and eukaryotes appeared

Life existed only in the sea

Precambrian time: simple anaerobic forms of life appeared

Paleozoic Era: lasted about 300 million years - fossil evidence shows that early in the Paleozoic Era, there was a diversity of marine life.

Many of the first known representatives of the animal phyla evolved

Ex. Invertebrates, brachiopods, and trilobites

Shells and outer skeletons appeared

Cambrian Period = Diversification during this period is termed the “Cambrian Explosion”

Some arthropods were the first animals to live on land

Some of the first vertebrates and land plants appeared

Ordovician/Silurian Periods – ancestors of modern octopi and squids appeared

Insects started to appear on land

Sharks appeared late in the period

Vertebrates began to invade the land – some of the first four-legged vertebrates evolved into the first amphibians

Devonian Period – Also called the “Age of the Fishes”

Reptiles evolved from amphibians

Winged insects appeared (dragonflies, cockroaches)

Plants formed vast swampy areas where eventually the remains formed thick deposits of sediment that changed into coal over time

Carboniferous/Permian Periods – life was expanding over Earth’s continents

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

Mesozoic Era – lasted about 180 millions years and included increasing dominance of dinosaurs and the appearance of flowering plants

Triassic Period – those that survived the mass extinction became the main forms of lifeImportant organisms

were fishes, insects, reptiles, and cone-bearing plants

Sometimes the Mesozoic is called the Age of the Reptiles

Some of the first dinosaurs appeared

Mammals also appeared late in the period, but were very small

Dinosaurs “ruled” the Earth for about 150 millions years

One of the first birds appeared during this time

Jurassic Period – dinosaurs became the dominant animals on land

Flying reptiles became extinct during this period

There were many reptiles in the sea with the fishes

Cretaceous Period – reptiles were still the dominant vertebrates

New plant life came about; leafy trees, shrubs, and small flowering plants

Another mass extinction occurred at the end of the Era – more than half of all plant and animal groups were wiped out, including all of the dinosaurs

Cenozoic Era – started about 65 million years ago to present - mammals evolved adaptations that allowed them to live in various environments – on land, in water, and even in the air – also termed the Age of the Mammals

Whales and dolphins evolved

Grasses evolved

Some mammals and birds became very large

Tertiary Period – the climate was warm and mild

Many of the animals we are familiar with became common

Quaternary Period – Earth’s climate was changing – Earth cooled and went through a series of ice ages About 20,000 years ago, Earth’s climate began to warm back up

The fossil record suggests the early human ancestors appeared about 4.5 million years ago – but they did not look entirely human

Homo sapiens may have appeared as early as 200,000 years ago in Africa

Section 17-4: Patterns of Evolution

Macroevolution refers to large-scale evolutionary patterns and processes that occur over long periods of time.

6 important topics in macroevolution are extinction, adaptive radiation, convergent evolution, coevolution, punctuated equilibrium, and change in developmental genes.

Extinction – it usually happens because species compete for resources and environments changeUnder the changing environment of a mass extinction, extinction isn’t related to ordinary natural selection

Mass extinctions usually resulted in a burst of evolution afterward that led to the production of many new species.

Adaptive Radiation – when a single species or small group of species has evolved, through natural selection and other processes, into diverse forms that live in different ways

On a larger scale, examples include dinosaurs, which resulted from adaptive radiation of reptiles

Ex. Darwin’s finches

Convergent evolution – the process by which unrelated organisms come to resemble one another

Different types of organisms start off with different “raw material” for natural selection, but share the same environmental demands like moving though air, moving through water, or eating similar foods

Natural selection could mold body structure to fit the environment; ie. arms and legs into wings or flippers

Ex. Many aquatic animals have streamlined bodies for swimming through water and have similar looking parts that do not share a common evolutionary history these are analogous structures

Coevolution – the process by which two species evolve in response to changes in each other over time

Punctuated equilibrium – some species have not evolved much through time, they are in a state of equilibrium

Sometimes something happens to upset the equilibrium and a change occurs

Many new species are produced by periods of rapid change when the equilibrium is upset

The rapid change can be caused by small populations becoming separated from the main part or even when a small population migrates to a new environment

Punctuated equilibrium describes the pattern of long, stable periods interrupted by brief periods of more rapid change

Developmental genes and body plans – changes in genes for growth can produce differentiation during embryological development and can produce transformations in body shape and size

Small changes in control genes can produce large changes in adult animals